IEEE C57.12.36

C57.12.36

TM

IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

IEEE Power Engineering Society Sponsored by the Transformers Committee

IEEE 3 Park Avenue New York, NY 10016-5997, USA

IEEE Std C57.12.36™-2007

7 March 2008

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IEEE Std C57.12.36TM-2007

IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers Sponsor

Transformers Committee of the IEEE Power Engineering Society Approved 27 September 2007

IEEE-SA Standards Board

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Abstract: Small power transformers have become a significant element in distribution systems supplying large commercial customers like major resort hotels and site-specific industrial customers that desire the local utility to own, operate, and maintain the serving transformer. These transformers can range in sizes from 112.5 kVA to 10 000 kVA with primary voltages at 69 000 V and below and secondary voltages from 34 500 V to 120 V. Transformers in this standard are generally for larger distribution customers often with special voltages or installation requirements like convention centers with large chiller plants and extensive exhibit space. There is often a desire to serve these transformers from underground systems using side-mounted bushings on the primary. This standard seeks to define the small power transformer that is applied as more than just a limited scope version of the power transformers covered by IEEE Std C57.12.10TM and as more than a large distribution class transformer covered by IEEE Std C57.12.34TM. Keywords: class I, distribution substation transformer, liquid-immersed, station type, transformer, unit substation

•

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ISBN 978-0-7381-5717-7 ISBN 978-0-7381-5718-4

STD95738 STDPD95738

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Introduction This introduction is not part of IEEE Std C57.12.36-2007, IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers.

This version is the initial release of IEEE Std C57.12.36. The conception of this standard was derived from the need to clarify the requirements for a unique class of transformer that was not adequately covered by existing standards for either pad-mounted distribution transformers or power transformers. The existing power transformers standard, IEEE Std C57.12.10TM,a covered ratings up to 100 000 kVA and 230 kV, which were far beyond the scope of this new standard. The basis for the development of this standard started from IEEE Std C57.12.13TM, NEMA standards 201 and 210,b along with using sections from both the power transformer standard, IEEE Std C57.12.10, and the newly developed three-phase pad-mount distribution transformer standard, IEEE Std C57.12.34TM. In conjunction with the development of this distribution standard, it was agreed to with NESCOM that the scope of IEEE Std C57.12.10 would continue to cover a broad product range such that power transformers with these same ratings would still be covered. This approach was taken to avoid creating a hole in the standards where small power transformers (e.g., rated 5 MVA with a 25 kV high voltage) would no longer be covered. During the development of this standard, there was a desire to include equipment coordination information for bushings and interface enclosures that would be useful when the transformer is directly connected to switchgear. Due to the variety of standards currently in place with existing switchgear manufactures, it was not possible to come to a consensus on what should or could be a transformer standard at this point in time. Instead an informative annex was created to provide the users of this standard some options for equipment coordination.

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Participants At the time this standard was submitted to the IEEE-SA Standards Board for approval, the Distribution Substation Transformer Working Group had the following membership: David A. Aho, Chair John R. Rossetti, Vice Chair Samuel Aguirre Jim Antweiler Javier Arteaga Israel Barrientos-Torres Wallace B. Binder Jr. Thomas P. Callsen John C. Crouse Marcel Fortin James M. Gardner Carlos Gaytan Ali Ghafourian Saurabh Ghosh Myron Gruber

Robert L. Grunert Kenneth S. Hanus Gary Hoffman Mohammad Hussain Sheldon Kennedy Brian Klaponski Stanley J. Kostyal Tommy Magee Phillip G. McClure Jerry R. Murphy Ray Nicholas Thomas Pekarek Donald W. Platts

Tom A. Prevost Jeffrey Schneider Stephen Schroeder Ibrahim Shteyh Stephen D. Shull Hyeong J. Sim Edward Smith Ronald Stahara Giuseppe Termini Robert Tillman Donnie Trivitt David A. Wiegand Richard vonGemmingen

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The following members of the individual balloting committee voted on this standard. Balloters may have voted for approval, disapproval, or abstention. William J. Ackerman Satish K. Aggarwal David A. Aho Paul R. Ahrens Steven C. Alexanderson I. J. Antweiler Carlo Arpino Paul D. Barnhart Barry L. Beaster Wallace B. Binder Jr. Thomas E. Blackburn III Stuart H. Borlase Troy D. Bredemeier Steven R. Brockschink Chris Brooks Paul A. Buchanan Carl L. Bush William A. Byrd Thomas P. Callsen Yunxiang Chen Tommy P. Cooper John C. Crouse Jorge E. Fernandez Daher Frank L. D’Amico Stephen Dare Dieter Dohnal Randall L. Dotson Mark M. Drabkin Donald G. Dunn Gary R. Engmann Mehrdad Eskandary Rabiz N. Foda Joseph Foldi Marcel Fortin Fredric A. Friend

Eduardo W. Garcia James M. Gardner Manuel M. Gonzalez Ron K. Greenthaler Randall C. Groves Robert L. Grunert Ajit K. Gwal Kenneth S. Hanus Adrienne M. Hendrickson Gary A. Heuston David A. Horvath Dennis Horwitz James D. Huddleston III David W. Jackson Clark A. Jacobson Gael Kennedy Morteza Khodaie Joseph L. Koepfinger Stanley J. Kostyal Jim Kulchisky Saumen K. Kundu John G. Lackey Scott R. Lacey Stephen R. Lambert Thomas W. La Rose Solomon Lee Boyd R. Leuenberger Thomas G. Lundquist G. L. Luri William A. Maguire J. Dennis Marlow John W. Matthews Omar S. Mazzoni Kenneth L. McClenahan Phillip G. McClure Mark F. McGranaghan

Joseph P. Melanson Gary L. Michel Charles A. Morse Daniel H. Mulkey Jerry R. Murphy Dennis K. Neitzel Arthur S. Neubauer Michael S. Newman Lorraine K. Padden Paulette A. Payne M. C. Pehosh Donald W. Platts Iulian E. Profir Jeffrey L. Ray Ryland B. Revelle Michael A. Roberts Charles W. Rogers John R. Rossetti Thomas J. Rozek Dinesh Sankarakurup Pranathy Steven Sano Bartien Sayogo Devki N. Sharma Stephen D. Shull Hyeong J. Sim Brian D. Sparling Malcolm V. Thaden T. P. Traub Joseph J. Vaschak Joe D. Watson David A. Wiegand Alan L. Wilks James W. Wilson Jr. William G. Wimmer Roland E. Youngberg

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When the IEEE-SA Standards Board approved this standard on 27 September 2007, it had the following membership: Steve M. Mills, Chair Robert M. Grow, Vice Chair Don Wright, Past Chair Judith Gorman, Secretary Richard DeBlasio Alex Gelman William R. Goldbach Arnold M. Greenspan Joanna N. Guenin Kenneth S. Hanus William B. Hopf Richard H. Hulett

Hermann Koch Joseph L. Koepfinger* John Kulick David J. Law Glenn Parsons Ronald C. Petersen Tom A. Prevost

Narayanan Ramachandran Greg Ratta Robby Robson Anne-Marie Sahazizian Virginia C. Sulzberger Malcolm V. Thaden Richard L. Townsend Howard L. Wolfman

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons: Satish K. Aggarwal, NRC Representative Michael H. Kelley, NIST Representative Michelle D. Turner IEEE Standards Program Manager, Document Development Matthew J. Ceglia IEEE Standards Program Manager, Technical Program Development

