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IEEE Standard Requirements for
Liquid-Immersed Power Transformers
IEEE Power & Energy Society
Sponsored by the
Transformers Committee
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IEEE
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IEEE Std C57.12.10™-2010
(Revision of
ANSI C57.12.10-1997)
6 January 2011
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Copyright The Institute of Electrical and Electronics Engineers, Inc.
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IEEE Std C57.12.10™-2010
IEEE Standard Requirements for
Liquid-Immersed Power Transformers
Sponsor
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(Revision of
ANSI C57.12.10-1997)
Transformers Committee
of the
IEEE Power & Energy Society
Approved 30 September 2010
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IEEE-SA Standards Board
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Abstract: This standard sets forth the requirements for power transformer application. This
standard is intended to be used as a basis for performance, interchangeability, and safety of the
equipment covered and to assist in the proper selection of such equipment. This document is a
product standard that covers certain electrical, dimensional, and mechanical characteristics of
50 Hz and 60 Hz, liquid-immersed power transformers and autotransformers. Such power
transformers may be remotely or integrally associated with either primary switchgear or
substations, or both, for step-down or step-up purposes and base rated as follows: 833 kVA and
above single-phase, 750 kVA and above three-phase. This standard applies to all liquidimmersed power transformers and autotransformers that do not belong to the following types of
apparatus: instrument transformers, step voltage and induction voltage regulators, arc-furnace
transformers, rectifier transformers, specialty transformers, grounding transformers, mobile
transformers, and mine transformers
Keywords: autotransformer, dimensional characteristics, electrical characteristics, load tap
changer, mechanical characteristics, power transformer, single-phase, three-phase
•
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IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating Committees of
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Introduction
This introduction is not part of IEEE Std C57.12.10-2010, IEEE Standard Requirements for Liquid-Immersed Power
Transformers.
This standard was prepared by the Revision of C57.12.10 Working Group of the Power Transformers
Subcommittee of the Transformers Committee of the IEEE Power and Energy Society. The purpose of this
standard is to cover the dimensional, electrical, and mechanical characteristics for liquid-immersed power
transformers and autotransformers.
This standard is a revision of ANSI C57.12.10-1997, American National Standard for Transformers—
230 kV and Below 833/958 through 8333/10 417 kVA, Single-Phase, and 750/862 through
60 000/80 000/100 000 kVA, Three-Phase Without Load Tap Changing; and 3750/4687 through
60 000/80 000/100 000 kVA with Load Tap Changing—Safety Requirements.
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The focus of this revision was to expand the scope of the standard and to include the requirements for
power transformers and autotransformers with high voltage up to 765 kV and with no limit on the
megavoltampere rating.
This revised standard includes the following significant changes:
⎯
The title was changed.
⎯
The scope was expanded to include autotransformers, increase the upper voltage limit to 765 kV,
and remove the maximum megavoltampere limit.
⎯
Distribution substation transformers, as defined in IEEE Std C57.12.36™ [B1],a were excluded from
this standard.
⎯
Most of the clauses were revised, rewritten, or rearranged.
⎯
Significant changes were made in the load tap changer (LTC) section. Additional requirements for
transformer paralleling operation were added.
⎯
An informative annex on LTC considerations was added.
This standard is a voluntary consensus standard. Its use may become mandatory only when required by a
duly constituted legal authority or when specified in a contractual relationship. To meet specialized needs
and to allow innovation, specific changes are permissible when mutually determined by the user and the
producer, provided that such changes do not violate existing laws and are considered technically adequate
for the function intended.
When this standard is used on a mandatory basis, the words shall and must indicate mandatory
requirements; the words should or may refer to matters that are recommended or permissive, but not
mandatory.
a
The numbers in brackets correspond to the numbers in the bibliography in Annex B.
