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IEEE Guide for SwitchgearUnit

SubstationRequirements

Sponsored by theSwitchgear Committee

IEEE3 Park AvenueNew York, NY 10016-5997USA

22 February 2013

IEEE Power and Energy Society

IEEE Std C37.121 2012

(Revision ofIEEE Std C37.121-1989)

Authorized licensed use limited to: University of Missouri-Kansas City. Downloaded on October 08,2015 at 19:06:07 UTC from IEEE Xplore. Restrictions apply.


2/42Authorized licensed use limited to: University of Missouri-Kansas City. Downloaded on October 08,2015 at 19:06:07 UTC from IEEE Xplore. Restrictions apply.


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IEEE Std C37.121-2012(Revision of

IEEE Std C37.121-1989)

IEEE Guide for SwitchgearUnitSubstationRequirements

Sponsor

Switchgear Committee

of theIEEE Power and Energy Society

Approved 5 December 2012

IEEE-SA Standards Board

Approved 28 October 2014

American National Standards Institute



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Abstract: The basis for the coordination of equipment in unit substations by assisting in theselection of components is intended as the use of this guide. A variety of designs for unitsubstations are possible using various combinations of incoming sections, transformer sections,outgoing sections, and transition sections. It is intended that the incoming, outgoing, transformer,and transition sections included in a unit substation meet the basic requirements of applicableindustry standards for those sections. This guide covers three-phase unit substations for step-down operation in the range of 112.5 kVA or greater at primary voltages of 601 V through 38 kV.

Keywords: control, dead-front switchboard, distribution, fuse, IEEE C37.121, metal-cladswitchgear, metal-enclosed switchgear, metering, mobile unit substation, molded-case circuitbreaker, motor control center, power circuit breaker, primary unit substation, radial substation,rectifier-type substation, secondary selective substation, secondary unit substation, spot-networksubstation, substation, surge protection, switchgear, transformer, transition section, unitsubstation

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ivCopyright 2013 IEEE. All rights reserved.

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vCopyright 2013 IEEE. All rights reserved.

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viCopyright 2013 IEEE. All rights reserved.

Introduction

This introduction is not part of IEEE Std C37.121-2012, IEEE Guide for SwitchgearUnit SubstationRequirements.

C37.121-1989 was originally developed by a working group sponsored by the Power Switchgear

Assemblies Technical Committee of the Switchgear Section (8SG) of the National Electrical Manufacturers

Association (NEMA/SG/5). The document was transferred from NEMA to the IEEE Power and EnergySociety Switchgear Committee, Switchgear Assemblies Subcommittee, in January of 2003. IEEE Std

C37.121 was reaffirmed by the IEEE Standards Association Standards Board in 2006.

The Switchgear Assemblies Subcommittee Task Force, created to review this document, determined thatthis document did not meet the intent of a standard as it references applicable IEEE Standards for all

requirements that must be met by each component of a Unit Substation. Based on this review, the Task

Force recommended that this document be changed from a Standard to a Guide. The recommendation wasaccepted by the Switchgear Assemblies Subcommittee.

In the revision of this document from a standard to a guide, the document has been revised to reflect needed

technical changes and to update the reference documents to the latest revisions. Other significant changesare as follows:

Subclauses 1.1 and 1.3 of IEEE Std C37.121-1989 have been combined to form the Scope andsubclauses 1.2 and 1.4 have been combined to form the Purpose of the new document.

Clause 2, Normative references, has been changed to remove dates and all informative standardreferences have been moved to Annex A.

Clause 3, Definitions, has been arranged in alphabetical order.

Table 1 has been redrawn and Table 2 through Table 5 have been combined into a new Table 2Primary unit substation transformers and Table 3Secondary unit substation transformers.

Metal-enclosed bus as described in IEEE Std C37.23 has been added to Clause 6Incomingsection, Clause 7Outgoing section, 11.2Unusual service conditions, and 11.5.2Loading

guides.

All of the existing referenced figures in Clause 6, Clause 7, and Clause 10 have been redrawn, andmoved into their respective sections. Figures that had previously shown a fused and unfused figure

have now been combined to show a figure with a fuse (when used) designation.

Clause 8, Ratings, has been updated to reflect the latest rating names and definitions.



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viiCopyright 2013 IEEE. All rights reserved.

Participants

At the time this guide was submitted to the IEEE-SA Standards Board for approval, the C37.121 Working

Group had the following membership:

James Smith,Chair

Douglas Edwards, Vice-Chair

P. BarnettPaul BarnhartJ. Baskins

Ted BurseL. Farr

M. FlackKeith FlowersS. Gohil

D. HrncirA. Jivanani

Harry JostenM. Lafond

D. LemmermanA. LivshitzD. MazumdarS. Meiners

A. MorganCharles MorseTed OlsenR. Parthasarathi

A. PatelP. SullivanC. TailorJ. Toney

The following members of the individual balloting committee voted on this guide. Balloters may have

voted for approval, disapproval, or abstention.

William AckermanPeter BalmaPaul BarnhartRobert Beavers

George BeckerSteven BeznerWallace BinderTed Burse

William BushWilliam Byrd

Stephen ConradGary Donner

Edgar DullniDouglas Edwards

Gary EngmannPatrick FitzgeraldKeith Flowers

Frank GerleveDavid GilmerMietek GlinkowskiJames Graham

Randall GrovesTimothy HaydenJeffrey HelzerGary Heuston

Scott Hietpas

Andrew JonesHarry JostenJohn Kay

Chad KennedyYuri KhersonskyJoseph L. KoepfingerJim Kulchisky

Saumen KunduChung-Yiu LamAlbert LivshitzFrank Mayle

Kenneth McClenahanGary Michel

Georges MontilletCharles MorseJerry Murphy

K. R. M. NairDennis NeitzelArthur NeubauerMichael S. Newman

Joe NimsTed OlsenLorraine Padden

Mirko Palazzo

Bansi PatelChristopher PetrolaIulian ProfirRobert Puckett

Reynaldo RamosJohn RoachMichael RobertsThomas Rozek

Bartien SayogoGil ShultzVeselin SkendzicJames Smith

James SmithJeremy Smith

Jerry SmithGary StoedterJames Swank

David TepenWayne TimmJoe UchiyamaJohn Vergis

Yingli WenKenneth WhiteLarry YonceJian Yu



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viiiCopyright 2013 IEEE. All rights reserved.