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Contents 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Mandatory requirements...................................................................................................................... 2 2. Normative references.................................................................................................................................. 2 3. Definitions .................................................................................................................................................. 3 4. Rating data.................................................................................................................................................. 3 4.1 Usual service conditions ...................................................................................................................... 3 4.2 Kilovolt-ampere ratings ....................................................................................................................... 3 4.3 Voltage applications ............................................................................................................................ 4 4.4 Standard levels for basic lightning impulse insulation ........................................................................ 5 4.5 Taps ..................................................................................................................................................... 5 4.6 Impedence voltage tolerance ............................................................................................................... 5 4.7 Routine tests ........................................................................................................................................ 6 4.8 Dielectric test levels............................................................................................................................. 6 5. Construction ............................................................................................................................................... 6 5.1 Accessories .......................................................................................................................................... 6 5.2 Bushings ............................................................................................................................................ 11 5.3 Enclosures.......................................................................................................................................... 14 5.4 Other neutral terminations ................................................................................................................. 15 5.5 Lifting, moving, and jacking facilities............................................................................................... 17 5.6 Nameplate.......................................................................................................................................... 18 5.7 Ground provisions ............................................................................................................................. 19 5.8 Polarity, angular displacement, and terminal markings ..................................................................... 19 5.9 Liquid preservation............................................................................................................................ 20 5.10 Tanks ............................................................................................................................................... 21 5.11 Auxiliary cooling equipment ........................................................................................................... 21 5.12 Power supply for transformer auxiliary equipment and controls..................................................... 23 5.13 Bushing-type current transformers .................................................................................................. 23 5.14 Surge arresters ................................................................................................................................. 24 5.15 Insulating liquid............................................................................................................................... 24 5.16 Loading............................................................................................................................................ 25 6. Installation and storage............................................................................................................................. 25 Annex A (informative) Substation Equipment Coordination for Secondary Unit Substations .................... 26

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IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers IMPORTANT NOTICE: This standard is not intended to assure safety, security, health, or environmental protection in all circumstances. Implementers of the standard are responsible for determining appropriate safety, security, environmental, and health practices or regulatory requirements. 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 This standard sets forth the requirements for indoor/outdoor distribution substation transformer application that is not covered by ANSI/IEEE distribution and power transformer standards. This standard is intended for use as a basis for performance and interchangeability as well as to assist in the proper selection of such equipment.

1.1 Scope This standard covers certain electrical, dimensional, and mechanical characteristics of 50 Hz and 60 Hz, two winding, liquid-immersed distribution substation transformers. Such transformers may be remotely or integrally associated with either primary and secondary switchgear or substations, or both, for step-down or step-up purposes rated as follows: a)

112.5 kVA through 10 000 kVA three-phase

b) 250 kVA through 6667 kVA single-phase c)

High voltage 69 000 V and below and low voltage 34 500 V and below

It is not intended that this standard shall apply to dry-type, regulating, pad-mounted, secondary-network, furnace, rectifier, mobile, railway, or mine transformers. 1 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

1.2 Mandatory requirements When this standard is used on a mandatory basis, the words “shall” and “must” indicate mandatory requirements, and the words “should” and “may” refer to matters that are recommended and permitted, respectively, but not mandatory.

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 C84.1, American National Standard for Electric Power Systems and Equipment—Voltage Ratings (60 Hertz). 1 ASME B1.1, Unified Inch Screw Threads (UN and UNR Thread Form). 2 ASME B1.20.1, Pipe Threads, General Purpose (Inch). IEEE Std 386TM, IEEE Standard for Separable Insulated Connector Systems for Power Distribution Systems above 600 V. 3, 4 IEEE Std C57.12.00TM, IEEE Standard for Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std C57.12.70TM, IEEE Standard Terminal Markings and Connections for Distribution and Power Transformers. IEEE Std C57.12.80TM, IEEE Standard Terminology for Power and Distribution Transformers. IEEE Std C57.12.90TM, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std C57.13TM, IEEE Standard Requirements for Instrument Transformers. IEEE Std C57.19.00TM, IEEE Standard General Requirements and Test Procedure for Outdoor Power Apparatus Bushings. IEEE Std C57.19.01TM, IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus Bushings. IEEE Std C57.19.100TM, IEEE Guide for Application of Power Apparatus Bushings. IEEE Std C57.91TM, IEEE Guide for Loading Mineral-Oil-Immersed Transformers. 1 ANSI 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/). 2 ASME publications are available from the American Society of Mechanical Engineers, P.O. Box 2300, Fairfield, NJ 07007-2300, USA (http://www.asme.org/). 3 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards/ieee.org/). 4 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

IEEE Std C57.93TM, IEEE Guide for Installation of Liquid-Immersed Power Transformers. IEEE Std C57.105TM, IEEE Guide for Application of Transformer Connections in Three-Phase Distribution Systems.

3. Definitions For the purposes of this document, the following terms and definitions apply. IEEE Std C57.12.80 5 and The Authoritative Dictionary of IEEE Standards Terms 6 should be referenced for terms not defined in this clause. 3.1 distribution substation transformer: A distribution transformer designed for installation in a station or substation. 3.2 liquid: Both less-flammable fluids and mineral transformer oil. 3.3 National Pipe Thread (NPT): See ANSI/ASME B1.20.1. 3.4 ONAN and ONAF: See IEEE Std C57.12.00, rating data section, for an explanation on the nomenclature established for these ratings. 3.5 safety factor: The ratio of the ultimate stress of the material used in the lifting, moving, and jacking provisions to the working stress. 3.6 working stress: The maximum combined stress developed in the lifting provisions by the static load of the completely assembled transformer.

4. Rating data

4.1 Usual service conditions Service conditions shall be in accordance with IEEE Std C57.12.00.

4.2 Kilovolt-ampere ratings Kilovolt-ampere ratings are continuous and based on not exceeding either a 65 °C average winding temperature rise or an 80 °C hot-spot conductor temperature rise. The temperature rise of the insulating fluid shall not exceed 65 °C when measured near the top of the tank. These kilovolt-ampere ratings are based on the usual temperature and altitude service conditions specified in IEEE Std C57.12.00. The common kilovolt-ampere ratings shall be per Table 1.

5

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Table 1 —Kilovolt-ampere ratings Single-phase transformers ONAFa ONAN — 250 333 — 500 — 833 958 1250 1437 1667 1917 2500 3125 3333 4167 5000 6250 6667 8334 — — — — — — — —

Three-phase transformers ONAFa ONAN 112.5 — 150 — 225 — 300 — 500 — 750 862 1000 1150 1500 1725 2000 2300 2500 3125 3750 4687 5000 6250 7500 9375 10 000 12 500

a

Single-phase and three-phase ONAF kilovolt-ampere values 2500 and above have a 25% increase over the ONAN rating. All other kilovolt-ampere ratings are increased by 15%. If specified, an additional percent increase in kilovolt-ampere rating can be achieved by adding fan capacity.

