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Glossary

Created By Operations Training Solutions (O-T-S)

June 25, 2015

Glossary 5/5/23

A-Index

A daily index of geomagnetic activity for a specific station or network of stations derived as the average of the 8, 3-hourly K indices. The A-Index varies from 0 to 400.

AP-Index

A 3-hour equivalent amplitude of magnetic activity based on KP-Index data from a planetary network of magnetic observatories. NOAA’s SWPC estimates the value of the AP index by measuring the geomagnetic field in near real time at magnetometer stations around the world. The real-time station indices are used to estimate the official KP & AP based on historical data.

Accelerating Power

In a generator, the difference between the mechanical input power & the electrical output power (assuming no losses). For a generator’s torque angle to change, the generator must have accelerating power.

Accumulated Inadvertent

The accumulation over time of inadvertent energy. The accumulated inadvertent account of a BA represents the amount of excess energy a BA has either supplied to the interconnection or absorbed from the interconnection. BAs monitor their accumulated inadvertent to ensure the accounts do not grow too large.

Active Power (MW)

The component of the complex power (MVA) that performs the work. The common unit of active power is the MW & the symbol is “P”. Active power is sometimes called real power.

Active Power Transmission Losses

The power consumed due to resistive characteristics of transmission elements. Calculated as I2R.

Actual Net Interchange (NIA)

Algebraic sum of actual MW transfers across all Tie Lines & includes pseudo-ties. BAs directly Operations Training Solutions 2015 1

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connected via asynchronous ties to another Interconnection may include or exclude MW transfers on those tie lines in their actual interchange, provided they are implemented in the same manner for net interchange schedule.

Actual Load

The actual MW drawn by a load from the power system. The actual load will be different from the rated or nominal load if the load’s voltage or frequency vary from nominal values.

Adequacy

The ability of the electric system to supply the aggregate electrical demand and energy requirements of the end-use customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements. (NERC Glossary of Terms)

Adequate Regulating Margin

The minimum on-line capacity that can be increased or decreased to allow the system to respond to all reasonable demand changes in order to be incompliance with the CPS.

Adjacent Balancing Authority

A Balancing Authority whose Balancing Authority Area is interconnected with another Balancing Authority Area either directly or via a multi-party agreement or transmission tariff. (NERC Glossary of Terms)

Adverse Reliability Impact

The impact of an event that results in Bulk Electric System instability or Cascading. (NERC Glossary of Terms)

After the Fact (ATF)

A time classification assigned to an RFI when the submittal time is greater than one hour after the start time of the RFI. (NERC Glossary of Terms)

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AGC Pulses

The AGC system will send signals to selected (regulating) generators to adjust their set-points. These signals are sent via telecommunication equipment. The signals are often called AGC pulses.

Aggregator

An entity responsible for planning, scheduling, accounting, billing, & settlement for energy deliveries from the aggregator’s choice of sellers &/or buyers. Aggregators seek to bring together customers or generators so they can buy or sell power in bulk, making a profit on the transaction.

Air-Gap

The air space between the rotor & stator of a motor or generator.

Alternating Current (AC)

A time varying electric current that changes periodically in magnitude & direction. In power systems, the changes follow the pattern of a sine wave having a frequency of 60 cycles per second in North America. AC is also used to refer to voltage, which follows a similar sine wave pattern.

Alternative Interpersonal Communication

Any Interpersonal Communication that is able to serve as a substitute for, and does not utilize the same infrastructure (medium) as, Interpersonal Communication used for day-today operation. (NERC Glossary of Terms)

Alternator

A rotating machine that develops alternating voltage & current outputs.

Aluminum Cable Steel Reinforced (ACSR)

A common type of transmission line conductor that comes in many different sizes & designs. The outer aluminum strands form the conductor & while the inner steel strands provide strength.


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Ambient Conditions

Common, prevailing, & uncontrolled atmospheric conditions at a particular location, either indoors or out. The term is often used to describe the temperature, humidity, & airflow or wind that equipment or systems are exposed to.

Amortisseur Winding

A machine winding consisting of a number of conducting bars attached between the magnetic poles of the machine’s rotor. Amortisseur windings may be used as starter windings or to help dampen power oscillations.

Ampere (A)

The unit of measurement of electrical current produced in a circuit by 1 volt acting through a resistance of 1 Ohm.

Amplitude

The value or magnitude that a waveform has at a specific point in time.

Analog Electronics

Electronic circuitry in which the magnitudes of quantities are used in a continuous manner to perform functions. For example, input current values may be amplified & used to perform work in an analog circuit. Analog is different than digital in the sense that analog circuits use voltage & current magnitudes continuously while digital circuits use voltage & current to determine the state (on or off) of elements.

Ancillary Service

Those services that are necessary to support the transmission of capacity and energy from resources to loads while maintaining reliable operation of the Transmission Service Provider's transmission system in accordance with good utility practice. (From FERC order 888-A.) (NERC Glossary of Terms)


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Angle Instability

The opposite of angle stability. When a power system loses angle stability it enters a period of angle instability. An angle unstable system has lost the magnetic bound that holds sections of an interconnected AC power system in synchronism with 1 another.

Angle Stability

An angle stable power system is 1 in which all elements of the power system are bound together via magnetic forces. For example, an angle stable generator’s internal magnetic field rotates in synchronism (in-step) with the magnetic field of the 3 power system to which it connects.

Anti-Aliasing Filter

An analog filter installed at a metering point to remove the high frequency components of the signal over the AGC sample period. (NERC Glossary of Terms)

Apparent Power

The product of the voltage & current phasors. Apparent power comprises both active & reactive power, usually expressed in kVA or MVA. Another term for apparent power is complex power.

Arc

The discharge of current through the air or in a gas.

Arcing Horns

An enhancement to a disconnect switch to increase the switch’s current interrupting capability.

Area Control Error (ACE)

Means the instantaneous difference between net actual and scheduled interchange, taking into account the effects of Frequency Bias including correction for meter error. (NERC Glossary of Terms)


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Area Interchange Methodology

The Area Interchange methodology is characterized by determination of incremental transfer capability via simulation, from which Total Transfer Capability (TTC) can be mathematically derived. Capacity Benefit Margin, Transmission Reliability Margin, and Existing Transmission Commitments are subtracted from the TTC, and Postbacks and counterflows are added, to derive Available Transfer Capability. Under the Area Interchange Methodology, TTC results are generally reported on an area to area basis. (NERC Glossary of Terms)

Armature

The component of an electrical machine in which the voltage is induced. In a synchronous generator the armature is usually the stator. The armature winding is wrapped about the armature.

Arranged Interchange

The state where a Request for Interchange (initial or revised) has been submitted for approval. (NERC Glossary of Terms)

Arrested Frequency

Point of maximum frequency excursion in the 1ST swing of the frequency excursion between time zero (point “A”) & time zero plus 20 seconds. Also called the frequency nadir.

Arresting Period Frequency Response

Combination of load damping & the initial primary control response acting together to limit the duration & magnitude of frequency change during the arresting period.

Asynchronous

To be out-of-step with a reference. An induction generator is an asynchronous generator as its rotor does not rotate in synchronism with the power system.


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Attaining Balancing Authority

A Balancing Authority bringing generation or load into its effective control boundaries through a Dynamic Transfer from the Native Balancing Authority. (NERC Glossary of Terms)

Aurora

Phenomenon associated with geomagnetic activity that is visible in the night sky. Aurorae occur within a band of latitudes known as the auroral oval, the location of which is dependent on geomagnetic activity. Aurorae are a result of collisions between atmospheric gases & charged particles (mostly electrons) guided by the geomagnetic field from the Earth’s magnetotail. Each gas (oxygen, nitrogen, etc.) emits a particular color depending on the energy of the particles & atmospheric composition. Auroral altitude range is 80 to 1,000 kM. Aurorae in the Northern Hemisphere are called Aurora Borealis while aurorae in the Southern Hemisphere are called Aurora Australis.

Aurora Oval

Elliptical band around each geo-magnetic pole ranging from about 75° magnetic latitude at local noon to about 67° at midnight under average conditions. These locations experience the maximum occurrence of aurorae. The aurora widens to both higher & lower latitudes during magnetic storms.

Auto-Load

A feature on some generators that automatically moves the generator to a target load (MW) level.

Automatic Generation Control (AGC)

Equipment that automatically adjusts generation in a Balancing Authority Area from a central location to maintain the Balancing Authority’s interchange schedule plus Frequency Bias. AGC may also accommodate automatic inadvertent payback and time error correction. (NERC Glossary of Terms)

Automatic Operating Systems

SPS, RAS, or other operating systems installed on electric systems that require no intervention on the part of system operators.


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Automatic Reclosing

Device that automatically recloses a CB after it has been tripped by a protective relay. Automatic reclosing is applied on both distribution & transmission overhead lines. Since many faults clear automatically, automatic reclosing improves service continuity. Automatic reclosing applied in less than 1 second is called high speed automatic reclosing. NERC includes both the reclosing device and its associated control circuitry in its definition of automatic reclosing.

Automatic Time Error Correction

A frequency control automatic action that a BA uses to offset its frequency contribution to support the Interconnection’s scheduled frequency.

Auto Transformer

A power transformer with a single coil for each electrical phase, as opposed to a conventional transformer, which has 2 coils per phase. In an auto-transformer, the entire coil acts as the primary winding while a portion of the same coil acts as the secondary winding.

Auxiliary Relay

A relay whose function is to supplement the actions of other types of relays. For example, a lockout relay is an auxiliary relay with numerous contacts that each can perform an action. ANSI has numbers assigned to 2 types of auxiliary relays; 86 (lock-out) & 94 (tripping).

Availability

A measure of time a generating unit, transmission line, or other facility is capable of providing service, whether or not it actually is in service. Typically, this measure is expressed as a percent available for the period under consideration.

Available Resource

The sum of existing generating capacity, plus new units scheduled for service, plus the net of equivalent firm capacity purchases & sales, less existing capacity unavailable due to planned outages.


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Available Flowgate Capability (AFC)

A measure of the flow capability remaining on a Flowgate for further commercial activity over and above already committed uses. It is defined as TFC less Existing Transmission Commitments (ETC), less a Capacity Benefit Margin, less a Transmission Reliability Margin, plus Postbacks, and plus counterflows. (NERC Glossary of Terms)

Available Margin

The difference between available resources & net internal demand, expressed as a percent of available resources. This is the capacity available to cover random factors such as forced outages of generating equipment, demand forecast errors, weather extremes, & capacity service schedule slippages.

Available Transfer Capability (ATC)

A measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses. It is defined as Total Transfer Capability less Existing Transmission Commitments (including retail customer service), less a Capacity Benefit Margin, less a Transmission Reliability Margin, plus Postbacks, plus counterflows. (NERC Glossary of Terms)

Average Demand

The electric energy delivered over any interval of time as determined by dividing the total energy by the units of time in the interval.

Avoided Cost

The cost the utility would incur but for the existence of an independent generator or other energy service option. Avoided cost rates have been used as the power purchase price utilities offer independent suppliers.

Back EMF

When AC current flows through a conductor an alternating magnetic field is created. This alternating magnetic field induces a voltage in the conductor. The induced voltage is called the conductor’s back EMF.


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Back-to-Back HVDC

An HVDC system in which AC is converted to DC & then immediately back to AC. The HVDC transmission path is very short, normally consisting of a short section (a few feet) of buswork.

Backup Power

Power provided by contract to a customer when that customer’s normal source of power is not available.

Balancing Area ACE Limit (BAAL)

A generation control standard in which the allowable ACE bounds vary as a function of the Interconnection frequency error. The BAAL Standard is part of BAL-001-2 & replaces CPS2.

Balancing Authority (BA)

The responsible entity that integrates resource plans ahead of time, maintains load-interchange-generation balance within a Balancing Authority Area, and supports Interconnection frequency in real time. (NERC Glossary of Terms)

Balancing Authority Area

The collection of generation, transmission, and loads within the metered boundaries of the Balancing Authority. The Balancing Authority maintains load-resource balance within this area. (NERC Glossary of Terms)

Bandwidth

The acceptable range of a quantity. For example, if it is desired to hold the voltage at a 345 kV bus between 355 & 360 kV then the bandwidth is 355 - 360 kV or 5 kV.

Banking

Energy delivered or received by a utility with the intent that it will be returned in kind (as energy) in the future.


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Base Load

The minimum amount of electric power delivered or required over a given period at a constant rate. (NERC Glossary of Terms)

Base Load Capacity

Capacity used to serve an essentially constant level of customer demand. Base load generating units typically operate whenever they are available, & they generally have a capacity factor that is above 60%.

Base Load Generating Units

Electric generating units that produce energy at a constant rate, usually at a low cost relative to other generating units available to the system. Baseload units are used to meet some or all of a given region’s continuous energy demand on a seasonal or daily basis, including at minimum load levels, & tend to operate non-stop except for maintenance or forced outages.

BES Cyber Asset

A Cyber Asset that if rendered unavailable, degraded, or misused would, within 15 minutes of its required operation, misoperation, or non-operation, adversely impact one or more Facilities, systems, or equipment, which, if destroyed, degraded, or otherwise rendered unavailable when needed, would affect the reliable operation of the Bulk Electric System. Redundancy of affected Facilities, systems, and equipment shall not be considered when determining adverse impact. Each BES Cyber Asset is included in one or more BES Cyber Systems. (A Cyber Asset is not a BES Cyber Asset if, for 30 consecutive calendar days or less, it is directly connected to a network within an ESP, a Cyber Asset within an ESP, or to a BES Cyber Asset, and it is used for data transfer, vulnerability assessment, maintenance, or troubleshooting purposes.) (NERC Glossary of Terms)

BES Cyber System

One or more BES Cyber Assets logically grouped by a responsible entity to perform one or more reliability tasks for a functional entity. (NERC Glossary of Terms)

Bilateral Agreement

A written statement signed by 2 parties that specifies the terms for exchanging energy.


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Bilateral Payback

Inadvertent interchange accumulations paid back via an interchange schedule with another BA.

Billing Demand

The demand upon which customer billing is based as specified in a rate schedule or contract. Billing demand may be based on the contract year, a contract minimum, or a previous maximum &, therefore does not necessarily coincide with the actual measured demand of the billing period.

Bipolar HVDC

An HVDC system that uses 2 conductors (poles). 1 pole is energized with a positive voltage & the other a negative voltage. The voltage between the poles may be 1,000 kV or greater.

Blackstart Capability Plan (BCP)

A documented procedure for a generating unit or station to go from a shutdown condition to an operating condition delivering electric power without assistance from the electric system. This procedure is only a portion of an overall system restoration plan. (NERC Glossary of Terms)

Blackstart Resource (BSR)

A generating unit(s) and its associated set of equipment which has the ability to be started without support from the System or is designed to remain energized without connection to the remainder of the System, with the ability to energize a bus, meeting the Transmission Operator’s restoration plan needs for real and reactive power capability, frequency and voltage control, and that has been included in the Transmission Operator’s restoration plan. (NERC Glossary of Terms)

Blackstart Unit (BSU)

A generating unit that has the ability to go from a shutdown condition to an operating condition & start delivering power without assistance from the power system the generator normally connects to.

Blade

The component of a steam turbine that is acted upon by the flow of steam. Blades in steam turbines are


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also referred to as buckets. Similarly, in gas, or combustion turbines, the blades are the components acted upon by the flow of the high pressure, high temperature gases produced in the combustor. In both steam & combustion turbines, the blades are arranged in multiple stages of varying diameter, with many blades per stage. Modern wind turbines, in contrast, typically utilize only 3 long blades. The purpose of the blades is to extract energy from the motion of the propelling fluid (steam, combustion gases, or air) & convert it into rotational form by direct coupling to a common spinning shaft that is in turn used to drive a generator.

Block Dispatch

A set of dispatch rules such that given a specific amount of load to serve, an approximate generation dispatch can be determined. To accomplish this, the capacity of a given generator is segmented into loadable “blocks,” each of which is grouped and ordered relative to other blocks (based on characteristics including, but not limited to, efficiency, run of river or fuel supply considerations, and/or “must -run” status). (NERC Glossary of Terms)

Blocked Governor

A governor control system that has somehow been blocked or prevented from responding to a frequency deviation. There are many ways to block a governor.

Boiler

A device for generating steam for power, processing, or heating purposes; or hot water for heating purposes or hot water supply. Heat from an external combustion source is transmitted to a fluid contained within the tubes found in the boiler shell. This fluid is delivered to an end-use at a desired pressure, temperature, & quantity.

Boiler Follow

A mode of operation of a fossil unit’s turbine/boiler control system in which the unit’s turbine is allowed to immediately respond to a load (MW) change request while the boiler response “follows” with the resulting temperature & pressure swings.

Broker

A third party who establishes a transaction between a seller & a purchaser. A Broker does not take title to the capacity or energy.


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British Thermal Unit (BTU)

The measurement of heat released by burning any material. The amount of energy necessary to raise the temperature of 1 pound of water by 1 degree Fahrenheit from 58.5 to 59.5 degrees Fahrenheit under standard pressure of 30 inches of mercury at or near its point of maximum density.

Brush

A sliding contact, usually made of carbon, located between the excitation current source & the rotor field winding leads of a synchronous generator.

Bulk Electric System (BES)

Unless modified by the lists shown below, all Transmission Elements operated at 100 kV or higher and Real Power and Reactive Power resources connected at 100 kV or higher. This does not include facilities used in the local distribution of electric energy.

Inclusions:

I1 - Transformers with the primary terminal and at least one secondary terminal operated at 100 kV or higher unless excluded by application of Exclusion E1 or E3.

I2 - Generating resource(s) including the generator terminals through the high-side of the step-up transformer(s) connected at a voltage of 100 kV or above with:

a) Gross individual nameplate rating greater than 20 MVA. Or,b) Gross plant/facility aggregate nameplate rating greater than 75 MVA.

I3 - Blackstart Resources identified in the Transmission Operator’s restoration plan. I4 - Dispersed power producing resources that aggregate to a total capacity greater than 75 MVA

(gross nameplate rating), and that are connected through a system designed primarily for delivering such capacity to a common point of connection at a voltage of 100 kV or above.

Thus, the facilities designated as BES are:

a) The individual resources, andb) The system designed primarily for delivering capacity from the point where those

resources aggregate to greater than 75 MVA to a common point of connection at a voltage of 100 kV or above.

I5 - Static or dynamic devices (excluding generators) dedicated to supplying or absorbing Reactive Power that are connected at 100 kV or higher, or through a dedicated transformer with a high-side voltage of 100 kV or higher, or through a transformer that is designated in Inclusion I1 unless excluded by application of Exclusion E4.

Exclusions:

E1 - Radial systems: A group of contiguous transmission Elements that emanates from a single point of connection of 100 kV or higher and:


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a) Only serves Load. Or,b) Only includes generation resources, not identified in Inclusions I2, I3, or I4, with an

aggregate capacity less than or equal to 75 MVA (gross nameplate rating). Or,c) Where the radial system serves Load and includes generation resources, not identified in

Inclusions I2, I3 or I4, with an aggregate capacity of nonretail generation less than or equal to 75 MVA (gross nameplate rating).

Note 1 – A normally open switching device between radial systems, as depicted on prints or one-line diagrams for example, does not affect this exclusion.

Note 2 – The presence of a contiguous loop, operated at a voltage level of 50 kV or less, between configurations being considered as radial systems, does not affect this exclusion.

E2 - A generating unit or multiple generating units on the customer’s side of the retail meter that serve all or part of the retail Load with electric energy if: (i) the net capacity provided to the BES does not exceed 75 MVA, and (ii) standby, back-up, and maintenance power services are provided to the generating unit or multiple generating units or to the retail Load by a Balancing Authority, or provided pursuant to a binding obligation with a Generator Owner or Generator Operator, or under terms approved by the applicable regulatory authority.

E3 - Local networks (LN): A group of contiguous transmission Elements operated at less than 300 kV that distribute power to Load rather than transfer bulk power across the interconnected system. LN’s emanate from multiple points of connection at 100 kV or higher to improve the level of service to retail customers and not to accommodate bulk power transfer across the interconnected system. The LN is characterized by all of the following:

a) Limits on connected generation: The LN and its underlying Elements do not include generation resources identified in Inclusions I2, I3, or I4 and do not have an aggregate capacity of non-retail generation greater than 75 MVA (gross nameplate rating);

b) Real Power flows only into the LN and the LN does not transfer energy originating outside the LN for delivery through the LN; and

c) Not part of a Flowgate or transfer path: The LN does not contain any part of a permanent Flowgate in the Eastern Interconnection, a major transfer path within the Western Interconnection, or a comparable monitored Facility in the ERCOT or Quebec Interconnections, and is not a monitored Facility included in an Interconnection Reliability Operating Limit (IROL).

E4 - Reactive Power devices installed for the sole benefit of a retail customer(s).

Note - Elements may be included or excluded on a case-by-case basis through the Rules of Procedure exception process. (NERC Glossary of Terms)

Bulk Power System (BPS)

A) facilities and control systems necessary for operating an interconnected electric energy transmission network (or any portion thereof); and (B) electric energy from generation facilities needed to maintain transmission system reliability. The term does not include facilities used in the local distribution of electric energy. (NERC Glossary of Terms)


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Bundled Load

The combined or aggregate usage of customers who purchase electric service as a single service rather than multiple services, including transmission service, which may be purchased separately.

Burden

Operation of the Bulk Electric System that violates or is expected to violate a System Operating Limit or Interconnection Reliability Operating Limit in the Interconnection, or that violates any other NERC, Regional Reliability Organization, or local operating reliability standards or criteria. (NERC Glossary of Terms)

Bus

An abbreviation of the word busbar. A bus is a location in an electrical network where 1 or more elements are connected together. CBs & disconnect switches are connected to buses.

Bus Tie Breaker

A circuit breaker that is positioned to connect two individual substation bus configurations. (NERC Glossary of Terms)

Business Practices

Those business rules contained in the Transmission Service Provider’s applicable tariff, rules, or procedures; associated Regional Reliability Organization or regional entity business practices; or NAESB Business Practices. (NERC Glossary of Terms)

Capacitance (C)

The property of an electrical circuit that opposes voltage changes by storing energy in its electric field. The symbol for capacitance is “C” & the unit is the Farad. All energized equipment has a natural capacitance.

Capacitive Load

A load that supplies lagging reactive power to the system.