When the IEEE-SA Standards Board approved this guide on 5 December 2012, it had the following membership:

Richard H. Hulett,Chair

John Kulick,Vice ChairRobert Grow,Past Chair

Konstantinos Karachalios, Secretary

Satish Aggarwal

Masayuki AriyoshiPeter BalmaWilliam BartleyTed Burse

Clint ChaplinWael DiabJean-Philippe Faure

Alexander Gelman

Paul HouzJim HughesYoung Kyun KimJoseph L. Koepfinger*

David J. LawThomas LeeHung Ling

Oleg Logvinov

Ted OlsenGary RobinsonJon Walter RosdahlMike Seavey

Yatin TrivediPhil WinstonYu Yuan

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Richard DeBlasio,DOE RepresentativeMichael Janezic,NIST Representative

Catherine Berger

IEEE Standards Program Manager, Document Development

Erin Spiewak

IEEE Program Manager, Technical Program Development



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ixCopyright 2013 IEEE. All rights reserved.

Contents

1. Overview .................................................................................................................................................... 11.1 Scope ................................................................................................................................................... 11.2 Purpose ................................................................................................................................................ 2

2. Normative references .......... ........... .......... ........... .......... ........... .......... ........... .......... ........... ........... .......... .... 2

3. Definitions .................................................................................................................................................. 4

4. Service conditions ...................................................................................................................................... 54.1 General ................................................................................................................................................ 54.2 Usual service conditions ...................................................................................................................... 54.3 Unusual service conditions .................................................................................................................. 6

5. Transformer section .................................................................................................................................... 65.1 Primary unit substation transformers ................................................................................................... 65.2 Secondary unit substation transformers ............................................................................................... 7

6. Incoming section ........................................................................................................................................ 76.1 High-voltage (or primary) bushings on the transformer cover ............................................................ 7 6.2 Primary terminal chamber on the transformer ..................................................................................... 8 6.3 Metal-enclosed bus .............................................................................................................................. 86.4 Metal-clad or metal-enclosed switchgear ............................................................................................ 86.5 Metal-enclosed interrupter switchgear ........... ........... .......... ........... ........... .......... ........... ............ .......... 96.6 Cutout, fuse, or fuse link.....................................................................................................................10

7. Outgoing section ........................................................................................................................................117.1 Metal-clad switchgear .........................................................................................................................117.2 Metal-enclosed interrupter switchgear ........... ........... .......... ........... ........... ........... .......... ............ .........11

7.3 Metal-enclosed bus .............................................................................................................................117.4 Metal-enclosed, low-voltage, power circuit breaker switchgear ........................................................117.5 Molded-case, circuit-breaker, dead-front switchboards ......................................................................117.6 Motor control centers ..........................................................................................................................12

8. Ratings .......................................................................................................................................................128.1 Rated power frequency .......................................................................................................................128.2 Rated kVA ..........................................................................................................................................128.3 Rated high voltage (or primary voltage) and rated low voltage (or secondary voltage) .....................12 8.4 Rated continuous current ....................................................................................................................128.5 Rated short-time withstand current .....................................................................................................128.6 Rated momentary withstand current ...................................................................................................138.7 Rated power frequency withstand voltages ........................................................................................13

8.8 Rated lightning impulse withstand voltage (BIL) ...............................................................................13

9. Construction ..............................................................................................................................................139.1 Phase and polarity arrangements ........................................................................................................139.2 Phase sequence ...................................................................................................................................149.3 Metal barriers ......................................................................................................................................149.4 Interlocks ............................................................................................................................................149.5 Grounding ...........................................................................................................................................149.6 Nameplates .........................................................................................................................................149.7 Drawings Diagrams Instructions ..................................................................................................15



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xCopyright 2013 IEEE. All rights reserved.

9.8 Coordination .......................................................................................................................................15

10. Typical arrangements ..............................................................................................................................1610.1 Primary unit substations ...................................................................................................................1610.2 Secondary unit substations ...............................................................................................................18

11. Guide for selection, application, installation and maintenance of unit substations .................................2211.1 Application considerations ...............................................................................................................2211.2 Unusual service conditions ...............................................................................................................2211.3 System conditions .............................................................................................................................2211.4 Location transformer selection ......................................................................................................2311.5 Load requirements ............................................................................................................................2311.6 Miscellaneous design considerations ................................................................................................2411.7 Installation, field-testing, operation, and maintenance .....................................................................26

Annex A (informative) Bibliography ............................................................................................................29



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1Copyright 2013 IEEE. All rights reserved.

IEEE Guide for SwitchgearUnitSubstationRequirements

IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, health, or

environmental protection, or ensure against interference with or from other devices or networks.

Implementers of IEEE Standards documents are responsible for determining and complying with all

appropriate safety, security, environmental, health, and interference protection practices and allapplicable laws and regulations.

This IEEE document is made available for use subject to important notices and legal disclaimers.

These notices and disclaimers appear in all publications containing this document and may

be found under the heading Important Notice or Important Notices and Disclaimers

Concerning IEEE Documents. They can also be obtained on request from IEEE or viewed at

http://standards.ieee.org/IPR/disclaimers.html.

1. Overview

1.1 Scope

This guide covers three-phase unit substations for step-down operation in the range of 112.5 kVA or greater

at primary voltages of 601 V through 38 kV.

This guide does not cover the following installations:

a) Substations in which the transformer section includes load-tap-changing equipment.

b) Substations in which the transformer section is described and defined as network, subway,vault, or underground in IEEE Std C57.12.24[B14] and IEEE Std C57.12.40[B18].

c) Substations in which the transformer section is described and defined as pad-mounted inANSI C57.12.22 and IEEE Std C57.12.27[B16].

d) Gas-insulated substations as described in IEEE Std C37.122[B13].

e) Rectifier-type substations.

f) Mobile unit substations.

g) Installations in ships, watercraft, railway rolling stock, aircraft, or automotive vehicles.

h) Installations for mines.

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IEEE Std C37.121-2012IEEE Guide for SwitchgearUnit SubstationRequirements


i) Installations of railways for generation, transformation, transmission, or distribution of power usedexclusively for operation of rolling stock, or for installations used exclusively for signaling and

railway communication purposes.

j) Installations of communication equipment that is under the exclusive control of communicationutilities, located outdoors or in building spaces used exclusively for such installations.

k) Installations under the exclusive control of electric utilities for the purpose of communication, ormetering; or for the generation, control, transformation, transmission, and distribution of electric

energy located in buildings used exclusively by utilities for such purposes or located outdoors on

property owned or leased by the utility or on public highways, streets, roads, etc; or outdoors by

established rights on private property.