4.3 Voltage applications For high-voltage ratings up to and including 34 500 V, the minimum phase voltage on the low voltage is 120 V. For high-voltage ratings above 34 500 V up to 69 000 V, the minimum phase voltage on the low voltage is 1386 V. For voltage ratios outside of these ranges, the transformer manufacturer should be consulted. Voltage limitations defined by the transformer kilovolt-ampere rating are shown in Table 2 for singlephase applications and Table 3 for three-phase applications. Table 2 —Winding voltage applications by single-phase kilovolt-ampere ratings kVA 250–333 500 833 1250 1667 2500–6667

High voltage (minimum) 1386 2400 4160 6900 6900 6900

High voltage (maximum) 46 000 46 000 46 000 69 000 69 000 69 000

Low voltage (minimum) 120 277 277 277 346 1386

Low voltage (maximum) 4160 4160 8320 15 000 15 000 34 500

Table 3 —Winding voltage applications by three-phase kilovolt-ampere ratings kVA 112.5–500 750–1000 1500–2000 2500 3750 5000 7500–10 000

High voltage (minimum) 600 1386 2400 4160 6900 6900 6900

High voltage (maximum) 34 500 46 000 46 000 46 000 69 000 69 000 69 000

Low voltage (minimum) 120 120 277 277 277 1386 1386

Low voltage (maximum) 2400 4160 4160 8320 15 000 15 000 34 500

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

4.4 Standard levels for basic lightning impulse insulation Minimum basic lightning insulation levels (BILs) shall be in accordance with Table 4. Table 4 —Winding insulation levels Basic lightning impulse insulation levels (BILs) 30 45 60 75 95 125 wye, 150 delta 150 wye, 200 delta 250 350

Nominal system voltage (kV rms) 1.2 2.5 5.0 8.7 15.0 25.0 34.5 46.0 69.0

NOTE—Refer to IEEE Std C57.12.00 for other ratings available. 7

4.5 Taps Unless otherwise specified, the standard high-voltage taps shall be 2 above the high-voltage nominal rating @ 2.5% each and 2 below the high-voltage nominal rating @ 2.5% each. Other common tap arrangements can be specified between the manufacturer and the user, such as 4 above, 4 below, 3 above, and 1 below, as well as 1 above and 3 below. Requirements may also be defined for a total of seven tap positions. User-defined tap requirements will depend on specific system requirements.

4.6 Impedance voltage tolerance The percent impedance voltage, as measured on the rated voltage connection, shall be Table 5 as follows (unless otherwise specified): Impedance selection should consider a fault current rating of downstream equipment. Table 5 —Percent impedance voltages kVA rating (1PH and 3PH) 112.5–749 750–4999 750–4999 750–4999 750–4999 750–4999 5000–10 000 5000–10 000 5000–10 000 5000–10 000

HV BIL (kV)

Impedance voltage LV 600 V and below LV above 600 V 1.70–5.75a 5.75 6.75 7.25 7.75 — — — — —

≤200 ≤110 150 200 250 350 ≤150 200 250 350

1.70–5.75a 5.75 6.5 7.0 7.5 8.0 6.5 7.0 7.5 8.0

a The minimum impedance listed in this table may produce available fault currents greater than the interrupting rating of down line equipment.

7

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

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

The tolerance on nominal impedance voltages shall be as specified in IEEE Std C57.12.00. The percent departure of the tested impedance voltage on any tap from the tested impedance voltage at rated voltage shall not be greater than the total tap voltage range and shall be expressed as a percentage of the rated voltage.

4.7 Routine tests Routine tests shall be made in accordance with IEEE Std C57.12.00 for distribution transformers and IEEE Std C57.12.90.

4.8 Dielectric test levels Dielectric test levels shall be in accordance with the distribution transformer test levels in IEEE Std C57.12.00.

5. Construction

5.1 Accessories When required or specified, the accessories listed in 5.1.1 through 5.1.9 shall be included and located as shown in Figure 1. 5.1.1 Tap changer On-load tap changers are not covered by this standard. When the high-voltage winding is specified to be designed with taps, a de-energized tap changer will be supplied. An externally operable handle shall be located on the side of the tank in segment 1 or 4 at a height convenient to the transformer design, or it shall be located on the cover. A tap-changer position indicating plate shall be provided and shall be marked with letters or Arabic numerals in sequence. The letter “A” or the Arabic numeral “1” shall be assigned to the voltage rating providing the maximum ratio of transformation. The tap-changer handle shall have provision for padlocking and shall provide visible indication of the tap position without unlocking. A hole with a minimum diameter of 9.5 mm (3/8 in) shall be provided for the padlock. The tap-changer handle or the area next to the handle shall be marked indicating suitability for deenergized operation only.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Figure 1 —Accessories and construction features Clause 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.1.6 5.1.7 5.1.8 5.1.9 5.2 5.5.2 5.5.3 5.5.4 5.6 5.7 5.8 5.9 5.11 5.11.1 5.11.2 5.12 5.13 5.14

Items

Location

Tap-changer Segment 1 or 4, or cover Liquid-level indicator Segment 1 Liquid-temperature indicator Segment 1 Winding-temperature indicator Segment 1 Pressure-vacuum gauge Segment 1 Pressure-vacuum bleeder valve Segment 1 Pressure relief Segment 1 or cover Drain valves, filter valves, and connections Segment 1 Sudden pressure relay Bushings Lifting facilities Moving facilities Jacking facilities Nameplate Segment 1 Ground provisions Polarity, angular displacement, and terminal markings Liquid preservation Auxiliary cooling equipment Segment 1 Controls for auxiliary cooling equipment Fans Auxiliary equipment power supply Bushing-type current transformers Surge arresters

Requirement As required Standard Standard Available when specified Standard As required Standard Standard Available when specified Standard Standard Standard Standard Standard Standard Standard Standard Available when specified Available when specified Available when specified Available when specified Available when specified Available when specified

5.1.2 Liquid-level indicator (LLI) A level gauge with a vertical face shall be mounted on the side of the tank and shall be readable to a person standing at the level of the base. The gauge shall have a dark-face dial with light markings and a light-colored indicating hand. The diameter of the dial (inside bezel) shall be 83 mm (3.3 in) ± 7 mm (0.28 in) when the 25 °C liquid level is 2500 mm (98.4 in) or less above the bottom of the base. 7 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Dial markings shall show the 25 °C level and the minimum and maximum operating levels. The words “Liquid Level” shall be on the dial or on a suitable nameplate adjacent thereto. The 25 °C liquid level shall also be shown by suitable permanent markings on the tank or by an indication on the nameplate of the distance from the liquid level to the highest point of the access hole flange surface. When contacts are required, the liquid-level indicator shall be supplied with one set of nonadjustable contacts set to close at the minimum safe operating level of the liquid. Each relay contact shall provide the ability to turn on an alarm device, contactor, or actuate another relay. Refer to 5.1.10 for contact ratings. 5.1.3 Liquid temperature indicator (LTI) A thermometer, which measures the top liquid temperature, shall be mounted such that the dial is readable to a person standing at ground or mounting pad level. Indicators having operating controls on their cases shall be mounted between 1200 mm (47.2 in) and 2200 mm (86.6 in) above the base. The minimum scale range shall be 0 oC to 120 oC. The thermal sensing element shall be mounted in a closed well at a suitable level to indicate the top-liquid temperature. The well shall be positioned such that it is covered by at least 25.4 mm (1.0 in) of fluid at the lowest permissible fluid level. The thermometer well dimensions can be found in IEEE Std C57.12.00. The thermometer dial shall have an analog (dial and pointer) or alphanumeric display that is readable in low and high ambient light conditions. The measurement title, “Liquid Temperature,” shall be marked or displayed on the dial or on a suitable nameplate mounted adjacent to the indicator. Indicators with analog displays shall have highly contrasting light markings on a dark dial or dark markings on a white dial. The dial diameter (inside the bezel) shall be 114 mm (4.5 in) ± 25.4 mm (1 in). Two indicating pointers, one for present temperature and one for peak (maximum recorded historical) temperature, shall be provided. The present temperature pointer may be light or dark, in contrast to the dial. The peak temperature pointer shall be orange-red and shall have a provision for resetting without opening any covers or windows. Digital indicators with numeric displays shall have characters at least 12 mm (0.5 in) in height. The measurement title may be shown on an alphanumeric display separately from the measured value, or the title may be marked on the dial or on a plate mounted adjacent to the gauge. A method of displaying the peak temperature, using externally operated controls, shall be provided. When contacts are required, the LTI shall have a minimum of two sets of contacts in accordance with 5.1.10 and have independent field-adjustable set point values over a minimum range of 65 °C to 110 °C. Each relay contact shall provide the ability to turn on a cooling stage, alarm device, contactor, or actuate another relay. Refer to 5.11.1 for switches and relays or contactors that allow for redundant manual control of cooling equipment. 5.1.4 Winding temperature indicator The winding temperature indicator is a device that provides a hot-spot temperature measurement. The measurement may be via direct sensors located in the winding or from a simulated and/or calculated method. The temperature indicator shall be mounted on the side of the tank and shall be readable to a person standing at ground or mounting pad level. Indicators having operating controls on their cases shall 8 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