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Capacitive Reactance (XC)

The opposition that capacitance provides to AC current. The capacitive reactance (XC) in a 60 HZ circuit is:

Capacitively Coupled Voltage Transformer (CCVT)

An instrument transformer that is similar in function to a potential transformer (PT). A capacitive voltage divider circuit is used in a CCVT to reduce power system voltage magnitudes to usable (120 Volt) levels. Capacitive Voltage Transformers (CVTs), Coupling Capacitor Potential Devices (CCPDs), & Bushing Potential Devices (BPDs) are similar in design & function to a CCVT.

Capacitor

A device intentionally designed to act as a capacitor & store energy in its electric field. Shunt capacitors are very common & act as a source of reactive power to the system. Series capacitors are less common devices that are used to reduce the inductive reactance of a transmission path.

Capacitor Bank

A capacitor is an electrical device that provides reactive power to the system & is often used to compensate for reactive load & help support system voltage. A bank is a collection of 1 or more capacitors at a single location.

Capacity

The rated continuous load-carrying ability, expressed in MW or MVA of generation, transmission, or other electrical equipment.

Capacity Benefit Margin (CBM)

The amount of firm transmission transfer capability preserved by the transmission provider for Load-Serving Entities (LSEs), whose loads are located on that Transmission Service Provider’s system, to enable access by the LSEs to generation from interconnected systems to meet generation reliability requirements. Preservation of CBM for an LSE allows that entity to reduce its installed generating

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capacity below that which may otherwise have been necessary without interconnections to meet its generation reliability requirements. The transmission transfer capability preserved as CBM is intended to be used by the LSE only in times of emergency generation deficiencies. (NERC Glossary of Terms)

Capacity Emergency

A capacity emergency exists when a Balancing Authority Area’s operating capacity, plus firm purchases from other systems, to the extent available or limited by transfer capability, is inadequate to meet its demand plus its regulating requirements. (NERC Glossary of Terms)

Capacity Factor

The ratio of the total energy generated by a generating unit for a specified period to the maximum possible energy it could have generated if operated at the maximum capacity rating for the same specified period, expressed as a percent.

Capacity Margin

The difference between net capacity resources & net internal demand expressed as a percent of net capacity resources.

Capacity Market

A market where LSEs purchase generating capacity (including adequate reserves) to cover their peak loads.

Cascading

The uncontrolled successive loss of system elements triggered by an incident at any location. Cascading results in widespread electric service interruption that cannot be restrained from sequentially spreading beyond an area predetermined by studies. (NERC Glossary of Terms)

Cavitation

Noise or vibration causing damage to a hydro unit’s turbine blades as a result of bubbles that form in the water as it goes through the turbine which causes a loss in capacity, head loss, efficiency loss, & the cavity or bubble collapses when they pass into higher regions of pressure.


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Centrifugal Ballhead Governor

A mechanical governor that uses rotating flyweights to sense shaft speed. A very common governor due to its durability & accuracy.

Charge

An atomic force. An electron has a negative charge. A proton has a positive charge. Like charges repel 1 another while opposite charges attract 1 another. Current is the flow of charge.

Charging

The MVAr production of a transmission line. For example, a 100-mile long 345 kV line creates approximately 75 MVAr.

Charging Current

The leading current that flows into the natural capacitance of a transmission line when it is first energized.

Chromosphere

Layer of the Sun’s atmosphere above the photosphere & beneath the corona.

Circuit

A conductor or a system of conductors through which electric current flows.

Circuit Breaker (CB)

A piece of power system equipment that is used to disconnect other equipment from the power system. CBs are grouped according to their insulating medium. For example, Air CB (ACB), Gas CBs (GCB), Oil CBs (OCB), etc.


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Circuit Switcher (CS)

An enhanced disconnect switch that is similar in function to a CB. A CS includes both an interrupting device (typically SF6 gas based) to enhance its current interrupting ability & a disconnect switch for electrical isolation.

Clock Hour

The 60-minute period ending at :00. All surveys, measurements, and reports are based on Clock Hour periods unless specifically noted. (NERC Glossary of Terms)

Cogeneration

Production of electricity from steam, heat, or other forms of energy produced as a by-product of another process. (NERC Glossary of Terms)

Coincidental Demand

The sum of 2 or more demands that occur in the same time interval.

Cold Load Pick-Up

The increase in a load’s magnitude during the initial period after it is energized. The cold load pick-up consists of a short-term (possibly several seconds) in-rush component & a longer-term (possibly many minutes) loss of load diversity component.

Combined Cycle

An electric generating technology in which electricity & process steam are produced from otherwise lost waste heat exiting from 1 or more combustion turbines. The exiting heat is routed to a conventional boiler or to a heat recovery steam generator for use by a steam turbine in the production of electricity. This process increases the overall efficiency of the electric generating unit.

Combustion Turbine Generator (CT)

A type of generator in which a fuel (oil, gas, etc.) is ignited & the combustion products are used to drive a turbine.


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Comision Federal de Electricidad (CFE)

A Mexican governmental entity that generates, transmits, distributes & sells electricity to more than 34.2 million customers, representing more than 100 million people annually. CFE interconnects to ERCOT via 3 HVDC ties & to WECC via AC transmission lines at the California border just south of San Diego.

Comma Separated Values (CSV)

Refers to a file containing the values in a table as a series of ASCII text lines organized so that each column value is separated by a comma from the next column’s value & each row starts a new line. A CSV is sometimes referred to as a “flat file”.

Commission

Normally refers to the Federal Energy Regulatory Commission (FERC).

Commonly or Jointly Owned Units (COU/JOU)

These terms may be used interchangeably to refer to a unit in which 2 or more Generation Owners share.

Commutation

The process of turning off 1 valve & turning on another in an HVDC converter. In a 12-pulse converter there are 12 commutations per cycle of the AC supply voltage.

Complex Power (S)

The vector sum of the MW & MVAr power. The common unit for complex power is MVA & the symbol is “S”. Complex Power is sometimes called apparent power.

Compliance Monitor

The entity that monitors, reviews, and ensures compliance of responsible entities with reliability standards. (NERC Glossary of Terms)


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Complimentary Currents

When a subsynchronous current flows in the stator winding of a synchronous machine it will induce 2 complimentary currents in the rotor of the machine. These complimentary currents will have frequencies of 60 the frequency of the subsynchronous stator current.

Composite Confirmed Interchange

The energy profile (including non-default ramp) throughout a given time period, based on the aggregate of all Confirmed Interchange occurring in that time period. (NERC Glossary of Terms)

Composite Protection System

The total complement of protection systems that function collectively to protect an element. Backup protection provided by a different element’s protection systems is excluded. (NERC Glossary of Terms)

Compressor

A machine that increases the pressure of a gas (typically air) or vapor.

Condenser

In a steam turbine generating station, the condenser is a type of heat exchanger that cools the steam exiting the turbine to the point where it condenses into water, thereby recovering the high quality feed water for reuse. The cooling is accomplished using separate cooling water. Surface condensers use a shell & tube assembly wherein the cooling water is circulated in the tubes, & the steam & condensate are contained in the tank-like housing, or shell, that surrounds & encloses the tubes.

Conductor

A material with a low resistance to current flow. A conductor is the opposite of an insulator.

Confirmed Interchange

The state where no party has denied and all required parties have approved the Arranged Interchange. (NERC Glossary of Terms)


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Congestion

The condition that exists when market participants seek to dispatch generation in a pattern that would result in power flows that cannot be physically accommodated by the transmission system. Although the transmission system will not normally be operated in an overload condition, it may be described as congested based on requested/desired schedules.

Congestion Contract

A financial instrument that provides a hedge against congestion price differences between zones.

Congestion Management Report

A report that the Interchange Distribution Calculator issues when a Reliability Coordinator initiates the Transmission Loading Relief procedure. This report identifies the transactions and native and network load curtailments that must be initiated to achieve the loading relief requested by the initiating Reliability Coordinator. (NERC Glossary of Terms)

Congestion Pricing

A system for pricing transmission service based in part on the instantaneous demand for transmission system capacity.

Consequential Load Loss

All Load that is no longer served by the Transmission system as a result of Transmission Facilities being removed from service by a Protection System operation designed to isolate the fault. (NERC Glossary of Terms)

Constant Frequency Control (CFC)

An operating mode of an AGC system. While in CFC an AGC system will determine the ACE value by considering only the frequency error.


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Constant Net Interchange (CNI) Control

An operating mode of an AGC system. While in CNI control an AGC system will determine the ACE value by considering only the interchange error.

Constrained Facility

A transmission facility (line, transformer, breaker, etc.) that is approaching, is at, or is beyond its System Operating Limit or Interconnection Reliability Operating Limit. (NERC Glossary of Terms)

Constraint

A limitation placed on interchange transactions that flow over a constrained facility.

Contiguous

To belong to a common area. Adjoining. A contiguous BA is self-contained. A non-contiguous BA may have pieces spread over an entire Interconnection. Pseudo Ties are used to link the different areas of a non-contiguous BA.

Contingency

The unexpected failure or outage of a system component, such as a generator, transmission line, circuit breaker, switch or other electrical element. (NERC Glossary of Terms)

Contingency Analysis (CA)

The process by which the effects of contingencies are identified. CA software is used to identify N-1 violations.

Contingency Reserve

The provision of capacity deployed by the Balancing Authority to meet the Disturbance Control Standard (DCS) and other NERC and Regional Reliability Organization contingency requirements. (NERC Glossary of Terms)


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Continuous Rating

The rating as defined by the equipment owner that specifies the level of electrical loading, usually expressed in MW or other appropriate units that a system, facility, or element can support or withstand indefinitely without loss of equipment life.

Contract Path

An agreed upon electrical path for the continuous flow of electrical power between the parties of an Interchange Transaction. (NERC Glossary of Terms)

Contractually Adjacent Balancing Authorities

Two BAs that are part of the same RTO.

Control Center

One or more facilities hosting operating personnel that monitor and control the Bulk Electric System (BES) in realtime to perform the reliability tasks, including their associated data centers, of: 1) a Reliability Coordinator, 2) a Balancing Authority, 3) a Transmission Operator for transmission Facilities at two or more locations, or 4) a Generator Operator for generation Facilities at two or more locations. (NERC Glossary of Terms)

Control Performance Standard (CPS)

The reliability standard that sets the limits of a Balancing Authority’s Area Control Error over a specified time period. (NERC Glossary of Terms)

Control System

A collection of electrical & mechanical components designed to produce a series of outputs based on a series of measured inputs. Common control systems related to power are excitation control & governor control systems.

Cooling Tower

A structure & associated equipment intended to facilitate the evaporative cooling of water by contact with


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air. In steam turbine generating stations, cooling water is routed through the cooling tower for cooling after having absorbed heat in the condenser.

Coordinated Control

A mode of operation of a fossil unit’s turbine/boiler control system in which the operation of the boiler & the turbine systems are coordinated. The coordination balances the pressure & temperature limits of the boiler system with the desire for a turbine to immediately respond to load (MW) change requests.

Core

The material used within a transformer. A transformer’s core is formed of a magnetic material such as iron. The purpose of the core is to confine the magnetic field to a target area.

Corrective Action Plan

A list of actions and an associated timetable for implementation to remedy a specific problem. (NERC Glossary of Terms)

Corona

(1) Condition that occurs on energized equipment when the surface potential (voltage) is so large that the dielectric strength of the surrounding air breaks down (ionization occurs). Symptoms of corona are a visible ring of light & a hissing sound. Corona is undesirable as energy losses & electrical interference are a consequence. Corona rings are used to reduce the gradient of the electric field & reduce the likelihood of corona occurring.

(2) Outermost layer of the Sun’s atmosphere, characterized by low densities & high temperatures.

Coronal Mass Ejection (CME)

Sun’s corona is formed by strong magnetic fields. Occasionally the solar atmosphere can violently release tongues of gas & magnetic fields called coronal mass ejections (CME). A large CME can contain 10,000,000,000 tons of matter that can be accelerated to several million miles per hour in a spectacular explosion. Solar material streaks out through space, impacting any planet or spacecraft in its path. CMEs are often associated with solar flares but may occur independently. If the CME strikes the Earth’s magnetic field, it can result in a GMD. A CME carries with it the magnetic field orientation of the Sun, which is either a North or South orientation. If the CME magnetic field has a southward orientation, GMDs are expected.


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Cosine

Cosine of either of the unknown angles of a right triangle is the ratio of the side adjacent to the unknown angle to the hypotenuse.

Cranking Path

A portion of the electric system that can be isolated and then energized to deliver electric power from a generation source to enable the startup of one or more other generating units. (NERC Glossary of Terms)

Cranking Power

Power delivered to the next generator to restart after a black-start unit has been started.

Critical Assets

Facilities, systems, and equipment which, if destroyed, degraded, or otherwise rendered unavailable, would affect the reliability or operability of the Bulk Electric System. (NERC Glossary of Terms)

Critical Cyber Assets (CCA)

Cyber Assets essential to the reliable operation of Critical Assets. (NERC Glossary of Terms)

Current (I)

The rate of flow of electrons in an electrical conductor. Symbol is “I” & unit is the ampere, or amp.

Current Sourced Converter (CSC)

Type of power converter in which the local AC system voltages are used as the voltage source to create DC voltages. 1 CSV operating as the inverter and 1 CSV operating as the rectifier are used to create an HVDC system.


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Current Transformer (CT)

A low power transformer (an instrument transformer) used to reduce normally high power system current magnitudes to lower values ( 5 amps). A CT has a low number of turns on the primary winding & a high number of turns on the secondary winding. The low magnitude secondary currents of a CT are typically input to protective relays, meters, etc.

Curtailability

The right of a TSP to interrupt all or part of a transmission service due to constraints that reduce the capability of the transmission network to provide that transmission service. Transmission service is to be curtailed only in cases where system reliability is threatened or emergency conditions exist.

Curtailment

A reduction in the scheduled capacity or energy delivery of an Interchange Transaction. (NERC Glossary of Terms)

Customer

The term applies to a customer for transmission, capacity or energy services (a PSE, Generator Owner, LSE, or end-use customer).

Cut-Out

A switching device typically found in the distribution system. Cut-outs often include a fuse so the device serves the dual purpose of providing electrical isolation & protection.

Cyber Assets

Programmable electronic devices, including the hardware, software, and data in those devices. (NERC Glossary of Terms)

Cyber Security Incident

A malicious act or suspicious event that:


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Compromises, or was an attempt to compromise, the Electronic Security Perimeter or Physical Security Perimeter or,

Disrupts, or was an attempt to disrupt, the operation of a BES Cyber System.

(NERC Glossary of Terms)

Cycle

A complete sequence of a repeating waveform. For example, the magnitude of a sine wave changes as the degrees progress from 0 to 360. After 360 the sine wave begins to repeat itself. 1 cycle of a sine-wave is therefore 360.

Damping

Forces or control system actions that reduce the magnitude of oscillations. Damping is provided by various natural phenomena & by various electrical equipment. Damping can either be positive (reduce oscillations) or negative (amplify oscillations).

Deadband

The distance about a target value within which a control system will not respond. For example, a governor control system may have a deadband of 0.036 HZ. Unless the frequency deviation exceeds 0.036 HZ, the governor will not respond.

Delayed Fault Clearing

Fault clearing consistent with correct operation of a breaker failure protection system and its associated breakers, or of a backup protection system with an intentional time delay. (NERC Glossary of Terms)

Delta Connection

A 3 electrical connection in which the 3 phases are connected in series with 1 another. The 3 phases form a closed triangle. Transformer windings are often connected in a Delta configuration.

Demand

1. The rate at which electric energy is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time.


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2. The rate at which energy is being used by the customer.


Demand-Side Management (DSM)

All activities or programs undertaken by any applicable entity to achieve a reduction in Demand. (NERC Glossary of Terms)

Derating (Generator)

Reduction in a generating unit’s net dependable capacity.

Deregulation

Elimination of some or all government regulations from a previously regulated industry or sector of an industry.

Deviation

Variance from the scheduled value. For example, if the scheduled value of voltage is 142 kV & the actual value is 138 kV then the voltage deviation is -4 kV.

Device Numbers

IEEE has adopted a numbering system for labeling various types of protective equipment. Often used numbers include:

IEEE Number Device Function

11 Multi-function relay Used to denote a protective relay with multiple functions.

21 Distance relay Operates when the ratio of voltage divided by current falls below a value.

25 Synchronizing Relay Operates when the phase angle across a CB is outside allowable limits.

27 Undervoltage relay Operates when voltage falls below a value.


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IEEE Number Device Function

49 Thermal Relay Operates when the temperature exceeds a value.

50 Instantaneous Overcurrent

Operates with no time delay when current rises above a value.

51 Time Overcurrent Operates with a time-delay when the current rises above a value.

52 CB The device that clears the fault current.

59 Overvoltage relay Operates when voltage exceeds a value.

63 Pressure Relay Operates when the oil pressure exceeds a value.

67 Directional Overcurrent Operates if current is above a value & flowing in the proper direction.

78 Out-of-step Operates if the phase angle exceeds a preset value.

79 Reclosing relay Initiates an automatic closing of a CB following a relay operation.

81 Frequency Relay Operates if frequency goes above or below a value.

86 Lockout Relay Auxiliary relay that locks-out after operation.

87 Differential relay Operates when there is a difference in the current entering & leaving the protected zone.

94 Tripping Relay Auxiliary tripping relay.

Dial-Up Connectivity

A data communication link that is established when the communication equipment dials a phone number and negotiates a connection with the equipment on the other end of the link. (NERC Glossary of Terms)

Dielectric

Insulating material used to separate & insulate. Capacitors use a dielectric between its 2 conducting plates.


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Differential Relay (87)

A protective relay that responds to the difference between the currents entering & leaving the protected zone of the relay. Differential relays are typically applied to transformers, substation buses, & generators.

Digital Electronics

Electronic circuitry that operates on data in the form of digits. Most digital circuits use the binary system in which digital components are either turned on (a data value of “1”) or off (a data value of “0”) based on circuit voltage magnitudes. Digital electronics have replaced analog electronics in most, but not all, applications.

Direct Control Load Management (DCLM)

Demand-Side Management that is under the direct control of the system operator. DCLM may control the electric supply to individual appliances or equipment on customer premises. DCLM as defined here does not include Interruptible Demand. (NERC Glossary of Terms)

Direct Current (DC)

Electric current that is steady & does not change in either magnitude or direction with time. DC is also used to refer to voltage &, more generally, to smaller or special purpose power supply systems utilizing direct current either converted from AC, from a DC generator, from batteries, or from other sources such as solar cells.

Disconnect Switch

A mechanical device that is used to isolate equipment from energized parts of the power system. Disconnect switches are not rated for interrupting current unless additional arc interrupting components are added to the disconnect.

Dispatch Order

A set of dispatch rules such that given a specific amount of load to serve, an approximate generation dispatch can be determined. To accomplish this, each generator is ranked by priority. (NERC Glossary of Terms)


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Dispatchable Generation

Generation available physically or contractually to respond to changes in system demand or to respond to transmission security constraints.

Distance Relay (21)

A distance relay is similar to an impedance relay with a key difference. Impedance relays activate if the ratio of the measured voltage divided by the current falls below a pickup point. A distance relay also monitors the angle between the voltage & current. Distance relays are inherently directional & more selective than impedance relays.

Distribution

For electricity, the function of distributing electric power using low voltage lines to retail customers.

Distribution Facilities

The facilities owned or controlled by a TO & used to provide wholesale distribution service.

Distribution Factor (DF)

The portion of an Interchange Transaction, typically expressed in per unit that flows across a transmission facility (Flowgate). (NERC Glossary of Terms)

Distribution Lines

Conductors used to distribute power to the utilities customers. Distribution lines may be 3 or 1.

Distribution Network

The portion of an electric system that is dedicated to delivering electric energy to an end user. The distribution network consists primarily of low voltage lines & transformers that “transport” electricity from the bulk power system to retail customers.


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Distribution Provider (DP)

Provides and operates the “wires” between the transmission system and the end-use customer. For those end-use customers who are served at transmission voltages, the Transmission Owner also serves as the Distribution Provider. Thus, the Distribution Provider is not defined by a specific voltage, but rather as performing the Distribution function at any voltage. (NERC Glossary of Terms)

Disturbance

1. An unplanned event that produces an abnormal system condition. 2. Any perturbation to the electric system. 3. The unexpected change in ACE that is caused by the sudden failure of generation or interruption of load. (NERC Glossary of Terms)

Disturbance Control Standard (DCS)

The reliability standard that sets the time limit following a Disturbance within which a Balancing Authority must return its Area Control Error to within a specified range. (NERC Glossary of Terms)

Disturbance Monitoring Equipment (DME)

Devices capable of monitoring and recording system data pertaining to a Disturbance. Such devices include the following categories of recorders:

Sequence of event recorders which record equipment response to the event Fault recorders, which record actual waveform data replicating the system primary voltages and

currents. This may include protective relays. Dynamic Disturbance Recorders (DDRs), which record incidents that portray power system

behavior during dynamic events such as low-frequency (0.1 Hz – 3 Hz) oscillations and abnormal frequency or voltage excursions


Diversity Factor

The ratio of the sum of the coincident maximum demands of 2 or more loads to their non-coincident maximum demand for the same period.

Droop

A characteristic of a governor control system that requires a decrease in generator shaft speed to produce an increase in the generator’s MW output. There are 2 types of droop; permanent & transient:Operations Training Solutions 2015 34

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Permanent Droop

Permanent droop is the droop used to enable all generators (with active governors) to share in frequency regulation & to ensure a MW response in proportion to unit size. Desired values of permanent droop are in the neighborhood of 5%. The 5% droop means that a 5% frequency change will result in the unit’s governor moving the fuel (steam, water, gas, etc.) valves across their full range.

Transient Droop

A feature implemented in some governor control systems to ensure generators do not enter into power oscillations following load changes. Power oscillations may occur due to the inherent time delay between a request for a load change by the governor & the ability of a generator to actually deliver the load change. This natural time delay could lead to excessive control action by the governor. A condition of oscillation called “hunting” could result. If a generator is on isochronous control, transient droop is a necessity. The transient droop function or “compensation” dampens a governor’s initial response following a speed change. The effect is temporary as opposed to a permanent droop function, which is permanent.

Droop Curve

A graphical method of representing the performance of a governor. The horizontal axis is typically generator output while the vertical axis is system frequency. When a governor with a % droop is plotted on such a curve the plot droops from left to right with increasing generator output.

Dynamic Rating

The process that allows a system element rating to vary with the changing environmental conditions in which the element is located.

Dynamic Reactive Reserve

Reactive power held in reserve in fast responding sources. Generators & static var compensators are possible sources of dynamic reactive reserve.