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Notice to users
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Patents
Attention is called to the possibility that implementation of this standard may require use of subject matter
covered by patent rights. By publication of this standard, no position is taken with respect to the existence
v
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or validity of any patent rights in connection therewith. The IEEE is not responsible for identifying
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Participants
At the time this standard was submitted to the IEEE-SA Standards Board for approval, the Revision of
C57.12.10 Working Group had the following membership:
Gary Hoffman, Chair
Saurabh Ghosh, Vice Chair
James Graham, Secretary
Don Anderegg
Javier Arteaga
Donald Ayers
Peter Balma
Stephen Beckman
Thomas Beckwith
Enrique Bentacourt
Wallace Binder
Carlos Bittner
Donald Cherry
Craig Colopy
Frank Damico
Ronald Daubert
Beth Dumas
Eduardo Garcia
Charles Garner
Everett Hager Jr.
James Harlow
David Harris
Roger Hayes
Martin Heathcoate
Rowland James Jr.
Marion Jaroszewski
Erwin Jauch
Sheldon Kennedy
Stanley Kostyal
Michael Lau
Gilbert Lemos
Thomas Lundquist
Dennis Marlow
John Mathiews
Vinay Mehrotra
Van Nhi Nguyen
Ray Nicholas
Gylfi Olafsson
Tony Pink
Donald Platts
Paulette Powell
Thomas Prevost
Scott Reed
John Rossetti
Steven Schapell
Stephen Schroeder
Devki Sharma
Thomas Spitzer
Craig Stiegemeier
Raman Surbramanian
Robert Tillman
Jane Ann Verner
Richard von Gemmingen
Peter Zhao
Thomas Blackburn
William Bloethe
W. Boettger
Paul Boman
Harvey Bowles
Steven Brockschink
Kent Brown
Steven Brown
Carl Bush
Donald Cash
Yunxiang Chen
Bill Chiu
Tommy Cooper
Jerry Corkran
William Darovny
Dieter Dohnal
Gary Donner
Donald Dunn
vi
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Fred Elliott
Gary Engmann
Joseph Foldi
George Forrest
Bruce Forsyth
Marcel Fortin
Eduardo Garcia
James Gardner
Saurabh Ghosh
Jalal Gohari
Eduardo Gomez-Hennig
James Graham
William Griesacker
Randall Groves
Bal Gupta
Ajit Gwal
J. Harlow
David Harris
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William J. Ackerman
Michael Adams
S. Aggarwal
Samuel Aguirre
Steven Alexanderson
Stephen Antosz
I. Antweiler
Stan Arnot
Donald Ayers
Peter Balma
Paul Barnhart
William Bartley
Barry Beaster
Thomas Beckwith
W. J. Bill Bergman
Steven Bezner
Wallace Binder
Thomas Bishop
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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.
Lee Matthews
Phillip McClure
Susan McNelly
Joseph Melanson
Gary Michel
Daleep Mohla
Kimberly Mosley
Jerry Murphy
Raymond Nicholas
Joe Nims
T. Olsen
Bansi Patel
Shawn Patterson
J. Patton
Brian Penny
Howard Penrose
Paul Pillitteri
Donald Platts
Alvaro Portillo
Gustav Preininger
Iulian Profir
Jeffrey Ray
Jean-Christophe Riboud
Michael Roberts
Charles Rogers
John Rossetti
Robert Hartgrove
Roger Hayes
William Henning
Steven Hensley
Gary Heuston
Gary Hoffman
R. Jackson
Erwin Jauch
James Jones
Stephen Jordan
Lars Juhlin
C. Kalra
Gael Kennedy
Sheldon Kennedy
Tanuj Khandelwal
Ethan Kim
J. Koepfinger
Neil Kranich
Jim Kulchisky
Saumen Kundu
John Lackey
Chung-Yiu Lam
Thomas la Rose
Thomas Lundquist
Richard Marek
J. Dennis Marlow
John W. Matthews
Marnie Roussell
Thomas Rozek
Dinesh Sankarakurup
Bartien Sayogo
Lubomir Sevov
Devki Sharma
Gil Shultz
Hyeong Sim
James Smith
Jerry Smith
Steve Snyder
Sanjib Som
Brian Sparling
Allan St. Peter
David Tepen
S. Thamilarasan
T. Traub
Joseph Tumidajski
Joe Uchiyama
John Vergis
Jane Verner
Loren Wagenaar
David Wallach
Barry Ward
Kenneth White
James Wilson
Murty V. V. Yalla
When the IEEE-SA Standards Board approved this standard on 30 September 2010, it had the following
membership:
Robert M. Grow, Chair
Richard H. Hulett, Vice Chair
Steve M. Mills, Past Chair
Judith Gorman, Secretary
Karen Bartleson
Victor Berman
Ted Burse
Clint Chaplin
Andy Drozd
Alexander Gelman
Jim Hughes
Young Kyun Kim
Joseph L. Koepfinger*
John Kulick
David J. Law
Hung Ling
Oleg Logvinov
Ted Olsen
Ronald C. Petersen
Thomas Prevost
Jon Walter Rosdahl
Sam Sciacca
Mike Seavey
Curtis Siller
Don Wright
*Member Emeritus
Also included are the following nonvoting IEEE-SA Standards Board liaisons:
Satish Aggarwal, NRC Representative
Richard DeBlasio, DOE Representative
Michael Janezic, NIST Representative
Lisa Perry
IEEE Standards Program Manager, Document Development
Matthew J. Ceglia
IEEE Standards Program Manager, Technical Program Development
vii
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Contents
1. Overview .................................................................................................................................................... 1
1.1 Scope ................................................................................................................................................... 1
1.2 Mandatory requirements ...................................................................................................................... 2
4. Rating data.................................................................................................................................................. 3
4.1 Usual service conditions ...................................................................................................................... 3
4.2 Kilovoltampere ratings ........................................................................................................................ 3
4.3 Voltage ratings..................................................................................................................................... 5
4.4 Insulation levels ................................................................................................................................... 5
4.5 Taps ..................................................................................................................................................... 5