be mounted between 1200 mm (47.2 in) and 2200 mm (86.6 in) above the base. The minimum scale range shall be 0 oC to 160 oC. Depending on the type of technology, the WTI may require that the transformer be equipped with a heated thermo-well, a load current signal from a bushing CT, or ports through the tank wall for sensor passage. When a top fluid temperature input is required, the thermal sensing element shall be mounted in a closed well at a suitable level to indicate the top-liquid temperature. When a heated thermo-well is required, the tank wall shall be ported to accept the specified heater. The well shall be positioned such that it is covered by at least 25.4 mm (1 in) of fluid at the lowest permissible fluid level. For dimensions of the well, see IEEE Std C57.12.00. The thermometer’s dial shall have an analog (dial and pointer) or alphanumeric display that is readable in low and high ambient light conditions. The measurement title, “Winding Temperature,” shall be marked or displayed on the dial or on a suitable nameplate mounted adjacent to the indicator. Indicators with analog displays shall have highly contrasting light markings on a dark dial or dark markings on a white dial. The dial diameter (inside the bezel) shall be 114 mm (4.5 in) ± 25.4 mm (1 in). Two indicating pointers, one for present temperature and one for maximum historical (peak) temperature, shall be provided. The present temperature pointer may be light or dark, in contrast to the dial. The peak temperature pointer shall be orange-red and shall have a provision for resetting without opening any covers or windows. Digital indicators with numeric displays shall have characters at least 12 mm (0.5 in) in height. A method of displaying the historical highest recorded temperature, through externally operated controls, shall be provided. The WTI shall have a minimum of three sets of contacts in accordance with 5.1.10 and shall be independently field adjustable over a minimum range of 95 °C to 125 °C. Each of the relay contacts shall provide the ability to turn on a cooling stage, alarm device, or actuate another relay or contactor. See 5.11.1 for switches and relays or contactors that allow for redundant manual control of cooling equipment. 5.1.5 Pressure-vacuum gauge A pressure-vacuum gauge with vertical face shall be mounted on the side wall of the tank and shall be readable to a person standing at the level of the base. The diameter of the dial (inside bezel) shall be 90 mm (3.5 in) ± 7 mm (0.28 in). The gauge shall have a dark-face dial with light-colored markings and a light-colored pointer. The scale range for the pressure-vacuum gauge shall be between 69 kPa or 10 psi, positive and negative. 5.1.6 Pressure-vacuum bleeder valve A pressure-vacuum bleeder device set to operate at the maximum operating pressures (positive and negative) indicated on the nameplate shall be furnished on transformers rated 2500 kVA and above. 5.1.7 Pressure relief A replaceable valve (PRV) shall be provided on transformers 2500 kVA and below to provide automatic relief of pressures that build up slowly in excess of normal operating pressures. These excess pressures 9 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

may be due to overloads, high ambient temperatures, external secondary faults, and incipient faults in the low-voltage winding. When relieving these excess pressures, the valve shall emit only a negligible amount of oil. The valve shall be furnished on the tank wall above the 140 °C top oil level, by the manufacturer’s calculation, and shall be located so as not to interfere with any other devices. The inlet port shall be 1/4 in or larger NPT (or NF thread with gasket), sized for a specified minimum flow rate. Exposed parts shall be of weather and corrosion-resistant materials. Gaskets and O-rings shall withstand oil vapor and a 105 °C temperature continuous under operating conditions as described in IEEE Std C57.91, without seizing or deteriorating, for the life of the transformer. The valve shall have a pull ring for manually reducing pressure to the atmosphere using a standard hookstick and shall be capable of withstanding a static pull force of 112 newtons (25 lb) for 1 min without permanent deformation. The valve shall withstand a static force of 445 newtons (100 lb) for 1 min applied normal to its longitudinal axis at the outermost extremity of the body. When specified, the venting port, on the outward side of the valve head set, shall be protected to prevent entry of dust, moisture, and insects before and after the valve has actuated; or a weather-cap-type indicator shall be provided, which will remain attached to the valve and provide positive indication to an observer that the valve has operated. Venting and sealing characteristics shall be as follows: a)

Cracking pressure: 69 kPa(gauge) ± 13 kPa(gauge}, (10 psig ± 2 psig).

b) Resealing pressure: 42 kPa(gauge) (6 psig) minimum. c)

Zero leakage from resealing pressure to –56 kPa(gauge) (–8 psig).

d) Flow at 103 kPA(gauge): 16.5 L/s minimum of air at a standard temperature and pressure. Standard temperature is 21 °C, and standard pressure is 101 kPa(absolute). (Flow at 15 psig: 35 SCFM minimum where SCFM is flow at cubic feet per minute, corrected for an air pressure of 14.7 psi and an air temperature of 21 °C.) A pressure-relief device (PRD) shall be provided on the cover for transformers rated above 2500 kVA or having an insulation level above 200 kV BIL. The minimum pressure relief rating for cover-mounted devices should be 5000 SCFM at 69 kPa (10 psi). In addition to the cover-mounted pressure relief device, the tank shall also have provisions to relieve tank pressure manually. 5.1.8 Drain valves, filter valves, and connections A combination drain and lower filter valve shall be located on the side of the tank in segment 1. This valve shall provide for drainage of the liquid to within 25.4 mm (1 in) of the bottom of the tank. The drain valve shall have a built-in 3/8 in sampling device, which shall be located in the side of the valve between the main valve seat and the pipe plug. The sampling device shall be supplied with a 5/16 in, 32 threads/in, male thread for the user’s connection and shall be equipped with a cap. The size of the drain valve shall be 1 in NPT, for transformers through 2500 kVA and 2 in NPT for the larger kilovolt-ampere ratings, and the drain valve shall have tapered pipe threads with a pipe plug in the open end. 10 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Transformers 2500 kVA and below shall have a 1 in NPT upper filter plug, or cap, located above the maximum liquid level in segment 1. Transformers above 2500 kVA shall have an upper filter valve in place of the upper filter plug. The size of the upper filter valve shall be 1 in NPT, with a pipe plug in the open end. 5.1.9 Sudden pressure relay When specified a sudden pressure relay shall be provided for the indication of transformer faults and to minimize damage to equipment. The relay shall not actuate under normal transformer operating pressures. The sudden pressure relay may be either a gas space or under fluid type. Under fluid types shall actuate under rapidly changing pressures of 10 kPa/s to 38 kPa/s (1.5 psi/s to 5.1 psi/s). The gas space-mounted relays shall actuate with a pressure change of 3.5 kPa/s to 21 kPa/s (0.5 psi/s to 3.1 psi/s). The relay shall actuate within three cycles. The sudden pressure relay shall be able to withstand full vacuum or positive pressure of 103.3 kPa (15 psi) without damage. The relay shall, as a minimum, be supplied with an alarm and trip contact. Refer to 5.1.10 for minimum contact ratings. 5.1.10 Temperature and liquid-level indicator contacts and wirings 5.1.10.1 Contact ratings Nongrounded alarm contacts shall be suitable for interrupting the following: a)