Dynamic Schedule

A time-varying energy transfer that is updated in Real-time and included in the Scheduled Net Interchange (NIS) term in the same manner as an Interchange Schedule in the affected Balancing Authorities’ control ACE equations (or alternate control processes). (NERC Glossary of Terms)


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Dynamic Transfer

The provision of the real-time monitoring, telemetering, computer software, hardware, communications, engineering, energy accounting (including inadvertent interchange), and administration required to electronically move all or a portion of the real energy services associated with a generator or load out of one Balancing Authority Area into another. (NERC Glossary of Terms)

Dynamic Transfer Signal

The electronic signal used to implement a pseudo-tie or dynamic schedule using either a metered value or a calculated value.

Earth Surface Potentials (ESP)

Quasi-DC voltages induced in the Earth’s surface during GMD events. ESP magnitude varies by location on the Earth’s surface, which causes geomagnetically induced currents (GICs) to flow in the Earth’s surface.

Economic Dispatch

The allocation of demand to individual generating units on line to effect the most economical production of electricity. (NERC Glossary of Terms)

Economy Energy

Electrical energy produced & supplied from a more economical source in 1 system & substituted for that being produced or capable of being produced by a less economical source in another system.

Electric Field

The invisible lines of force that surround an energized piece of equipment. An electric field is created when a conductor is energized by a voltage. Energy is stored in an electric field.


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Electric System Losses

Total electric energy losses in the electric system. The losses consist of transmission, transformation, & distribution losses between supply sources & delivery points. Electric energy is lost primarily due to heating of transmission & distribution elements.

Electric Utility

Person, agency, authority, or other legal entity that owns or operates facilities for the generation, transmission, distribution, or sale of electric energy primarily for use by the public, & is defined as a utility under the statutes & rules by which it is regulated. An electric utility can be investor owned, cooperatively owned, or government owned (by a federal agency, crown corporation, state, provincial government, municipal government, & public power district).

Electrical Circuit

An electrical circuit consists of a voltage source, a conducting path to a load, a load, & a return path from the load to the voltage source. All 4 of these ingredients are necessary for current to flow in the electrical circuit.

Electric Reliability Organization (ERO)

The Energy Policy Act of 2005 required the creation of an independent ERO to be certified by FERC & tasked with developing & enforcing mandatory reliability standards applying to the bulk power system. NERC currently fills the role of ERO.

Electrical Distance

The impedance of a transmission line is a measure of the electrical distance. For example, if a line has a 100 impedance then 50 is ½ the line’s electrical length. Impedance relays are often called distance relays in reference to the use of impedance as a measure of electrical distance.

Electrical Energy

The generation or use of electric power by a device over a period of time, expressed in kilowatthours (kWh), megawatthours (MWh), or gigawatthours (GWh). (NERC Glossary of Terms)


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Electrodes

The connection to ground of an HVDC converter. The grounding electrode provides a voltage reference & may be part of the current return path.

Electro Hydraulic Control (EHC)

A form of a control system in which electrical devices are typically used to measure a quantity magnitude & hydraulics used to perform a control action. For example, a governor control system may consist of a simple electrical machine that measures the shaft’s speed of rotation (frequency) & a hydraulic system that changes the positions of fuel valves.

Electrojet

An auroral electrojet is a current that flows in the Earth’s ionosphere in the auroral zone. An equatorial electrojet is a current layer in the ionosphere above the equator.

Electromagnet

Temporary magnet created by passing electric current through a coil. The coil is typically wound about a magnetic core.

Electromagnetic Induction

The creation of a voltage in a conductor due to a relative movement between the conductor & a magnetic field. Electromagnetic induction is the basic principle of operation of transformers & generators.

Electromotive Force (EMF)

The voltage produced by a generator is called an electromotive force or EMF. The symbol “E” is often used to indicate an EMF.

Electronic Access Control or Monitoring Systems (EACMS)

Cyber Assets that perform electronic access control or electronic access monitoring of the Electronic Security Perimeter(s) or BES Cyber Systems. This includes Intermediate Systems. (NERC Glossary of Terms)


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Electronic Access Point (EAP)

A Cyber Asset interface on an Electronic Security Perimeter that allows routable communication between Cyber Assets outside an Electronic Security Perimeter and Cyber Assets inside an Electronic Security Perimeter. (NERC Glossary of Terms)

Electronic Security Perimeter (ESP)

The logical border surrounding a network to which BES Cyber Systems are connected using a routable protocol. (NERC Glossary of Terms)

Element

Any electrical device with terminals that may be connected to other electrical devices such as a generator, transformer, circuit breaker, bus section, or transmission line. An element may be comprised of one or more components. (NERC Glossary of Terms)

Emergency or BES Emergency

Any abnormal system condition that requires automatic or immediate manual action to prevent or limit the failure of transmission facilities or generation supply that could adversely affect the reliability of the Bulk Electric System. (NERC Glossary of Terms)

Emergency Assistance (EA)

Energy &/or capacity provided to a utility to assist that utility during a capacity emergency.

Emergency Energy

Electrical energy purchased by a member system whenever an event on that system causes insufficient operating capability to cover its own demand requirement.

Emergency Rating

The rating as defined by the equipment owner that specifies the level of electrical loading or output, usually expressed in megawatts (MW) or Mvar or other appropriate units, that a system, facility, or element can support, produce, or withstand for a finite period. The rating assumes acceptable loss of


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equipment life or other physical or safety limitations for the equipment involved. (NERC Glossary of Terms)

Emergency Response Rate

The rate of load change that a generating unit can achieve under emergency conditions, such as loss of a unit, expressed in MW per minute.

Emergency Voltage Limits

The operating voltage range on the interconnected systems that is acceptable for the time, sufficient for system adjustments to be made following a facility outage or system disturbance.

End-Use Customer

The party served by an LSE (energy) & DP (wire service).

Energy

The power used over a period of time. Electrical energy is measured in watt-hours, kWh, or MWh.

Energy Conversion

The conversion of energy from 1 form to another. For example, a coal fired steam turbine/generator converts the coal’s chemical energy to thermal energy by burning the coal. The thermal energy is then converted to mechanical energy by heating water & turning the turbine with steam. The mechanical energy of the turbine is then converted to electrical energy via electromagnetic induction.

Energy Emergency

A condition when a Load-Serving Entity or Balancing Authority has exhausted all other resource options and can no longer meet its expected load obligations. (NERC Glossary of Terms)

Energy Emergency Alert (EEA)

NERC Reliability Standard EOP-002 prescribes the use of an energy emergency alert (EEA) procedure when a BA is unable to meet its customers’ expected energy requirements. These energy emergencies


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are declared by the BA’s RC, & are categorized by level of severity, i.e., EEA1, 2, or 3, with level 3 being the most severe.

Energy Exchange

Transaction whereby the receiver accepts delivery of energy for a supplier’s account & returns energy later at times, rates, & in amounts as mutually agreed.

Energy Imbalance Service

Provides energy correction for any hourly mismatch between a transmission customer’s energy supply & the demand served.

Energy “In-Kind” Payback

Inadvertent interchange accumulated during “on-peak” hours shall only be paid back during “on-peak” hours. Inadvertent interchange accumulated during “off-peak” hours shall only be paid back during “off-peak” hours.

Energy Management System (EMS)

An EMS system is a computer system used by an energy company. The EMS includes subfunctions of SCADA & AGC.

Energy Only Market

A market for electric energy that pays resources only for delivered energy & ancillary services, & does not pay for installed capacity.

Envelope

The imaginary boundary that surrounds the fundamental frequency oscillations waveshape. The frequency of lower frequency oscillations can be determined by noting the frequency of the oscillation envelope.


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Epsilon 1 (ε1I)

Epsilon 1 is the 1-minute average of the root mean square (RMS) targeted frequency error for each Interconnection, as recommended by the NERC Resources Subcommittee & approved by the NERC Operating Committee. Epsilon 1 values for each Interconnection are unique. Epsilon 1 is used in the calculations for CPS1 & BAAL.

Epsilon 10 (ε10)

Epsilon 10 is the 10-minute average of the root mean square (RMS) targeted frequency error for each Interconnection, as recommended by the NERC Resources Subcommittee & approved by the NERC Operating Committee. Epsilon 10 values for each Interconnection are unique. Epsilon 10 is used in the calculation for the L10 bound.

Equal Area Criterion

A method of determining the angle stability or instability of a simple power system. The Equal Area Criterion states that the decelerating area of a power-angle curve must be at least as large as the accelerating area for the power system to be angle stable.

Equipment

An electrical device with terminals that may be connected to other electrical devices.

Equipment Rating

The maximum and minimum voltage, current, frequency, real and reactive power flows on individual equipment under steady state, short-circuit and transient conditions, as permitted or assigned by the equipment owner. (NERC Glossary of Terms)

ERCOT

Acronym for the Electrical Reliability Council of Texas. The ERCOT ISO operates the Texas Interconnection.

E-Tag

Electronic Tagging, or E-Tag, is used to schedule an interchange transaction in wholesale electricity markets. NERC &/or Regional Entities collect all E-Tag data in near real-time to assist RCs in identifying Operations Training Solutions 2015 42

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transactions to be curtailed to relieve overload when transmission constraints occur.

Excitation System

A generator control system used to control the production of reactive power. The excitation system’s main components are the voltage regulator & the exciter.

Exciter

The DC power source for an excitation system.

Exciting Current

The current drawn by a transformer to magnetize its core & supply the core losses. The exciting current is typically 1-2% of the transformer’s full load current.

Exempt Wholesale Generator (EWG)

Wholesale generators created under the 1992 Energy Policy Act that are exempt from certain financial & legal restrictions stipulated in the Public Utilities Holding Company Act of 1935.

Existing Transmission Commitments (ETC)

Committed uses of a Transmission Service Provider’s Transmission system considered when determining ATC or AFC. (NERC Glossary of Terms)

External Routable Connectivity

The ability to access a BES Cyber System from a Cyber Asset that is outside of its associated Electronic Security Perimeter via a bi-directional routable protocol connection. (NERC Glossary of Terms)

Extinction Advance Angle ()

In an HVDC converter operating as an inverter, the angle by which the valve firing is advanced from its normal voltage based commutation point. The Greek lower case letter gamma ( ) is the symbol for the extinction advance angle. The extinction advance angle is used to control the magnitude of the converter’s (when operated as a inverter) output voltage.


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Facilities Study

An engineering study conducted by the TSP to determine the required modifications to the TSP’s transmission system, including the cost & scheduled completion date for such modifications, that will be required to provide the requested transmission service.

Facility

A set of electrical equipment that operates as a single Bulk Electric System Element (e.g., a line, a generator, a shunt compensator, transformer, etc.) (NERC Glossary of Terms)

Facility Rating

The maximum or minimum voltage, current, frequency, or real or reactive power flow through a facility that does not violate the applicable equipment rating of any equipment comprising the facility. (NERC Glossary of Terms)

Farad (F)

The unit of capacitance. Symbol is “F”.

Fast Valving

A method of reducing the accelerating energy in a steam turbine/generator. Fast valving involves the rapid adjustment of turbine valves when a generator starts to accelerate. Fast valving may assist in maintaining the angle stability of a power system.

Fault

An event occurring on an electric system such as a short circuit, a broken wire, or an intermittent connection. (NERC Glossary of Terms)

Federal Energy Regulatory Commission (FERC)

A U.S. Federal agency that, among other responsibilities, regulates the transmission & wholesale sales of electricity in interstate commerce.


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Feedback Loop

A control loop in which current control actions are influenced by the responses to previous control actions. In a feedback loop (such as in a generator’s excitation system) the response of the controlled variable is constantly monitored to determine what new control actions should be taken.

Ferranti Rise

A phenomena in which a transmission line, with 1 end closed & 1 end open, is exposed to its highest voltage magnitude at the open-end of the line. The Ferranti Rise Effect is due to the absorption of a leading charging current when a transmission line is energized but open-ended.

Ferroresonance

A resonance condition in which a portion of the inductance is provided by an iron-core inductance (Ferro is Latin for iron.). Iron-core inductances change magnitude when the iron is saturated. Ferroresonance is possible in the transmission or distribution system but is much more common in the distribution system.

Field Winding

The winding wrapped about the rotor of a synchronous machine. DC excitation current is fed to the field winding to produce the rotor’s magnetic field.

Filament

Mass of gas suspended over the Sun’s chromosphere by magnetic fields & seen as dark ribbons threaded over the solar disk. A filament on the edge of the Sun seen in emission against the dark sky is called a prominence.

Filter

A combination of capacitors, inductors, & resistors used to encourage or block the flow of a specific frequency or band of frequencies of energy.


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Firm Capacity

Capacity that is as firm as the seller’s native load unless modified by contract. Associated energy may or may not be taken at option of purchaser. Supporting reserve is carried by the seller.

Firm Demand

That portion of the Demand that a power supplier is obligated to provide except when system reliability is threatened or during emergency conditions. (NERC Glossary of Terms)

Firm Transmission Service

The highest quality (priority) service offered to customers under a filed rate schedule that anticipates no planned interruption. (NERC Glossary of Terms)

Flare

A sudden eruption of energy from the sun’s surface lasting minutes to hours, from which radiation & particles are emitted.

Flashover

An electrical discharge through air around or over the surface of insulation, between objects of different potential, caused by placing a voltage across the air space that results in the ionization of the air space. (NERC Glossary of Terms)

Flat Frequency Control

An alternative name for constant frequency control.

Flat Tie-line Control

An alternative name for constant net interchange control.


Glossary 5/5/23

Flowgate

1.) A portion of the Transmission system through which the Interchange Distribution Calculator calculates the power flow from Interchange Transactions. 2.) A mathematical construct, comprised of one or more monitored transmission Facilities and optionally one or more contingency Facilities, used to analyze the impact of power flows upon the Bulk Electric System. (NERC Glossary of Terms)

Flowgate Methodology

The Flowgate methodology is characterized by identification of key Facilities as Flowgates. Total Flowgate Capabilities are determined based on Facility Ratings and voltage and stability limits. The impacts of Existing Transmission Commitments (ETCs) are determined by simulation. The impacts of ETC, Capacity Benefit Margin (CBM) and Transmission Reliability Margin (TRM) are subtracted from the Total Flowgate Capability, and Postbacks and counterflows are added, to determine the Available Flowgate Capability (AFC) value for that Flowgate. AFCs can be used to determine Available Transfer Capability (ATC). (NERC Glossary of Terms)

Flyweights

The portion of a centrifugal ballhead governor that rotates.

Force Majeure

A superior force, act of God or unexpected & disruptive event, which may serve to relieve a party from a contract or obligation.

Forced Outage

1. The removal from service availability of a generating unit, transmission line, or other facility for emergency reasons. 2. The condition in which the equipment is unavailable due to unanticipated failure. (NERC Glossary of Terms)

Forced Outage Rate

An indication of the percentage of time a generating unit, transmission line or other energy facility is forced out of service for emergency reasons. This percentage excludes the time during which the unit experiences a planned service shutdown (for example, scheduled maintenance). This number can be used to describe a facility’s reliability. A weighted average of forced outage rates for all generating


Glossary 5/5/23

facilities in a utility’s system may be used to describe system reliability. Forced Outage Hours + In-Service Hours x 100 = Forced Outage Rate.

Forecast

Predicted demand for power. A forecast may be short term (for example, 15 minutes) for system operation purposes, long-term (5 to 20 years) for generation planning purposes, or for any range in between. A forecast may include peak demand, energy, reactive power, or demand profile. A forecast may be made for total system demand, transmission loading, substation/feeder loading, individual customer demand, or appliance demand.

Fourier Analysis

A scientific process in which the various frequency components (harmonics) of a waveform are identified. For example, a waveform may have a fundamental frequency of 60 HZ but also contain 3rd & 5th harmonic components. Fourier analysis is used to identify the order of the harmonic components.

FRCC

Acronym for the Florida Reliability Coordinating Council. FRCC is 1 of the eight NERC Regional Entities.

Frequency (F)

The rate at which a repeating waveform repeats itself. Frequency is measured in cycles per second or in Hertz (HZ). The symbol if “F”.

Frequency Bias

A value, usually expressed in megawatts per 0.1 Hertz (MW/0.1 Hz), associated with a Balancing Authority Area that approximates the Balancing Authority Area’s response to Interconnection frequency error. (NERC Glossary of Terms)

Frequency Bias Setting (B)

A number, either fixed or variable, usually expressed in MW/0.1 Hz, included in a Balancing Authority’s Area Control Error equation to account for the Balancing Authority’s inverse Frequency Response contribution to the Interconnection, and discourage response withdrawal through secondary control systems. (NERC Glossary of Terms)


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Frequency Deviation

A change in Interconnection frequency. (NERC Glossary of Terms)

Frequency Error (F)

The difference between the actual and scheduled frequency. (FA – FS ) (NERC Glossary of Terms)

Frequency Regulation

The ability of a Balancing Authority to help the Interconnection maintain Scheduled Frequency. This assistance can include both turbine governor response and Automatic Generation Control. (NERC Glossary of Terms)

Frequency Response (Equipment)

The ability of a system or elements of the system to react or respond to a change in system frequency. (NERC Glossary of Terms)

Frequency Response (System)

The sum of the change in demand, plus the change in generation, divided by the change in frequency, expressed in megawatts per 0.1 Hertz (MW/0.1 Hz). (NERC Glossary of Terms)

Frequency Response Characteristic (FRC or β)

The MW response of the power system (or a section of the power system) to a frequency deviation. The FRC is typically stated in terms of MW per 0.1 HZ. For example, a BA may have an FRC of 200 MW/0.1 HZ. This value of FRC indicates that for a frequency deviation of 0.1 HZ this BA would respond with 200 MW. The FRC of a system varies with changing system conditions.

Frequency Response Measure (FRM)

The median of all the Frequency Response observations reported annually by Balancing Authorities or Frequency Response Sharing Groups for frequency events specified by the ERO. This will be calculated as MW/0.1Hz. (NERC Glossary of Terms)


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Frequency Response Obligation (FRO)

The Balancing Authority’s share of the required Frequency Response needed for the reliable operation of an Interconnection. This will be calculated as MW/0.1Hz. (NERC Glossary of Terms)

Frequency Response Sharing Group (FRSG)

A group whose members consist of two or more Balancing Authorities that collectively maintain, allocate, and supply operating resources required to jointly meet the sum of the Frequency Response Obligations of its members. (NERC Glossary of Terms)

Frequency Swings

Constant changes in frequency from its nominal or steady-state value.

Function

A set of related reliability tasks.

Functional Entity

The term used in the Functional Model which applies to a class of entity that carries out the tasks within a function.

Fundamental Frequency

The base frequency for a system. For example, the fundamental frequency of North American power systems is 60 HZ while a large portion of the world uses 50 HZ as the fundamental frequency.

Gallery

A passageway within a water dam created to allow inspection of the dam’s structure.


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Gate/Grid Control

The means of controlling a mercury arc valve (MAV) or a thyrister valve. A pulse of current or voltage is applied to the grid of a MAV or the gate of a thyrister. The pulse will turn the valve on if it is forward biased. Gate/grid control is typically only used to turn a valve on.

Generating Unit Capability

The MW production rating of a generator.

Generation

The process of producing electrical energy from other forms of energy; also, the amount of electric power produced, usually expressed in kW or MW.

Generation Control

The process by which the generation supply is adjusted to both maintain system frequency & keep a close match between the actual tie-line flows & the scheduled tie-line flows.

Generator

Generally, an electromechanical device used to convert mechanical power to electrical power.

Generator-to-Load Distribution Factor (GLDF)

The algebraic sum of a Generator Shift Factor and a Load Shift Factor to determine the total impact of an Interchange Transaction on an identified transmission facility or Flowgate. (NERC Glossary of Terms)

Generator Operator (GOP)

The entity that operates generating unit(s) and performs the functions of supplying energy and Interconnected Operations Services. (NERC Glossary of Terms)


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Generator Owner (GO)

Entity that owns and maintains generating units. (NERC Glossary of Terms)

Generator Runback

The intentional rapid reduction of the output level of an generating unit or an entire generating station, either manually or automatically via plant controls, due to any of a variety of problems in the plant that limit the plant’s capacity to generate power, or problems on the transmission system external to the plant which limit the capability of the system to accept the plant’s power output.

Generator Shift Factor (GSF)

A factor to be applied to a generator’s expected change in output to determine the amount of flow contribution that change in output will impose on an identified transmission facility or Flowgate. (NERC Glossary of Terms)

Geomagnetic Disturbance (GMD)

A worldwide disturbance of the Earth’s magnetic field. Also called solar magnetic disturbance (SMD) & geomagnetic storm. GMDs occur 1 to 4 days after a flare or other eruption (such as a CME) on the Sun.

Geomagnetic Induced Currents (GIC)

Quasi-DC currents induced in the Earth’s surface as a result of GMDs & the ESPs produced. GICs enter the power system via the grounded neutrals of equipment. High GIC levels can thermally damage transformers, lead to increases in Mvar usage, & create high harmonic levels that can further harm the power system. The extra high voltage (EHV) portion of the grid (≥ 345 kV) is especially vulnerable to GICs due to its low impedance & common usage of grounded autotransformers.

Geomagnetic Storm

See Geomagnetic Disturbance.


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Good Utility Practice

Any of the practices, methods & acts engaged in or approved by a significant portion of the electric utility industry during the relevant time period, or any of the practices, methods & acts which, in the exercise of reasonable judgment in light of the facts known at the time the decision was made, could have been expected to accomplish the desired result at a reasonable cost consistent with good business practices, reliability, safety & expedition. Good Utility Practice is not intended to be limited to the optimum practice, method, or act to the exclusion of all others, but rather to be acceptable practices, methods, or acts generally accepted in the region.

Governor

The electronic, digital or mechanical device that implements primary frequency response of generating units or other system elements.

Governor Characteristic Curve

A graphical method of representing the performance of a governor / generator combination. The horizontal axis is typically generator output while the vertical axis is system frequency. When a governor with a % droop is plotted on such a curve the plot droops from left to right with increasing generator output.

Greek Alphabet

Upper & Lower case letters from the Greek alphabet are typically used by electrical engineers to designate angles & represent variables. The following Greek letters are commonly used:

alpha delta ε epsilon mu

Phi beta theta pie

omega gamma lambda rho

Omega

Grid

An electrical transmission &/or distribution network.


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Gross Generation

The output power (in MW) at the stator terminals of a generator.

