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P1818/D1.7, October 2014

Draft Guide for the Design of Low Voltage Auxiliary Systems for Electric Power Substations

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DOCVARIABLE "txtGorRPorSTD" \* MERGEFORMAT Guide for the Design of Low Voltage Auxiliary Systems for Electric Power Substations

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Introduction

This introduction is not part of P1818/D1.7, Draft

DOCVARIABLE "txtGorRPorSTD" \* MERGEFORMAT Guide for the Design of Low Voltage Auxiliary Systems for Electric Power Substations .

Contents

11. Overview

11.1 Scope

11.2 Purpose

22. Normative references

23. Definitions

24. Design of substation AC systems

34.1 Design criteria

44.2 Station power source requirements

74.3 Load requirements

94.4 Conductor selection

134.5 Station Power Transformer:

224.6 Transfer Switch

254.7 Bus layout and distribution circuits configuration

334.8 AC distribution panelboards for electrical substations

354.9 Circuit Protection

364.10 Equipment Specifications

374.11 Operation and maintenance considerations

405. Design of substation DC system

405.1 Design Criteria

425.2 Typical equipment served by the DC system

435.3 One-line diagram

445.4 DC batteries

485.5 Battery chargers

515.6 DC Panels

515.7 Load transfer methods

535.8 Design considerations

595.9 Maintenance Provisions

60Annex A (informative) Bibliography

62Annex B (informative) Conductor selection examples

62B.1 Example XX-1

63B.2 Example XX-2

Draft

DOCVARIABLE "txtGorRPorSTD" \* MERGEFORMAT Guide for the Design of Low Voltage Auxiliary Systems for Electric Power Substations

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1. Overview

1.1 Scope

This guide will consider the components of both the AC and DC systems and provide guidelines and recommendations for designing the appropriate systems for the substation under consideration. This guide includes the low voltage auxiliary systems from the source(s) to the distribution point(s). Reliability requirements and load characteristics are discussed and distribution methods are recommended.

1.2 Purpose

The low voltage AC and DC auxiliary systems comprise very important parts of the substation equipment. The design of the AC and DC auxiliary systems facilitates the safe and reliable operation of the substation. This guide considers various factors that affect the design of the AC and DC auxiliary systems such as reliability, load requirements, system configurations, personal safety and protection of auxiliary systems equipment.

2. Normative references

The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies.

Coming soon

3. Definitions

For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause.

Coming soon

4. Design of substation AC systems

The objective of this section is to provide the required information for the substation engineer to design safe and economical AC station auxiliary system as applicable for each substation. The station power for most substations can be represented by the block diagram shown in figure 4-1 below. Detailed information is given in the section given next to each part.

Figure 1 represents an ultimate station power configuration that can be applied to any substation depending on substation size reliability and load requirements. One source is designated as the normal feed, and a second and/or third source is designated as a backup. A loss of the normal or preferred source will result in transferring the load to the backup source. In substations with multiple sources, the sources are normally connected to a transferring scheme. One or more AC panels are used to serve the substation load as required.

As a first step to the design process, the design engineer must review the design criteria for the station power; the number of sources, source type: three phase or single phase, required transformer rating, connection and other factors that may affect the final configuration of the station power for the applicable substation. The design criteria is discussed in 4.1.

Figure 1 Block Diagram for Typical Substation AC Station Power

4.1 Design criteria

In general, the design criteria of the AC auxiliary system are determined by the demand load of the connected KVA of the substation loads, as well as the voltage ratings and number of phases of the substation equipment to be supplied. Auxiliary transformers and other station power components should consider substation expansion and/or anticipated growth rate. Timing of any proposed expansion may dictate initial installation or deferral of station power components. Some loads may be identified as critical, which requires AC service to be maintained at all times. Depending upon such critical loads, the substation may require two or three AC station service sources with the ability to transfer loads between sources.

Due to the importance of the station power to the operation and reliability of the substation, the following factors must be considered in order to determine the required station power configurations:

4.1.1 System stability

System stability considerations are important for the reliability requirements of the station power. If the loss of a certain substation will result in system disturbance not only within the owner utility system but might also have a cascading effect on neighboring utilities which may result in a blackout condition in the area.

4.1.2 Customer service and loss of revenue

Some substations serve critical loads such as hospitals, manufacturing complexes, government offices, schools and others, or serve large blocks of load where the substation reliability requirements are high. Some substations are connected to power plants where the loss of the substation equipment may result in tripping the plant which results in loss of revenue for the utility. More than one station power source may be warranted for these types of substations. Other less critical substations may have limited effect on the customers service and one source for the station power may be justified.

4.1.3 Equipment protection

Substation equipment protection considerations must be given to all substations regardless of the size, however high and extra high voltage substations contain high cost equipment such as transformers where the cooling system is considered very important to the operation of this equipment and therefore a backup source is generally required. Protective relays or other electronic control equipment located in high temperature areas may require a continuous cooling system and therefore the second source is generally required.

4.1.4 Design considerations

The designer may consider the following list when designing an AC system for Substation:

a) Location of AC equipment indoor or outdoor

b) Number of AC panels

c) Essential loads connections

d) Non-essential load connections

e) Conductor type and size

f) Voltage drop calculations

g) National Electrical Code (NEC) [B21] requirements.

4.1.5 Selection of auxiliary system voltage

Several secondary voltage levels are available for AC auxiliary systems. When determining the voltage level needed, the designer may use a standard voltage level determined by the designers power system or use a variation to maintain the voltage levels of the equipment being supplied. Either way, the designer needs to consider the factors in 4.3.

4.2 Station power source requirements

Three sources are represented in Figure 1. One source, preferably the most reliable source, is designated and used as the primary or normal source. The second source is designated as a backup source and is typically used only when the primary source has been lost. The third source if available will be used as a second back up and is typically utilized only if both the primary and secondary sources have been lost. There are four sources that are commonly used as substation AC auxiliary power sources:

h) Power transformer tertiary

i) Substation bus

j) Distribution line

k) Standby generators

Each source has advantages and disadvantages. Substation location, substation equipment, and bus configurations may dictate which source is preferred.

4.2.1 Power transformer tertiary:

When used in substations, the tertiary of a three winding or autotransformer can provide a good source for station power applications. When the primary and secondary windings are connected wye, a third winding connected in delta is typically used for stabilizing purposes. A tertiary winding presents a low impedance path to zero sequence currents and harmonics, thereby reducing the zero sequence impedance presented to the outside world, while avoiding the problem of tank heating. The tertiary winding typically has a volt-ampere rating between 20-30% of the volt-ampere rating of the primary winding, and typically has a medium voltage rating up to 34.5 kV. If there are plans to use the transformer tertiary for station power purposes, the tertiary winding is brought out of the transformer otherwise, the tertiary winding is buried.

The volt-ampere rating of the tertiary winding typically exceeds the maximum volt-ampere requirement of a substations AC auxiliary power load and is an adequate AC auxiliary power source.

Consideration should be given to the available fault current at the tertiary bus. In the case that the fault current magnitude exceeds the interrupting rating of the protective equipment such as fuses or circuit breakers, several options can be employed to mitigate the fault current. These options include installing, current limiting fuses, resistors, or reactors or increasing the transformer tertiary impedance.