1.2 Purpose

The guide is intended for use as the basis for the coordination of equipment in unit substations by assisting

in the selection of components. A variety of designs for unit substations are possible using various

combinations of incoming sections, transformer sections, outgoing sections, and transition sections.

It is intended that the incoming, outgoing, transformer, and transition sections included in a unit substation

shall meet the basic requirements of applicable industry standards for those sections. In addition, this guideprovides suggested requirements when used as part of a unit substation.

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.

ANSI C37.51, American National Standard for SwitchgearMetal-Enclosed Low-Voltage AC Power

Circuit Breaker Switchgear AssembliesConformance Test Procedures.1

ANSI C84.1, American National Standard for Electric Power Systems and EquipmentVoltage Ratings

(60 Hz).2

ANSI/UL 845, Motor Control Centers.3

ANSI/UL 891, Dead-Front Switchboards.

IEEE Std C37.010, IEEE Application Guide for AC High-Voltage Circuit Breakers Rated on a

Symmetrical Current Basis.4,5

IEEE Std C37.20.1, IEEE Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear.

1NEMA publications are available from Global Engineering Documents, 15 Inverness Way East, Englewood, CO 80112, USA.

(http://global.ihs.com).2ANSI publications are available from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor,

New York, NY 10036, USA (http://www.ansi.org).3UL Standards are available from Comm-2000, 1414 Brook Drive, Downers Grove, IL 60515, USA (http://www.comm-2000.com).

4IEEE publications are available from the Institute of Electrical and Electronic Engineers, Service Center, 445 Hoes Lane, Piscataway,

NJ 08854, USA (http://www.standards.ieee.org ).5The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc.

http://global.ihs.com/http://global.ihs.com/http://global.ihs.com/http://www.ansi.org/http://www.ansi.org/http://www.ansi.org/http://www.comm-2000.com/http://www.comm-2000.com/http://www.comm-2000.com/http://www.standards.ieee.org/http://www.standards.ieee.org/http://www.standards.ieee.org/http://www.standards.ieee.org/http://www.comm-2000.com/http://www.ansi.org/http://global.ihs.com/


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IEEE Std C37.20.2, IEEE Standard for Metal-Clad Switchgear.

IEEE Std C37.20.3, IEEE Standard for Metal-Enclosed Interrupter Switchgear.

IEEE Std C37.23, IEEE Standard for Metal-Enclosed Bus.

IEEE Std C37.24, IEEE Guide for Evaluating the Effect of Solar Radiation on Outdoor Metal-EnclosedSwitchgear.

IEEE Std C37.40, IEEE Standard Service Conditions and Definitions for High-Voltage Fuses,

Distribution Enclosed Single-Pole Air Switches, Fuse Disconnecting Switches, and Accessories.

IEEE Std C57.12.00, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power,

and Regulating Transformers.

IEEE Std C57.12.01, IEEE Standard General Requirements for Dry-Type Distribution and Power

Transformers Including Those with Solid Cast and/or Resin Encapsulated Windings.

IEEE Std C57.12.10, IEEE Standard Requirements for Liquid-Immersed Power Transformers.

IEEE Std C57.12.51, IEEE Standard for Ventilated Dry-Type Power Transformers, 501 kVA and Larger,

Three-Phase, with High-Voltage 601 V to 34 500 V; Low-Voltage 208Y/120 V to 4160 VGeneral

Requirements.

IEEE Std C57.12.52, Standard for Sealed Dry-Type Power Transformers, 501 kVA and Larger, Three-

Phase, with High-Voltage 601 to 34500 Volts, Low-Voltage 208Y/120 to 4160 VoltsGeneral

Requirements.

IEEE Std C57.91, IEEE Guide for Loading Mineral-Oil-Immersed Transformers.

IEEE Std C57.94, IEEE Recommended Practice for Installation, Application, Operation, and

Maintenance of Dry-Type General Purpose Distribution and Power Transformers.

IEEE Std C57.96, IEEE Guide for Loading Dry-Type Distribution and Power Transformers.

IEEE Std C57.142, IEEE Guide to Describe the Occurrence and Mitigation of Switching Transients

Induced by Transformers, Switching Device, and System Interaction.

IEEE Std C62.22, IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating

Current Systems.

NEMA C37.57, SwitchgearMetal-Enclosed Interrupter Switchgear AssembliesConformance Testing.

NEMA C37.55, SwitchgearMedium-Voltage Metal-Clad AssembliesConformance Test Procedures.

NFPA 70, National Electrical Code (NEC).6

NFPA 70B, Recommended Practices for Electrical Equipment Maintenance.

UL 1562, Transformers, Distribution, Dry-TypeOver 600 Volts.

6The NEC is published by the National Fire Protection Association, Batterymarch Park, Quincy, MA 02269, USA

(http://www.nfpa.org ). Copies are also available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane,Piscataway, NJ 08854, USA (http://standards.ieee.org ).

http://www.nfpa.org/http://www.nfpa.org/http://www.nfpa.org/http://standards.ieee.org/http://standards.ieee.org/http://standards.ieee.org/http://standards.ieee.org/http://www.nfpa.org/


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3. Definitions

For the purposes of this document, the following terms and definitions apply. The IEEE Standards

Dictionary: Glossary of Terms & Definitionsshould be referenced for terms not defined in this clause. 7

articulated unit substation: A unit substation in which the incoming, transforming, and outgoing sections

are manufactured as one or more subassemblies intended for connection in the field.

barrier: A partition within the enclosure and part of the contained equipment, used for the insulation or

isolation of electric circuits or electric arcs.

conformance tests: Certain performance tests to demonstrate compliance with the applicable standards.

The test specimen is normally subjected to all planned routine tests prior to initiation of the conformance

test program.

NOTEThe conformance tests may, or may not, be similar to certain design tests. Demonstration of margin

(capabilities beyond the standards) is not required.8

design tests: Tests performed to determine the adequacy of the design of a particular type, style, or model

of any unit of equipment, or its component parts, to meet its assigned ratings and to operate satisfactorily

under normal service conditions or under any specified conditions. Such tests may also be used todemonstrate compliance with applicable standards of the industry.