4.6 Impedance voltage ............................................................................................................................... 6
4.7 Top-liquid temperature-range limits .................................................................................................... 7
4.8 Routine tests ........................................................................................................................................ 7
5. Construction ............................................................................................................................................... 7
5.1 Accessories .......................................................................................................................................... 7
5.2 Bushings ............................................................................................................................................ 13
5.3 Lifting, moving, and jacking facilities ............................................................................................... 15
5.4 Nameplate .......................................................................................................................................... 17
5.5 Ground pads....................................................................................................................................... 18
5.6 Polarity, angular displacement, and terminal markings ..................................................................... 18
5.7 Liquid preservation system ................................................................................................................ 19
5.8 Tanks ................................................................................................................................................. 21
5.9 Auxiliary cooling equipment ............................................................................................................. 22
5.10 Power supply for transformer auxiliary equipment and controls ..................................................... 23
5.11 Terminal board ................................................................................................................................ 24
5.12 Junction boxes ................................................................................................................................. 24
5.13 Disconnecting switches with interlocks and terminal chambers...................................................... 24
5.14 Throat connection ............................................................................................................................ 25
5.15 Current transformers ........................................................................................................................ 25
5.16 Surge arresters ................................................................................................................................. 26
5.17 Other insulating liquid ..................................................................................................................... 26
5.18 Loading ............................................................................................................................................ 26
5.19 “Other” tests .................................................................................................................................... 27
6. LTC equipment – basic construction features .......................................................................................... 27
6.1 Load tap changer (LTC) .................................................................................................................... 27
6.2 Tap selector switch ............................................................................................................................ 27
6.3 Motor and drive mechanism .............................................................................................................. 28
6.4 Position indicator ............................................................................................................................... 28
6.5 Control equipment and accessories.................................................................................................... 29
Annex A (informative) LTC considerations ................................................................................................. 36
A.1 Constant and variable flux LTC applications ................................................................................... 36
A.2 Transformer paralleling .................................................................................................................... 38
A.3 Control of the high-voltage voltage or the low-voltage voltage ....................................................... 41
Annex B (informative) Bibliography............................................................................................................ 48
viii
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3. Definitions .................................................................................................................................................. 3
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2. Normative references.................................................................................................................................. 2
IEEE Standard Requirements for
Liquid-Immersed Power Transformers
IMPORTANT NOTICE: This standard is not intended to ensure safety, security, health, or
environmental protection. 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
1.1 Scope
This document is a product standard that covers certain electrical, dimensional, and mechanical
characteristics of 50 Hz and 60 Hz, liquid-immersed power transformers and autotransformers. Such power
transformers may be remotely or integrally associated with either primary switchgear or substations, or
both, for step-down or step-up purposes and base rated as follows: 833 kVA and above single-phase,
750 kVA and above three-phase.