Another consideration should be given to the detection of the ground fault on the tertiary bus. The tertiary buses on three phase power transformers are generally short and typically do not require any ground fault protection. However, when single phase power transformers are used to construct a three phase bank, bus runs that connect and form the tertiary bus become much longer and are more likely to be subject to a phase to ground fault. A single phase to ground fault on the tertiary will not generate fault current in the delta and will not trip the distribution transformer high side protection, nor will the fault trip a typical power transformer bank differential scheme. Some utilities choose to install a ground detection network on the tertiary delta to detect this single line to ground fault to either signal an alarm or trip the power transformer bank. A common method for constructing this ground detection network is to install three small delta connected transformers with a grounding resistor. There will be zero voltage drops across this grounding resistor under normal operating conditions. There will be a voltage drop across this grounding resistor during a phase to ground fault and that indicates that a phase to ground fault on the delta tertiary has occurred.

4.2.2 Substation bus:

Another available source for station power is to use the substation voltage bus as a source. This possible source is normally used when other sources are not available due to its relatively high cost. A Power Voltage Transformer (PVT) or Station Service Voltage Transformer (SSVT), is the device that may be used to transform the bus voltage to the AC auxiliary voltage. These devices are available for voltages between 34.5 to 230 kV. One or more PVTs might be required depending on required station power load. See Figure 2 for possible connections.

Figure 2 Possible SSVT locations

The PVT is normally located within the line or bus relays zones of protection. A fault on the PVT will be cleared by the protective relay faster than any high side fuse. Also the size of the required fuse may not be available for certain voltage levels. The protection engineer should be consulted for the final location when determining the required PVT protection. Low side over current protection of the secondary conductors used for auxiliary station service are typically applied as close to the secondary terminals as possible. Surge protection is typically needed on the high side connection of the PVT. If arresters protecting other equipment in the station are close enough to protect the PVT, a dedicated arrester for the PVT may not be required. Guidance on surge protection and separation effects can be found in IEEE Std C62.22 [B7].

4.2.3 Distribution feeders:

Another source for the station power is the use of nearby distribution feeders. The feeder primary, if owned by the substation owner, is normally brought into the substation underground and is connected to a step-down transformer located inside the substation preferably near the control building. If the feeder is owned by another utility, a revenue meter is installed before it can be connected to the step down transformer. Since the feeder has more exposure to faults, it is typically used as a backup to the primary source.

4.2.4 Standby generators

Generators may also be used as an AC auxiliary power source. In substations, generators are typically used as an emergency power source instead of a permanent power source. This is due to the disadvantages of using generators as a permanent AC auxiliary source. Choosing to use generators as a permanent AC auxiliary source will require additional design considerations. Fire protection systems will need to be designed to protect the substation equipment from a generator file. Fuel storage systems will need to be installed to house the fuel needed to run the generators. The generators may also be housed in a separate building structure, which requires the installation of a ventilation system.

Generators used as an emergency AC auxiliary power source have more merit than as a permanent source. As an emergency source, there is not the same need for fire protection installation, fuel storage system, or building ventilation (if the generators are located outdoors in the switchyard).

4.3 Load requirements

In order to design a reliable and safe station service system, the AC load must be defined and calculated. The calculated load current is used to select the station service transformer size and the conductor rating. These loads consist but not limited to the following:

In order to design a reliable and safe station service system, the AC load must be defined and calculated. The calculated load current is used to select the station service transformer size and the conductor rating. These loads consist but not limited to the following:

a) Substation ultimate plan: In order to account for the ultimate load of the substation, any future loads must be considered and included in calculating the maximum station load. This include the loads for additional power transformers, additional cooling and heating, additional breakers of all voltage level.

b) Maintenance equipment load: For large substations, maintenance crews may select to use a diesel generator as a source for the maintenance equipment. This generator can be permanently installed in a convenient location in the substation or can be carried on a truck as required. If on the other hand, the maintenance equipment will be serviced from the station power transformer, the load of the maintenance equipment must be included in the calculations of the station power transformer rating.

c) External load: Loads that are not associated with the substation equipment but may be required to be serviced by the substation station service transformer. This can include external building heating or cooling loads, building lighting and any communication equipment loads associated with equipment other than substation load.

d) Substation maintenance and storage building: Large substations may require additional building for maintenance and or equipment storage. The load requirements for the building must be included in the station power transformer KVA calculations. The load for this building may include the heating/cooling equipment building light and any other machinery that might be required for performing maintenance.

e) Substation AC Loads: Substation load consists of the following loads:

1) Major equipment loads: This load is related to equipment operation and as a result is considered essential loads.

i) Transformer cooling fans

ii) Transformer pumps.

iii) Load tap changer motor drive

iv) Breaker AC charging motor

v) Equipment heaters.

2) Yard Loads - This load includes yard lighting and any receptacle load for equipment testing. Also transformer oil retention pit pumps load should be considered in the calculations of the Station Service KVA rating.

3) Control Building Loads: The control building is normally used to house critical equipment used for the protection and operation of the substation and also other elements of power system associated with the substation such as transmission lines connected to the substation. This equipment includes substation protective relays, metering, and the battery system and control equipment. For optimum operations of this equipment, the cooling and heating of the control building must be operated within a specified temperature. As a result the control building heating and cooling load is considered essential load. Other load such as battery chargers, control building lights and expected receptacle load must be considered in the transformer kVA rating.

4) Construction load: In both transmission and distribution substations, the substation AC auxiliary systems are typically used to supply loads such as, but not limited to:

i) Essential load

ii) Non-essential load

iii) Maintenance load

iv) Construction load

4.3.1 Essential load

These loads are related to equipment operation and are necessary to the proper function of the substation.

a) Transformer cooling, oil pumps, and load tap changers

b) Substation battery charging systems

c) Circuit breaker air compressors and charging motors

d) Power circuit breaker control circuits

e) Power equipment heating circuits

f) Communications equipment

g) Relaying, supervisory, alarm, and control equipment

h) AC/DC converter uninterruptable power supplies

i) AC powered motor operated disconnect switches

4.3.2 Non-essential load

Provide description

a) Outdoor lighting, security systems and receptacles

b) Control building or switchgear building lighting, HVAC, and receptacles

4.3.3 Maintenance load

4.3.4 Construction load

4.4 Conductor selection

This section presents general information useful in the selection of both line and load conductors. It describes various characteristics essential to conductor selection: conductor type, insulation type, insulation voltage rating, insulation temperature rating, conductor terminations, and conductor size.

A process flow chart has been developed to aid the designer/engineer in the conductor selection process. Refer to Figure 3 below, as well as Sections 4.4.1 -4.4.6 of this document, for guidance on conductor selection based on these characteristics. However, please make a note that any rules/restriction set forth by the Authority Having Jurisdiction (AHJ) in the area that work is being performed, will supersede any documents referenced in this section.