NOTE 1Design tests are performed on representative apparatus or prototypes to verify the validity of design analysisand calculation methods, and to substantiate the ratings assigned to all other apparatus of basically similar design.

These tests are not intended to be performed on every design or during normal production. The applicable portion ofthese design tests may also be used to evaluate modifications of a previous design to assure that performance has not

been adversely affected. Test data from previous similar designs may be used for current designs when appropriate.Once made, design tests need not be repeated unless the design is so changed as to modify performance.

NOTE 2Design tests are sometimes called type tests.

enclosure: A surrounding case or housing used to provide a degree of protection to the enclosed conductors

or equipment, and to provide a degree of protection to personnel against incidentally contacting live parts.

high voltage: A general term that pertains to the primary voltage, or primary-voltage side, of a transformer

or of a unit substation.

incoming section: Equipment that includes necessary mechanical and electrical connecting parts for

coordination in a unit substation on the high-voltage (or primary) side of a transformer section.

low voltage: A general term that pertains to the secondary voltage, or secondary-voltage side, of a

transformer or of a unit substation.

other tests: Tests, so identified in individual product standards, that may be specified by the user in

addition to routine tests. (Examples: Impulse; insulation power factor; audible sound)

outgoing section: Equipment that includes necessary mechanical and electrical connecting parts for

coordination into a unit substation on the low-voltage (or secondary) side of a transformer section.

primary unit substation: A unit substation in which the low-voltage section is rated above 1000 V.

7TheIEEE Standards Dictionary: Glossary of Terms & Definitionsis available athttp://shop.ieee.org.

8Notes in text, tables, and figures are given for information only and do not contain requirements needed to implement the standard.

http://shop.ieee.org/http://shop.ieee.org/http://shop.ieee.org/http://shop.ieee.org/


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production tests: Tests performed during production for quality control by the manufacturer(s) on every

device, or on representative samples, or on parts or materials, as required to verify that the manufactured

product meets the design specifications and applicable standards.

NOTE 1Certain quality assurance tests on identified critical parts of repetitive high-production devices may beperformed on a planned statistical sampling basis.

NOTE 2Production tests are sometimes called routine tests.

secondary unit substation: A unit substation in which the low-voltage section is rated 1000 V and below.

substation: An assemblage of equipment in which the incoming, transforming, and outgoing sections are

manufactured as one or more subassemblies, through which electric energy in bulk is passed for the

purpose of switching or modifying its characteristics.

transformer section: A three-phase power transformer used for step-down operation that includes

necessary mechanical and electrical connecting parts for coordination in a unit substation.

transition (throat) section: A mechanical, electrical, and coordinated connection between a transformer

section and an incoming section, or between a transformer section and an outgoing section, or between

different types of incoming sections, or between different types of outgoing sections. A transition (throat)section may be:

a) Integral parts of two adjacent sections,

b) An integral part of one section, or

c) A separate section.

unit substation: A substation consisting primarily of one or more transformers that are mechanically and

electrically connected to, and coordinated in design with, incoming and outgoing equipment.

NOTEFor this guide, the term unit substation shall be limited to mean articulated unit substation only.

4. Service conditions

4.1 General

A variety of designs for unit substations are possible using various combinations of incoming sections,

transformer sections, and outgoing sections. The design of individual sections of a unit substation must be

compatible with the service conditions in which they are applied.

4.2 Usual service conditions

Unit substations conforming to this guide should be suitable for operation at their nameplate ratings underthe following usual conditions:

a) Temperature

1) The effect of solar radiation is negligible. (The principles stated in IEEE Std C37.24 may beused for guidance.)

2) C except that, when the transformer is liquid-

immersed, the minimum temperature of the liquids uppermost layers is C.



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3) The maximum ambient air temperature is +40C.

4) The average ambient air temperature in any 24-hour period is not more than +30C.

b) Altitude. The maximum altitude is 1000 m (3300 ft). See IEEE Std C37.20.1, IEEE Std C37.20.2,

IEEE Std C37.20.3, and IEEE Std C57.12.00 or IEEE Std C57.12.01 for corrections to dielectric

strength and continuous-current rating at altitudes greater than 1000 m (3300 ft).

4.3 Unusual service conditions

For unit substations that are to be applied under conditions other than those in 4.2,see11.2 for guidance.

5. Transformer section

Transformer sections should meet the requirements of this guide and shall comply with the requirements of

their applicable standards, including the standard kVA ratings listed inTable 1.Furthermore, the rated high

voltage and rated low voltage shall be used in combination with the standard kVA ratings that are listed in

Table 2 andTable 3.Primary unit substation transformers should be of the types in 5.1.Secondary unit

substation transformers should be of the types in5.2.

5.1 Primary unit substation transformers

5.1.1 Liquid-immersed without load-tap-changing

These transformers shall meet the following requirements:

a) Voltage and kVA combinations as indicated in Column 1, Column 2, and Column 5 ofTable 2

b) The requirements of IEEE Std C57.12.10 and IEEE Std C57.12.36[B17]

5.1.2 Liquid-immersed with load-tap-changing

These transformers are not covered in this guide.

5.1.3 Ventilated dry-type, including those with solid-cast or resin-encapsulated windings



b) The requirements of IEEE Std C57.12.01 and IEEE Std C57.12.50 and the requirements for solidcast-resin units

5.1.4 Sealed dry-type


a) Voltage and kVA combinations as indicated in Column 1, Column 2, and Column 4 of Table 2

b) The requirements of IEEE Std C57.12.01, IEEE Std C57.12.52, and UL 1562



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5.2 Secondary unit substation transformers

5.2.1 Liquid-immersed without load-tap changing



b) The requirements of IEEE Std C57.12.10 and IEEE Std C57.12.36[B17]

5.2.2 Ventilated dry-type, including those with solid-cast or resin-encapsulated windings



b) The requirements of IEEE Std C57.12.51

5.2.3 Sealed dry-type



b) The requirements of IEEE Std C57.12.01 or IEEE Std C57.12.52, and UL 1562

6. Incoming section

Incoming sections should meet the requirements of this guide and shall comply with the requirements of

their applicable specifying standards. Incoming sections may include the components described in 6.1 to

6.6.

NOTEFigure 1 to Figure 10 are shown with the high-voltage section on the left of the figures and the low-voltagesection on the right.

6.1 High-voltage (or primary) bushings on the transformer cover

These bushings shall meet the thermal, mechanical, and dielectric requirements of the applicable

transformer standard. A typical example is shown inFigure 1.