Ground

A conducting connection between an electrical circuit or device & the earth. A ground may be intentional, as in the case of a safety ground, or accidental, which may result in high overcurrents.

Half-Cycle Saturation

A magnetic saturation of a transformer’s core due to the presence of a DC current in the transformer windings. The operating point of the transformer on its saturation curve is shifted such that for a portion of ½ of each cycle the transformer saturates.

Harmonics

Integer multiples of the fundamental frequency. If the fundamental frequency is 60 HZ then the 2nd Harmonic has a frequency of 120 HZ, 3rd Harmonic 180 HZ, etc.

Head

Vertical change in elevation, expressed in either feet or meters, between the head water level & the tailwater level of a hydro-electric facility.

Heat Rate

An expression for the efficiency of a thermal power plant. The heat rate is the amount of heat (measured in British Thermal Units or BTU) that is required to produce a kWh or electrical output. The lower the heat rate, the more efficient the power plant.

Heat Tracing

The application of a heat source to pipes, lines, & other equipment which, in order to function properly, must be kept from freezing. Heat tracing typically takes the form of a heating element running parallel with & in direct contact with piping.


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Henry (H)

The unit of inductance. The symbol for a Henry is “H”.

Hertz (HZ)

Hertz is the unit of frequency equal to 1 cycle per second.

High Side Winding (HS)

The high voltage winding of the transformer.

High Voltage Direct Current (HVDC)

In electric power systems, the term HVDC (high voltage DC) is used to describe a transmission-level facility that interconnects between 2 portions of a power system, 2 different power systems, or 2 different Interconnections. The HVDC Tie consists of: (1) a converter station to convert 3 phase AC power to DC; (2) a DC connection to a second converter station; & (3) a second converter station that reconverts the DC power back to 3-phase AC. The DC connection between the 2 converter stations may be either a long HVDC transmission line or, in the case of “back-to-back” converters at the same location, a simple set of bus bars. The power flow in DC ties is not free-flowing as it is in AC lines, but rather is controlled precisely by control systems on the converters. Unlike AC lines, DC ties can interconnect between asynchronous interconnections such as ERCOT, the Eastern Interconnection, & the Western Interconnection because concerns about frequency, phase angle, & voltage differences are rendered immaterial by the AC-to-DC-to-AC conversion process.

Host Balancing Authority

1. A Balancing Authority that confirms and implements Interchange Transactions for a Purchasing Selling Entity that operates generation or serves customers directly within the Balancing Authority’s metered boundaries. 2. The Balancing Authority within whose metered boundaries a jointly owned unit is physically located. (NERC Glossary of Terms)

Hourly Value

Data measured on a Clock Hour basis. (NERC Glossary of Terms)


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HVDC

Acronym for High Voltage Direct Current. The term HVDC is commonly used when the DC voltage is above 100 kV.

HVDC Converter

An arrangement of equipment designed & operated to convert between AC & DC power. A converter can be operated as an inverter (DC to AC) or a rectifier (AC to DC).

HVDC Modulation

A feature added to the controls of an HVDC system. HVDC modulation modulates (adjusts) the power flow into an HVDC converter in order to dampen power oscillations in the AC supply system. HVDC modulation assists with damping AC system power oscillations.

Hydraulics

The use of fluid forces to perform work. For example, hydraulics are often used in governor control systems to develop the large forces required to move steam or water valves.

Hypotenuse

The side of a right triangle which is opposite the 90 angle.

IEEE

Acronym for the Institute of Electrical & Electronic Engineers. The IEEE is an international standards organization that publishes guidelines for, among other areas, power systems.

Igneous Rock

Rock that was created by volcanic activity.


Glossary 5/5/23

Ignition Delay Angle ()

In an HVDC converter operating as a rectifier, the angle by which the valve firing is delayed from its normal voltage based commutation point. The Greek lower case letter alpha () is the symbol for the ignition delay angle. The ignition delay angle is used to control the magnitude of the converter’s (when operated as a rectifier) output voltage.

Imbalance

A condition where the generation & interchange schedules do not match demand.

Impedance (Z)

The total opposition to the current flow in an electrical circuit. The symbol for the impedance is “Z”. The impedance includes the resistance (R), capacitance (C), & the inductance (L).

Impedance Relay

A protective relay that activates if the ratio of the measured voltage divided by the current (the impedance) falls below a pickup point. Impedance relays were first developed in the 1920’s. Impedance relays have largely been replaced by distance relays. Distance relays monitor voltage, current, & the angle between voltage & current.

Implemented Interchange

The state where the Balancing Authority enters the Confirmed Interchange into its Area Control Error equation. (NERC Glossary of Terms)

Impulse Turbine

A water turbine in which high velocity water is directed through nozzles at the turbine buckets. A Pelton Wheel is an example of an impulse turbine.

Inadvertent Energy

When inadvertent interchange exists for a period of time, inadvertent energy will be accumulated.


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Inadvertent Energy Balancing

A BA’s accounting of its inadvertent interchange, which is the accumulated difference between actual & scheduled interchange.

Inadvertent Energy Payback

When the inadvertent energy that a BA accumulates exceeds a specified value, the BA should arrange for an inadvertent payback.

Inadvertent Interchange

The difference between the Balancing Authority’s Net Actual Interchange and Net Scheduled Interchange. (IA - IS) (NERC Glossary of Terms)

Incremental Cost (λ)

The cost associated with producing an additional MWh of energy from a generating unit. Incremental cost is typically stated in $/MWh or Mills/kWh. The Greek letter Lambda (λ) is often used as the symbol for incremental cost.

Incremental Heat Rate

The amount of additional heat that must be added to a thermal generating unit at a given loading to produce an additional unit of output. It is usually expressed in British thermal units per kilowatt hour (BTU/kWh) of output.

Incremental Losses

The increase in losses due to an increase in power flow. For example, assume the power flow on a transmission line is initially 100 MW. If the power flow is increased to 101 MW there will be incremental energy losses associated with the 1 MW increase in power flow. The percentage of the incremental loss increases with increasing levels of power flow.


Glossary 5/5/23

Independent Power Producer (IPP)

Any entity that owns or operates an electricity generating facility that is not included in an electric utility’s rate base. This term includes, but is not limited to, cogenerators and small power producers and all other nonutility electricity producers, such as exempt wholesale generators, who sell electricity. (NERC Glossary of Terms)

Independent System Operator (ISO)

An organization responsible for the reliable operation of the power grid in a particular region & for providing open access transmission access to all market participants on a nondiscriminatory basis. ISOs in the U.S. include the California ISO, ISO New England, the New York ISO, PJM, the Midwest ISO, & ERCOT. These ISOs dispatch generation in their respective geographic territories.

Indirect Demand-Side Management

Programs such as conservation, improvements in efficiency of electrical energy use, rate incentives, rebates, & other similar activities to influence the customer’s demand.

Inductance (L)

The property of an electrical circuit that opposes a change in current flow. The symbol for inductance is the letter “L” & the unit is the Henry (symbol “H”).

Induction Machine

An AC machine that can be operated as a generator or as a motor. When operated as a generator the induction machine’s rotor is driven at a speed greater than synchronous speed. When operated as a motor the induction machine’s rotor is driven at a speed less than synchronous speed. Induction generators are rarely used by large scale power generators. Induction motors are the most common type of AC motor. Induction machines absorb reactive power (always a lagging load) & cannot be used to produce reactive power as a synchronous machine can.

Inductive Load

A load that absorbs lagging reactive power from the system.


Glossary 5/5/23

Inductive Reactance (XL)

The opposition that inductance provides to AC current. The inductive reactance (XL) in a 60 HZ circuit is:

Inertia

The property of an object that resists changes to the motion of the object. For example, the inertia of a rotating object resists changes to the object’s speed of rotation. The inertia of a rotating object is a function of its mass, diameter, & speed of rotation.

Inertial Energy

Energy stored within a rotating mass. For example, a spinning generator contains inertial energy. The terms inertial energy, stored energy, & rotational energy are often used interchangeably to refer to the energy stored in the rotating elements (motors & generators) of the power system.

In-Rush Current

The sudden rush of current when a transformer or a motor is first energized. The peak magnitudes of the in-rush current last only a few cycles but can reach levels more than 10 times the device’s full load current.

Installed Reserve

The difference between a utility’s expected annual peak MW generation capability & their annual peak MW load. Installed reserves are a rough approximation of a utility's spare or reserve generation.

Instantaneous Demand

The rate of energy delivered at a given instant.


fLX L 2

60

Glossary 5/5/23

Instrument Transformers (IT)

A low power transformer classification. Instrument transformers (IT) include current transformers (CTs), potential transformers (PTs) & capacitive devices.

Insulator

A material with high impedance to current flow. An insulator is the opposite of a conductor.

Integrated Demand

The average of the instantaneous demands (MW) over the demand interval (usually hours).

Integration

In the terms for dynamic schedule and pseudo-tie above means the value could be mathematically calculated or determined mechanically with a metering device.

Inter Area Mode

A power oscillation mode (frequency) in which a large section of an interconnected power system oscillates with respect to another large section of the same interconnection. The Inter-area mode ranges from 0.05 to 0.5 HZ.

Inter Control Center Communications Protocol (ICCP)

ICCP is a communications protocol used by utility organizations throughout the world to provide data exchange over wide area networks (WANs).

Interchange

Energy transfers that cross Balancing Authority boundaries. (NERC Glossary of Terms)


Glossary 5/5/23

Interchange Arrangement

The process of finding a seller & buyer for an interchange transaction, plus reserving transmission services.

Interchange Authority (IA)/Interchange Coordinator (IC)

The responsible entity that authorizes implementation of valid and balanced Interchange Schedules between Balancing Authority Areas, and ensures communication of Interchange information for reliability assessment purposes. (NERC Glossary of Terms)

Interchange Block Accounting

Energy accounting that assumes a beginning & ending ramp time of zero minutes. For accounting purposes, this moves the energy associated with the starting & ending ramps into the adjacent starting & ending clock time of the Interchange.

Interchange Confirmation

Agreement of the terms of the interchange schedule prior to its implementation.

Interchange Distribution Calculator (IDC)

The mechanism used by Reliability Coordinators in the Eastern Interconnection to calculate the distribution of Interchange Transactions over specific Flowgates. It includes a database of all Interchange Transactions and a matrix of the Distribution Factors for the Eastern Interconnection. (NERC Glossary of Terms)

Interchange Implementation

The physical initiation of the interchange schedule by entering it into the BA’s energy management system or by approving a schedule that has been electronically transferred into the energy management system.

Interchange Meter Error (IME)

The meter error correction factor, which represents the difference between the integrated hourly average


Glossary 5/5/23

of the actual net interchange (NIA) & the cumulative hourly net interchange energy measurement (in MWH).

Interchange Schedule

An agreed-upon Interchange Transaction size (megawatts), start and end time, beginning and ending ramp times and rate, and type required for delivery and receipt of power and energy between the Source and Sink Balancing Authorities involved in the transaction. (NERC Glossary of Terms)

Interchange Transaction

An agreement to transfer energy from a seller to a buyer that crosses one or more Balancing Authority Area boundaries. (NERC Glossary of Terms)

Interchange Transaction Cancellation

The complete withdrawal of an interchange transaction by a purchasing-selling entity prior to the start time of the transaction.

Interchange Transaction Curtailment

The complete or partial interruption of an interchange transaction that has started or “holding” of a new interchange transaction that has not yet started by a TSP, RC, or BA to maintain operating security.

Interchange Transaction Tag

The details of an Interchange Transaction required for its physical implementation. (NERC Glossary of Terms)

Interchange Transaction Termination

The complete interruption of an interchange transaction by a PSE after the start time of the transaction.

Interconnect

When 2 power systems tie together with AC transmission, the transmission line is referred to as an interconnect.


Glossary 5/5/23

Interconnected Operations Service (IOS)

A service (exclusive of basic energy and transmission services) that is required to support the reliable operation of interconnected Bulk Electric Systems. (NERC Glossary of Terms)

Interconnected System

A system consisting of 2 or more individual electric systems that normally operate in synchronism & have connecting tie lines.

Interconnection

When capitalized, any one of the four major electric system networks in North America: Eastern, Western, ERCOT and Quebec. (NERC Glossary of Terms)

Interconnection Reliability Operating Limit (IROL)

A System Operating Limit that, if violated, could lead to instability, uncontrolled separation, or Cascading outages that adversely impact the reliability of the Bulk Electric System. (NERC Glossary of Terms)

Interconnection Reliability Operating Limit TV

The maximum time that an IROL can be violated before the risk to the Interconnection or other RC areas becomes greater than acceptable. Each IROL’s TV shall be less than or equal to 30 minutes.

Interconnection Time Monitor

An entity that monitors time error & initiates & terminates time error corrections.

Interface

The specific set of transmission elements between 2 areas or between 2 areas comprising 1 or more electrical systems.


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Intermediate Balancing Authority

A Balancing Authority on the scheduling path of an Interchange Transaction other than the Source Balancing Authority and Sink Balancing Authority. (NERC Glossary of Terms)

Interpersonal Communication

Any medium that allows two or more individuals to interact, consult, or exchange information. (NERC Glossary of Terms)

Inter-Regional Security Network (ISN)

The telecommunications & data system used to share operating information, including security data, among the data recipients.

Interruptible Load or Interruptible Demand

Demand that the end-use customer makes available to its Load-Serving Entity via contract or agreement for curtailment. (NERC Glossary of Terms)

Interruptible Responsive Reserve

Interruptible load controlled by high-set underfrequency relaying.

Interruption

A reduction in non-firm transmission service due to economic reasons.

Intra-Area Mode

A power oscillation mode (frequency) in which a pocket of generation in a power system oscillates with respect to another pocket of generation in the same power system. The intra-area mode ranges from 0.4 to 1 HZ.


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Intra-Plant Mode

A power oscillation mode (frequency) in which several generators in a multi-unit power station oscillate with respect to 1 another. The intra-plant mode ranges from 1.5 to 3 HZ.

Inverter

An HVDC converter operated to convert DC power to AC power.

Ionization

Ionization occurs when an atom (or group of atoms) receives enough energy to split into 1 or more free electrons & a positive ion. Ionization is a special case of charging. The visible corona effect that often surrounds energized equipment is due to ionization of the air by the strong electric field surrounding the energized conductor.

Ionosphere

Region of the Earth’s upper atmosphere containing free electrons & ions produced by ionization of the atmosphere by solar radiation.

Island

An electrically isolated portion of an Interconnection. An islanded system maintains its own frequency. Islands are frequently formed after major disturbances or during restoration following a major disturbance.

Isochronous Governor Control

A governor operated with a 0% droop. When in isochronous control, a governor will try to maintain 60 HZ. Isochronous control may be used during a system restoration.

Isolated

To be electrically separated from the remainder of the Interconnection. An isolated system does not have transmission ties to the rest of the Interconnection. An isolated system is an electrical island.


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Joint Control

Automatic Generation Control of jointly owned units by two or more Balancing Authorities. (NERC Glossary of Terms)

Jointly Owned Unit (JOU)

This term refers to a unit in which 2 or more entities share ownership.

Journal Bearing

The bearing that supports the actual weight of a generator.

K-Index

Earth’s geomagnetic field can be measured with instruments called magnetometers. Data from many magnetometers allow observers to track the current state of geomagnetic conditions. Magnetometer data are often recorded in the form of 3-hourly indices (units of nano-Tesla) that provide a quantitative measure of the level of geomagnetic activity. One such index is called the K-Index. K-Index values range from 0 to 9 & are directly related to the amount of fluctuation (relative to a quiet day) in the geomagnetic field over a 3-hour interval. A K-Index is tied to a specific observatory location.

K-Index Alert

K-Index Alerts are an increased level from K-Index Warnings and are issued by SWPC for K-Index from 4 to 9 based on increasing likelihood of GMD effects using data from the ACE satellite which monitors the solar wind.

K-Index Warning

K-Index Warnings are issued by SWPC for any period with expected K-Index ≥ 4.

KP-Index

KP (K-Planetary) index is used to indicate the severity of a planetary magnetic disturbance. KP is an index based on the average of weighted K-Indices at multiple ground magnetic field observatories & was first implemented in 1932. KP is based on the range of the magnetic field variation within 3-hour intervals.


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KP values range from 0 (very quiet) to 9 (severely disturbed).

Kilo Var (KVAr)

Unit of reactive power equal to 1,000 Var.

Kilo Volt (kV)

Unit of electrical potential equal to 1,000 Volts.

Kilo Volt Amperes (kVA)

Unit of complex power equal to 1,000 volt amperes. Here, complex power is in contrast to active power. On AC systems the voltage & current will not be in phase if reactive power is being transmitted.

Kilo Watt Hour (kWh)

Unit of energy equaling 1 thousand watt hours, or 1 kilowatt used over 1 hour. This is the normal quantity used for metering & billing electricity customers. The retail price for a kWh varies from approximately 4 cents to 20 cents. At a 100% conversion efficiency, 1 kWh is equivalent to about 4 fluid ounces of gasoline, 3/16 pound of liquid petroleum, 3 cubic feet of natural gas, or ¼ pound of coal.

Kirchhoff’s Current Law

A basic electrical law that states that the sum of all the currents at any point in an electrical circuit equal zero amps.

Kirchhoff’s Voltage Law

A basic electrical law that states that the sum of all the voltages around any closed electrical circuit equal zero volts.

L Sub-Ten (L10)

The bandwidth that ACE is bounded in order to comply with CPS2. An ACE value (±) whose width is proportional to a BA’s size.


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Lagging

Term used when comparing voltage & current waves. The wave that is heading positive & crosses zero last is the lagging wave. In a lagging load the current wave lags the voltage wave.

Lambda ()

The incremental cost of generation. Lambda is commonly expressed in $/MWh or Mils/kWh. The symbol for lambda is “” (the Greek letter lambda).

Leading

Term used when comparing voltage & current waves. The wave that is heading positive & crosses zero first is the leading wave. In a leading load the current wave leads the voltage wave.

Leap Second

A Leap Second is a second of time added to coordinated universal time (UTC) to make it agree with astronomical time to within 0.9 seconds. Historically, Leap Seconds are implemented as needed on June 30th or December 31st.

Lightning Arrester

A piece of equipment that is designed to protect the power system from high voltages. Lightning arresters activate when transient over voltages (TOVs) occur & harmlessly shunt the voltage surge to ground.

Limiting Element

The element that is: 1.) Either operating at its appropriate rating, or 2.) Would be following the limiting contingency. Thus, the Limiting Element establishes a system limit. (NERC Glossary of Terms)

Line Trip

Refers to the automatic opening of a transmission line by its CBs. These openings or “trips” are controlled by protective relays & are designed to protect the power system during faulted conditions.


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Load

An end-use device or customer that receives power from the electric system. (NERC Glossary of Terms)

Load Cycle

The normal pattern of demand over a specified time period associated with a device or circuit.

Load Damping

The damping effect of the load to a change in frequency due to the physical aspects of the load such as the inertia of motors & the physical load to which they are connected.

Load Duration Curve

A non-chronological, graphical summary of demand levels with corresponding time durations using a curve, which plots demand magnitude (power) on 1 axis & percent of time that the magnitude occurs on the other axis.

Load Factor

A measure of the degree of uniformity of demand over a period of time, usually 1 year, equivalent to the ratio of average demand to peak demand expressed as a percentage. Load factor is calculated by dividing the total energy provided by a system during the period by the product of the peak demand during the period & the number of hours in the period.

Load Following

Commitment of energy based resources (generation or energy schedules) to match the forecast load level for a given period. This is a form of course control for moment-to-moment resource/load matching.

Load Overshoot

A short-term increase in load magnitude due to an increase in the customer’s voltage. Load overshoot results from downstream tap changers boosting the customer’s voltage prior to the upstream tap changer responding.


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Load Reference Set-Point

In governor control systems this setting determines the position of the controlled valve when the frequency is at the scheduled value. From a system operations perspective, the load reference set-point is the MW a generator will produce when the frequency is 60 HZ.

Load Rejection

The rejection of load by a generator. If a generator suddenly loses its transmission path, it has undergone a load rejection. The generator will speed up until its mechanical power input can be removed or the unit tripped.

Load/Frequency Relationship

The relationship between frequency deviations & the load magnitude. In general, the load magnitude varies with the frequency. If the frequency rises the load magnitude rises & vice versa.

Load Serving Entity (LSE)

Secures energy and transmission service (and related Interconnected Operations Services) to serve the electrical demand and energy requirements of its end-use customers. (NERC Glossary of Terms)

Load Shedding

The process of deliberately removing (either manually or automatically) pre-selected customer demand from a power system in response to an abnormal condition, to maintain the integrity of the system & minimize overall customer outages.

Load Shift Factor (LSF)

A factor to be applied to a load’s expected change in demand to determine the amount of flow contribution that change in demand will impose on an identified transmission facility or monitored Flowgate. (NERC Glossary of Terms)


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Local Mode

A power oscillation mode (frequency) in which a generator oscillates with respect to the remainder of the power system. The local mode ranges from 0.8 to 2 HZ.

Lockout

A state of a transmission line following CBs operations where the condition detected by the protective relaying was not eliminated by temporarily opening & reclosing the line. In this state, the CBs cannot generally be reclosed without resetting a lockout device.

Logistics

The handling of the details of an operation.

Long-Term Firm Point-To-Point Transmission Service

Firm point-to-point transmission service under Part II of the Tariff with a term of 1 year or more.

Long Term Transmission Planning Horizon

Transmission planning period that covers years six through ten or beyond when required to accommodate any known longer lead time projects that may take longer than ten years to complete. (NERC Glossary of Terms)

Loop Flow

See unscheduled power flow.

Loss of Load Diversity

An increase in the total load that occurs due to a majority of the customer’s load drawing power from the system at the same time. During normal system operations only a percentage of the customer’ total load is drawing power at any 1 time. When load diversity is lost a larger percentage of the customer load draws power simultaneously.


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Loss of Load Expectation (LOLE)

The expected number of days in the year when the daily peak demand exceeds the available generating capacity. The LOLE is obtained by calculating the probability of daily peak demand exceeding the available capacity for each day & adding these probabilities for all the days in the year. The index is referred to as Hourly Loss-of-Load-Expectation if hourly demands are used in the calculations instead of daily peak demands. LOLE also is commonly referred to as Loss-of-load-probability.

Loss of Synchronism

The loss of the magnetic bond between elements of a power system. Loss of synchronism & out-of-step refer to the same concept.

Losses

The energy losses in the power system. The total system losses consist of the transmission, transformation, & distribution system losses.

Low Side Winding (LS)

The low voltage winding of the transformer.