This section covers the selection of both line and load conductors. There are six main characteristics to consider when selecting a conductor conductor type, insulation type, cable insulation voltage rating, cable insulation temperature rating, the terminations being connected to (temperature rating, ampacity, etc.), and conductor size; these are the topics that will be covered within this section. The engineer performing the conductor selection can use the process flow chart shown below in Figure 3 and this clause of the document for guidance on conductor selection based on these characteristics

Select Conductor Type

Select Cable Insulation Voltage Rating

Select Cable Temperature Rating

Adjust Allowable Ampacity of Conductor based

on Ambient Temperature

,

Depth of Conductors

Below Ground

,

and Proximity to other

Conductors

(

if necessary

)

Is Ampacity of the Conductor higher than what

is required of the Load

?

NO

Redesign Load Distribution

Y

E

S

N

O

Conductor Size is Acceptable

Perform Voltage Drop Calculations

Select Cable Insulation Type

Y

E

S

Are the Voltage Drops in all of the Conductor

Acceptable per Applicable Standards

?

Make Initial Conductor Size Selection based

on Required Ampacity

Consider Characteristics of Termination Points

and Connected Equipment

YES

Check Short Circuit Withstand Capability

Is the Calculated Short Circuit Capability of the

Conductor higher than the Available Fault

Current

?

N

O

Adjust Conductor Size or Fault Clearing Time

to Allow for Withstand of Available Fault

Current

Figure 3 Conductor selection process flow chart

For specific examples on selecting conductors (control, instrument, power and communication), refer to the Annexes in the latest version of IEEE Std 525 [B12].

4.4.1 Conductor type

The first step in conductor selection is to determine the type of conductor to be used. The conductor material and stranding type are the most important factors to consider (i.e. aluminum vs. copper and stranded vs. solid). There are advantages to using each type of conductor depending on the application. Consider characteristics such as their weight, conductivity and surrounding environmental conditions for the application of the conductors. Copper has historically been used for conductors of insulated cables due to its desirable electrical and mechanical properties. The need for mechanical flexibility usually determines whether a solid or a stranded conductor is used, and the degree of flexibility is a function of the total number of strands. A single insulated or bare conductor is defined as a conductor, whereas an assembly of two or more insulated conductors, with or without an overall covering, is defined as a cable. All of this information is typically available from cable manufacturer. For additional information on conductor material and stranding see IEEE Std 141[B8] and IEEE Std 525 [B12].

4.4.2 Cable insulation voltage rating

Cable insulation voltage rating is selected based on the phase to phase operating voltage, and the expected fault clearing time. Further guidance on selecting the voltage rating of cables should be provided by the AHJ, or by specific product literature provided by the manufacturer. For an example of selecting cable insulation type, see B.1.

4.4.3 Cable insulation type

For jacketed conductors, the insulaton type should be selected to meet the local environmental conditions- such as dry, wet or both and chemical resistance. Typically, information on the application of different insulation types will be provided by the manufacturer; there may also be requirements by the AHJ. IEEE Std 141[B8] provides general guidance on cable insulation selection. For an example of selecting cable insulation type, see B.1.

4.4.4 Cable temperature rating

The temperature rating of the cable should be selected to withstand the ambient temperature of the environment in which it is installed, in addition to any self-heating that may occur. The engineer selecting the conductor should note that the conductor installation may cross multiple environments, all of which must be considered. Typical conductor temperature ratings are 60, 75, 90, and 105C.

4.4.5 Consideration for the characteristics of termination points & connected equipment

The temperature rating of the conductor may need to be derated in some cases, depending on the availability and type of termination points that it is connected to. If the temperature rating of a conductor is higher than its connected termination point, the conductor must be rated to match the temperature of the termination point. The lowest temperature rating of any one component in a circuit is the temperature that should be used for the circuits design. This will affect the allowable ampacity of a conductor. Typically the termination point limit specified by the manufacturer is 75 C. For an example involving cable selection based on termination ratings, see B.1.

4.4.6 Conductor Size Calculations

The following factors should be considered when selecting the conductor size:

l) Required ampacity

m) Ambient temperature correction

n) Adjustment factors for multiple conductors in a raceway or cable

o) Voltage drop

p) Short circuit withstand

4.4.6.1 Required ampacity (initial conductor size selection)

The initial selection of a conductor as required by ampacity is based on the load assessment of the circuit that it is supplying. A design criterion (also known as a load study) should be completed, identifying and classifying all substation loads to be supplied, in order to determine the required ampacity. For more information refer to 4.3.

All conductors should be initially sized based on the ampacity of the load(s) they are supplying, and the available fault current at the point in the system in which they are located. The size of the conductor may be based on requirements provided by the AHJ, or by specific product literature provided by the manufacturer. For an example involving the initial conductor size selection, see Annex B. Once the initial conductor type and size selection is made, verification should be made to ensure that the conductor has been sized to avoid overheating and unacceptable levels of voltage drop in the circuit. These verifications and any associated calculations will be discussed in the forthcoming sections. If the verifications prove the conductor size to be inadequate, then the engineer should make an economically and practically sound decision to redesign the load distribution scheme. The redesign decision could involve any of the following options:

q) Resize the conductor

r) Reallocate loads to or rebalance loads among different circuits to adjust load distribution

s) Add additional circuits to accommodate new loads (may require a larger AC panel)

t) Decrease the distance of circuit run (impacting voltage drop)

4.4.6.2 Temperature, burial depth, and bundling corrections

The ampacity rating of a cable can decrease based on its ambient temperature, burial depth, and proximity to other current-carrying conductors. Typically the manufacturer will provide different ampacity ratings based on either a range of or a specific ambient temperature. If the ambient temperature in the area of a particular cable installation is not within range of the ambient temperature specified by the manufacturer, then the ampacity should be adjusted. Guidance for this type of ampacity adjustment factor should be obtained from the cable manufacturer or the AHJ. For an example involving ampacity adjustment based on ambient temperature, see B.2.

4.4.6.3 Voltage drop verifications

Losses by means of voltage drop across a conductor are directly proportional to the length and impedance (resistance) of the conductor. By nature of Ohms law, it is apparent that the higher the current a conductor is carrying, and the higher the resistance of a conductor (ohm/kFT), the greater the voltage drop will be. Voltage drop can create under-voltage issues for substation equipment, leading to various malfunctions, depending on the type of equipment.

Voltage drop calculation for a 1- conductor or one phase of a 3- circuit:

V

pf

XL

pf

RL

I

V

D

2

2

))

arccos(

(

)

(

2

%

+

=

LISTNUM STDS_EQ \* MERGEFORMAT

Voltage drop calculation for a 3- conductor:

V

pf

XL

pf

RL

I

V

D

2

2

))

arccos(

(

3

)

(

3

%

+

=

LISTNUM STDS_EQ \* MERGEFORMAT

where

VDis the line to neutral voltage drop of the conductor expressed in Volts, for a 1- conductor; or the line to line voltage drop of the conductor expressed in Volts, for a 3- conductor.