Figure 1Bushings on transformer cover9

9Figure 1toFigure 10 are shown with the high-voltage section on the left of the figures and the transformer section on the right.



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6.2 Primary terminal chamber on the transformer

This chamber shall meet the thermal, mechanical, and dielectric requirements of the applicable transformer

standard. A typical example is shown inFigure 2.

Figure 2 Terminal chamber9

6.3 Metal-enclosed bus

Metal-enclosed bus shall meet the requirements of IEEE Std C37.23.

6.4 Metal-clad or metal-enclosed switchgear

This switchgear shall meet the requirements of IEEE Std C37.20.2 and NEMA C37.55 or IEEE Std

C37.20.1 and ANSI C37.51 as applicable. Typical Metal-Clad or Metal-Enclosed Switchgear examples are

shown inFigure 3 throughFigure 6.

Figure 3 Metal-clad or metal-enclosed switchgear For one incoming line9



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Figure 4 Metal-clad or metal-enclosed switchgear For two incoming lines9

Figure 5 Metal-clad or metal-enclosed switchgear For two incoming lines, selector function

9

Figure 6 Metal-clad or metal-enclosed switchgear For one incoming line, looped9

6.5 Metal-enclosed interrupter switchgear

This switchgear shall meet the requirements of IEEE Std C37.20.3 and NEMA C37.57. Typical examples

are shown inFigure 7 throughFigure 9.



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Figure 7 Interrupter or disconnect switches Single circuit9,10

Figure 8 Interrupter or disconnect switches Dual circuit9, 10

Figure 9 Interrupter or disconnect switches Selector circuit9,10

6.6 Cutout, fuse, or fuse link

This equipment shall meet the applicable service requirements of the components covered in IEEE Std

C37.40. A typical example is shown inFigure 10.

10Metal-Enclosed Interrupter Switchgear, the switching devices and fuses may be non-draw-out.



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Figure 10 Interrupter cutouts Fused or unfused9, 10

7. Outgoing section

Outgoing sections should meet the requirements of this guide and shall comply with the requirements oftheir applicable specifying standards. Because of the complexity of outgoing arrangements, it is not

practical to present typical examples here. Outgoing sections may include the components listed in 7.1

through7.6.

7.1 Metal-clad switchgear

This switchgear shall meet the requirements of IEEE Std C37.20.2 and NEMA C37.55.

7.2 Metal-enclosed interrupter switchgear

This switchgear shall meet the requirements of IEEE Std C37.20.3 and NEMA C37.57.

7.3 Metal-enclosed bus

Metal-enclosed bus shall meet the requirements of IEEE Std C37.23.

7.4 Metal-enclosed, low-voltage, power circuit breaker switchgear

This switchgear shall meet the requirements of IEEE Std C37.20.1 and ANSI C37.51.

7.5 Molded-case, circuit-breaker, dead-front switchboards

These switchboards shall meet the requirements of ANSI/UL 891.



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7.6 Motor control centers

These control centers shall meet the requirements of ANSI/UL 845.

8. Ratings

The rating of each section of a unit substation shall comply with the applicable standards for its

components (referenced in Clause5,Clause6,and Clause7)and shall be equal to or greater than the rating

of the unit substation. The kVA, high-voltage, and low-voltage ratings of the transformer section shall be

the basis for those ratings of the unit substation. Other sections shall be coordinated with those ratings. The

unit substation shall have the ratings listed in8.1 through8.3.

8.1 Rated power frequency

The rated power frequency of a unit substation shall be the frequency of the circuit for which it is designed.

8.2 Rated kVA

The rated kVA of a unit substation shall be the maximum rated kVA of the three-phase power transformer

in accordance withTable 1.The kVA rating of a double-ended unit substation shall be the total kVA of the

two transformers.

8.3 Rated high voltage (or primary voltage) and rated low voltage (or secondaryvoltage)

In combination with the rated kVA of a unit substation, the rated voltages shall be as follows:

a) For primary unit substations, as indicated inTable 2

b) For secondary unit substations, as indicated inTable 3

8.4 Rated continuous current

The rated continuous current for high-voltage and low-voltage equipment of a unit substation shall be equal

to the respective high-voltage and low-voltage full-load currents of the transformer section.

8.5 Rated short-time withstand current

The rated short-time withstand current of a unit substation is the rms short-circuit current that is intended to

be carried for a specified period of time without causing electrical, thermal, or mechanical damage. The

current shall be the rms value determined over the specified period of time.

The rated short-time withstand current rating of a unit substation shall be the rated short-circuit current

(carrying) rating of the high-voltage interrupting device. If no high-voltage interrupting device is present,

the rated short-time withstand current rating of the unit substation shall be the through-fault current of the

transformer, in terms of primary amperes.



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8.6 Rated momentary withstand current

The rated momentary withstand current of a unit substation is the maximum rms total current that it is

required to withstand. The current is the rms value, including the dc component, at the major peak of the

maximum cycle as determined from the envelope of the current wave of the maximum offset phase during

a test period of at least 10 cycles. The symmetrical current is the rated short-time current and the peak

current value is 2.6 times its rated short-time current at the major peak of the maximum cycle (this is alsoreferred to as the peak withstand current).

The rated momentary withstand current of a unit substation shall be the rated short circuit current withstand

rating of the high-voltage interrupting device. If no high-voltage interrupting device is present, the rated

momentary withstand current rating of the unit substation shall the through-fault current of the transformer,

in terms of primary amperes.

8.7 Rated power frequency withstand voltages

The rated power frequency withstand voltage of a unit substation is the maximum alternating-current

voltage that it is intended to withstand for one minute. The alternating-current voltage shall have a crest

value equal to 1.41 times the rms value, shall be as close to a sine wave as practical, and shall have afrequency not less than the rated frequency.

The rated power frequency withstand voltage of the unit substation, on its high-voltage end, shall be the

lesser rating of adjacent high-voltage sections.

The rated power frequency withstand voltage of the unit substation, on its low-voltage end shall be the

lesser rating of adjacent low-voltage sections.

8.8 Rated lightning impulse withstand voltage (BIL)

The rated lightning impulse withstand voltage of the unit substation on its high-voltage end shall be thelesser rating of adjacent high-voltage sections. The rated lightning impulse withstand voltage of the unit

substation on its low-voltage end shall be the lesser rating of adjacent low-voltage sections. Lightning

impulse voltage ratings levels are not applicable to low-voltage equipment below 1000 V.