This standard applies to all liquid-immersed power transformers and autotransformers that do not belong to
the following types of apparatus:
a)
Instrument transformers
b)
Step voltage and induction voltage regulators
c)
Arc-furnace transformers
d)
Rectifier transformers
e)
Specialty transformers
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This voluntary consensus standard sets forth the requirements for power transformer application. This
standard is intended to be used as a basis for performance, interchangeability, and safety of the equipment
covered and to assist in the proper selection of such equipment.
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
f)
Grounding transformers
g)
Mobile transformers
h)
Mine transformers
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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.
NOTE—The introduction of this standard describes the circumstances under which the document may be used on a
mandatory basis.1
2. Normative references
The following referenced documents are indispensable for the application of this document (i.e., they must
be understood and used; therefore, 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 referenced, 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).2
ASME B1.1, American National Standard for Unified Inch Screw Threads (UN and UNR Thread Form).3
ASME B1.20.1, American National Standard for Pipe Threads, General Purpose, Inch.
IEC 60038:2009, IEC standard voltages, ed7.0.4
IEEE Std C37.90.1™, IEEE Standard for Surge Withstand Capability (SWC) Tests for Relays and Relay
Systems Associated with Electric Power Apparatus.5, 6
IEEE Std C57.12.00™, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power and
Regulating Transformers.
IEEE Std C57.12.70™, IEEE Standard Terminal Markings and Connections for Distribution and Power
Transformers.
IEEE Std C57.12.80™, IEEE Standard Terminology for Power and Distribution Transformers.
IEEE Std C57.13™, IEEE Standard Requirements for Instrument Transformers.
1
Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement
the standard.
2
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/).
3
ASME publications are available from the American Society of Mechanical Engineers, 3 Park Avenue, New York, NY 10016-5990,
USA (http://www.asme.org/).
4
IEC publications are available from the Sales Department of the International Electrotechnical Commission, Case Postale 131, 3, rue
de Varembé, CH-1211, Genève 20, Switzerland/Suisse (http://www.iec.ch/). IEC publications are also available in the United States
from the Sales Department, American National Standards Institute, 25 West 43nd Street, 4th Floor, New York, NY 10036, USA.
5
IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854,
USA (http://standards.ieee.org/).
6
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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Not for Resale
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
IEEE Std C57.19.00™, IEEE Standard General Requirements and Test Procedure for Power Apparatus
Bushings.
IEEE Std C57.19.01™, IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus
Bushings.
IEEE Std C57.91™, IEEE Guide for Loading Mineral-Oil-Immersed Transformers.
IEEE Std C57.131™, IEEE Standard Requirements for Load Tap Changers.
3. Definitions
For the purpose of this document, the following terms and definitions shall apply. For other terms, the
standard transformer terminology in IEEE Std C57.12.807 shall apply. Other electrical terms are defined in
The IEEE Standards Dictionary: Glossary of Terms & Definitions.8
product standard: An industry product manufacturing or performance specification.
4. Rating data
4.1 Usual service conditions
Service conditions shall be in accordance with IEEE Std C57.12.00.
4.2 Kilovoltampere ratings
4.2.1 General
Kilovoltampere ratings are continuous and based on not exceeding 65 °C average winding temperature rise
by resistance and 80 °C hottest spot temperature rise. The temperature rise of the insulating fluid shall not
exceed 65 °C when measured near the top of the tank. These kilovoltampere ratings are based on the usual
temperature and altitude service conditions specified in IEEE Std C57.12.00.
