Glossary

Glossary7/7/15

Glossary

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

June 25, 2015

A-36

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. NOAAs 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 generators 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 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 loads 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)

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 aggregators 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.

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 machines 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)

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 generators 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 switchs 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)

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.

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 Earths 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 Authoritys 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.

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 Interconnections 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.

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 conductors back EMF.

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 customers 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.

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 regions 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.

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 Operators restoration plan needs for real and reactive power capability, frequency and voltage control, and that has been included in the Transmission Operators 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 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 units turbine/boiler control system in which the units 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.

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 Operators 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, and

b) 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:

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 customers 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. LNs 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)

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 Providers 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.

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 Providers 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 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 Areas 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 units 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.

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 Suns 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.

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 loads 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.

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 columns 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)

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 elements 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)

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.

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)

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 Authoritys 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 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 units 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 transformers 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 Suns atmosphere, characterized by low densities & high temperatures.

Coronal Mass Ejection (CME)

Suns 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 Earths 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.

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.

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:

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.

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

(NERC Glossary of Terms)

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 units 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.

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.

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)

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.

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

(NERC Glossary of Terms)

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 generators MW output. There are 2 types of droop; permanent & transient:

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 units 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 governors 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)

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 Earths surface during GMD events. ESP magnitude varies by location on the Earths surface, which causes geomagnetically induced currents (GICs) to flow in the Earths 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.

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 the lines 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)

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 shafts 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 Earths 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)

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 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 coals 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 are declared by the BAs 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 suppliers 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 customers 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.

Epsilon 1 (1I)

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 (10)

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 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 systems 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 transformers 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 Providers 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 converters (when operated as a inverter) output voltage.

Facilities Study

An engineering study conducted by the TSP to determine the required modifications to the TSPs 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.

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 generators 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 rotors magnetic field.

Filament

Mass of gas suspended over the Suns 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.

Firm Capacity

Capacity that is as firm as the sellers 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 suns 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.

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 facilitys reliability. A weighted average of forced outage rates for all generating facilities in a utilitys 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 Areas 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 Authoritys Area Control Error equation to account for the Balancing Authoritys inverse Frequency Response contribution to the Interconnection, and discourage response withdrawal through secondary control systems. (NERC Glossary of Terms)

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 )

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)

Frequency Response Obligation (FRO)

The Balancing Authoritys 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 dams structure.

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)

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 plants capacity to generate power, or problems on the transmission system external to the plant which limit the capability of the system to accept the plants power output.

Generator Shift Factor (GSF)

A factor to be applied to a generators 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 Earths 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 Earths 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.

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:

alphadeltaepsilon mu

Phibetatheta pie

omega gammalambdarho

Omega

Grid

An electrical transmission &/or distribution network.

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 transformers 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 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.

Henry (H)

The unit of induc