9. Construction

The construction of all sections of a unit substation should be coordinated and the sections shall be

compatible with each other.

9.1 Phase and polarity arrangements

As viewed from the main switching-device side of the operating mechanism, the phase arrangement on

buses and primary connections of all sections should be 1, 2, 3 counting from front to back, top to bottom,

or left to right.

However, the following exception should be noted. For other arrangements of unit substations, the

transformer bushing phasing may be other than 1, 2, 3 counting from front to back, top to bottom, or left to



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right. With these arrangements, a phase transposition shall be made at the connection to the outgoing and

incoming sections so that 1, 2, 3 phasing is maintained in these sections.

Panel devices should be mounted with 1, 2, 3 phasing counting from left to right and top to bottom, as

viewed from the front of the panel.

9.2 Phase sequence

The phase sequence on connection diagrams (see9.7)should be such that, when considering voltages to

neutral on a polyphase system with respect to the element of time, the voltage of phase 1 will reach a

maximum before the voltage of phase 2, phase 2 before phase 3, and so forth, in numerical order. This

sequence should be designated as phase sequence in the order 1, 2, 3 and so forth.

9.3 Metal barriers

Metal barriers should be provided to segregate the incoming section from the transformer and the outgoing

sections from the transformer.

9.4 Interlocks

Where transformer line currents exceed the load-interrupting capability of the incoming line-section

equipment, interlocking should be provided to prevent opening the equipment on currents in excess of its

rating. Such interlocking should be effected either mechanically or by a combination of mechanical and

electrical devices.

9.5 Grounding

The ground bus for each section of the unit substation should have a provision for connection to a station

ground by suitable conductors.

9.6 Nameplates

Each incoming, outgoing, and transformer section shall have a nameplate in accordance with its applicable

standard. The nameplate should be mounted so as to be visible after normal installation.



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9.7 Drawings Diagrams Instructions

The drawings, diagrams, and instructions supplied with each unit substation should include, as applicable,

all or a combination of the following drawings, depending on the type and complexity of the equipment:

a) General arrangement (outline) and foundation plan

b) One-line and/or three-line diagram11

c) Schematic/elementary diagram11

d) Connection/wiring diagram11

e) Interconnection diagram11

f) Terminal diagram

g) Control-metering-relay panel arrangement and bills of material

h) Instruction books containing information about receiving, handling, storage, installation, operating,

and maintenance, covering all sections and all devices mounted on or within the substation

9.8 Coordination

9.8.1 Mechanical coordination and connection

If the transition (throat) section is connected to a metal-enclosed bus, it should be the responsibility of the

manufacturer of the bus duct to match the termination facilities provided by the manufacturers of the

transformer and incoming or outgoing section, unless otherwise mutually determined by the affected

manufacturers, with the approval of the purchaser.

If the transition (throat) section consists of a close-coupled throat arrangement, it should be the

responsibility of the transformer manufacturer to match the termination facilities provided by the

manufacturers of the incoming and outgoing sections unless otherwise mutually predetermined by the

affected manufacturers, with the approval of the purchaser.

If the transition (throat) section is a compartment between equipment, such as low-voltage metal-enclosed

switchgear, metal-clad switchgear, or metal-enclosed-interrupter switchgear, the responsibility for

matching should be mutually determined by the affected manufacturers of this equipment, with the

approval of the purchaser.

Flexible connections should be provided for the connection between the incoming, outgoing, or transition

(throat) sections and the bushings of the transformer except for exposed cover-mounted transformer

bushings.

9.8.2 Secondary and control wire interconnections between sections

For adjacent and close-coupled sections, wiring and necessary details should be provided, and wire should

be isolated from power circuits rated over 600 volts ac, nominal. Isolation should be by grounded metal

enclosures, metal barriers, metal conduit, electrical metallic tubing, or other approved means, except for

short lengths of wire at, for example, instrument transformer terminals, temperature-measuring terminals,

secondary devices, and fan-control equipment.

11These diagrams are defined in IEEE Std 315[B9].



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For remote sections, each section should be provided with terminal blocks and terminal-block details for

users external interconnections.

10. Typical arrangements

Because of the varied ratings and types of equipment, there are many possible arrangements for unit

substations. These are listed in10.1 and10.2,and illustrated inFigure 11 throughFigure 23.

10.1 Primary unit substations

A primary unit substation is usually one of the following types:

a) Radial (seeFigure 11)

b) Distributed-Network (seeFigure 12)

c) Spot-Network (seeFigure 13)

d) Secondary (low voltage) selective (seeFigure 14)

e) Duplex (seeFigure 15)

Also seeFigure 16 for typical alternate arrangements of two-transformer sections.

Figure 11 Primary unit substation Radial type12

12Figure 11 andFigure 12 are shown with the transformer section on the left of the figure and the outgoing section on the right of the

figure.



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Figure 12 Primary unit substation Distributed network type12

Figure 13 Primary unit substation Spot network type13

Figure 14 Primary unit substation Secondary selective type13

13Figure 13 toFigure 15 are shown with the transformer sections on the left and right of the figure and the outgoing section in the

middle of the figure.



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Figure 15 Primary unit substation Duplex type (breaker-and-a-half scheme)13

Figure 16 Primary unit substation Transformer unit substation (alternate arrangements)

10.2 Secondary unit substations

A secondary unit substation is usually one of the following types:

a) Radial (seeFigure 17 andFigure 18)

b) Distributed-Network (seeFigure 19)

c) Spot-Network (seeFigure 20)

d) Secondary (low voltage) selective (seeFigure 21,Figure 22,andFigure 23)



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Figure 17 Secondary unit substation Radial type

Figure 18 Secondary unit substation Radial type with reverse arrangement



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Figure 19 Secondary unit substation Distributed network type with indoor-outdoor arrangement

Figure 20 Secondary unit substation Spot network type

Figure 21 Secondary unit substation Secondary select type



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Figure 22 Secondary unit substation Secondary select type with common rrawout aisle arrangement

Figure 23 Secondary unit substation

Secondary select type with indoor-outdoor arrangement



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11. Guide for selection, application, installation, and maintenance of unitsubstations

11.1 Application considerations

In applying unit substations, the following subjects may require consideration:

a) Service conditions, that is, environmental considerations

b) System conditions, for example, voltage, frequency, and available short-circuit currents

c) Installation conditions, for example, restricted access or exposure to the general public

d) Load requirements, including duty cycles if applicable

11.2 Unusual service conditions

a) Significant solar radiation

b) Ambient temperature outside the limits in4.2

c) Altitudes above 1000 m (3300 ft)

d) Surrounding environment containing volatile organic compounds, moisture and dust.

e) Exposure to abnormal vibration, natural or man-made

f) Local conditions affecting grounding resistance

For metal-enclosed switchgear equipment (which may be used as the incoming or outgoing sections of unit

substations) guidelines and requirements for responding to unusual service conditions are given in

IEEE Std C37.20.1, IEEE Std C37.20.2, IEEE Std C37.20.3, and IEEE Std C37.23. Information concerning

unusual service conditions for transformers is included in IEEE Std C57.12.00 and IEEE Std C57.12.01.