V is the nominal voltage of the circuit

R is the Alternating-Current resistance in to neutral per 1000 ft

XL is the Alternating-Current reactance in to neutral per 1000 ft

I is the Amperes: The load in amperes at 100 percent

L is the length of the conductor being considered for the voltage drop

pf is the equivalent power factor being considered for the circuit. If this factor has been accounted for in the load study, then a value of 1.0 should be used in the voltage drop calculation

After a conductor size is selected with an acceptable level of voltage drop, verify the terminal voltage delivered compared to the operating voltage range of the load. Its important to ensure adequate voltage is delivered to critical loads (i.e. trip coils, battery chargers, etc). It is common for the AHJ to provide standards that provide acceptable levels of voltage drop. For an example involving voltage drop calculations, see B.2.

4.4.6.4 Short circuit calculations

The conductor size should be verified to ensure that it can withstand the available short circuit current at its termination point. Sizing of the conductor based on available fault current is a function of initial or continuous conductor operating temperature, the final conductor temperature after a fault, the maximum possible fault clearing time based on protective devices, and available fault current at the circuits termination point. The conductor final temperature limits should be provided obtained from the cable manufacturer. Suggested methods and values for sizing a conductor based on short circuit current can be found in IEEE Std 525 [B12] , and IEEE Std 242 [B9]. However, any rules and/or standards provided by the AHJ should be considered before any other methods of calculation are applied. For an example involving sizing a conductor based on short circuit calculations, see Annex B

4.5 Station Power Transformer:

The objective of this section is to provide information to help the substation engineer select the appropriate station service transformer for the substation under consideration. This section discusses the required number of transformers, transformer kVA rating, transformer connections, transformer short circuit rating and some other items of consideration.

4.5.1 Number of the Transformers Requirements:

The number of station power transformers required for a substation can be determined based on the design criterion discussed in section 4.1.4. One transformer can be acceptable for small low load substation, for substations with high reliability requirements; two or more station power transformers will be required. An important factor that can affect the number of station power transformers is the available sources for station power. For some sites locations such as remote switchyards only one source is available. In this case one transformer is connected to the high voltage bus and a Generator is used as a backup.

Many utilities and power producers have developed standards and guide lines that help determine the number of station power transformers that are required for a particular substation. These guide lines is based on the utility system conditions and reliability requirements.

4.5.2 Single or Three Phase Transformer Requirements

Station load dictate whether single phase or three phase transformers are required. In general single phase transformers have been used for distribution substations when the load is single phase and has a low current rating. Three phase transformers have been used for high voltage and extra high voltage substations when the load is high and some station load requires three phase voltage input. Using a single phase transformer to serve large station load may result in a high level of secondary current. This could result in equipment with higher current rating as well as larger conductors due to excessive voltage drop. Other loads such as maintenance and construction equipment may also dictate if three phase transformers will be required for station power.

4.5.3 Station Service Transformer Rating:

Station service transformer ratings are specified by the kVA rating, transformer primary and secondary voltages, the short circuit rating and the BIL rating.

4.5.3.1 Transformer KVA and Voltage Rating:

The capacity of a transformer is determined by the amount of current it can carry continuously at rated voltage without exceeding the design temperature. Transformer rating is given in kilovolt-amperes (kVA) since the capacity is limited by the load current which is proportional to the kVA regardless of the power factor.

As a first step to determine the kVA rating of the transformer, the engineer must consider all possible loads that will be serviced by the transformer. The load can be calculated as described in 4.3 and is used to determine the transformer rating as described below:

Calculations of Transformer KVA Rating:

The kVA rating of the transformer must be selected to account for the maximum expected load which the transformer is required to serve. The following methods have been used to determine the transformer kVA rating:

Small substation with light load requirements:

For small substation with light load requirements, the kVA rating of the single phase transformer is determined by calculating the ultimate connected load and adding a safety margin of 20%.

Medium to Large Substations:

For large substations with high load requirements, the station must be classified as follows:

a) Continuous Loads: Loads that continue to operate for three hours and more are defined by the NEC as continuous loads. In substations the following loads can be considered continuous:

1) Control building HVAC and lightning.

2) Transformer fans and/or pumps

3) Battery chargers

4) Equipment heaters

5) Yard lighting

6) Illuminated signs

7) Miscellaneous inverters

8) Receptacle loads.

b) Non continuous Loads: Loads that are momentary are considered non continuous loads. In substation the following loads can be considered non continuous:

1) Breakers AC motor spring chargers running current. Since this load type is momentarily and the possibility of more than one breaker charging motor starts at the same time is remote, it is suggested the load of only two motors loads are added to the transformer kVA rating calculations.

2) Maintenance loads including transformer and breaker processing equipment.

3) Construction loads including construction trailers and equipment.

The transformer kVA rating is then calculated by the following:

)

(

2

.

1

ousLoad

NonContinu

ousLoad

TtlContinu

rating

rKVA

Transforme

+

=

LISTNUM STDS_EQ \* MERGEFORMAT

Once the recommended transformer kVA rating is calculated, Table 1 can be used to select the appropriate transformer size for the application. Normally a transformer rating greater than the calculated value is selected.

Table 1 Standard kVA ratings for distribution transformers (source?)

Overhead Type

Pad Mounted Type

Single Phase

Three Phase

Single Phase

Three Phase

5

15

25

112.5

10

30

37.5

150

15

45

50

225

25

75

75

300

37.5

112.5

100

500

50

150

167

750

75

225

1000

100

300

1500

167

500

2000

250

2500

333

500

4.5.3.2 Transformer Voltage Rating:

The primary and secondary voltage of the transformer must be specified. The following factors affect both the primary and secondary voltage:

u) Available source

v) Transformer type single phase or three phase

w) Transformer connection

x) Load voltage requirements

For a single phase transformer, the phase to ground voltage is specified. For a transformer with a delta connected primary or for three wire system, a phase to phase voltage is specified. For a four wire source or for a transformer with a Wye connected transformer both phase to phase and phase to ground voltage is specified.

The following are some of the substation secondary voltage can be one of the following configurations to be considered:

a) For single phase system 480/240 or 240/120

1) 480/240 volts, three phase, four wire, mid-tap, delta.

When using this voltage configuration larger equipment loads such as three-phase transformer fans and oil pumps need to be specified at 480 volts. Other system loads can be specified at either 480 or 240 volts, single phase. This type of system is ungrounded and will require a ground detection system. If one phase of the system becomes grounded an alarm is initiated to indicate a ground. If a second phase becomes grounded then a phase to phase fault condition exists and a trip is initiated. This system has a high leg and requires that panelboards are labeled to identify that such a condition exists. This system is very uncommon.

2) 240/120 volts, single-phase, three wire.

This is basically a residential service but applicable to small to medium sized substations. Panelboards that combine both power and lighting requirements can be used with this system thus reducing the required number of panelboards.

b) For three phase system 480/277, 208/120 or 240/120 when a Delta connected secondary with center tap grounded.

1) 480/277 volts, grounded wye connected, three-phase, four-wire

When using this voltage configuration larger equipment loads such as three-phase transformer fans and oil pumps need to be specified at 480 volts. One advantage of this system is that luminaires can be equipped with 277 volt ballasts reducing voltage drop for larger runs over the use of the more common 120 volt lights. In this system receptacles are fed through dry-type 480-120/240 volt single phase transformers.