0.02 A direct-current inductive load

b) 0.20 A direct-current noninductive load c)

2.5 A alternating-current noninductive or inductive load

d) 250 V maximum in all classes 5.1.10.2 Contact wiring and wire color coding Contacts shall be wired with cable having the color coding shown in Figure 2 or with cable having permanent labeling. Terminal blocks shall be provided in a weatherproof enclosure located in segment 1 for terminating contacts. Where transformers are at a location remote from the switchgear, terminal blocks shall be provided for the user’s external interconnections.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

MULTICONDUCTOR CABLE COLOR SEQUENCE

CONTACT ARRANGEMENT

BLACK 1st SWITCH

RED BLUE* ORANGE YELLOW

2nd SWITCH

BROWN* RED/BLACK TRACER 3rd SWITCH

BLUE/BLACK TRACER ORANGE/BLACK TRACER*

* if provided

Figure 2 —Contact wiring and wire color coding

5.2 Bushings The insulation level of line bushings shall be equal to or greater than the insulation level of the windings to which they are connected. The insulation level of the high-voltage or low-voltage neutral bushing on three-phase transformers having a grounded wye-connected winding shall be the same as that of the low-voltage line bushings for windings 15 kV and below. For windings above 15 kV, a 15 kV neutral bushing with 95 kV BIL shall be provided. 5.2.1 Bushing locations and bushing phasing Bushing locations and bushing phasing must be specified. Certain kilovolt-ampere and voltage ratings may impose design limitations on bushing location, orientation, height, and spacing. For reference, segment 1 is referred to as the frontplate and is defined by the location of the nameplate. For minimum electrical clearances, refer to IEEE Std C57.12.00. Four bushings shall be provided for each permanently connected wye-winding on three-phase transformers and labeled as H0, H1, H2, H3 or X0, X1, X2, and X3. For delta-connected windings, the H0 and X0 bushing would be omitted. For single-phase transformers, omit H0, H3, X0, and X3 as necessary.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

5.2.1.1 Sidewall bushings A standard sidewall bushing arrangement (unit substation) has the high-voltage bushings in segment 2 and low-voltage bushings in segment 4 per Figure 1. Unless otherwise specified, bushings will be in-line and arranged as follows [refer to part a) of Figure 3]: Bushing arrangements Side view

Three-phase

Single-phase

High voltage (left to right)

H3-H2-H1-H0

H1-H2

Low voltage (left to right)

X0-X1-X2-X3

X1-X2

NOTE—Eliminate the H0 and X0 bushings for delta-connected applications.

5.2.1.2 Cover bushings A standard cover bushing arrangement (station type substation) has the high-voltage bushings in segment 3 and low-voltage bushings in segment 1 (along the longer sides of the tank) with the frontplate defined as segment 1. Due to packaging considerations for required tank cooling and controls, the frontplate tank components (i.e., controls or nameplate) may be located on the end of the tank in segment 4 as defined in Figure 1. Unless otherwise specified, bushings will be in-line and arranged as follows [refer to part b) of Figure 3]: Bushing arrangements Side view

Three-phase

Single-phase

High voltage (left to right)

H0-H1-H2-H3

H1-H2

Low voltage (left to right)

X0-X1-X2-X3

X1-X2

NOTE—Eliminate the H0 and X0 bushings for delta-connected applications.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

H0 and/or X0 WHEN REQUIRED

H3 H2 H1

Tank bottom

a)

b)

Figure 3 —a) Sidewall bushing arrangements and b) cover bushing arrangements 5.2.2 Bushing and cable support requirements

For sidewall live front bushings, supports may be needed to prevent excessive cantilever forces on the bushings. These supports may be applied directly to the bushings or provisions may be supplied to support the incoming connections (i.e., cable or bus). The actual type and location of the supports will depend on the strength of the bushing, type of enclosure, and the characteristics of the bushing terminations (i.e., cable, bus, or entrance location). Typically low-voltage bushings with 10 or more holes should have support provisions for the bushings or the cables. The number of positions (i.e., 8-hole or 10-hole) is determined by the size of cable, number of cable runs, and connected load needed to meet ampacity requirements. Specific bushing and cable support requirements should be specified by the user. 5.2.3 Electrical characteristics

The electrical characteristics of outdoor transformer bushings shall be as listed in IEEE Std C57.12.00 for bushings with a nominal system voltage of 34.5 kV and below. For applications requiring power apparatus bushings or with a system voltage above 34.5 kV, refer to information listed in IEEE Std C57.19.00 and IEEE Std C57.19.01. 5.2.4 Other bushings

High-voltage dead-front bushings may also be used on products covered by this document. Refer to IEEE Std 386 for information on available bushings. These high-voltage connectors shall consist of either bushing wells and bushing inserts or integral bushings as specified. Separable insulated high-voltage connectors that are designed for operation after the transformer is in place shall be located so that they can be operated with hot-line tools.

5.3 Enclosures Where practical, enclosures must be securely attached to the side of the tank, or cover, to allow the transformer to be shipped with the enclosure installed. Product covered by this standard is intended for use 14 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

in installations in secured areas generally inaccessible to unauthorized and untrained persons. The enclosures shall provide a degree of protection to any enclosed equipment and conductors and shall provide a degree of protection against incidental contacting of live parts. Security requirements or integrity ratings must be specified for enclosures intended for ground-level use in installations accessible to the unsupervised general public. 5.3.1 Junctions boxes (chamber)

Junction boxes (cabinets) may be provided for the cable or bus entrance for windings of 200 kV BIL or less. Certain kilovolt-ampere and voltage ratings may impose design limitations on the availability or location of these items. The high-voltage and low-voltage junction boxes, if specified, shall be mounted on the side of the tank in either segment 2 or 4 corresponding to the location of the high-voltage and low-voltage bushings. The junction box may be secured to the tank wall directly or via an intermediate flange. The junction box shall be supplied with a sufficiently sized opening for access to make the required bushing and arrester terminations. Secondary access panels may also be supplied on the sides of the cabinet. The main panel shall be bolted in place around the perimeter or hinged door with a latching handle. The weather resistance of the cabinet shall meet the requirements of the indoor or outdoor environment as specified. Any unique security requirements or integrity ratings must be specified. 5.3.2 Throat connections

A throat connection or connections may be provided for windings of 200 kV BIL or less. Certain kilovoltampere and voltage ratings may impose design limitations on the availability or location of these items. The high-voltage and low-voltage throats, if specified, shall be mounted on the side of the tank in either segment 2 or 4, or on the cover, corresponding to the location of the high-voltage and low-voltage bushings. Due to space limitations if located on the cover, typically only a high-voltage or low-voltage throat would be provided, not both. The throat connections may be removable or permanently attached to the tank. 5.3.3 Disconnecting switches with interlocks and terminal chambers

Disconnecting switches with interlocks and terminal chambers may be provided for windings of 200 kV BIL or less (see 5.4.3 when neutral termination is required). Certain kilovolt-ampere and voltage ratings may impose design limitations on the availability or location of these items. The high-voltage terminal chamber shall be mounted on the side of the tank in segment 2 or 4. The low-voltage terminal chamber shall be mounted on the side of the tank in segment 2 or 4. An auxiliary nameplate may be specified to identify a one-line illustration showing disconnecting switches with interlocks.