Magnetic Field

The invisible lines of force between the north & south poles of a magnet. A magnetic field is created when current flows through a conductor. Energy is stored in a magnetic field.

Magnetism

A property of matter associated with moving charges. A material may be a permanent magnet or it may acquire magnetic characteristics due to current flow through the material.

Magnetosphere

The magnetic cavity surrounding a magnetized body. The magnetosphere is carved out of the passing solar wind, which impedes the direct entry of solar wind plasma into the cavity. The Earth has a magnetosphere.


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Magnetotail

Extension of the Earth’s magnetosphere in the opposite direction of the Sun as a result of the magnetosphere’s interaction with the solar wind.

Margin

The difference between net capacity resources & net internal demand. Margin is usually expressed in MW.

Market Assessment

The evaluation & verification of the commercial details of arranged Interchange (required purchase, sale, & transmission service arrangements) during initial request for interchange & the evaluation & verification of the commercial details of a market adjustment to confirmed & implemented Interchange.

Market Clearing Price

The price at a location at which supply equals demand - all demand at or above this price has been satisfied, & all supply at or below this price has been purchased.

Market Operator

An entity that is responsible for the implementation of an organized market & submits market adjustments based on market outcomes. A Market Operator must be registered in the Electric Industry Registry (currently TSIN) in order to submit market adjustments.

Market Participant

An entity participating in the energy marketplace by buying/selling transmission rights, energy, or ancillary services into, out of, or through an ISO-controlled grid.

Marketer

An entity that has the authority to take title to electrical power generated by itself or another entity & re-market that power at market-based rates.


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Mechanical Energy

Energy of a mechanical nature. For example, a rotating mass possesses mechanical energy.

Mechanical Input Power

Power input of a mechanical nature. For example, a steam or water turbine input mechanical power to the rotor of a generator.

Mega Watt hour (MWh)

1 million watt hours.

Mercury Arc Valve (MAV)

A high power switch that utilizes older “tube” based technologies. MAVs were commonly used in HVDC converters & other power converter applications. MAVs have largely been replaced by thyristers.

Metered Value

A measured electrical quantity that may be collected by telemetering, SCADA, or other means.

Metering

A device for measuring a quantity. For example, meters are used to measure power flows, voltages, current, frequency, etc.

Microprocessor

An arithmetic, logic, & control unit all contained on 1 integrated circuit chip. 1 microprocessor contains thousands of transistors.

Mill

A unit of currency equal to 1/10 of a cent.


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Minimum Vegetation Clearance Distance (MVCD)

The calculated minimum distance stated in feet (meters) to prevent flash-over between conductors and vegetation, for various altitudes and operating voltages. (NERC Glossary of Terms)

Misoperation

The failure of a composite protection system to operate as intended for protection purposes. Any of the following is a misoperation:

1. Failure to trip - during fault - a failure of a composite protection system to operate for a fault condition for which it is designed. The failure of a protection system component is not a misoperation as long as the performance of the composite protection system is correct

2. Failure to trip - other than fault - a failure of a composite protection system to operate for a non-fault condition for which it is designed, such as a power swing, undervoltage, overexcitation, or loss of excitation. The failure of a protection system component is not a misoperation as long as the performance of the composite protection system is correct.

3. Slow trip - during fault - a composite protection system operation that is slower than required for a fault condition if the duration of its operating time resulted in the operation of at least 1 other element’s composite protection system.

4. Slow trip - other than fault - a composite protection system operation that is slower than required for a non-fault condition, such as a power swing, undervoltage, overexcitation, or loss of excitation, if the duration of its operating time resulted in the operation of at least 1 other element’s composite protection system.

5. Unnecessary trip - during fault - an unnecessary composite protection system operation for a fault condition on another element.

6. Unnecessary trip - other than fault - an unnecessary composite protection system operation for a non-fault condition. A composite protection system operation that is caused by personnel during on-site maintenance, testing, inspection, construction, or commissioning activities is not a misoperation. (NERC Glossary of Terms)

Mode

A specific oscillation frequency. For example, a steam/turbine generator’s shaft has specific frequencies at which it is susceptible to SSR. These frequencies are called modes.

Monopolar HVDC

An HVDC system that uses 1 conductor energized with either a positive or negative voltage & a current return path.


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Motor Load

A simplified grouping of all spinning type load. Motor load includes air conditioner compressors, motor drives, etc. Total load is composed of motor load plus non-motor load.

MRO

Acronym for the Midwest Reliability Organization. MRO is 1 of the eight NERC REs.

Must-Run Generation

Generation designated to operate at a specific level & not available for dispatch.

Native Balancing Authority

A Balancing Authority from which a portion of its physically interconnected generation and/or load is transferred from its effective control boundaries to the Attaining Balancing Authority through a Dynamic Transfer. (NERC Glossary of Terms)

Native Load (NL)

The end-use customers that the Load-Serving Entity is obligated to serve. (NERC Glossary of Terms)

Natural Frequency

Every mechanical device has a natural frequency of oscillation. For example, when a force is applied to a bridge the bridge will oscillate at its natural frequency. Electrical circuits also have a natural frequency of oscillation. An electrical circuit’s natural frequency is determined by its resistance, inductance, & capacitance.

Near Term Transmission Planning Horizon

The transmission planning period that covers Year One through five. (NERC Glossary of Terms)


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NERC

An acronym for the North American Electric Reliability Corporation. The purpose of NERC is to enhance the reliability of the interconnected power systems of North America. NERC manages the development of Reliability Standards that provide mandatory requirements to ensure reliable operation of the power system.

Net Actual Interchange (NIA)

The algebraic sum of all metered interchange over all interconnections between two physically Adjacent Balancing Authority Areas. (NERC Glossary of Terms)

Net Capacity

The maximum capacity (or effective rating), modified for ambient limitations, that a generating unit, power plant, or electric system can sustain over a specified period, less the capacity used to supply the demand of station service or needs.

Net Energy for Load

Net Balancing Authority Area generation, plus energy received from other Balancing Authority Areas, less energy delivered to Balancing Authority Areas through interchange. It includes Balancing Authority Area losses but excludes energy required for storage at energy storage facilities. (NERC Glossary of Terms)

Net Generation

The net power available from a generator to be fed to the power system. Net generation is equal to gross generation minus the generator’s internal power usage (station service).

Net Interchange Schedule (NIS)

The algebraic sum of all Interchange Schedules with each Adjacent Balancing Authority. (NERC Glossary of Terms)


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Net Internal Demand

Total of all end-use customer demand & electric system losses within specified metered boundaries, less direct control management & interruptible demand.

Net Schedule

The algebraic sum of all Interchange Schedules across a given path or between Balancing Authorities for a given period or instant in time. (NERC Glossary of Terms)

Network Customer

An entity receiving transmission service pursuant to the terms of the TSP’s Network Integration Transmission service.

Network Integration Transmission Service

Service that allows an electric transmission customer to integrate, plan, economically dispatch and regulate its network reserves in a manner comparable to that in which the Transmission Owner serves Native Load customers. (NERC Glossary of Terms)

Network Resource

Any designated generating resource owned, or purchased or leased by a network customer under the network integration transmission service tariff. Network resources do not include any resource, or any portion thereof, that is committed for sale to third parties or otherwise cannot be called upon to meet the network customer’s network load on a non-interruptible basis.

Nodal Market

Prices are assessed at points (i.e., nodes) where electricity enters or leaves the grid. Transmission lines throughout the grid may be subject to congestion rents, which means generators may receive different prices based on how they contribute to or relieve congestion on the grid. For example, ERCOT transitioned from a zonal to a nodal market on December 1, 2010. Their nodal market calculates transmission costs from the point of generation from roughly 4,000 delivery points. Nodal pricing is intended to provide a more detailed & accurate picture of transmission & generation than zonal pricing. ERCOT’s nodal system reduces the time interval for which the market-clearing price is calculated to five minutes (from fifteen minutes in their former zonal market).


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No Load Tap Changer (NLTC)

A tap changer that is designed to change the turns ratio only when the transformer has no current flow across its windings. The term offload tap changer (OLTC) is also used to refer to this type tap changer.

NOAA

The acronym for the National Oceanic & Atmospheric Administration of the U.S. Government. NOAA is the source of the K-Indices for geomagnetic activity.

Nominal

The design or rated value. Not necessarily the value that is intended or that occurs. For example, the nominal voltage for a piece of equipment would be the design or rated voltage but in operation the equipment may be operated at a different value of voltage.

Nominal Load

The rated or nameplate load. For example, 100 MW of customer load may be fed from a utility bus. This load will draw 100 MW if the voltage & frequency at the bus are at nominal values. If voltage or frequency should vary then the actual load will be different than the nominal load.

Non-Coincident Demand

The sum of 2 or more demands that occur in different demand intervals.

Non-Consequential Load Loss

Non-Interruptible Load loss that does not include: (1) Consequential Load Loss, (2) the response of voltage sensitive Load, or (3) Load that is disconnected from the System by end-user equipment. (NERC Glossary of Terms)

Non-Firm Energy

Electrical energy that may be interrupted by either the provider or the receiver of the energy by giving advance notice to the other party to the transaction. This advance notice period is equal to or greater


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than the minimum period agreed to in the contract. Non-firm energy may also be interrupted to maintain system reliability of third-party TSPs. Non-firm energy must be backed up by reserves.

Non-Firm Point-To-Point Transmission Service

Point-to-point Transmission service under the Tariff that is reserved & scheduled on an as-available basis & is subject to curtailment or interruption.

Non-Firm Transmission Service

Transmission service that is reserved on an as-available basis and is subject to curtailment or interruption. (NERC Glossary of Terms)

Non-Motor Load

A simplified grouping of all non-spinning type load. Non-motor load includes resistive heaters, lighting, etc. Total load is composed of motor load plus non-motor type load.

Non-Spinning Reserve

1. That generating reserve not connected to the system but capable of serving demand within a specified time. 2. Interruptible load that can be removed from the system in a specified time. (NERC Glossary of Terms)

Normal Clearing

A protection system operates as designed and the fault is cleared in the time normally expected with proper functioning of the installed protection systems. (NERC Glossary of Terms)

Normal Excitation

A condition in which the generator’s excitation system is supplying exactly the excitation current needed to maintain the magnetic field of the generator. A normally excited generator is neither supplying nor absorbing reactive power from the system.


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Normal (Pre-Contingency) Operating Procedures

Operating procedures that are normally invoked by the system operator to alleviate potential facility overloads or other potential system problems in anticipation of a contingency.

Normal Rating

The rating as defined by the equipment owner that specifies the level of electrical loading, usually expressed in megawatts (MW) or other appropriate units that a system, facility, or element can support or withstand through the daily demand cycles without loss of equipment life. (NERC Glossary of Terms)

Normal Voltage Limits

The operating voltage range on the interconnected systems that is acceptable on a sustained basis.

North American Energy Standards Board (NAESB)

The acronym for the North American Energy Standards Board. NAESB is responsible for business practice standards in the wholesale electric energy industry.

Notice of Proposed Rulemaking (NOPR)

In the case of a Federal agency, a written announcement published in the Federal Register which includes information regarding the time, place, & nature of a public rulemaking proceeding, the legislative authority under which the agency is proposing the rule, either the terms or substance of the proposed rule or a description of the subjects & issues involved & an invitation to interested persons to submit comments to the agency regarding the proposed rule. The publication of a Notice of proposed rulemaking in the Federal Register is usually the first step in a rulemaking proceeding eventually leading up to the agency adopting of a Final Rule.

NPCC

Acronym for the Northeast Power Coordinating Council. NPCC is 1 of the eight NERC REs.


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Nuclear Plant Interface Requirements (NPIR)

The requirements based on NPLRs and Bulk Electric System requirements that have been mutually agreed to by the Nuclear Plant Generator Operator and the applicable Transmission Entities. (NERC Glossary of Terms)

Off-Peak

Those hours or other periods defined by NAESB business practices, contract, agreements, or guides as periods of lower electrical demand. (NERC Glossary of Terms)

On-Peak

Those hours or other periods defined by NAESB business practices, contract, agreements, or guides as periods of higher electrical demand. (NERC Glossary of Terms)

Ohm’s Law

A basic electrical law that relates the voltage (V), current (I), & impedance (Z). Ohm’s law is commonly stated as:

V = I x Z

Open Access

FERC Order No. 888 requires public utilities to provide non-discriminatory transmission service over their transmission facilities to third parties to move bulk power from 1 point to another on a nondiscriminatory basis for a cost-based fee. Order 890 expanded Open Access to cover the methodology for calculating ATC; improvements that opened a coordinated transmission planning processes; standardization of energy & generation imbalance charges; & other reforms regarding the designation & un-designation of transmission network resources.

Open Access Same Time Information Service (OASIS)

An electronic posting system that the Transmission Service Provider maintains for transmission access data and that allows all transmission customers to view the data simultaneously. (NERC Glossary of Terms)


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Open Access Transmission Tariff (OATT)

Electronic transmission tariff accepted by the U.S. Federal Energy Regulatory Commission requiring the Transmission Service Provider to furnish to all shippers with non-discriminating service comparable to that provided by Transmission Owners to themselves. (NERC Glossary of Terms)

Operating Criteria

The fundamental principles of reliable interconnected systems operation.

Operating Instruction

A command by operating personnel responsible for the Real-time operation of the interconnected Bulk Electric System to change or preserve the state, status, output, or input of an Element of the Bulk Electric System or Facility of the Bulk Electric System. (A discussion of general information and of potential options or alternatives to resolve Bulk Electric System operating concerns is not a command and is not considered an Operating Instruction.) (NERC Glossary of Terms)

Operating Plan

A document that identifies a group of activities that may be used to achieve some goal. An Operating Plan may contain Operating Procedures and Operating Processes. A company-specific system restoration plan that includes an Operating Procedure for black-starting units, Operating Processes for communicating restoration progress with other entities, etc., is an example of an Operating Plan. (NERC Glossary of Terms)

Operating Procedure

A document that identifies specific steps or tasks that should be taken by one or more specific operating positions to achieve specific operating goal(s). The steps in an Operating Procedure should be followed in the order in which they are presented, and should be performed by the position(s) identified. A document that lists the specific steps for a system operator to take in removing a specific transmission line from service is an example of an Operating Procedure. (NERC Glossary of Terms)

Operating Process

A document that identifies general steps for achieving a generic operating goal. An Operating Process includes steps with options that may be selected depending upon Real-time conditions. A guideline for controlling high voltage is an example of an Operating Process (NERC Glossary of Terms)


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Operating Reserve

That capability above firm system demand required to provide for regulation, load forecasting error, equipment forced and scheduled outages and local area protection. It consists of spinning and non-spinning reserve. (NERC Glossary of Terms)

Operating Reserve - Spinning

The portion of Operating Reserve consisting of:

Generation synchronized to the system and fully available to serve load within the Disturbance Recovery Period following the contingency event; or

Load fully removable from the system within the Disturbance Recovery Period following the contingency event.

Operating Reserve - Supplemental

The portion of Operating Reserve consisting of:

Generation (synchronized or capable of being synchronized to the system) that is fully available to serve load within the Disturbance Recovery Period following the contingency event; or

Load fully removable from the system within the Disturbance Recovery Period following the contingency event.

Operating Security

The ability of a power system to withstand or limit the adverse effects of any credible contingency to the system including overloads beyond emergency ratings, excessive or inadequate voltage, loss of stability or abnormal frequency deviations.

Operating Security Limit

The value of a system operating parameter (for example, total power transfer across an interface) that satisfies the most limiting of prescribed pre- & post- contingency operating criteria as determined by equipment loading capability & acceptable stability & voltage conditions.


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Operating Transmission Limit

The maximum value of the most critical system operating parameters which meets: (a) pre-contingency criteria as determined by equipment loading capability & acceptable voltage conditions, (b) transient performance criteria, or, (c) post-contingency loading & voltage criteria.

Operating Voltage

The voltage level by which an electrical system is designated and to which certain operating characteristics of the system are related; also, the effective (root-mean-square) potential difference between any two conductors or between a conductor and the ground. The actual voltage of the circuit may vary somewhat above or below this value. (NERC Glossary of Terms)

Operational Planning Analysis

Evaluation of projected system conditions to assess anticipated (pre-contingency) and potential (post-contingency) conditions for next-day operations. Evaluation shall reflect applicable inputs including, but not limited to, load forecasts; generation output levels; Interchange; known protection system and special protection system status or degradation; Transmission outages; generator outages; facility ratings; and identified phase angle and equipment limitations. (Operational planning analysis may be provided through internal systems or through third-party services.) (NERC Glossary of Terms)

Operations Support Personnel

Individuals who perform current day or next day outage coordination or assessments, or who determine SOLs, IROLs, or operating nomograms, in direct support of Real-time operations of the Bulk Electric System. (NERC Glossary of Terms)

Oscillations

Cyclic variations in voltage, current, frequency, or power flows. The voltage & current of the power system naturally oscillates at 60 HZ. The term oscillations is typically used to refer to low frequency (a few HZ) oscillations that may occur.

Oscillatory Stability / Instability

An angle stability classification. The oscillatory stability limit of a power system is the maximum amount of MW that can be transmitted across the system without excessive power oscillations leading to a loss of


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synchronism. No large disturbance need occur. The response of generator control systems (governor & exciter) are very important to oscillatory stability / instability.

Outage

The period during which a generating unit, transmission line, or other facility is out of service. Outages are typically categorized as forced, due to unanticipated problems that render a facility unable to perform its function &/or pose a risk to personnel or to the system, or scheduled / planned for the sake of maintenance, repairs, or upgrades.

Out-of-Step (OOS)

To lose synchronism. Out-of-step is best viewed in terms of rotating magnetic fields. When a generator is out-of-step with the power system it connects to, the generator’s rotating magnetic field is no longer in-step or in synchronism with the rotating magnetic field of the system.

Out-of-Step Protective Relay (78)

A protective relay that is designed to detect out-of-step conditions & initiate a user determined response. Out-of-step protective relays are often based on the same principles as impedance relays. An out-of-step relay differentiates between an out-of-step condition & a fault condition by measuring the speed at which the measured impedance changes.

Outage

The period during which a generating unit, transmission line, or other facility is out of service.

Outage Transfer Distribution Factor (OTDF)

In the post-contingency configuration of a system under study, the electric Power Transfer Distribution Factor (PTDF) with one or more system Facilities removed from service (outaged). (NERC Glossary of Terms)

Overcurrent Relay

A protective relay that activates in response to a high current magnitude. Overcurrent relays can be either timed or instantaneous & directional or non-directional.


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Overexcite

A generator is overexcited when the applied excitation is greater than that needed to support the generator’s magnetic field. The excess reactive power produced by the over-excitation condition flows to the system. An overexcited generator supplies reactive power to the system. The terms “lagging” & “boosting” are also used to refer to an overexcited generator. A transformer may also overexcite. Over-excitation of a transformer results from either applying to high a voltage magnitude or operating at to low of a frequency.

Overlap Angle ()

When commutating between 2 valves in an HVDC converter, a natural overlap period occurs in which both valves are simultaneously conducting. The length of the overlap period is measured in degrees & is called the overlap angle. The symbol for the overlap angle is the Greek lower case letter mu ().

Overlap Regulation Service

A method of providing regulation service in which the Balancing Authority providing the regulation service incorporates another Balancing Authority’s actual interchange, frequency response, and schedules into providing Balancing Authority’s AGC/ACE equation. (NERC Glossary of Terms)

Parallel Circuit

An electrical circuit in which all the positive terminals are connected to a common point. All the negative terminals are connected to a second point. The voltage drop is the same across each full element of the parallel circuit.

Parallel Path Flows

The difference between the scheduled & actual power flow, assuming zero inadvertent interchange, on a given transmission path.

Parallel Resonance

A resonance condition in a circuit with a parallel combination of inductance & a capacitance. At resonance a parallel circuit reaches its maximum impedance equal to a multiple of the circuit’s resistance value.


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Parking

Parking is a commercial term that defines energy or transmission that is not complete. The transaction sink, source, & path are identified when tagged – prior to transaction implementation. The priority of the transaction is also identified when tagged. The merchant buys a “link” of the final transmission path & “parks” the generation without identifying source or sink (but must show POR & POD). The merchant can also buy a “link” of energy & park it. The ATC for a parked transaction should be decremented – there is an impact on a flowgate, but it is not calculated until the source & sink are known. Parking applies to both firm & non-firm energy & transmission.

Participation Factors

A set of dispatch rules such that given a specific amount of load to serve, an approximate generation dispatch can be determined. To accomplish this, generators are assigned a percentage that they will contribute to serve load. (NERC Glossary of Terms)

Parties

The TSP & transmission customer receiving service under the Tariff.

Peak Demand

1. The highest hourly integrated Net Energy For Load within a Balancing Authority Area occurring within a given period (e.g., day, month, season, or year). 2. The highest instantaneous demand within the Balancing Authority Area. (NERC Glossary of Terms)

Peak Load

As defined by NERC, the highest hourly integrated net energy for load (generation plus imports minus exports) within a BA area occurring within a given period (e.g., day, month, season, or year), or the highest instantaneous demand within the BA area.

Peak Period

A period consisting of 2 or more calendar months but less than 7 calendar months, which includes the period during which the responsible entity’s annual peak demand is expected to occur.


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Peaking Capacity

Capacity used to serve peak demand. Peaking generating units operate a limited number of hours per year, & their capacity factor is normally less than 20%.

Peaking Generating Units

Peaking plants operate primarily during times when load or demand increases rapidly to a maximum level & remains there for only a short time, e.g., on hot summer afternoons when air conditioning causes electricity usage to reach its highest level in the daily cycle. Peaking plants are often powered by natural gas, but they can also be powered by water at hydroelectric dams or by fuel oil. These plants can be brought online & taken offline quickly, in response to changing demand.

Penstock

A water pipe or conduit that carries water from the upper water reservoir to the turbine at a hydroelectric unit.

Performance-Reset Period

The time period that the entity being assessed must operate without any violations to reset the level of non-compliance to zero. (NERC Glossary of Terms)

Period

The time for 1 complete cycle of a repeating wave. For example, a 60 HZ current steps through 60 cycles per second. The period of 1 complete cycle is therefore 1/60th of a second.

Permanent Magnet Generator (PMG)

A simple generator that uses a permanent magnet (a material that retains its magnetism) to provide the required magnetic field. PMGs are often part of governor control systems. The PMG is used to produce an output voltage whose magnitude is representative of a generator’s shaft speed.


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Per-Unit (P.U.)