2) 208/120 volts, grounded wye-connected, three-phase, four-wire

In this system either 208 volts single phase and three-phase or 120 volt single-phase equipment can be used. Panelboards that combine both power and lighting requirements can be used thus reducing the required number of panelboards. The saving realized by using fewer panelboards may be offset by higher conductor costs caused by increased size due to voltage drop issues compared to a 480 volt system. In this type of system the receptacles can be served directly from 120 volts.

3) 240 volts, three-phase, three wire, delta

When using this voltage configuration, three-phase or single-phase transformer fans and oil pumps need to be specified at 240 volts. This type of system is ungrounded and will require a ground detection system. If one phase of the system becomes grounded an alarm is initiated to indicate a ground. If a second phase becomes grounded then a phase to phase fault condition exists and a trip is initiated.

4)240/120 volts, three-phase, four-wire, mid-tap, closed delta

This voltage configuration is the most common for small to mid-sized substations. With this system one phase of the auxiliary transformer is center tapped to obtain 120 volts. Panelboards that combine both power and lighting requirements can be used thus reducing the required number of panelboards. This system has a high leg and requires that panelboards are labeled to identify that such a condition exists.

5) 240/120 volts, three-phase, four-wire, mid-tap, open delta

This is essentially the same as the closed delta system with the exception that with only two transformers the kVA rating is only 58 percent of the kVA capacity of when three transformers are used. This system is more economical for a medium-sized installation or when used for a temporary installation. This system uses single-phase transformers and the third transformer can be added in the future to increase the overall kVA capacity. This type of system is commonly used when there is a small three phase load and a large single phase 120/240 volt load. This system has a high leg and requires that panelboards are labeled to identify that such a condition exists.

4.5.3.3 Transformer Short Circuit Rating:

The short-circuit ratings for distribution transformers are set by IEEE Std C57.12.00 [B3]. The maximum magnitude required for units with secondary voltages rated less than 600 V is given in the table below:

Table 2 Distribution transformer short-circuit withstand capability

Single Phase KVA

Three Phase KVA

Rating (times normal

5-25

15-75

40

37.5-100

112.5-300

35

167-500

500

25

750-2500

1/ZT

Two winding distribution transformers with secondary voltages rated above 600 volts are required to withstand short-circuits limited only by the transformers impedance.

The duration of the short-circuit current is determined by

For Transformer rated 500 kVA or below:

2

1250

I

t

=

LISTNUM STDS_EQ \* MERGEFORMAT

For transformer 750 to 2500kVA:

1

=

t

LISTNUM STDS_EQ \* MERGEFORMAT

where

tis the duration in seconds

Iis the symmetrical short-circuit current (per unit)

4.5.3.4 Transformer Impedance:

The station service transformer impedance should be considered when evaluating the AC system equipment rating. The AC equipment must withstand the maximum fault current and the circuit breakers must be capable of interrupting and clearing the fault. The impedance of a transformer has a major effect on system fault levels. It determines the maximum value of current that will flow under fault conditions.

The percentage impedance, can be specified as low as 2% for small distribution transformers and as high as 20% for large power transformers. Impedance values outside this range are generally specified for special applications.

4.5.3.5 Transformer BIL Rating:

The BIL rating of the transformer is its ability to withstand overvoltage conditions resulting due to fault condition, lighting surges or any over voltage due to switching surges. Table 3 meets IEEE Std C57.12.20 [B5] and can be used to specify the BIL rating of the transformer.

Table 3 Transformer BIL ratings [B5]

Voltage Range Volts

Insulation Class KV

BIL

480-600

1.2

30

2160-2400

5.0

60

4160-4800

8.7

75

7200-12470

15

95

13200-14400

18

125

19920-22900

25

150

34400

34.5

200

4.5.4 Transformer Types:

The following transformer types are used in the substation:

y) Pole or structure mounted transformer: The primary is connected overhead to the bus and the secondary can be brought to the main panel via conduit or trench. This transformer type is easiest used when the load is single phase and less than 100kVA and the required secondary voltage is 120/240V or 277/480V. However, three phase installations are common as well.

z) Pad Mounted Transformer: To limit the voltage drop and reduce the length of the secondary conductors, the transformers are best located near the control building. The location must not interfere with the vehicle movement within the substation and located near the cable for easy access to the control building. The primary cables are connected to the bus/ transformer tertiary and brought under ground to the transformer. The secondary cables are connected to the AC system as required. This transformer type is typically used when medium voltage is available, the connected load is predominantly three phase and the total load is greater than 100kVA.

aa) Station Service Voltage Transformer (SSVT): This transformer type combines the characteristics of a voltage transformer with convenient power capability. Used in the substation application if no low or medium voltage bus is available or there is no nearby distribution feeder exists or the cost of installing the feeder is high. One or three transformers can be installed depending the required kVA rating. The primary is normally connected from phase to ground. Typical secondary ratings available 120/240V, 277/480V, 240/480V and 600 VAC.

4.5.5 Transformer Connections:

Depending on number of transformer selected for station power applications two types of connections are employed:

4.5.5.1 Single Phase Transformer Application:

Single-phase distribution transformers are manufactured with one or two primary bushings. The single-primary-bushing transformers can be used only on grounded wye systems. For this connection the H1 bushing is connected to one of the available phases while the other bushing is connected to ground as shown in Figure 4.

Figure 4 Single phase to ground connection (ABB)

When a primary delta system is available, a phase to phase voltage is applied between the two bushings H1 and H2 as shown in Figure 5.

Figure 5 Single phase transformer with phase to phase connections ABB

The secondary voltage can also be 480/240V if required.

4.5.5.2 Three Phase Transformer Connections:

Three phase transformer connection can be achieved by using two or three single phase transformers and connected as required. When a three phase transformer is required a pad mounted three phase transformer is normally used for the station power applications. A pad mounted three phase transformer is applicable to below grade connection from both the primary and the secondarys sides. The following transformer connections have been used for the substation station service applications:

Delta Delta Connection:

The delta-delta connection shown in Figure 6 is suitable for both ungrounded and effectively grounded sources. Phase to phase voltage is applied to H1, H2 and H3 terminals of the transformer. For substation application when the required voltage is 240 or 480, a 3- wire connection is used. When the required voltage is 240/120 or 480/240V a 4-wire service can be used. The Delta- Delta four 4-wire service is accomplished by grounding the midtap of one of the transformer windings. The single phase rating is normally 5% of the transformer rating, the three phase rating of the transformer will be derated also. As an example for 300kVA transformer, the single phase rating is .05*300 or 15kVA or 15000/120= 125A. The maximum rating of each transformer winding is equal to (300/(1.73*.48))/1.73 or 417A. The taped winding rating is equal to 417-125A= 292A or (292*.24) = 70kVA, for three windings 70*3=210kVA.

Figure 6 Delta-Delta connection ABB

The advantages of the Delta- Delta Connection are as follows:

ab) System voltages are more stable in relation to unbalanced load

ac) When three single phase transformers are used to form the phase bank, If one transformer is failed, The remaining two transformer can be used at 58% of the total kVA rating.

ad) The delta connection provides a closed path for circulation of third harmonic component of current. The flux remains sinusoidal which results in sinusoidal voltages.