5.4 Other neutral terminations When specified, other neutral terminations shall be provided. 15 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

5.4.1 Neutral termination of wye-connected high-voltage windings

When specified, designated neutral terminations of wye-connected high-voltage windings shall be one of the following: a)

The neutral shall be ungrounded and not accessible.

b) The neutral shall be brought through the cover. c)

The neutral shall be brought through the tank side.

d) The neutral shall be grounded internally and accessible for separation from ground for testing. e)

Provision for a future high-voltage neutral bushing shall be made on the cover in segment 2. A fully insulated neutral shall be brought to a terminal board for isolated neutral operation of the transformer.

High-voltage windings of transformers with a wye−delta terminal board supplied in accordance with 5.8.4 shall be available in one of the following constructions: ⎯ Neutral ungrounded and not accessible. ⎯ Neutral brought through the cover. 5.4.2 Neutral termination of wye-connected low-voltage windings

When specified, one of the following neutral terminations of wye-connected low-voltage windings shall be provided: a)

Permanently wye-connected low-voltage windings shall have the low-voltage neutral bushing furnished as provided for in 5.2.3.

b) Low-voltage windings of a three-phase transformer with a low-voltage wye−delta terminal board supplied in accordance with 5.8.4 shall be provided in one of the following constructions: 1) Without neutral bushing. 2) With a neutral bushing of the same voltage class as that of the winding to which it is connected. 5.4.3 Constructions for neutral terminations

Neutral terminations, when furnished in accordance with 5.4.1 and 5.4.2, shall be provided in one of the following constructions: a)

Cover-mounted.

b) When the transformer construction is in accordance with 5.3: 1) Cover-mounted. 2) Located in the junction box, terminal chamber, or throat, respectively.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

5.5 Lifting, moving, and jacking facilities 5.5.1 Safety factor

Lifting and jacking facilities shall be designed to provide a safety factor of 5. Moving facilities shall be designed to provide a safety factor of 2 since the unit is not suspended. 5.5.2 Lifting facilities

Separate lifting facilities shall be provided for lifting the cover from the transformer, for lifting the core and coil assembly from the tank, and for lifting the complete transformer (with the cover securely fastened in place). Note that the use of a lifting beam or spreader may be required for large transformers. The bearing surfaces of the lifting facilities shall be free of sharp edges. For units 500 kVA and larger, lifting provisions shall be designed for lifting with four vertical slings. 5.5.3 Moving facilities

The base of the transformer shall be of heavy plate or have members forming a rectangle that will permit rolling or skidding in the directions of the centerlines of the segments. Provisions shall be provided on or adjacent to the base for moving the transformer parallel to these centerlines. The points of support during movement shall be so located that the center of gravity of the transformer as prepared for shipment will not fall outside these points of support when the base is tilted 15° or less from the horizontal, with or without insulating liquid in the transformer. 5.5.4 Jacking facilities

Jacking facilities shall be located near the extreme ends of the junctions of the segments. For transformers above 2500 kVA, dimensions and clearances for jacking provisions shall be as shown in Figure 4.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

CL OF JACK CLEARANC TANK SIDEWALL F

E

JACKING POINT

TUBES, ETC

F

TANK BASEPLATE

A FLOOR LINE

B

A B E F G H

Weight 35 000 lb (15 900 kg) or less 88.9 mm 3-1/2 in 63.5 mm 2-1/2 in 686 mm 27 in 127 mm 5 in 76.2 mm 3 in 127 mm 5 in

A B E F G H

G

G H

H

Weight 35 000–65 000 lb (15 900–29 500 kg) 127 mm 5 in 63.5 mm 2-1/2 in 686 mm 27 in 127 mm 5 in 76.2 mm 3 in 127 mm 5 in

A B E F G H

Weight over 65 000 lb (29 500 kg) 457 mm 18 in 102 mm 4 in 508 mm 20 in 127 mm 5 in 76.2 mm 3 in 127 mm 5 in

NOTE 1—Dimensions E, F, G, and H are free clearances. NOTE 2—Where required in manufacturer’s standard designs, any dimensions may be in excess of those shown. NOTE 3—E applies to nonremovable coolers only. NOTE 4—Weight includes completely assembled transformer and fluid.

Figure 4 —Provision for jacking (transformers above 2500 kVA)

5.6 Nameplate The nameplate shall conform to the requirements of nameplate C as described in IEEE Std C57.12.00. It shall be located in segment 1 near the centerline and near eye level. Note that for units with the tap changer located in segment 4, the nameplate may be located in the same segment as the tap changer. Voltage and current ratings shall be given as follows: 0 100 1 000 10 000

to to to to

99.9 999 9 999 99 999

to nearest 0.1 to nearest 1 to nearest 5 to nearest 10

The change in liquid level per each 10 °C change in temperature shall be indicated on the nameplate. 18 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

5.7 Ground provisions 5.7.1 Tank ground pad

A tank-grounding pad shall consist of a stainless-steel or copper-faced steel pad, 50.8 mm (2.0 in) × 88.9 mm (3.5 in), with two holes horizontally spaced on 44.5 mm (1.75 in) centers and drilled and tapped for 0.5 in –13 Unified National Coarse thread (UNC) (as defined in ANSI B1.1). The minimum thickness of the copper facing shall be 0.4 mm (0.016 in). The minimum threaded depth of the holes shall be 12.7 mm (0.5 in). Thread protection for the ground pad shall be provided. The ground pad shall be welded on the base or on the tank wall near the base. If the base is detachable, a ground pad shall be located on both the tank wall and the removable base. All transformers shall have a minimum of two ground pads located toward the extreme ends of the base and diagonally opposite, located in such a way as not to interfere with the jacking facilities. 5.7.2 Neutral bushing ground

When neutral bushings are furnished, a ground pad shall be welded on the tank or cover near the neutral bushing. 5.7.3 Unit substation grounding

Provisions shall be made for the connection of each section of the unit substation to a station ground. Where removable compartments are provided, they shall be electrically bonded to the transformer tank.

5.8 Polarity, angular displacement, and terminal markings 5.8.1 Polarity

All single-phase transformers shall have subtractive polarity. 5.8.2 Angular displacement

The angular displacement between high-voltage and low-voltage terminal voltages of three-phase transformers with delta–delta or wye–wye connections shall be 0°. The angular displacement between high-voltage and low-voltage terminal voltages of three-phase transformers with wye–delta or delta–wye connections shall be 30°, with the low voltage lagging the high voltage as shown in Figure 5. Reference IEEE Std C57.105 for information on phase relations for three-phase distribution systems. 5.8.3 Terminal markings

External terminals shall be marked in accordance with IEEE Std C57.12.70. 19 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

The high-voltage and low-voltage bushing arrangements shall be as shown in Figure 3. 5.8.4 Terminal board

Only one of the following types of terminal boards may be selected for a transformer: a)

A terminal board that provides for a series-multiple connection.

b) A wye−delta terminal board that provides angular displacements as shown in Figure 5 for transformers with three-phase windings of 110 kV BIL (15 kV nominal system voltage) or less. The other winding of the transformer shall be permanently delta-connected.