A system for reporting quantity magnitudes. A per-unit number is stated in terms of a base quantity. For example if the base voltage is 345 kV then a voltage of 359 kV is 1.04 per-unit (359/345=1.04).

Petersen Coil

A grounding reactor, used in otherwise ungrounded AC transmission systems, designed to limit the current flowing to ground at the location of a fault almost to zero by setting up a reactive current to ground that balances the capacitive current to ground flowing from the lines. Also known as an arc-suppression coil.

Phase

AC power systems use 3 conductors to efficiently generate & transmit large amounts of power. Each of the conductors is called a phase. The phases are each assigned a letter designation; “A”, “B”, & “C”. Customer load can be connected as single-phase (1), 2-phase (2), or 3-phase (3).

Phase Angle

The angle by which 1 waveform leads or lags another waveform. A phase angle can exist between 2 voltages, 2 currents, or between a current & a voltage.

Phase Shifting Transformer (PST)

A transformer designed to shift the phase of the incoming voltages. PSTs use a tap changing winding in the similar manner to a conventional tap changer. The voltage of a PSTs tap changer is added in quadrature to the incoming voltage to create a phase shift. PSTs are also referred to as phase angle regulators or PARs.

Phasors

Similar to a vector but also includes information about the frequency of the quantity. A phasor diagram is a collection of lines that, like vectors, illustrate a quantities magnitude & direction. However, phasor diagrams must also specify the frequency of the quantity. AC voltages & currents can be represented by phasors.


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Photosphere

The lowest visible layer of the Sun’s atmosphere. Sunspots are observed in the photosphere.

Physical Access Control Systems (PACS)

Cyber Assets that control, alert, or log access to the Physical Security Perimeter(s), exclusive of locally mounted hardware or devices at the Physical Security Perimeter such as motion sensors, electronic lock control mechanisms, and badge readers. (NERC Glossary of Terms)

Physical Security Perimeter

The physical border surrounding locations in which BES Cyber Assets, BES Cyber Systems, or Electronic Access Control or Monitoring Systems reside, and for which access is controlled. (NERC Glossary of Terms)

Physically Adjacent Balancing Authorities

2 BAs that are directly interconnected with each other.

Pick-Up Point

An operating setting for a protective relay that determines at what point the relay will activate.

Pilot Relay

A protective relay system typically used to protect high voltage transmission lines. Pilot relays use telecommunication systems to communicate between the terminals of the transmission line.

Planned Derating

The removal of a component for repairs that is scheduled well in advance & has a predetermined duration.


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Planned Outage

An outage that is planned well in advance.

Planning

The process by which the performance of the electric system is evaluated & future changes & additions to the bulk electric systems are determined.

Planning Assessment

Documented evaluation of future Transmission System performance and Corrective Action Plans to remedy identified deficiencies. (NERC Glossary of Terms)

Planning Authority (PA)/Planning Coordinator (PC)

The responsible entity that coordinates and integrates transmission facility and service plans, resource plans, and protection systems. (NERC Glossary of Terms)

Planning Reserve

The difference between an entity’s expected annual peak capability & its expected annual peak demand expressed as a percentage of the annual peak demand.

Point of Delivery (POD)

A location that the Transmission Service Provider specifies on its transmission system where an Interchange Transaction leaves or a Load-Serving Entity receives its energy. (NERC Glossary of Terms)

Point of Receipt (POR)

A location that the Transmission Service Provider specifies on its transmission system where an Interchange Transaction enters or a Generator delivers its output. (NERC Glossary of Terms)


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Point-To-Point (PTP) Transmission Service

The reservation and transmission of capacity and energy on either a firm or non-firm basis from the Point(s) of Receipt to the Point(s) of Delivery. (NERC Glossary of Terms)

Poles

(1) The electrical circuits formed by the field winding on the rotor of a synchronous generator. When DC current is passed through the field winding the poles become magnetic north or south poles. (2) The opposite ends of a magnet where the field is most concentrated, designated as the north & south poles. In a synchronous generator, the magnetic poles are established by DC current passing through the field winding on the rotor which is essentially the coil of an electromagnet. (3) In AC electrical equipment, particularly in switches & circuit breakers, poles refer to the contact assemblies associated with a particular phase. For example, it is common to refer to pole A, B, or C of a 3 phase disconnect switch.

Potential Difference

A difference in voltage magnitudes between 2 locations. Current can flow along a closed path if a potential difference exists across the path.

Potential Transformers (PT)

A low power transformer (an instrument transformer) used to reduce normally high power system voltages to low values ( 110 Volt). A PT has a high number of turns on the primary winding & a low number of turns on the secondary winding. The low magnitude secondary voltages of a PT are typically input to meters, relays, etc.

Power (P)

The rate at which energy is expended to do work. Power is measured in watts, kW, MW, or GW.

Power Angle (δ)

The phase angle between 2 voltage waveforms. A power angle is the same as a voltage phase angle difference. The power angle is a major factor in determining the amount of MW flow between 2 locations. The Greek letter delta (δ) is the symbol for power angle.


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Power Converter

A mechanical or solid-state device for converting AC power to DC power or vice versa. Modern power converters are thyrister based devices that are typically strong sources of harmonics.

Power Factor (PF)

The ratio of the MW to MVA. The cosine of the phase angle between a load’s voltage & current is the power factor of the load. A unity power factor load draws no MVar, just MW.

Power Flow Program

A computerized algorithm that simulates the steady-state behavior of the power system under a given set of conditions.

Power Pool

2 or more interconnected electric systems planned & operated to supply power for their combined demand requirements.

Power System

The collective name given to the elements of the electrical system. The power system includes the generation, transmission, distribution, substations, etc. The term power system may refer to 1 section of a large interconnected system or to the entire interconnected system.

Power System Stabilizer (PSS)

A feature added to an excitation system that is designed to assist with the damping of low frequency (1 HZ) power system oscillations. A typical PSS provides positive damping to power oscillations by ensuring that voltage corrections made by the excitation system are in-phase with detected frequency oscillations.

Power Transfer Distribution Factor (PTDF)

In the pre-contingency configuration of a system under study, a measure of the responsiveness or change in electrical loadings on transmission system Facilities due to a change in electric power transfer from one


Glossary 5/5/23

area to another, expressed in percent (up to 100%) of the change in power transfer. (NERC Glossary of Terms)

Power-Angle Curve

A graphical representation of the MW transfer equation. The power-angle curve is a plot of the active power transfer as the power angle is varied between 0 & 180. The power-angle curve is a good tool for analyzing the angle stability of a simple (2 bus) power system.

Power-Circle Diagram

A graphical method of illustrating how MW & MVAr flows change as the power angle changes. Power-circle diagrams are composed of circular characteristics of the power flow out of the sending end & into the receiving end of a 2 bus system.

Primary Frequency Control

The power delivered to the interconnection in response to a frequency deviation through generator governor response, load response (typically from motors), demand response (designed to arrest frequency excursions), & other devices that provide an immediate response to frequency based on local (device‐level) control systems, without human or remote intervention.

Primary Frequency Response

The immediate proportional increase or decrease in real power output provided by generating units/generating facilities and the natural real power dampening response provided by Load in response to system Frequency Deviations. This response is in the direction that stabilizes frequency. (NERC Glossary of Terms)

Primary Inadvertent Interchange

The component of area inadvertent interchange caused by the regulating deficiencies of the area.

Primary Winding

The winding of a transformer that is connected to the power input or source end of the transformer.


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Prime Mover

A mechanism that converts thermal or hydraulic energy into mechanical power. For example, a coal fired boiler with a steam turbine is a prime mover as it converts the thermal energy of coal into the mechanical power to turn the turbine.

Pro Forma Tariff

Usually refers to the standard OATT and/or associated transmission rights mandated by the U.S. Federal Energy Regulatory Commission Order No. 888. (NERC Glossary of Terms)

Production Cost

The costs associated with starting, operating, & stopping generating units.

Protected Cyber Assets (PCA)

One or more Cyber Assets connected using a routable protocol within or on an Electronic Security Perimeter that is not part of the highest impact BES Cyber System within the same Electronic Security Perimeter. The impact rating of Protected Cyber Assets is equal to the highest rated BES Cyber System in the same ESP. A Cyber Asset is not a Protected Cyber Asset if, for 30 consecutive calendar days or less, it is connected either to a Cyber Asset within the ESP or to the network within the ESP, and it is used for data transfer, vulnerability assessment, maintenance, or troubleshooting purposes. (NERC Glossary of Terms)

Protection System

Protective relays which respond to electrical quantities, Communications systems necessary for correct operation of protective functions Voltage and current sensing devices providing inputs to protective relays, Station dc supply associated with protective functions (including station batteries, battery

chargers, and non-battery based dc supply), and Control circuitry associated with protective functions through the trip coil(s) of the circuit breakers

or other interrupting devices.


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Protection System Maintenance Program (PSMP)

An ongoing program by which Protection System and automatic reclosing components are kept in working order and proper operation of malfunctioning components is restored. A maintenance program for a specific component includes one or more of the following activities:

Verify — Determine that the component is functioning correctly. Monitor — Observe the routine in-service operation of the component. Test — Apply signals to a component to observe functional performance or output behavior, or to

diagnose problems. Inspect — Examine for signs of component failure, reduced performance or degradation. Calibrate — Adjust the operating threshold or measurement accuracy of a measuring element to

meet the intended performance requirement.


Protective Relay

A mechanical or electronic device used to sense abnormal power system conditions & respond to isolate the abnormal equipment typically by opening CBs.

Provider of Last Resort

A legal obligation (traditionally given to utilities) to provide service to a customer where competitors have decided they do not want that customer’s business.

Pseudo-Tie

A time-varying energy transfer that is updated in Real-time and included in the Actual Net Interchange term (NIA) in the same manner as a Tie Line in the affected Balancing Authorities’ control ACE equations (or alternate control processes).

Pumped Storage Hydro Electric

A plant that usually generates electric energy during peak load periods by using water previously pumped into an elevated storage reservoir during off-peak periods when excess generating capacity is available to do so. When additional generating capacity is needed, the water can be released from the reservoir through a conduit to turbine generators located in a power plant at a lower level.


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Public Utilities Regulatory Policies Act (PURPA)

The Public Utility Regulatory Policies Act of 1978, passed by the U.S. Congress. This statute requires States to implement utility conservation programs & create special markets for co-generators & small producers who meet certain standards, including the requirement that States set the prices & quantities of power the utilities must buy from such facilities.

Purchasing-Selling Entity (PSE)

The entity that purchases or sells, and takes title to, energy, capacity, and Interconnected Operations Services. Purchasing-Selling Entities may be affiliated or unaffiliated merchants and may or may not own generating facilities. (NERC Glossary of Terms)

P-V Curve

A power versus voltage curve. A plot of the power transferred to a bus versus the voltage at that bus. P-V curves are a graphical tool used to analyze a power system’s voltage stability.

Pythagorean Theorem

A mathematical relationship which states that in a right triangle the square of the hypotenuse length is equal to the sum of the squares of the lengths of the remaining 2 sides.

Quadrature (Q)

At an angle of 90. When 2 vectors are in quadrature they are perpendicular to 1 another. The symbol “Q” for reactive power is derived from the word Quadrature.

Qualified Controllable Device (WECC)

A controllable device installed in the Interconnection for controlling energy flow; the WECC Operating Committee has approved using the device for controlling the USF on the qualified transfer paths.

Qualifying Facility (QF)

A cogeneration or small power production facility that meets certain ownership, operating, & efficiency criteria established by the FERC pursuant to the PURPA.


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Quality (Q)

A factor for measuring the frequency response of an electrical circuit. A circuit’s quality is dependent upon the relative magnitudes of its reactive & resistive elements.

Radiation Storm

Space radiation storm happens when an explosion on the Sun accelerates solar protons toward Earth. These protons stream past our planet where they are (mostly) deflected by Earth's magnetic field. NOAA’s SWPC has defined 5 types of radiation storms (S-1 to S-5), ranging from mild (S-1) to extreme (S-5). Radiation storms have minor impacts on power systems but can impact telecommunication systems.

Ramp Period

The time between generation ramp start & end times usually expressed in minutes.

Ramp Rate or Ramp

(Schedule) The rate, expressed in megawatts per minute, at which the interchange schedule is attained during the ramp period.

(Generator) The rate, expressed in megawatts per minute, that a generator changes its output. (NERC Glossary of Terms)

Ramp Window

The time period that occurs each hour for adjusting a BA’s generation. A typical ramp window is from 10 minutes to the hour till 10 minutes after the hour. If all BAs use the same ramp windows, frequency deviations will be reduced.

Rate Base

The value of property upon which a utility is permitted to earn a specified rate of return as established by a regulatory authority. The rate base generally represents the value of property used by the utility in providing service & may be calculated by any 1 or a combination of the following accounting methods: fair value, prudent investment, reproduction cost, or original cost. Depending on which method is used, the rate base includes cash, working capital, materials & supplies, & deductions for accumulated


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provisions for depreciation, contributions in aid of construction, customer advances for construction, accumulated deferred income taxes, & accumulated deferred investment tax credits.

Rate-of-Change (Protective Relay)

A type of protective relay that monitors the rate at which a quantity changes. For example, a rate-of-change relay may monitor the rate at which the MW flow along a transmission line varies. The relay could be set to trip the line if the rate of MW flow change exceeds a specified value.

Rated System Path Methodology

The Rated System Path Methodology is characterized by an initial Total Transfer Capability (TTC), determined via simulation. Capacity Benefit Margin, Transmission Reliability Margin, and Existing Transmission Commitments are subtracted from TTC, and Postbacks and counterflows are added as applicable, to derive Available Transfer Capability. Under the Rated System Path Methodology, TTC results are generally reported as specific transmission path capabilities. (NERC Glossary of Terms)

Rating

The operational limits of a transmission system element under a set of specified conditions. (NERC Glossary of Terms)

Reach

The extent of protection that an impedance relay provides to a transmission line. The reach is typically defined in terms of the impedance of the line. For example, a zone #1 impedance relay may reach 85% into the protected line.

Reaction Turbine

A water turbine in which the pressure difference across the turbine blades causes the blades to turn. A Francis Turbine is an example of a reaction turbine.

Reactive Capability Curve

A graphical method of illustrating the complex power output limits of a synchronous generators. The reactive capability curve is sometimes called a “D-Curve” as it is typically shaped like the letter “D”.


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Reactive power (Q)

The portion of electricity that establishes and sustains the electric and magnetic fields of alternating-current equipment. Reactive power must be supplied to most types of magnetic equipment, such as motors and transformers. It also must supply the reactive losses on transmission facilities. Reactive power is provided by generators, synchronous condensers, or electrostatic equipment such as capacitors and directly influences electric system voltage. It is usually expressed in kilovars (kvar) or megavars (Mvar). (NERC Glossary of Terms)

Reactor

A device intentionally designed to act as an inductor & store energy in its magnetic field. A shunt reactor acts as a sink (absorber) of reactive power from the system. Series reactors are devices that are used to increase the inductive reactance (XL) of a transmission path.

Real Power (P)

The portion of electricity that supplies energy to the load. (NERC Glossary of Terms)

Real Time Assessment

An evaluation of system conditions using real-time data to assess existing (pre-contingency) and potential (post-contingency) operating conditions. The assessment shall reflect applicable inputs including, but not limited to: load, generation output levels, known protection system and special protection system status or degradation, transmission outages, generator outages, Interchange, facility ratings, and identified phase angle and equipment limitations. (Real time assessment may be provided through internal systems or through third-party services.) (NERC Glossary of Terms)

Reallocation

The total or partial curtailment of Transactions during TLR Level 3a or 5a to allow Transactions using higher priority to be implemented. (NERC Glossary of Terms)

Recallable Available Transmission Capability (RATC)

TTC less the TRM, less recallable transmission service, less non-recallable transmission service (including the CBM). RATC must be considered differently in the planning & operating horizons. In the planning horizon, the only data available are recallable & non-recallable transmission service reservations, whereas in the operating horizon transmission schedules are known.Operations Training Solutions 2015 102

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Recallability

The right of a TSP to interrupt all or part of a transmission service for any reason, including economic, that is consistent with FERC policy & the TSP’s transmission service tariffs or contract provisions.

Receiving Balancing Authority

The Balancing Authority importing the Interchange. (NERC Glossary of Terms)

Reclosing Relay

A relay that automatically (after a few cycles or a few seconds) recloses a transmission line following a fault.

Rectifier

An HVDC converter operated to convert AC power to DC power.

Region

1 of the 8 NERC REs.

Regional Entity (RE)/Regional Reliability Organization

1. An entity that ensures that a defined area of the BES is reliable, adequate & secure. 2. A member of the North American Electric Reliability Council. The RE organization can serve as the compliance monitor (NERC Glossary of Terms)

Regional Transmission Group (RTG)

Voluntary organization of transmission owners, transmission users, & other entities interested in coordinating transmission planning & expansion & use on a regional & interregional basis.


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Regional Transmission Organization (RTO)

An organization that is independent from all generation & power marketing interests & has exclusive responsibility for electric transmission grid operations, short-term electric reliability, & transmission services within a multi-State region. To achieve those objectives, the RTO manages transmission facilities owned by different companies & encompassing 1, large, contiguous geographic area.

Regulating Reserve

An amount of reserve responsive to Automatic Generation Control, which is sufficient to provide normal regulating margin. (NERC Glossary of Terms)

Regulating Transformer

A transformer used to regulate voltage or phase angle. Conventional tap changing (ULTC) transformers & phase shifting transformers (PSTs) are regulating transformers.

Regulating Unit

A generator used for the regulation of system frequency. To serve as a regulating unit the generator must have available spinning reserve.

Regulation

The ability to maintain a quantity within acceptable limits. For example, frequency regulation is the control or regulation of the system frequency to within a tight bandwidth of 60 HZ. Voltage regulation is the control of a voltage level within a set bandwidth.

Regulation Reserve Sharing Group (RRSG)

A group whose members consist of two or more Balancing Authorities that collectively maintain, allocate, and supply the Regulating Reserve required for all member Balancing Authorities to use in meeting applicable regulating standards. (NERC Glossary of Terms)


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Regulation Service

The process whereby one Balancing Authority contracts to provide corrective response to all or a portion of the ACE of another Balancing Authority. The Balancing Authority providing the response assumes the obligation of meeting all applicable control criteria as specified by NERC for itself and the Balancing Authority for which it is providing the Regulation Service. (NERC Glossary of Terms)

Relative Acceleration

For torque & power angles to change, a relative acceleration must exist for a period of time. 1 part of the system must accelerate with respect to another part. Once relative acceleration occurs, any speed difference that has developed will continue the increase or decrease in torque or power angles. Torque & power angles will not stop changing until all sections of the system are running at the same frequency.

Relay

An electrical or mechanical device that responds to a measured input with a user determined output. Types of relays include auxiliary relays, monitoring relays, regulating relays, & protective relays.

Reliability

The degree of performance of the elements of the BES that results in electricity being delivered to customers within accepted standards & in the amount desired. Reliability may be measured by the frequency, duration, & magnitude of adverse effects on the electric supply.

Reliability Adjustment Arranged Interchange (RAAI)

Request to modify confirmed interchange or implemented interchange for reliability purposes. (NERC Glossary of Terms)

Reliability Assessment

The evaluation & verification of the reliability details of arranged interchange (path contiguity, ramping ability, & transmission system availability) during initial request for interchange & the evaluation & verification of the reliability details of a reliability adjustment arranged Interchange to confirmed & implemented interchange.


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Reliability Coordinator (RC)

The entity that is the highest level of authority who is responsible for the reliable operation of the Bulk Electric System, has the Wide Area view of the Bulk Electric System, and has the operating tools, processes and procedures, including the authority to prevent or mitigate emergency operating situations in both next-day analysis and real-time operations. The Reliability Coordinator has the purview that is broad enough to enable the calculation of Interconnection Reliability Operating Limits, which may be based on the operating parameters of transmission systems beyond any Transmission Operator’s vision. (NERC Glossary of Terms)

Reliability Coordinator Area

The collection of generation, transmission, and loads within the boundaries of the Reliability Coordinator. Its boundary coincides with one or more Balancing Authority Areas. (NERC Glossary of Terms)

Reliability Coordinator Information System (RCIS)

The system that Reliability Coordinators use to post messages and share operating information in real time. (NERC Glossary of Terms)

Reliability Criteria

Principles used to design, plan, operate, & assess the actual or projected reliability of an electric system.

Reliability Directive

A communication initiated by a Reliability Coordinator, Transmission Operator, or Balancing Authority where action by the recipient is necessary to address an Emergency or Adverse Reliability Impact. (NERC Glossary of Terms)

Reliability Must Run (RMR) Unit

A unit that must run for operational or reliability reasons, regardless of economic considerations. An RMR unit would not otherwise be operated unless it is necessary to provide voltage support, stability or management of localized transmission constraints.


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Reliability Standard

A requirement, approved by the United States Federal Energy Regulatory Commission under this Section 215 of the Federal Power Act, or approved or recognized by an applicable governmental authority in other jurisdictions, to provide for reliable operation [Reliable Operation] of the bulk-power system [Bulk-Power System]. The term includes requirements for the operation of existing bulk power system [Bulk-Power System] facilities, including cyber security protection, and the design of planned additions or modifications to such facilities to the extent necessary to provide for reliable operation [Reliable Operation] of the bulk-power system [Bulk-Power System], but the term does not include any requirement to enlarge such facilities or to construct new transmission capacity or generation capacity. (NERC Glossary of Terms)

Reliable Operation

Operating the elements of the bulk-power system [Bulk-Power System] within equipment and electric system thermal, voltage, and stability limits so that instability, uncontrolled separation, or cascading failures of such system will not occur as a result of a sudden disturbance, including a cyber security incident, or unanticipated failure of system elements. (NERC Glossary of Terms)

Remedial Action Scheme (RAS)

Scheme designed to detect predetermined system conditions & automatically take corrective actions that may include, but are not limited to, adjusting or tripping generation (MW & MVar), tripping load, or reconfiguring a system. RAS accomplish objectives such as:

Meet requirements identified in the NERC Reliability Standards Maintain BES stability Maintain acceptable BES voltages Maintain acceptable BES power flows Limit the impact of cascading or extreme vents

The following do not individually constitute a RAS:

a) Protection systems installed for purpose of detecting faults on BES Elements & isolating the faulted elements

b) Schemes for automatic UFLS & UVLS comprised of only distributed relaysc) OOS tripping & power swing blockingd) Automatic reclosing schemese) Schemes applied on an element for non-fault conditions, such as, but not limited to, generator

loss-of-field, transformer top-oil temperature, overvoltage, or overload to protect element against damage by removing it from service

f) Controllers that switch or regulate 1 or more of the following: series or shunt reactive devices, FACTS devices, PSTs, VFTs, or tap-changing transformers; &, that are located at & monitor quantities solely at the same station as the element being switched or regulated

g) FACTS controllers that remotely switch static shunt reactive devices located at other stations to regulate the output of a single FACTS device


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h) Schemes or controllers that remotely switch shunt reactors & shunt capacitors for voltage regulation that would otherwise be manually switched

i) Schemes that automatically deenergize a line for a non-fault operation when 1 end of the line is open

j) Schemes that provide anti-islanding protection (e.g., protect load from effects of being isolated with generation that may not be capable of maintaining acceptable frequency and voltage)

k) Automatic sequences that proceed when manually initiated solely by a System Operatorl) Modulation of HVDC or FACTS via supplementary controls, such as angle damping or

frequency damping applied to damp local or inter-area oscillationsm) SSR protection schemes that directly detect subsynchronous quantities (e.g., currents or

torsional oscillations)n) Generator controls such as, but not limited to, AGC, generation excitation [e.g. AVR & PSS],

fast valving, & speed governing

Remote Terminal Unit (RTU)

An electronic device, installed in a substation or generator, which acts as an interface between a company’s EMS system & the generator or substation.