The disadvantages of the Delta- Delta Connection are as follows:

The disadvantage of / connection is the absence of a neutral terminal on either side. Another drawback is that the electrical insulation is stressed to the line voltage. Therefore, a -connection winding requires more expensive insulation than a Y-connected winding for the same power. The delta connection is susceptible to ferroresonance.

DELTA-WYE Connection

The delta-wye connection shown in Figure 7 is suitable for both ungrounded and effectively grounded sources. The transformer primary is connected Delta and therefore phase to phase voltage voltages are connected to H1, H2 and H3 transformer terminals. The secondary is suitable for 3-wire service or if neutral is grounded 4- wire grounded service. In substation application 4- wire service is normally used. Typical substation secondary voltages for this transformer connection are 480/277or 208/120V.

When the neutral is grounded the transformer acts as ground source for the secondary system. Fundamental and harmonic frequency zero-sequence currents in the secondary lines supplied by the transformer do not flow in the primary lines. Instead these zero-sequence currents circulate in the closed delta primary windings. When supplied from effectively grounded primary system, ground relay for primary system does not see load unbalances and ground faults in the secondary system.

Figure 7 Delta-wye connection ABB

When used in 25 and 35 kV three-phase 4-wire primary systems, ferroresonance can occur when energizing or de-energizing the bank using single pole switches located at the primary terminals. With smaller kVA transformers in the bank, the probability of ferroresonance is higher.

WYE-WYE Connection

The wye-wye connection shown in Figure 8 is best applied at four wire primary and secondary were both the primary and secondary neutrals are grounded. The high voltage terminals H1, H2 and H3 are connected to the three phases and the H0 Neutral is connected to ground. In a grounded wye-wye 120 and 240 V or 480 and 240 V cannot be supplied, 208/120V or 480/277V can be supplied by this connection.

Figure 8 Grounded wye grounded wye transformer connection

The following operating conditions should be considered when this transformer connection is selected:

ae) Excessive tank heating can result depending on the transformer construction. For threelegged core construction, excessive tank heating is probable. For fivelegged transformers, tank heating is possible if the load unbalance is high. Tank heating can be limited if the transformer bank is made from three single phase transformers.

af) Zero sequence currents and harmonics will transfer to the primary. The secondary can act as high impedance ground source.

ag) Ferroresonance condition is unlikely if the transformer bank is made from three single phase transformers but possible for a four or five legged construction transformer.

ah) Coordination between the source ground protective device and the secondary ground protective device is required because the secondary current can pass to the primary.

4.6 Transfer Switch

4.6.1 General

The need for an auxiliary power system transfer switch is related to the criticality of the substation. If only one station service power source is available, a transfer switch is not be required. If there are no critical AC system requirements, the DC battery system may be sufficient to operate the critical DC systems until the AC station service power is restored.

Most substations are provided with two sources of station service AC power. The two sources of station service power are generally designated as the primary source and the alternate (or backup or secondary) source. Both sources should be of equal reliability.

To simplify the operation of the transfer between sources, a break before make operation is suggested. This will ensure that sources that are out of phase with one another do not operate in parallel. In the case of manual operation of the transfer switch, it may be desirable to disable or lock out one source while the other source is being used. In both cases, sufficient training should be provided to operators to ensure that sources are not paralleled.

Since the auxiliary power sources can be supplied at different voltages than the utilization voltage in the substation, the transfer switch can be applied at either the primary or secondary voltage. The higher voltage application results in lower current rated equipment. 13.8kV, 12.47kV, 4.16kV, 480V and 240/120V are common auxiliary power voltages and the transfer switch can be applied at any of these voltages. The auxiliary power source can be either three-phase or single-phase depending on the station service requirements. Transfer switches typically can be purchased with two, three or four poles. A four pole switch has the ability to switch the neutral and is necessary on a system that has separately derived system.

Smaller rated transfer switches can be wall mounted. Floor mounted switches are common. Transfer switches can be purchased for indoor or outdoor mounting.

The transfer switch may be as simple as two input sources with switching devices and one output to the load. The transfer system may be as elaborate as a unit switchgear consisting of two input switching devices, two transformers, two main circuit breakers, one tie circuit breaker and multiple branch circuit breakers.

Figure 9 Simple transfer switch (Figure 4.9-1)

Figure 10 Complex transfer System (Figure 4.9-2)

Another consideration when designing the transfer system is the reliability of the transfer switch. It may be prudent to make provisions to bypass the switch in the event of the switchs failure, maintenance or replacement. This may be accomplished by having a third source routed to the substation AC load center that is left normally open and locked out until it is needed. It may be more cost effective to route another set of conductors from either or both of the primary and alternate source to the substation AC load center. Similar to the transfer operation, training and procedure should be provided to the operator so that it will be unlikely to parallel sources for a bypass option.

4.6.2 Manual Transfer Switch

For less critical substations a manual transfer switch will provide the capability of transferring from the primary to the alternate source. The manual transfer switch will be a much simpler and lower cost switch than an automatic transfer switch. However, the use of the manual transfer switch will require station alarms to alert operations personnel of the loss of the primary source and dispatching personnel to the substation to operate the manual transfer switch. Proper design of the DC battery system is required to provide continuous operation of critical systems (system protection functions, control and breaker tripping) while personnel responds and manually operates the transfer switch.

If the substation has only one source of AC power, a manual transfer switch may still be desirable as a connection point for a temporary AC alternate source, such as a portable generator.

The manual transfer switch consists of two manually operated switching devices (usually circuit breakers) capable of interrupting the load current of the transfer switch. The two switching devices are typically mechanically interlocked to prevent both AC sources from being connected in parallel. Fault current interruption capability is not required in the transfer switch, but could be included or provided separately. Indication of source status (hot or dead) is not typically provided. Some point of alarm is necessary to detect the loss of the primary AC source.

4.6.3 Automatic Transfer Switch

Critical substations or substations with critical AC loads will require a transfer switch that will automatically transfer from the primary source to the alternate source when the primary source is lost.

Transfer should occur under the following conditions:

ai) There should be a time delay on loss of the primary source. This is to prevent transfer for momentary problems with the primary source.

aj) The alternate source is available. This is to prevent transfer to a dead source.

Automatic return to the primary source should occur only after the primary source has been restored for a specified time period to prevent return to an unstable source.

The automatic transfer switch consists of two electrically operated switching devices (usually circuit breakers) capable of interrupting the load current of the transfer switch. The two switching devices can be electrically and/or mechanically interlocked to prevent both AC sources from being connected in parallel. Fault current interruption capability is not required in the transfer switch, but could be provided or added separately. Detection and indication of source status (hot or dead) is required. Time delays and control sequencing is necessary to prevent transferring to a dead alternate source or to prevent nuisance transferring to unstable sources. Indicating lights and relays are usually provided. Alarm indication of transfer should be provided. Close and latch capability must also be consider in equipment rating.

4.6.4 Alternate Methods

If both sources are designated as primary sources, the AC load can be divided between the two sources with the transfer switch system consisting of the two normally closed primary circuit breakers and a normally open transfer circuit breaker.