H2

GROUP 1 ANGULAR DISPLACEMENT 0°

H1

X2

H3 X1

H2

X3

H1

H3 X1

DELTA-DELTA CONNECTION

H2

GROUP 2 ANGULAR DISPLACEMENT 30°

X2

Y-Y CONNECTION

H2

X2

X2 X1

X1 H1

X3

H3

X3

DELTA-Y CONNECTION

H1

H3

X3

Y-DELTA CONNECTION

Figure 5 —Angular displacement

5.9 Liquid preservation 5.9.1 Sealed tank system

A sealed-tank system shall be provided. The transformer shall be of sealed-tank construction. The transformer shall remain effectively sealed for a top-liquid temperature of –5 °C to +105 °C continuous and under operating conditions as described in IEEE Std C57.91. NOTE—A sealed-tank system is one in which (1) the interior of the transformer will be sealed from the atmosphere throughout a top-liquid temperature range of 100 °C and (2) the gas plus liquid volume will remain constant so that the internal gas pressure will not exceed 69 kPa (10 psi) gauge positive or 55.2 kPa (8 psi) gauge negative. Operation at these temperatures may cause the mechanical pressure-vacuum bleeder device, when supplied, to function to relieve excessive positive or negative pressures.

5.9.2 Other oil preservation systems

When specified, other systems of oil preservation shall be provided on transformers rated above 5000 kVA as follows. 20 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers NOTE—Inert-gas pressure system. An inert-gas pressure system is a system in which by means of a positive pressure of inert gas maintained from a separate inert-gas source and reducing valve system, the interior of the transformer shall be sealed from the atmosphere through a top-oil temperature range of 100 °C. The internal gas pressure shall not exceed 55.2 kPa (8 psi) gauge.

5.10 Tanks 5.10.1 Pressure withstand

Maximum operating pressures (positive and negative) for which the transformer is designed shall be indicated on the nameplate. The tank shall be of sufficient strength to withstand a gauge pressure of 50 kPa (7 psi) without permanent distortion and 104 kPa (15 psi) without rupturing. 5.10.2 Vacuum withstand

Tanks shall be designed for vacuum filling (external pressure of one atmosphere, essentially full vacuum) in the field on all transformers with high-voltage insulation levels of 350 kV BIL or with a rating of 10 000 kVA. 5.10.3 Access

A bolted or welded main cover construction shall be provided. Removable handholes or manholes shall be provided in the cover. Handholes, if circular, shall be a minimum of 229 mm (9 in) in diameter. If rectangular, they shall have minimum dimensions of 114 mm (4.5 in) × 367 mm (14.5 in). Manholes, if circular, shall be a minimum of 381 mm (15 in) in diameter. If rectangular or oval, they shall have minimum dimensions of 254 mm (10 in) × 406 mm (16 in). 5.10.4 Tank base

Provisions must be provided in the base for use with hold-down devices on the pad.

5.11 Auxiliary cooling equipment 5.11.1 Control of auxiliary cooling equipment

The equipment for automatic control of auxiliary cooling equipment controlled from the top-liquid temperature shall consist of: a)

A thermally operated control device per 5.1.3.

b) A manually operable switch connected in parallel with the automatic control contacts and enclosed in a weather-resistant cabinet located on the side of the tank of segment 1, at a height not greater than 1.52 m (59.8 in) above the base. c)

A relay for control shall be mounted inside the cabinet.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

When specified, the equipment for automatic control of auxiliary cooling equipment shall be controlled from the winding temperature and shall consist of: ⎯

A winding-temperature indicator device per 5.1.4.

⎯

A manually operable switch connected in parallel with the automatic control contacts and enclosed in a weather-resistant cabinet located on the side of the tank in segment 1, at a height not greater than 1.52 m (59.8 in) above the base.

⎯

A relay for control shall be mounted inside the cabinet.

5.11.2 Fans

Fan motors shall be either 120 V or 240 V, 60 Hz, single phase, without centrifugal switch, and they shall be individually fused or otherwise thermally protected. If the power supply for either 120 V or 240 V single-phase motors is not available, provision shall be made to accommodate another motor supply voltage in accordance with ANSI C84.1-1989, not in excess of 600 V. A control power transformer will need to be specified if the fan supply voltage is to be tapped off of the secondary voltage. Forced-air-cooled transformers shall include a sufficient number of automatically-controlled fans to give the kilovolt-ampere ratings shown in Table 1. The kilovolt-ampere ratings are continuous and are based on the maximum temperature rises. The fans shall be controlled by a suitable temperature control relay. 5.11.3 Provision for future air cooling 5.11.3.1 Provision for future forced-air cooling under LTI control

When class ONAN transformers are to have provision for future forced-air cooling and the control of the forced-air equipment is to be by the liquid temperature, the following equipment shall be provided: a)

The necessary mechanical arrangement.

b) A thermally operated liquid temperature control device per 5.1.3 with the thermal element mounted in a well. c)

Provision for mounting the control cabinet.

d) Provision for mounting the fans. e)

The ONAF (future) kilovolt-ampere rating shall be indicated on the nameplate.

5.11.3.2 Provision for future forced-air cooling under WTI control

When class ONAN transformers are to have provision for future forced-air cooling and the control of the forced-air equipment is to be by the winding temperature, the following equipment shall be provided: a)

The necessary mechanical arrangement.

b) A thermally operated winding-temperature control device per 5.1.4. c)

Provision for mounting the control cabinet.

d) Provision for mounting the fans. e)

The ONAF (future) kilovolt-ampere rating shall be indicated on the nameplate. 22 Copyright © 2008 IEEE. All rights reserved.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

5.12 Power supply for transformer auxiliary equipment and controls The power supply voltage for the transformer auxiliary equipment and controls should be specified and provided by the user. It should be in accordance with ANSI C84.1. The voltage rating for auxiliary equipment and controls supplied with the transformer should also be in accordance with ANSI C84.1.

5.13 Bushing-type current transformers 5.13.1 Physical location and attachment

There shall be a maximum of two current transformers per bushing, not including current transformers for winding-temperature indicators. If provisions for the addition of future current transformers are required, the user shall so specify. Provisions shall be made on transformers 2500 kVA and larger for removing bushing-type current transformers from the transformer tank without removing the tank cover. Practical guide: Typically, bushing-type current transformers are only furnished on transformers rated above 2500 kVA that are equipped with 15 kV class and higher, cover-mounted bushings. When using bushing-type current transformers with sidewall-mounted bushings, special arrangements are necessary for mounting the current transformers and routing of lead assemblies. With limited room on the tank wall due the bushing spacing, the current transformers need to be mounted elsewhere in the tank. When externally mounted window-type current transformers are specified, consideration must be given to bushing selection. These considerations include bushing centerline spacing, weight limits for bushing supports, spade length for incoming cable connections, and required depth of enclosure. 5.13.2 Secondary lead termination and arrangement

All secondary leads shall be brought to terminal blocks inside a weatherproof enclosure, which will be located in segment 1 of the transformer as shown in Clause 5. Shorting-type terminal blocks shall be used to terminate the secondary circuit of all current transformers. 5.13.3 Accuracy and certification