Reportable Cyber Security Incident

A Cyber Security Incident that has compromised or disrupted one or more reliability tasks of a functional entity. (NERC Glossary of Terms)

Reportable Disturbance

Any event that causes an ACE change greater than or equal to 80% of a Balancing Authority’s or reserve sharing group’s most severe contingency. The definition of a reportable disturbance is specified by each Regional Reliability Organization. This definition may not be retroactively adjusted in response to observed performance. (NERC Glossary of Terms)

Reporting ACE

The scan rate values of a Balancing Authority’s Area Control Error (ACE) measured in MW, which includes the difference between the Balancing Authority’s Net Actual Interchange and its Net Scheduled Interchange, plus its Frequency Bias obligation, plus any known meter error. In the Western Interconnection, Reporting ACE includes Automatic Time Error Correction (ATEC).

Reporting ACE is calculated as follows:

Reporting ACE = (NIA − NIS) − 10B (FA − FS) − IME

Reporting ACE is calculated in the Western Interconnection as follows: Operations Training Solutions 2015 108

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Reporting ACE = (NIA − NIS) − 10B (FA − FS) − IME + IATEC

Where:

NIA (Actual Net Interchange) is the algebraic sum of actual megawatt transfers across all Tie Lines and includes Pseudo‐Ties. Balancing Authorities directly connected via asynchronous ties to another Interconnection may include or exclude megawatt transfers on those Tie lines in their actual interchange, provided they are implemented in the same manner for Net Interchange Schedule.

NIS (Scheduled Net Interchange) is the algebraic sum of all scheduled megawatt transfers, including Dynamic Schedules, with adjacent Balancing Authorities, and taking into account the effects of schedule ramps. Balancing Authorities directly connected via asynchronous ties to another Interconnection may include or exclude megawatt transfers on those Tie Lines in their scheduled Interchange, provided they are implemented in the same manner for Net Interchange Actual.

B (Frequency Bias Setting) is the Frequency Bias Setting (in negative MW/0.1 Hz) for the Balancing Authority.

10 is the constant factor that converts the frequency bias setting units to MW/Hz.

FA (Actual Frequency) is the measured frequency in Hz.

FS (Scheduled Frequency) is 60.0 Hz, except during a time correction.

IME (Interchange Meter Error) is the meter error correction factor and represents the difference between the integrated hourly average of the net interchange actual (NIA) and the cumulative hourly net Interchange energy measurement (in megawatt‐hours).

IATEC (Automatic Time Error Correction) is the addition of a component to the ACE equation for the Western Interconnection that modifies the control point for the purpose of continuously paying back Primary Inadvertent Interchange to correct accumulated time error. Automatic Time Error Correction is only applicable in the Western Interconnection.

IATEC shall be zero when operating in any other AGC mode.

Y = B / BS. H = Number of hours used to payback Primary Inadvertent Interchange energy. The value of H is

set to 3. BS = Frequency Bias for the Interconnection (MW / 0.1 Hz). Primary Inadvertent Interchange (PIIhourly) is (1-Y)

o (IIactual - B * ΔTE/6) IIactual is the hourly Inadvertent Interchange for the last hour. ΔTE is the hourly change in system Time Error as distributed by the Interconnection Time

Monitor.

Where:

ΔTE = TEend hour – TEbegin hour – TDadj – (t)*(TEoffset)Operations Training Solutions 2015 109

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TDadj is the Reliability Coordinator adjustment for differences with Interconnection Time Monitor control center clocks.

t is the number of minutes of Manual Time Error Correction that occurred during the hour. TEoffset is 0.000 or +0.020 or -0.020. PIIaccum is the Balancing Authority’s accumulated PIIhourly in MWh. An On-Peak and Off-Peak

accumulation accounting is required.

Where:

All NERC Interconnections with multiple Balancing Authorities operate using the principles of Tie-line Bias (TLB) Control and require the use of an ACE equation similar to the Reporting ACE defined above. Any modification(s) to this specified Reporting ACE equation that is (are) implemented for all BAs on an Interconnection and is (are) consistent with the following four principles will provide a valid alternative Reporting ACE equation consistent with the measures included in this standard.

1. All portions of the Interconnection are included in one area or another so that the sum of all area generation, loads and losses is the same as total system generation, load and losses.

2. The algebraic sum of all area Net Interchange Schedules and all Net Interchange actual values is equal to zero at all times.

3. The use of a common Scheduled Frequency FS for all areas at all times.

4. The absence of metering or computational errors. (The inclusion and use of the IME term to account for known metering or computational errors.) (NERC Glossary of Terms)

Request for Interchange

A collection of data as defined in the NAESB Business Practice Standards submitted for the purpose of implementing bilateral Interchange between Balancing Authorities or an energy transfer within a single Balancing Authority. (NERC Glossary of Terms)

Request Status

The overall status of a transaction/tag, generated by the Tag Authority Service. (For example; pending, implemented, or dead.)

Requesting PSE

The PSE submitting the interchange transaction tag.


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Rerating

A change in the capability of a generator due to a change in conditions such as age, upgrades, auxiliary equipment, cooling, etc.

Reserve Sharing Group (RSG)

A group whose members consist of two or more Balancing Authorities that collectively maintain, allocate, and supply operating reserves required for each Balancing Authority’s use in recovering from contingencies within the group. Scheduling energy from an Adjacent Balancing Authority to aid recovery need not constitute reserve sharing provided the transaction is ramped in over a period the supplying party could reasonably be expected to load generation in (e.g., ten minutes). If the transaction is ramped in quicker (e.g., between zero and ten minutes) then, for the purposes of Disturbance Control Performance, the Areas become a Reserve Sharing Group. (NERC Glossary of Terms)

Resistance (R)

The property of a material that opposes or resists current flow by converting electric energy to heat. The symbol for resistance is the letter “R”.

Resistivity

A measure of the ability of a material to conduct electricity. The greater the resistivity, the greater the opposition to current flow. An insulator has a high resistivity.

Resonance

In an electrical circuit, resonance is achieved when the magnitudes of the inductive & capacitive elements match. Resonance may be achieved by inputting energy at the circuit’s resonant frequency or by varying the size of the inductive or capacitive elements. A radio receiver is tuned to resonance at the channel the operator wants to receive.

Resonant

To achieve resonance.


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Resonant Frequency (FR)

The frequency at which resonance is achieved in an electrical circuit. The formula for the resonant frequency is:

Resource Planner

The entity that develops a long-term (generally one year and beyond) plan for the resource adequacy of specific loads (customer demand and energy requirements) within a Planning Authority Area. (NERC Glossary of Terms)

Resources

Resources is typically used to refer to available generation. For example, load must match resources to maintain frequency.

Response Rate

The Ramp Rate that a generating unit can achieve under normal operating conditions expressed in megawatts per minute (MW/Min). (NERC Glossary of Terms)

Responsive Reserves

Reserve capacity that is available to respond to system frequency disturbances.

Responsive Spinning Reserve

That portion of spinning reserve available to respond to frequency disturbances as a result of the generator’s governor response.


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Restoration

The process of returning generators & transmission system elements & restoring load following an outage on the electric system.

Restrike

The re-ignition of an electric arc.

RFC

Acronym for the Reliability First Corporation. RFC is 1 of the 8 NERC REs.

Right-of-Way (ROW)

The corridor of land under a transmission line(s) needed to operate the line(s). The width of the corridor is established by engineering or construction standards as documented in either construction documents, pre-2007 vegetation maintenance records, or by the blowout standard in effect when the line was built. The ROW width in no case exceeds the applicable Transmission Owner’s or applicable Generator Owner’s legal rights but may be less based on the aforementioned criteria. (NERC Glossary of Terms)

Right Triangle

A triangle in which 1 of the 3 internal angles is equal to 90.

Rolling Blackout

Also known as rotating outages, these are controlled, temporary interruptions of service to customers, most commonly initiated by switching off selected distribution circuits intended to reduce load during times of capacity shortfalls due to significant forced outages of generation &/or transmission facilities. The service interruptions are transferred from 1 group (or block) of customers to another over time so that no 1 group bears the entire burden of the necessary reduction in load.

Root Mean Square (RMS)

The effective value of an AC voltage or current. The RMS value of an AC quantity would produce the same amount of heat in a DC resistive circuit. For example if an AC circuit has an RMS current of 10


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amps, then 10 amps of DC current would have the same heating effect. Most AC meters read RMS values.

Rotational Energy

Energy stored within a rotating mass. For example, a spinning generator contains rotational energy. The terms inertial energy, stored energy, & rotational energy are often used interchangeably to refer to the energy stored in the rotating elements (motors & generators) of the power system.

Rotor

The rotating component of a motor or generator.

Runner

The rotating element of a hydro turbine.

Saturation

Condition reached in a transformer where the magnetic flux has reached its maximum value. Transformers can saturate from exposure to excessive voltage, excessive current, or exposure to DC currents. Damaging effects of saturation include a failure to follow the turns ratio, excessive heat production & harmonic creation.

Scenario

Possible event. (NERC Glossary of Terms)

Schedule (Noun)

An interchange schedule. (NERC Glossary of Terms)

Schedule (Verb)

To set up a plan or arrangement for an interchange transaction. (NERC Glossary of Terms)


Glossary 5/5/23

Schedule Confirmation

The process of verifying the accuracy of an interchange schedules between all the entities to the transaction.

Schedule Implementation

The process of entering the details of a negotiated schedule into the control systems of BAs involved in a transaction of power & energy.

Schedule Period

The length of time between the nominal starting & ending time of each schedule.

Scheduled Frequency

60.0 Hertz, except during a time correction.

Scheduled Interchange

Electric power scheduled to flow between entities, usually the net of all sales, purchases, & wheeling transactions between those areas at a given time.

Scheduled Losses

The scheduled power transfer to a TSP for compensation of losses incurred on that TSP’s transmission system due to a transfer of power between PSEs.

Scheduled Net Interchange (NIS)

Algebraic sum of all scheduled MW transfers, including dynamic schedules, with adjacent BAs, & taking into account the effects of schedule ramps. BAs directly connected via asynchronous ties to another Interconnection may include or exclude MW transfers on those tie lines in their scheduled interchange, provided they are implemented in the same manner for actual net interchange.


Glossary 5/5/23

Scheduled Total Interchange

The net of all interchange schedules with all Adjacent BAs. It is, in essence, the scheduled interchange with the Interconnection.

Scheduled Value

The desired or target value. For example, for a 345 kV bus, the scheduled value of voltage may be 358 kV. System operators would try to hold this bus voltage at 358 kV.

Scheduling Path

The Transmission Service arrangements reserved by the Purchasing-Selling Entity for a Transaction. (NERC Glossary of Terms)

Scroll Case

A spiral-shaped steel intake guiding the flow of water into the wicket gates located just prior to the turbine of a hydro-electric facility.

Scrubber

A pollution control system used in fossil fuel units for removing sulfur from the exhaust gases.

Seams

The boundaries between adjacent electricity-related organizations. Differences in regulatory requirements or operating practices may create “seams problems”.

Secondary Frequency Control

Actions provided by an individual BA or its RSG intended to restore primary control response & restore frequency from the arrested frequency back to scheduled frequency, or to maintain scheduled frequency deployed in the minutes time frame. Secondary control comes from either manual or automated dispatch from a centralized control system. Secondary control also includes initial reserve deployment for disturbances & maintains the minute‐to‐minute balance throughout the day & is used to restore frequency to normal following a disturbance & is provided by both spinning & non‐spinning reserves.


Glossary 5/5/23

Secondary Inadvertent Interchange

The component of area inadvertent interchange caused by the regulating deficiencies of area. (NERC Glossary of Terms)

Secondary Winding

The winding of a transformer that is connected to the power output or load end of the transformer.

Sectionalizing

The process of reducing the load on a distribution feeder using switching actions.

Self-Excitation

A possible operating condition for a generator in which the charging current from a high voltage transmission system takes over as the excitation current source of the generator. Self-excitation is a dangerous condition as high voltages can develop in the generator & in the generator auxiliaries.

Self-Provision

An alternate means (other than purchase from the TSP) by which a transmission customer may fulfill its IOS obligations. Alternatives for Self-Provision may include: 1) use of the transmission customer’s own IOS Resource, or 2) purchase of an IOS Resource from a third party IOS supplier.

Sending Balancing Authority

The Balancing Authority exporting the Interchange. (NERC Glossary of Terms)

SERC

Acronym for the Southeastern Electric Regional Reliability Council. SERC is 1 of the eight NERC REs.


Glossary 5/5/23

Series Circuit

Electrical circuit in which elements are connected end to end. The same current flows through each element of a series circuit.

Series Resonance

A resonance condition in a circuit with a series combination of inductance & capacitance. At resonance a series circuit reaches its minimum impedance equal to the circuit’s resistance value.

Servomotor

A device used to create a force based on a control signal input. For example, in a governor control system a control signal is first developed using a control valve. The control signal is input to a servomotor. The servomotor then drives steam valves, wicket gates, etc. The servomotor may be an electric motor, an hydraulic piston, or any other means of developing a force.

Settling Frequency

Refers to the point when the frequency stabilizes following a generation trip. Point B represents the interconnected system frequency at the point immediately after the frequency stabilizes due to governor action but before the contingent BA takes corrective AGC action.

Shield Wires

Conductors strung across the top of transmission lines that are designed to protect the transmission line from lightning strikes.

Short Circuit

The introduction of a low impedance path between conductors energized at different voltages. A short circuit is the same as a fault.

Short Circuit Ratio (SCR)

The ratio between the short circuit MVA of the local system & the MVA rating of a piece of equipment. For example, the SCR of a generator is equal to the MVA that would flow to a 3 fault applied at the generator’s high side bus divided by the MVA rating of the generator. SCRs can also be calculated for HVDC systems & other electrical devices.Operations Training Solutions 2015 118

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Shunt Capacitor Bank

Shunt capacitors are capacitors connected from the power system to an electrical ground. They are used to supply KVAr to the system at the point where they are connected. A shunt capacitor bank is a group of shunt capacitors.

Sine

The sine of either of the unknown angles of a right triangle is the ratio of the side directly opposite the unknown angle to the hypotenuse.

Sine Wave

The graphical representation of a mathematical function that describes the smooth, symmetrical, & periodic variation of a quantity that oscillates in magnitude or amplitude. In AC electric power systems, the voltage & current are characterized by sine waves having a frequency of 60 Hz. These waveforms, starting from a zero baseline, traverse a path that increases to a crest (positive maximum), then falls back to zero, continues downward to a trough (equal but opposite to the crest, i.e., in the negative direction), & back to zero in 1/60th of a second.

Single Contingency

The sudden, unexpected failure or outage of system facilities or elements (generating unit, transmission line, transformer, etc.). Elements removed from service as part of the operation of a RAS are considered part of a single contingency.

Sink

Final POD for the transaction: the actual load.

Sink Balancing Authority

The Balancing Authority in which the load (sink) is located for an Interchange Transaction and any resulting Interchange Schedule. (NERC Glossary of Terms)


Glossary 5/5/23

Six-Pulse Converter

An HVDC converter that has six pulses to the DC output voltage for each cycle of the AC supply voltage.

Sliding Pressure

A method of operation available in some thermal units. Normally a unit’s steam valves are adjusted to control the amount of steam sent to the turbine stages. In a sliding pressure mode of operation, the steam valves are operated wide open to minimize valve throttling losses. The boiler pressure is then varied to adjust the MW output of the unit.

Slip

The difference between the synchronous speed of an induction motor & the speed at which its rotor actually rotates.

Slip-Rings

Metal rings attached to the shaft of a synchronous machine. The rotor’s field winding terminates on the inner surface of the slip-rings while the brushes ride on the smooth outer surface of the slip-rings.

Small Power Producer

Under the PURPA, a small power production facility (or small power producer) generates electricity using waste, renewable (water, wind, & solar), or geothermal energy as a primary energy source. Fossil fuels can be used, but renewable resource must provide at least 75 percent of the total energy input.

Smoothing Reactor

A series reactor attached to an HVDC transmission line that smoothes the ripple of an HVDC converter’s DC output voltage & assists with the power conversion process.

Software

A series of instructions written to enable computer hardware to perform useful tasks.


Glossary 5/5/23

Solar Flare

Intense, short-lived releases of energy from the surface of the Sun. Solar flares are seen as bright areas on the Sun in optical wavelengths & as bursts of noise in radio wavelengths. Solar flares can last from minutes to hours. Flares are our solar system’s largest explosive events. The primary energy source for flares appears to be the tearing & reconnection of strong magnetic fields. CMEs are often associated with large flares.

Solar Magnetic Disturbance (SMD)

See Geomagnetic Disturbance (GMD)

Solar Maximum

Months during a sunspot cycle when the sunspot number reaches a maximum.

Solar Minimum

Months during a sunspot cycle when the sunspot number reaches a minimum.

Solar Storm

Energy disturbance on the surface of the Sun.

Solar Wind

Outward flow of solar particles & magnetic fields from the Sun. Solar wind speed varies between 300 & 1,000 kM/sec.

Solenoid

An electro-magnet that, when energized, is used to perform a mechanical function such as opening a switch.


Glossary 5/5/23

Solid State Relay

A relay that uses semiconductor components.

Source

The initial point of receipt for the transaction: the actual generation facility.

Source Balancing Authority

The Balancing Authority in which the generation (source) is located for an Interchange Transaction and for any resulting Interchange Schedule. (NERC Glossary of Terms)

Special Protection System (SPS)

An automatic protection system designed to detect abnormal or predetermined system conditions, and take corrective actions other than and/or in addition to the isolation of faulted components to maintain system reliability. Such action may include changes in demand, generation (MW and Mvar), or system configuration to maintain system stability, acceptable voltage, or power flows. An SPS does not include (a) underfrequency or undervoltage load shedding or (b) fault conditions that must be isolated or (c) out-of-step relaying (not designed as an integral part of an SPS). Also called Remedial Action Scheme. (NERC Glossary of Terms)

Spinning Reserve

Unloaded generation that is synchronized and ready to serve additional demand. (NERC Glossary of Terms)

SPP

Acronym for the Southwest Power Pool. SPP is 1 of the 8 NERC REs.

Stability

The ability of an electric system to maintain a state of equilibrium during normal and abnormal conditions or disturbances. (NERC Glossary of Terms)


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Stability Limit

The maximum power flow possible through some particular point in the system while maintaining stability in the entire system or the part of the system to which the stability limit refers. (NERC Glossary of Terms)

State Estimator (SE)

Computer software that takes measurements of quantities related to a system state as input & provides an estimate of the complete system state (as its output). The SE is used to confirm that the monitored power system is operating in a secure state by simulating the system both at the present time & 1 step ahead, for a particular network topology & loading condition. With the use of an SE & its associated contingency analysis software, system operators can review each critical contingency to determine whether each possible future state is within reliability limits.

Static Var Compensator (SVC)

A combination of shunt reactors & shunt capacitors that use thyrister based switches & thyrister control to regulate the var output.

Static Var System (SVS)

A combination of an SVC & other reactive power equipment. A common control system controls both the SVC & the other reactive equipment.

Station

A location in an electrical network where 1 or more elements are connected. Examples include generating stations & substations.

Station Service

The electric supply for the ancillary equipment used to operate a generating station or substation.

Station Service Generator

A generator (usually found in hydro plants) used to normally supply electric energy for station service equipment.


Glossary 5/5/23

Stator

The stationary component of a motor or generator.

Steady State Time Period

Time period before a disturbance occurs. It is a stable pre-event condition for the existing system configuration, which includes all existing BES elements, including elements on maintenance, planned or unplanned outage.

Steady State Stability / Instability

An angle stability classification. The steady state stability limit of a power system is the maximum amount of active power that can be transmitted across the system without a loss of synchronism occurring. No large disturbance need occur.

Storage

Energy transferred from 1 entity to another entity that has the ability to conserve the energy (that is, stored as water in a reservoir, coal in a pile, etc.) with the intent that the energy will be returned at a time when such energy is more useable to the original supplying entity.

Stored Energy

Energy stored within a rotating mass. For example, a spinning generator contains stored energy. The terms inertial energy, stored energy, & rotational energy are often used interchangeably to refer to the energy stored in the rotating elements (motors & generators) of the power system.

Subregion

A subdivision of a RE.

Substation

An element of the power system that contains CBs, disconnect switches, transformers, reactors, capacitors, & other equipment. A central control house is often provided to house control & protective equipment.


Glossary 5/5/23

Subsynchronous

A frequency below synchronous speed.

Subsynchronous Resonance (SSR)

An electric power system condition where the electric network exchanges energy with a turbine/generator at 1 or more of the natural frequencies of the combined system. The frequency of the energy exchange is below the synchronous frequency (subsynchronous) of the system.

Subtransmission

A functional or voltage classification relating to lines at voltage levels between the distribution & transmission levels.

Sunspot

Dark areas on the Sun’s surface. Sunspots contain strong magnetic fields that are constantly shifting. A moderate sized sunspot is about as large as the Earth. Sunspots form & dissipate over periods of days or weeks. Sunspots occur when strong magnetic fields emerge through the Sun’s surface & allow the area to cool slightly, from a background value of 6000 ° C down to about 4200 ° C. This area appears as a dark spot in contrast with the Sun.