4.7 Bus layout and distribution circuits configuration

4.7.1 Simple radial system

In this system, a single primary service and station light and power transformer supply all auxiliary AC load. There is no duplication of equipment. System cost is the lowest of all the circuit arrangements.

The simplest version of this system is shown in Figure 11. It has panelboards supplied directly from station light and power transformer through a single main breaker. One of the panelboards (A) is used to connect a feed to another panelboard (B).

Cable

Note

:

Fused switch can be used

instead of circuit breaker

Panel

Board A

Panel

Board B

Figure 11 Simplest panelboards

A variation of this system is shown in Figure 12 where a power block is used to split a power supply coming from transformer breaker into cables feeding both panelboards A and B.

Note

:

Fused switch can be used

instead of circuit breaker

Cable

Panel

Board A

Panel

Board B

Power

Block

Figure 12 Variation of simplest panelboard

Another version of a simple radial system is shown in Figure 13, where a main panelboard is connected directly to a station light and power transformer breaker is cascading power supply through breakers to sub-panelboards.

Cable

Panel

Board

Sub Panel

Board

Note

:

Fused switch can be used

instead of circuit breaker

Figure 13 Sub-panelboard

The main deficiency of the systems shown in Figure 11, Figure 12, and Figure 13 is that the panelboards do not have independent feeds from the main system and are connected to a station light and power transformer breaker through a single common cable, which may be several hundred feet long and because of that, susceptible to failures. In the case of the cable fault or a failure of one of the panelboards ahead of the internal main breaker, the whole auxiliary AC power system becomes de-energized.

To make a simple radial system more reliable and flexible, the auxiliary bus with feeder breakers, shown in Figure 14 may be used. In this system, the auxiliary bus is connected directly to transformer breaker through a bus or a short cable run and panelboards are connected to the bus via feeder breakers and separate individual cables. A failure of any panelboard or a feeding it cable will result in a tripping of a corresponding feeder breaker, leaving the rest of the AC system intact

Panel

Board

Cable

Note

:

Fused switch can be used

instead of circuit breaker

Figure 14 Reliable and flexible panelboard system

Further improvement of redundancy of a simple radial system may be achieved thru installation of emergency generator, which starts upon loss of the station light and power transformers feed to the auxiliary bus, tripping transformer breaker and closing of the generator breaker as shown in Figure 15.

Panel

Board

Note

:

Fused switch can be used

instead of circuit breaker

EO

Electrically Operated

NO

Normally Open

Cable

EO

NC

EO

NO

Figure 15 Panelboards with emergency generator

The main advantages of a simple radial system are its low cost and simplicity of its operation and expansion. However, it has a low reliability, because loss of a primary supply, main cable or station light and power transformer will result in the interruption of auxiliary AC service for the entire substation. Another drawback of a simple radial system is the necessity to de-energize it to perform any routine maintenance of its main elements (transformer, transformer breaker, auxiliary bus etc.). Thats why this system is acceptable only for small substations, feeding low priority loads where a low reliability of auxiliary AC supply system and extended down time required to perform its adequate maintenance are acceptable.

4.7.2 Expanded radial systems

If a simple radial A.C. system is applied to a larger substation, its expanded version with two station power and light transformers may be used. See Figure 16.

The advantages and disadvantages of expanded radial system are the same as those described for the simple one. However, by having two transformers, a better redundancy of power supply is achieved. The panelboards can be fed through automatic or manual transfer switches, which can also provide added flexibility in the continuity of power supply to the load if one of the transformers or busses is out of service.

Panel

Board

Cable

Panel

Board

Cable

Note

:

Fused switch can be used

instead of circuit breaker

Figure 16 Expanded radial system

4.7.3 Primary selective system

Protection against loss of a primary power supply can be gained through the use of a primary selective system shown in Figure 17. Each station light and power transformer is connected to two separate primary feeders through switching equipment to provide normal and alternate sources of power supply. Upon failure of the normal source, the transformer is switched to the alternate source. Switching can be either manual or automatic.

Each panelboard can be fed through automatic or manual transfer switches, which provides the continuity of power supply to the load if one of the transformers or the busses is out of service.

Cable

Cable

Note

:

An alternate uses primary selector

switch with a single fuse

interrupter switch

Fused switch can be used instead

of circuit breaker

Panel

Board

Panel

Board

Figure 17 Primary selective system

4.7.4 Secondary selective systems

If a pair of station power and light transformers is connected through a secondary tie circuit breaker or automatic transfer switch, it will result in a secondary selective system shown in Figure 18. If any of the primary feeders or transformers fails, power supply from the remaining source will be maintained through the corresponding transformers secondary breaker and a tie breaker. Tie breaker may be normally open. If this is the case, after failure of one of the sources and opening of affected transformers secondary breaker, a tie breaker should be closed either manually or automatically to provide a power supply for the bus section normally connected to the failed source. When a power supply from this source is restored, a manual opening of the tie breaker and closing of the returning to service transformers breaker are recommended.

Each panelboard can be fed through automatic or manual transfer switches, which provides the continuity of power supply to the load if one of the transformers or the busses is out of service.

Note

:

Fused switch can be used

instead of circuit breaker

EO

NO

Cable

Cable

Panel

Board

Panel

Board

EO

NC

EO

NC

Figure 18 Secondary selective system

4.7.5 Secondary selective system with emergency generator

If the level of redundancy provided by a secondary selective system shown in Figure 18 is not sufficient, an emergency generator with a circuit breaker may be added to it as shown in Figure 19. Normally, generators breaker is open, and for a loss of a single primary feeder or transformer, this scheme works exactly like the one shown in Figure 18. But upon the loss of both transformer feeds (both transformer secondary breakers are open) the emergency generator starts automatically and its breaker closes restoring power to both busses. Manual closing of transformer breaker is recommended upon restoration of any primary feed after stopping the emergency generator.

Each panelboard can be fed through automatic or manual transfer switch, which can allow the continuity of power to the load if one of the transformer or the bus is out of service.

Cable

Cable

Panel

Board

Panel

Board

Emergency

Generator

Note

:

Fused switch can be used

instead of circuit breaker

EO

NC

EO

NO

EO

NC

EO

NO

Figure 19 Secondary selective system with emergency generator

If even more redundancy is needed, Figure 19 may be developed into a system with two tie breakers and possibly three (3) transformers and an emergency generator as shown in Figure 20. The operational logic for this scheme is consistent with the one described for schemes shown in Figure 17and Figure 18.

Cable

Panel

Board

Panel

Board

Emergency

Generator

Note

:

Fused switch can be used

instead of circuit breaker

Cable

EO

NO

EO

NC

EO

NC

EO

NO

EO

NO

EO

NC

Figure 20 Secondary selective system with emergency generator and additional redundancy

The size of cable feeding any load or panelboard is required to be selected in accordance with NEC Article 310 or any applicable code and to be protected by upstream breaker or protective device.

4.8 AC distribution panelboards for electrical substations

Typically, there is some introduction information here.