11.3 System conditions

11.3.1 Voltage and frequency

The voltage operating ranges of a unit substation are dependant on the high-voltage and low-voltage ratings

of the included transformer. When the voltage variation is greater than acceptable for the application, then

the use of equipment load regulation should be considered. (The application of that equipment is not

covered in this guide.)

11.3.2 Short-circuit considerations

Short-circuit current valuesshort-time and momentarymust be calculated for the incoming and

outgoing sections to permit the selection and coordination of switching and protective devices, and also to

provide proper protection for the transformer.

Procedures for making the calculations are included in standards such as IEEE Std C37.010 and IEEE Std

141[B6].



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The E/X Simplified Method, described in IEEE Std C37.010, is usually satisfactory.

Short-circuit values for common substations and conditions are given in IEEE Std 141[B6].

While interrupting ratings of protective devices are based on symmetrical current, it is only necessary when

applying these devices to consider the maximum symmetrical short-circuit current, since the product

standards covering testing take into account the maximum possible asymmetry.

The short-time withstand current and momentary withstand current ratings of a unit substation are defined

in Clause8. In planning the unit substation, its rating should be made at least equal to the calculated

available short-circuit current values. Due consideration should be given to possible future system changes

that might demand increases in the calculated available values.

11.4 Locationtransformer selection

The physical location of the unit substation is of major importance in determining the type of transformer to

be used for a particular installation.

11.4.1 Indoor and outdoor locations

Less-flammable liquid-insulated transformers, nonflammable fluid-insulated transformers and sealed

transformers are, in general, suitable for use both indoors and outdoors.

Liquid-immersed transformers insulated with flammable liquids are normally suitable only for outdoor

locations. They may be installed indoors only when located in fireproof vaults and where local codes

permit. See electrical code information in NFPA 70 and NFPA 70B. Ventilated dry-type transformers are

normally suitable only for indoor locations, except when mounted in a specifically designed enclosure for

outdoor application.

11.4.2 Environmental conditions

Environmental conditions such as dust, moisture, and fumes may be important factors in the selection of

transformer types.

11.4.3 Audible sound level

If the normal sound level is a factor at the location and operation of any transformer, special consideration

should be given to the sound abatement in the form of barriers or low-noise transformers.

11.4.4 Grounding

It is necessary to take grounding considerations into account when installing unit substations. Refer to

IEEE Std 141[B6] and IEEE Std 142[B7] for grounding instructions.

11.5 Load requirements

The load capacity of a unit substation is determined by the kVA rating of the included transformer(s).



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11.5.1 Transformer kVA rating

The kVA rating of the transformer section should, at a minimum, be sufficient to handle the immediate

required load. Consideration should be given to any future load growth that might be required by expanding

or changing facilities. When substantial load growth is anticipated, it is usually desirable to install, initially,

a single transformer substation with a main circuit breaker that can be expanded readily to a two-

transformer (double-ended) substation when the additional capacity is needed.

For application of demand factors and diversity factors to the total connected load, refer to IEEE Std 141

[B6] and IEEE Std 142[B7].Fan-cooling may be used to provide higher kVA ratings to meet load growth.

For secondary-selective systems or network systems, fan-cooling is often used to provide higher ratings

during periods when one or more transformers are out of service.

Other requirements for the selection and rating of equipment could involve unusual loading conditions, for

example, the starting of large motors, or the operation of welding equipment or sensitive loads such as

computers and scientific instruments.

11.5.2 Loading guides

Consult the following:

a) For liquid-immersed transformers, consult IEEE Std C57.91.

b) For dry-type transformers, consult IEEE Std C57.96.

c) For metal-enclosed switchgear assemblies, consult IEEE Std C37.20.1, IEEE Std C37.20.2,

IEEE Std C37.20.3 and IEEE Std C37.23.

11.6 Miscellaneous design considerations

11.6.1 Incoming section

In unit substation applications, it is desirable to be able to electrically isolate the transformer from the

incoming circuit. An exception occurs when the transformer of the unit substation is the only load on the

incoming circuit; in this case the switching device at the source of the circuit may serve to isolate the

transformer electrically.

However, the switching device, whether a fused switch or a relayed circuit breaker, must provide proper

protection for the transformer to prevent damage from short-circuit current (short-time withstand or

momentary withstand) due to abnormal secondary fault conditions.

11.6.2 Transformer section

The selection of transformers for unit substations is based on such factors as location (see 11.4)and load

requirements (see11.5). Other considerations include surge protection, supplementary cooling, and

maintenance requirements.

11.6.2.1 Surge protection

When connected to circuits that are subject to lightning or other transients, surge protection should be

considered. Surge protection should limit voltage surges to values below the impulse-withstand ratings of



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the unit substation. See IEEE Std C62.22 for specific information concerning surge protection. See IEEE

Std C57.142 for information on mitigation of switching transients.

If the unit substation is to operate in an environment that requires a greater lightning impulse-withstand

capability, surge arresters should be used to ensure that the equipment is properly protected at the required

lightning impulse level.

11.6.2.2 Forced-air cooling

The forced-air cooling rating of the transformer is often a factor in determining the transformer to be

selected. For single-ended unit substations, the forced-air cooling rating of the transformer is based on the

system total load. For double-ended unit substations, the load conditions when each transformer may have

to supply the total load of the substation during an emergency or during maintenance should be considered.

11.6.2.3 Maintenance requirements

For information on maintenance requirements and techniques, see the manufacturers literature and the

following guides:

a) For liquid-immersed transformers: IEEE Std C57.93 [B22], IEEE Std C57.104 [B23], and

IEEE Std C57.106[B24].

b) For dry-type transformers: IEEE Std C57.94.