Sunspot Cycle

Over the last 300 years, the average number of sunspots has regularly waxed & waned in an 11-year sunspot cycle. During the Sun’s 11 year cycle the polarities of its north & south poles reverse. The sunspot cycle is a useful way to mark changes in the Sun. Solar Minimum refers to the several Earth years when the number of sunspots is lowest. Solar Maximum occurs in the years when sunspots are most numerous. During Solar Maximum, activity on the Sun & its effects on our terrestrial environment are high. Sunspot cycle is also called solar cycle.

Sunspot Number

A daily index of sunspot activity.


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Supercritical Boiler

A boiler system that is operated at a much higher temperature & pressure than a conventional drum type boiler. Supercritical boilers are also called “once through” type boilers. A supercritical boiler does not have any significant steam storage, which affects its response to governor commands.

Supersynchronous

A frequency above synchronous speed.

Supervisory Control

A form of remote control comprising an arrangement for the selective control of remotely located facilities by an electrical means over 1 or more communication media.

Supervisory Control & Data Acquisition (SCADA)

A system of remote control and telemetry used to monitor and control the transmission system. (NERC Glossary of Terms)

Supplemental Regulation Service

A method of providing regulation service in which the Balancing Authority providing the regulation service receives a signal representing all or a portion of the other Balancing Authority’s ACE. (NERC Glossary of Terms)

Surge

A transient variation of current, voltage, or power flow in an electric circuit or across an electric system. (NERC Glossary of Terms)

Surge Impedance Loading (SIL)

The MW loading on a transmission line at which the line’s natural reactive power production equals its reactive power usage.


Glossary 5/5/23

Sustained Outage

The de-energized condition of a transmission line resulting from a fault or disturbance following an unsuccessful automatic reclosing sequence and/or unsuccessful manual reclosing procedure. (NERC Glossary of Terms)

Switching Order

A sequence of steps to accomplish a desired switching action.

Switching Process

The process by which the status (open, closed, etc.) of power system equipment (switches, CBs, etc.) is adjusted to perform maintenance or enhance operations.

Switching Station

Power system equipment used to tie together 2 or more electric circuits through switches. The switches are selectively arranged to permit a circuit to be disconnected, or to change the electric connection between the circuits.

Switching Surge

The sudden changes to voltage & current waveforms that accompany transmission system switching events.

Synch-Check Relay (25)

A protective relay that will not allow a CB to be closed unless the frequency difference, voltage magnitude difference, & voltage angle across the open CB are within acceptable limits.

Synchronize

The process of bringing 2 electrical systems together by closing a circuit breaker at an interface point when the voltages & frequencies are properly aligned. Also, when generators are brought on-line, they are said to be synchronized to the system.


Glossary 5/5/23

Synchronous

To be in-step with a reference. A synchronous generator rotates in synchronism with the power system.

Synchronous Condenser

A synchronous machine that operates as a synchronous motor. The MW to turn the machine’s shaft is drawn from the power system. The full capabilities of the machines excitation system (to absorb & supply MVAr) are then available for voltage control purposes. Hydroelectric generators can often be operated in synchronous condenser mode. The unit’s water turbine is typically de-watered & the unit’s rotor turned as if it were a motor.

Synchronous Machine

An AC machine whose rotor rotates in synchronism with the power system to which it is attached. Synchronous machines can be either generators or motors. A synchronous machine also includes a source of DC excitation current (the excitation system).

Synchronous Speed

The speed at which the rotor of a synchronous generator must rotate in order to stay in synchronism with the rotating magnetic field of the system. The synchronous speed is determined by the frequency of the power system & the number of magnetic poles in the rotor. For example, the synchronous speed of a 2 pole steam-turbine generator in a 60 Hz system is 3600 RPM, while the synchronous speed of a 24 pole hydro generator is 1/12th of that, or 300 RPM.

Synchroscope

A device for comparing the frequency difference & voltage angle across an open CB.

System

A combination of generation, transmission, and distribution components. (NERC Glossary of Terms)

System Operating Limit (SOL)

The value (such as MW, MVar, Amperes, Frequency or Volts) that satisfies the most limiting of the


Glossary 5/5/23

prescribed operating criteria for a specified system configuration to ensure operation within acceptable reliability criteria. System Operating Limits are based upon certain operating criteria. These include, but are not limited to:

Facility Ratings (Applicable pre- and post-Contingency equipment or facility ratings) Transient Stability Ratings (Applicable pre- and post-Contingency Stability Limits) Voltage Stability Ratings (Applicable pre- and post-Contingency Voltage Stability) System Voltage Limits (Applicable pre- and post-Contingency Voltage Limits)


System Operator

An individual at a Control Center of a Balancing Authority, Transmission Operator, or Reliability Coordinator, who operates or directs the operation of the Bulk Electric System (BES) in Real-time. (NERC Glossary of Terms)

System Personnel

Those people who have the capability to affect system operations & who must abide by the authority vested in the system operator. May include power plant operators, system maintenance personnel, power schedulers, power marketers, etc.

Tag

Refers to the collection of information in the electronic request for an energy schedule & subsequent responses utilized in the electronic Transaction Information System (TIS) implemented by NERC.

Tag ID

A unique identifying number assigned to each tag by the Tag Agent, currently represented by combining the codes for the Source BA, PSE, & Sink BA together with a unique number.

Taps

Fixed electrical contacts at different positions on a transformer’s winding. Taps are adjusted to change the voltage ratio of a transformer.


Glossary 5/5/23

Target

An indicator on a relay that is displayed when the relay operates. The term “flag” is often used to refer to a target.

Task

1 of the elements that make up a function in the Functional Model.

Telemetering

The process by which measurable electrical quantities from substations and generating stations are instantaneously transmitted to the control center, and by which operating commands from the control center are transmitted to the substations and generating stations. (NERC Glossary of Terms)

Telemetry

Equipment for measuring a quantity (amps, volts, MW, etc.) & transmitting the result via a telecommunication system (radio, microwave, etc.) to a remote location for indication or recording.

Tertiary Frequency Control

Encompasses actions taken to get resources in place to handle current & future changes in load or contingencies. Reserve deployment & reserve restoration following a disturbance is a common type of tertiary frequency control.

Tertiary Winding

An additional, often third, winding added to a power transformer. The tertiary winding may be used to connect a reactor, capacitor, or to provide station service.

Thermal Limit

A power flow limit based on the possibility of damage by heat. Heating is caused by the electrical losses that are equal to I2R.


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Thermal Rating

The maximum amount of electrical current that a transmission line or electrical facility can conduct over a specified time period before it sustains permanent damage by overheating or before it sags to the point that it violates public safety requirements. (NERC Glossary of Terms)

Theta ()

A Greek symbol used to indicate an angular measurement.

3-Part Communication

A communications protocol where information is verbally stated by a party initiating a communication, the information is repeated back correctly to the party that initiated the communication by the second party that received the communication, & the same information is verbally confirmed to be correct by the party who initiated the communication.

Thrust Bearing

The bearing that opposes axial forces in the generator shaft.

Thyrister

A solid-state electronic component whose ability to conduct current is controlled via its voltage polarity or a gate signal. Thyristers are also called silicon controlled rectifiers (SCR). Thyristers are combined in series/parallel arrangements to perform rapid switching actions. Thyristers are used in modern static var compensators (SVC), adjustable speed drives (ASD), & HVDC converters.

Tie-Line

A circuit connecting two Balancing Authority Areas. (NERC Glossary of Terms)

Tie-Line Bias (TLB)

A mode of Automatic Generation Control that allows the Balancing Authority to 1.) maintain its Interchange Schedule and 2.) respond to Interconnection frequency error. (NERC Glossary of Terms)


Glossary 5/5/23

Tie-Line Telemetry

Telemetry equipment used to measure power flow data on a tie-line connecting BAs. The power flow data is then transmitted to both BAs.

Time Error

The difference between the Interconnection time measured at the Balancing Authority(ies) and the time specified by the National Institute of Standards and Technology. Time error is caused by the accumulation of Frequency Error over a given period. (NERC Glossary of Terms)

Time Error Correction (TEC)

An offset to the Interconnection’s scheduled frequency to return the Interconne ction’s Time Error to a predetermined value. (NERC Glossary of Terms)

Title Transfer

An exchange of energy ownership; may or may not be associated with a physical delivery of energy.

TLR Log

Report required to be filed after every TLR Level 2 or higher in a specified format. The NERC IDC prepares the report for review by the issuing Reliability Coordinator. After approval by the issuing Reliability Coordinator, the report is electronically filed in a public area of the NERC Web site. (NERC Glossary of Terms)

Torque

The torque is a force that produces a rotating or twisting action.

Torque Angle ()

The angle by which the rotating magnetic field of synchronous machine leads or lags the rotating magnetic field of the system to which it connects. A generator has a positive torque angle while a motor has a negative torque angle. The symbol for the torque angle is the letter “” (Greek lower case letter delta).


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Torque-Speed Curves

A graphical means of illustrating the relationship between the torque developed by a motor & the speed of rotation of the motor shaft.

Torsional

A twisting force.

Total Actual Interchange

The algebraic sum of all interchange metered with all Physically Adjacent BAs. It is, in essence, the actual interchange with the Interconnection.

Total Flowgate Capability (TFC)

The maximum flow capability on a Flowgate, is not to exceed its thermal rating, or in the case of a flowgate used to represent a specific operating constraint (such as a voltage or stability limit), is not to exceed the associated System Operating Limit. (NERC Glossary of Terms)

Total Harmonic Distortion (THD)

A computed value used to quantify the harmonic content of a waveform. The THD is a measure of the percent of the harmonic components content as compared to the magnitude of the fundamental component.

Total Transfer Capability (TTC)

The amount of electric power that can be moved or transferred reliably from one area to another area of the interconnected transmission systems by way of all transmission lines (or paths) between those areas under specified system conditions. (NERC Glossary of Terms)

Transfer Capability

The measure of the ability of interconnected electric systems to move or transfer power in a reliable manner from 1 area to another over all transmission lines (or paths) between those areas under specified system conditions. The units of transfer capability are in terms of electric power, generally expressed in


Glossary 5/5/23

MW. The transfer capability from “Area A” to “Area B” is not generally equal to the transfer capability from “Area B” to “Area A”

Transfer Capability

The measure of the ability of interconnected electric systems to move or transfer power in a reliable manner from one area to another over all transmission lines (or paths) between those areas under specified system conditions. The units of transfer capability are in terms of electric power, generally expressed in megawatts (MW). The transfer capability from “Area A” to “Area B” is not generally equal to the transfer capability from “Area B” to “Area A.” (NERC Glossary of Terms)

Transfer Limit

The maximum amount of power that can be transferred in a reliable manner from 1 area to another over all transmission lines (or paths) between those areas under specified system conditions.

Transformer

A device that operates on magnetic principles to increase (step up) or decrease (step down) voltage.

Transient Time Period

Transitional time period beginning after a disturbance in which high-speed automatic actions occur in response to the disturbance. This time period starts at the time of the disturbance & can continue for seconds or until a new steady state is achieved.

Transient Stability

The ability of an electric system to maintain synchronism between its parts when subjected to a disturbance & to regain a state of equilibrium following that disturbance.

Transmission

An interconnected group of lines and associated equipment for the movement or transfer of electric energy between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric systems. (NERC Glossary of Terms)


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Transmission Constraint

A limitation on one or more transmission elements that may be reached during normal or contingency system operations. (NERC Glossary of Terms)

Transmission Customer

1. Any eligible customer (or its designated agent) that can or does execute a transmission service agreement or can or does receive transmission service. 2. Any of the following responsible entities: Generator Owner, Load-Serving Entity, or Purchasing-Selling Entity. (NERC Glossary of Terms)

Transmission Line

A system of structures, wires, insulators and associated hardware that carry electric energy from one point to another in an electric power system. Lines are operated at relatively high voltages varying from 69 kV up to 765 kV, and are capable of transmitting large quantities of electricity over long distances. (NERC Glossary of Terms)

Transmission Loading Relief (TLR)

A procedure used to manage congestion on the electric transmission system.

Transmission Operator (TOP)

The entity responsible for the reliability of its “local” transmission system, and that operates or directs the operations of the transmission facilities. (NERC Glossary of Terms)

Transmission Owner (TO)

The entity that owns and maintains transmission facilities. (NERC Glossary of Terms)

Transmission Overload

A state where a transmission line has exceeded either a normal or emergency rating of the electric conductor.


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Transmission Planner (TP)

The entity that develops a long-term (generally one year and beyond) plan for the reliability (adequacy) of the interconnected bulk electric transmission systems within its portion of the Planning Authority Area. (NERC Glossary of Terms)

Transmission Reliability Margin (TRM)

The amount of transmission transfer capability necessary to provide reasonable assurance that the interconnected transmission network will be secure. TRM accounts for the inherent uncertainty in system conditions and the need for operating flexibility to ensure reliable system operation as system conditions change. (NERC Glossary of Terms)

Transmission Service

Services provided to the Transmission Customer by the Transmission Service Provider to move energy from a Point of Receipt to a Point of Delivery. (NERC Glossary of Terms)

Transmission Service Provider (TSP)

The entity that administers the transmission tariff and provides Transmission Service to Transmission Customers under applicable transmission service agreements. (NERC Glossary of Terms)

Traveling Wave

Energy can be viewed as a wave traveling through space. When a disturbance occurs in the power system the disturbance can be viewed as a wavefront of current & voltage propagating out from the disturbance point. A frequency disturbance can also be viewed in terms of a frequency deviation propagating out from the disturbance point in the shape of a wave.

TRE

Acronym for the Texas Reliability Entity. TRE is 1 of the 8 REs.

Trip

The opening of CBs on an electric system, normally to electrically isolate a particular element of the system to prevent it from being damaged by fault current or other potentially damaging conditions.


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Trigonometry

A branch of mathematics that deals with the relationships between the lengths of sides & the angles between the sides of triangles.

Triplen Harmonics

The odd numbered harmonic orders that are evenly divisible by the number 3. For example, the 3rd, 9th, 15th, etc. are Triplen Harmonics.

Turbine

A rotating mechanical device. A turbine is rotated by the force of a working fluid. The working fluid is typically steam, water, or combustion gas.

Turbine Follow

A mode of operation of a fossil unit’s turbine/boiler control system in which the unit’s response to a load (MW) change request is delayed if the boiler’s temperature &/or pressure moves outside set boundaries. The MW response of the unit’s turbine “follows” the response of the boiler.

Turning Gear

A method of rotating the shaft of a horizontally mounted turbine/generator. The turning gear may rotate the shaft at 2 RPM to prevent shaft warp.

Turns Ratio

The ratio of the number of turns in the primary winding of a transformer to the number of turns in the secondary winding.

Twelve Pulse Converter

An HVDC converter that has 12 pulses to the DC output voltage for each cycle of the AC supply voltage.


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Unbundling

Separating electric utility service into its basic components & offering each component separately for sale with separate rates for each component.

Under Load Tap Changer (ULTC)

A tap changer that is designed to change the turns ratio when the transformer has current flow across its windings. The term load tap changer (LTC) is also used to refer to this type tap changer.

Underexcite

A generator is underexcited when the applied excitation is less than that needed to support the generator’s magnetic field. The deficiency in the reactive power needs of the generator is supplied by the system. An underexcited generator absorbs reactive power from the system. The terms “leading” & “bucking” are also used to refer to an underexcited generator.

Underfrequency Load Shedding (UFLS)

The tripping of customer load based on magnitudes of system frequency. For example, a utility may dump 5% of their connected load if frequency falls below 59.3 HZ, dump an additional 10% if frequency falls below 58.9 HZ, & dump a final 10% if frequency falls below 58.5 HZ. These 3 steps of load shedding would form this utilities UFLS plan. The purpose of UFLS is a final effort to arrest a frequency decline.

Undervoltage Load Shedding Program (UVLS)

Automatic load shedding program, consisting of distributed relays and controls, used to mitigate undervoltage conditions impacting the BES, leading to voltage instability, voltage collapse, or cascading. Centrally controlled undervoltage-based load shedding is not included.

Unilateral Payback

Inadvertent interchange accumulations paid back unilaterally controlling to a target of non-zero ACE. Controlling to a non-zero ACE ensures that the unilateral payback is accounted for in the CPS calculations. The unilateral payback control offset is limited to BA’s L10 limit & shall not burden the Interconnection.


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Unit Commitment

The process of selecting which generating units will be placed on line to serve the load & reserve requirements.

Universal Coordinated Time (UTC)

UTC is time standard commonly used across world.

World’s timing centers have agreed to keep their time scales closely synchronized or coordinated Therefore name Coordinated Universal Time

o French acronym is UTC Used as time reference in transmission of Tag data to/from Tag services Tag software may permit entering data in local time

Unscheduled Power Flow

Power does not flow where it is scheduled but rather according to the relative impedance of the available paths. When power is scheduled to flow from system “A” to system “B” some of the power may flow through an adjoining system “C”. The power that flows through system “C” is called unscheduled power flow. Also called parallel flow or loop flow.

Valve

General name given to a mercury-arc or thyrister based device that is used to conduct current when a control signal is applied. A valve is turned on by a gate or grid pulse. When turned on a valve conducts current. When turned off a valve blocks the flow of current. A valve is turned off by removing the forward biased voltage & stopping the current flow. Power converters are composed of several valves connected in different arrangements depending on the converter design.

Vectors

Line segments that are used to represent the magnitude & direction of physical quantities. DC voltages & currents can be represented by vectors.

Vegetation Inspection

The systematic examination of vegetation conditions on a Right-of-Way and those vegetation conditions under the applicable Transmission Owner’s or applicable Generator Owner’s control that are likely to Operations Training Solutions 2015 139

Glossary 5/5/23

pose a hazard to the line(s) prior to the next planned maintenance or inspection. This may be combined with a general line inspection. (NERC Glossary of Terms)

Viscosity

A property of a fluid that tends to prevent the fluid from flowing when subjected to an applied force. High-viscosity fluids resist flow; low-viscosity fluids flow easily.

Voltage (V)

The electrical force (a separation of charge) that causes current to flow. Symbol is “V” & units are volts.

Voltage Collapse

A process in which a voltage unstable system experiences an uncontrollable reduction in system voltage.

Voltage Control

The control of transmission voltage through adjustments in generator reactive output & transformer taps, & by switching capacitors & inductors on the transmission & distribution systems.

Voltage Limits

A hard limit above or below which is an undesirable operating condition. Normal limits are between 95 & 105 percent of the nominal voltage at the bus under discussion.

Voltage Phase Angle

Phase angle between 2 voltages. Voltage phase angle is the same as the power angle. (In a generator the voltage phase angle between the internal generator voltage & the stator terminal voltage is the equivalent of the torque angle.)

Voltage Ratio

Ratio between primary & secondary voltages of a transformer. There is a strong (but not identical) relationship between a transformer’s voltage ratio & its turns ratio.


Glossary 5/5/23

Voltage Reduction

A procedure designed to deliberately lower the voltage at a bus. It is often used as a means to reduce demand by lowering the customer’s voltage.

Voltage Regulator

A component of an excitation system. The voltage regulator monitors the generator’s output voltage & causes an adjustment in excitation current when required. Voltage regulators can be operated in either a manual or automatic mode.

Voltage Relay (27 & 59)

A protective relay that activates in response to either a high (59) or low (27) voltage.

Voltage Source

A device capable of producing a voltage. Generators & batteries are voltage sources.

Voltage Sourced Converter (VSC)

Type of power converter in which a DC capacitor is used as the voltage source to create a three-phase AC voltage. A single VSC can be used as a STATCOM (which can create or absorb MVar) or 2 VSCs can be used to create a back-to-back HVDC converter.

Voltage Stability

The ability of a power system to maintain voltage so that when the system nominal load is increased the actual power transferred to that load will increase. In a voltage stable power system the power transfer & the system voltages are controllable by the system operators.

V-Q Curve

A voltage versus reactive power curve. A plot of the voltage at a bus versus the reactive power injected into that bus. V-Q curves are a graphical tool used to analyze a power system’s voltage stability.


Glossary 5/5/23

Watthour (Wh)

A unit of measure of electrical energy equal to 1 watt of power supplied to, or taken from, an electric circuit steadily for 1 hour.

WebSAS

Acronym for a web based security analysis system. This software tool is used in the WECC to assist with congestion management. WebSAS is the WECC’s wide area reliability tool or WART.

WECC

Acronym for the Western Electricity Coordinating Council. WECC is 1 of the 8 NERC REs.

Wheeling

The movement of electricity from 1 system to another over transmission facilities of interconnecting systems. Wheeling service contracts can be established between 2 or more systems.

Wheeling Charges

Payments required for providing wheeling services.

Wicket Gate

Adjustable elements that control the flow of water to the turbine of a hydro-electric facility.

Wide Area

The entire Reliability Coordinator Area as well as the critical flow and status information from adjacent Reliability Coordinator Areas as determined by detailed system studies to allow the calculation of Interconnected Reliability Operating Limits. (NERC Glossary of Terms)


Glossary 5/5/23

Wide Area Reliability Tool (WART)

This generic term is intended to reflect in a tool neutral manner those wide-area reliability assessment tools (such as the IDC in the Eastern Interconnection or WebSAS in the Western Interconnection) acknowledged by NERC as a decision making tool among various reliability entities.

Wind Generating Station

A collection of wind turbines electrically connected together & injecting energy into the grid at 1 point, sometimes known as a wind farm. (NERC Glossary of Terms)

Wye Connection

A 3 electrical connection in which 1 end of each of the 3 phases is connected to a common point. The common point is often grounded. Transformer windings are often connected in a Wye configuration.

Year 1

The first twelve month period that a Planning Coordinator or a Transmission Planner is responsible for assessing. For an assessment started in a given calendar year, Year One includes the forecasted peak Load period for one of the following two calendar years. For example, if a Planning Assessment was started in 2011, then Year One includes the forecasted peak Load period for either 2012 or 2013. (NERC Glossary of Terms)

Zonal Market

A market for electric energy divided into regional pricing zones. Generators within a zone receive the same price for the power they provide, & transmission lines crossing zonal boundaries are assessed additional costs due to market congestion when the power flowing through them reaches operational constraints.

Zones of Protection

The zone or area within which a protective relay can sense abnormal conditions.


Documents

Glossary Dynamics of Disturbance DOD Glossary.d… · Web viewThe HVDC transmission path is very short, ... & convert it into rotational form by direct coupling to a common spinning