4.8.1 AC distribution panelboard application

AC distribution panelboards are utilized for termination of service and feeder cable circuits and for origination of feeder and branch cable circuits for distribution of auxiliary AC electrical power to loads in electrical substations in 50 or 60 Hz AC systems at or below 600 volts.

An AC distribution panelboard is an assembly of bus bars, switching over-current protection devices and connections housed in an enclosure with the purpose to control and distribute auxiliary AC power to substations loads. AC distribution panelboards have a main bus bar for each phase, main lug terminals or a main device such as a switch, fuses or molded-case circuit breaker (MCCB), and neutral and/or ground buses, if appropriate. Depending on voltage rating, AC distribution panelboards can utilize a switch and/or overcurrent devices such as plug or cartridge fuse branch circuit devices or MCCB branch circuit devices.

Bus bars in AC distribution panelboards are current density rated and meet temperature rise limitations established in UL Standard 67 [B23]. Standard bus bar current densities are 750 amperes per square inch for aluminum bus bars and 1000 amperes per square inch for copper bus bars. Some AC distribution panelboard manufacturers offer reduced current densities of 600 amperes per square inch for aluminum bus bars and 800 amperes per square inch for copper bus bars.

Because AC distribution panelboards utilized in electrical substations might utilize oversized conductors because of voltage drop concerns, and might utilize heavier than normal insulation and jackets; cabinets and enclosures for AC distribution panelboards utilized in electric substations should be selected with adequate wire bending and gutter spaces. Care must be taken to assure that connectors on terminals of MCCB and other protective devices are selected to be suitable for use with such oversized conductors.

4.8.2 AC distribution panelboard ratings

Panelboards can be single phase or three phase as required for the application.

Typical voltage ratings for AC panelboards for different AC systems are given in Table 4.

Table 4 Panelboard voltage ratings source??

VOLTAGE RATINGS OF AC PANELBOARDS

NUMBER OF PHASES

NUMBER OF WIRES

AC VOLTAGE RATING-VOLTS

1

2

120, 240, 277

1

3

120/240

3*

3

208Y/120, 480Y/277

3

3

120, 240, 480, 600

3

4-wire with neutral

208Y/120, 400Y/230, 480Y/277, 600Y/347

3

4-wire delta with neutral connected at midpoint of one phase

240/120

*Derived from 3-phase, 4-wire system

Although more ratings are available, typical nominal continuous root-mean-square (rms) current ratings of AC distribution panelboard main buses, main terminal lugs, main fuse and holder, and main MCCB utilized in applications in electrical substations range between 100 A and 800 A. The maximum main current rating in AC panelboards is usually less than 1600 A. The current rating of an AC distribution panelboard should not be less than the feeder and branch circuit capacity required for the load.

Typical nominal continuous rms current ratings of feeder and branch circuits range between 20 A and 400 A. The maximum feeder and branch circuit current rating in AC distribution panelboards is usually 1200 A.

Unless marked to indicate otherwise, the provisions for cable terminations provided in AC distribution panelboards are based on the use of 60 degrees C temperature rise for wire sizes #14 to No. 1 AWG and 75 degrees temperature rise for wire sizes 1/0 AWG and larger.

Unless rated for 100 percent continuous load at its rated current, the total load on any overcurrent device utilized in an AC distribution panelboard should not exceed 80 percent of its nominal current rating.

4.8.3 AC distribution panelboard short-circuit rating and protection

AC distribution panelboards shall be protected from short-circuit current on the supply side by an over-current device having a nominal continuous rating not greater than that of the panelboard.

The rms symmetrical and asymmetrical short-circuit current at an AC distribution panelboard location can be determined in accordance with methods provided in IEEE Std 141 [B8]. The rated rms symmetrical and asymmetrical interrupting current of an AC distribution panelboard should exceed the available short-circuit current. Consideration should be given to possible future increases in available short circuit current. Most AC distribution panelboards are selected to have a fully integrated short-circuit interrupting rating where the AC distribution panelboard and all overcurrent devices enclosed in the AC distribution panelboard have ashort-circuit current rating greater than the available short-circuit current, but series ratings may be utilized. Selectivity between overcurrent devices should be considered, if possible.

4.8.4 AC distribution panelboard standards

AC distribution panelboards are designed and manufactured in accordance with NEMA Standard PB 1[B17] and UL Standard 67 [B23], and are usually supplied in suitable cabinets or enclosures which are manufactured in accordance with NEMA Std 250 [B19] or UL 50 [B22] and designed to be mounted in or on a wall or other support structure and accessible only from the front. In general, AC distribution panelboards should be specified and applied in accordance with the NFPA 70 [B21], Article 408, or other similar national or local standards, including all provisions for grounding.

Usual service conditions for AC distribution panelboards are ambient temperature of -5 C to 40 C for AC distribution panelboards utilizing MCCB and -30 C to 40 C for AC distribution panelboards utilizing enclosed switches. Usual altitude is not greater than 2000 m (6600 ft). AC distribution panelboards for outdoor application can have a greater rated ambient temperature range and must be provided with enclosures with a suitable weatherproof rating.

AC distribution panelboards tested and certified to meet requirements of International Building Code (IBC) [B2] Zone 3 or Zone 4 or other similar standards should be considered in seismically active areas.

An AC distribution panelboard utilized for service equipment to provide main control and means of cutoff of the supply conductors near the point of entrance of supply conductors of a building, structure or other area or premises should meet all requirements for service equipment required in NFPA 70 [B21], UL 67 [B23], and UL 869A [B26].

MCCBs are covered by NEMA AB-1[B16] and UL 489 [B25], and fusible switches are covered by UL 98 [B24].

4.9 Circuit Protection

4.9.1 Panel protection

The type of panel protection will be dependent on how the incoming supply cables are terminated.

In the case of panels with incoming supply cables terminating on terminal blocks with no protective device, the panel should be located within close proximity of the point of delivery. The disconnecting device (fused safety switch, fused cutout, etc.) upstream of the panel should be sized to protect the panel based on the ampacity of the panel bus bars. The number of disconnecting devices (breakers, fuses) within the panel should be limited to a maximum of six.

For panels with incoming supply cables terminating on main breakers, breaker should be sized to protect the panel based on the ampacity of the panel bus bars. In this case, the number of interrupting devices is not limited as in the case of the terminal block panel (at one time, this was limited to 42 circuits, and may still be in certain jurisdictions).

4.9.2 Feeder protection

Feeder and branch circuit protection should be determined based on the terminal and load ampacity.

Feeders are circuit conductors between the service equipment or power supply and the final branch-circuit overcurrent device(s). Feeder circuit conductors and associated protective device (circuit breaker, fuses) are sized to meet or exceed the size of the branch circuit protective device (circuit breaker, fuse). Typically this is based on 100% of the non-continuous and 125% of the continuous load.

Branch circuits are the conductors and associated protective device between the final overcurrent device protecting the circuit and the equipment. The branch circuit is sized based on 100% of the non-continuous load and 100% of the continuous load.

4.9.3 Selection of Circuit breakers

Appropriate circuit breaker selection is important for the protection and fault clearing coordination of the AC auxiliary power system. There are th