Differences in the maintenance requirements of the various types of transformer sections may be a factor in

determining the type of transformer to be used for a particular application.

Maintenance of liquid-immersed transformers should at least include periodic checking of the tank for

pressure-tightness, and sample-testing of the liquid for dielectric strength and other characteristics.

Dielectric strength can sometimes be restored by filtering.

Maintenance of sealed transformers should include periodic inspections for pressure-tightness. An

indication of positive pressure on the transformer pressure gauge is generally sufficient, since these units

are usually sealed with positive gas-pressure.

If a unit substation component, such as switchgear or dry-type transformer, is to be stored or, after

installation, deenergized for a significant period of time in humid conditions, a space-heater should be

provided within the housing, and left energized from a suitable power source.

11.6.3 Outgoing section

11.6.3.1 Selection of low-voltage protective devices

Circuit breakers are generally recommended for circuit protection for flexibility in operation. They can be

reclosed quickly where a sustained interruption of power cannot be tolerated.

Current-limiting devices can be utilized where reenergizing speed is not critical, or where high interrupting

capability (up to 200 000 A) is needed, or both. Their speed of operation and current-limiting action can

provide protection to coordinate equipment in circuits that have high available short-circuit currents. When

current-limiting devices are used, the ratings of protected equipment must be coordinated with the



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maximum let-through energy and the peak let-through current of the device. Where there may be damage

from single-phasing, the protective device needs to open all three phases.

11.6.3.2 Transformer main secondary fused switch or circuit breaker

A transformer main secondary protective device should be used for the following reasons:

a) In case of emergency, it is quicker and simpler to deenergize the entire load by opening a

transformer main secondary protective device at each substation than to open each feeders

protective device. Conversely, in case of trouble in the primary cable or transformer, it is often

desirable to disconnect the low-voltage bus from the transformers and to supply the bus from an

alternate source.

b) It provides fault-protection for the bus and backup-protection for the feeder devices. In addition,

where a main secondary protective device is present, it may provide overload protection for a

transformer, and less-sensitive primary protection may be used. This is particularly true for ground-

fault protection.

c) Where interlocking is used, it provides for, and simplifies, key-interlocking between the primary

switch and secondary switchgear.d) When a transformer main secondary relayed circuit breaker is used, coordination with primary

protective devices is more readily obtainable.

e) It provides flexibility. Note that a main secondary protective device is essential for several of the

circuit arrangements inFigure 11 throughFigure 23.A radial-type unit substation that includes a

main fused switch or circuit breaker can later be expanded into one of the more elaborate circuit

arrangements without extensive field modification.

11.6.3.3 Continuous-current rating of transformer secondary protective devices andconnections

The transformer main secondary circuit breakers, or fuse switches and connections, should have

continuous-current ratings that are approximately 25 percent greater than the continuous-current rating ofthe transformer. This is recommended because transformers often carry short-time loads above their

nameplate ratings due to short duty cycles or low-ambient temperatures.

When selecting the continuous-current rating of the transformers main secondary protective devices and

connections, consideration should also be given to whether the transformer has, or will have, a continuous

supplementary-cooled rating.

11.6.4 Protective relaying

See IEEE Std C37.90[B10] and IEEE Std C37.91[B11],for general guidance in the applications of

protective relays for unit substations.

11.7 Installation, field-testing, operation, and maintenance

The installation and field-testing of unit substations should, in general, be performed in accordance with

manufacturers instructions (refer to NFPA 70B and IEEE Std C37.20.1, IEEE Std C37.20.2, and IEEE Std

C37.20.3. Meggering is recommended when equipments have been stored in damp and dirty locations. If

the megger readings are less than the manufacturers recommendations, a drying-out and cleaning of the



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insulation system is recommended. When dielectric testing is performed, it should be conducted at the

reduced values specified for field- testing in the applicable referenced standards.

When various sections of the unit substation have to be assembled in the field, the connections should be

made in strict accordance with the manufacturers drawings and instructions.

Operation and maintenance instructions are also furnished by the manufacturer and should be followed.Note that additional maintenance may be required when equipment is installed in unusual service

environments (see11.2). General maintenance requirements for transformers are shown in 11.6.2.3.

Table 1 Standard three-phase transformer kVA ratings from 112.5 kVA 10 000 kVAa

Self-Cooled

(kVA)

Forced-Air-Cooledc

(kVA)

Liquid-

Immersed

Ventilated Dry-

Type

Sealed Dry-

Type

Liquid-

Immersed

Ventilated Dry-

Type

Sealed Dry-

Type

Col. 1 Col. 2 Col. 3 Col. 4 Col. 5 Col. 6

112.5 112.5

150 150

225 225

300 300 400 500 500 667

750 750 750 862 1 000

1 000 1 000 1 000 1 150 1 333

1 500 1 500 1 500 1 725 2 000

2 000 2 000 2 000 2 300 2 667

2 500 2 500 2 500 3 125 3 333

3 750 3 750 3 750 4 687 5 000

5 000 5 000 5 000 6 250 6 667

7 500 7 500 7 500 9 375 10 000

10 000 10 000 10 000 12 500 13 333

a Based on an average winding temperature rise, due to resistance, of: 65 C rise for liquid-immersedtransformers; 150 C rise for dry-type transformers. (Other rises of 115 C and 80 C are also available forventilated-dry-type transformers.)

b For transformers rated above 2500 kVA, the forced-air-cooled rating is generally 125% for liquid immersion,and 133% for ventilated dry-types.

c When a future forced-air-cooled rating is planned, self-cooled transformers shall be specified to have provisionfor future addition of equipment for fan-cooling.



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Table 2 Primary unit substation transformersa b c

Voltages

(V)

Transformer type

(kVA)

Rated HV Rated LV Ventilated Dry-

Type

Sealed Dry-Type Liquid-

Immersed

Col. 1 Col. 2 Col. 3 Col. 4 Col.5

6 9007 200 2 4004 160

4 160Y/2 400

---------

---------

1 0003 7501 0003 7501 0003 750

12 00012 47013 20013 800

2 4004 160

4 160Y/2 400

7507 5007507 5007507 500

7505 0007505 0007505 000

1 0007 5001 0007 5001 0007 500

23 000 2 4004 160

4 160Y/2 400

1 5007 5001 5007 5001 5007 500

1 5005 0001 5005 0001 5005 000

1 0007 5001 0007 5001 0007 500

34 500 2 4004 160