Current transformers shall have the full winding accuracy class as listed in Table 6, for either metering or relay accuracy class. Unless otherwise specified, the transformer manufacturer will provide relay accuracy class when current transformers are required. When metering accuracy class current transformers are specified, a certified test report shall be provided at the time of supply. If taps are required, the current transformer taps shall be as specified by IEEE Std C57.13, and the manufacturer of these current transformers shall supply the accuracy test results at specified burdens. Standard bushing current transformer ratios are specified in Table 6.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Table 6 —Recommended accuracy classification of bushing current transformers Bushing insulation class (kV)

Bushing current transformer ratio

Metering accuracy class at full winding ratio

Relay accuracy class at full winding ratio

46 and below

600:5 1200:5, 2000:5, 3000:5 4000:5 and higher

.3B0.9 .3B1.8 .3B1.8

C200 C400 C800

69

600:5 1200:5 2000:5 and higher

.3B0.9 .3B1.8 .3B1.8

C200 C400 C800

5.14 Surge arresters Surge arresters and/or mounting provisions and/or surge arrester ground pads shall not be provided unless otherwise specified by the user. When connected to circuits that are subject to lightning or switching surges, transformers should be equipped with surge protection equipment that has been selected to limit voltage surges to values below the surge ratings of the transformers and other equipment. When surge arresters are required on singlephase units, one surge arrester connected to the H1 or X1 bushing will be supplied unless otherwise specified. The following types of construction are available for surge protection, and the specific requirements must be specified: a)

Provision only for the mounting of surge arresters.

b) Mounting complete with surge arresters. c)

A surge arrester ground pad consisting of a tank grounding pad (in accordance with 5.7) that is mounted near the base of the arresters and that may be specified for each set of arresters, except that individual ground pads may be supplied where the separation of the arresters are such that individual pads for grounding each phase arrester represent a better design.

Mounting provisions will depend on the specific class of arrester specified (i.e., distribution, intermediate, or station) and the location of the arrester (i.e., cover mounted or inside of a bushing enclosure). When arresters are located inside of bushing enclosures, the cable entrance point must be specified in order to locate the arresters and avoid interference. Consideration must also be given to sizing the enclosure depth properly. NOTE—Material for connecting surge arresters to live parts and to ground pads is not included in item a) through item c).

5.15 Insulating liquid Some insulating liquids may have technical limitations (i.e., voltage level) that may limit their scope of application.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

5.16 Loading IEEE Std C57.91 provides guidance and information concerning loading under various conditions, some of which may be limited by the capability of the ancillary components of the transformer. When specified, ancillary components and other construction features (cables, bushings, tap changers, liquid expansion space, and the like) shall be supplied in such a way that they will not limit the loading to less than the capability of the windings. In some cases, load on a transformer can change from low load to close to maximum. If the forced cooling control is based on either liquid or winding temperature indicators, the differences in time constants of these devices and windings can create delayed operation of the cooling devices. When specified, other means to start the cooling devices shall be provided. NOTE—IEEE Std C57.91 is a guide, and it provides the best-known general information for loading transformers under various conditions based on typical winding insulation systems and is based on the best engineering information available at the time of preparation. It discusses the “limitations” of ancillary components other than windings that may limit the capability of transformers to meet its guidelines.

6. Installation and storage The transformer shall be stored in a vertical position on its base and shall remain essentially in that position at all times, including transport to the site and during installations. Equipment manufactured to this specification may be installed in areas where environmental and climatic conditions make operation at varying angles of tilt from the horizontal an important consideration. Under these circumstances, the users may wish to make a particular “angle of tilt” part of their specifications. Refer to IEEE Std C57.93 for more details concerning recommendations for installation and storage.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Annex A (informative) Substation equipment coordination for secondary unit substations Currently very few dimensional standards for the product scope are covered by this standard. Therefore, it was impossible to agree on and enforce a newly defined set of dimensional standards that would be different from current manufacturing practices. Thus, it was decided to try and include some useful information in an informative annex for a user to refer to when defining requirements and for a manufacturer to consider when creating designs. The usefulness of this information will need to be determined once this standard is released to industry. Changes will inevitably be made to this annex, and possibly over time some of this information will be incorporated into the body of the standard. Bushing centerline height: Typical sidewall bushing centerline heights are shown below. For equipment coordination purposes, end users must specify their requirements or the manufacturer must provide as built dimensions. Different tolerances on these heights may apply depending on the type of bushings connections (i.e., cable or bus). Equipment retrofit requirements may require special dimensions. kVA rating

Sidewall bushing centerline height (typical)

Below 750

1143 mm (45 in)

750 through 2500

1397 mm (55 in)

Above 2500 through 5000

1651 mm (65 in)

Above 5000

1905 mm (75 in)

Bushing centerline spacing: Sidewall bushings shall be separated sufficiently to provide electrical clearance, space for thermal imaging, and the necessary electrical connections. The minimum sidewall bushing centerline spacing requirements are shown below. For equipment coordination purposes, end users must specify their requirements or the manufacturer must provide as built dimensions. Different tolerances on these centerline spacings may apply depending on the actual bushing used and on the type of bushings connections (i.e., cable or bus). Equipment retrofit requirements may require special dimensions. kV rating of winding

Bushing centerline spacing (minimum)

1.2 and below (1200 A)

127 mm (5.0 in)

1.2 and below (4515 A)

152 mm (6.0 in)

1.2 and below (6000 A)

178 mm (7.0 in)

Above 1.2 through 15

178 mm (7.0 in)

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Low-voltage bushing connections ≤1.2 kV: Maximum current 3600 6000 5400

Hole pairs (minimum) 2 3 3

Pattern orientation Square Horizontal Vertical

Hole spacing

Hole diameter

44.5 mm (1.75 in) 44.5 mm (1.75 in) 44.5 mm (1.75 in)

14 mm (0.56 in) 14 mm (0.56 in) 14 mm (0.56 in)

High-voltage bushing connections ≤15 kV: Maximum current 600 600 600

Hole pairs (minimum) 1 1 2

Pattern orientation Horizontal Vertical Square

Hole spacing

Hole diameter

44.5 mm (1.75 in) 44.5 mm (1.75 in) 44.5 mm (1.75 in)

14 mm (0.56 in) 14 mm (0.56 in) 14 mm (0.56 in)

NOTE—Eyebolt connections are a typical termination on high-voltage bushings. These may also be necessary for equipment coordination.

Bushing flange options ≤15 kV:

Option 1: Width: Inside = 34.0

Outside = 38.0

Hole spacing: 4 spaces @ 9.00 in

Height: Inside = 40.0

Outside = 44.0

Hole spacing: 4 spaces @ 10.5 in

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Bushing throat options:

Option 1: Width: Inside = 34.0

Outside = 38.0

Hole spacing: 4 spaces @ 9.00 in

Height: Inside = 16.0

Outside = 20.0

Hole spacing: 2 spaces @ 9.00 in

Option 2: (request to verify dimensions) Width: Inside = 40.0

Outside = 44.0

Hole spacing: 11 spaces @ 3.89 in

Height: Inside = 17.5

Outside = 22.0

Hole spacing: 6 spaces @ 3.46 in

Tank layout: The tank layouts in Figure A.1 are typical configurations for equipment coordination, but obviously they are not fully inclusive of all situations. The dimensions referenced can be used as needed to specify requirements for a particular application. In some instances, only the flange-toflange and bushing height dimensions are critical, whereas in other cases, all of the dimensions shown, and possibly others, are critical to some degree. The required dimensional information could be specified by the end user, or requested of the manufacturer, at time of quote or order.

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IEEE Std C57.12.36-2007 IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

a)

b)

Figure A.1—a) HV left (segment 2) and b) HV right (segment 4)

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