The kilovoltampere rating of the transformer shall be based on its capacity at ONAN cooling stage. When
fans and/or pumps are added to the transformer (forced cooling), its rating shall be increased by the
percentage indicated in Table 1.
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4.2.2 Kilovoltampere rating base
7
8
Information on references can be found in Clause 2.
The IEEE Standards Dictionary: Glossary of Terms & Definitions is available at http://shop.ieee.org/.
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IEEE Standard Requirements for Liquid-Immersed Power Transformers
Table 1 — Transformer kilovoltampere rating
ONAN < 2500 kVA three-phase
ONAN< 833 kVA single-phase
ONAN
100%
Forced cooling
1st stage
2nd stage
115%
N/A
2500 ≤ ONAN ≤ 10 000 kVA three-phase
833 ≤ ONAN ≤ 3333 kVA single-phase
ONAN
100%
Forced cooling
1st stage
2nd stage
125%
N/A
ONAN > 10 000 kVA three-phase
ONAN > 3333 kVA single-phase
ONAN
100%
Forced cooling
1st stage
2nd stage
133%
167%
For a transformer without a self-cooled rating, the applicable multiplying factor given in Table 20 of
IEEE Std C57.12.00-2006 shall be applied to the maximum nameplate kilovoltampere rating to obtain the
equivalent base kilovoltampere rating.
Typical transformers ratings are given in Table 2. Actual ratings shall be mutually agreed between the user
and manufacturer.
In transformers with concentric winding arrangement, two or more separate windings may be situated one
above the other. In this case, the average winding temperature rise limit shall apply to the average of the
individual readings for the stacked windings if they are of equal size and kilovoltampere rating and similar
design. If they are not, the evaluation should be subject to agreement between the user and the
manufacturer. For all rated loading conditions that are evaluated, a hot spot temperature rise limit of 80 °C
shall apply to all windings.
Table 2 — Typical transformer kilovoltampere rating
ONAN
Forced cooling
1st stage
Three-phase transformers
Forced cooling
ONAN
1st stage
2nd stage
833
1041
750
862
—
1250
1562
1000
1150
—
1667
2500
2084
3125
1500
2000
1725
2300
—
—
3333
5000
4167
6250
2500
3750
3125
4688
—
—
6667
8333
—
8333
10 417
—
5000
7500
10 000
6250
9375
12 500
—
—
—
—
—
12 000
16 000
20 000
—
—
—
—
—
—
15 000
20 000
25 000
20 000
26 667
33 333
25 000
33 333
41 667
—
—
30 000
40 000
50 000
—
—
—
—
37 500
50 000
50 000
66 667
62 500
83 333
—
—
60 000
80 000
100 000
An autotransformer with a tertiary winding for external loading has no standard basis for megavoltampere
rating. All simultaneous loading conditions including megavoltampere rating and power factor shall be
specified by the user.
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Single-phase transformers
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
An equivalent two-winding kilovoltampere rating of an autotransformer is the rated power of the autoconnected winding multiplied by the auto-factor. Auto-factor is also known as “reduction factor” or
“co-ratio.”
Co-ratio = (N – 1)/N = (HV – LV)/HV
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As an example, a 138/69 kV, 100 MVA autotransformer has a co-ratio of (138 – 69)/138 = 0.5 and an
equivalent two-winding rating equal to 100 × 0.5 = 50 MVA.
If the transformer in addition is provided with a nonautoconnected tertiary winding of 35 MVA rated
power, then its equivalent two-winding rating will be (50 + 50 + 35)/2 = 67.5 MVA.
4.3 Voltage ratings
Voltage ratings for power transformers shall conform to the nominal and maximum system voltages
defined in Table 4 and Table 5 of IEEE Std C57.12.00-2006.
4.4 Insulation levels
Basic impulse insulation levels (BILs) for transformers shall conform to the BIL levels in Table 4 of
IEEE Std C57.12.00-2006.
4.5 Taps
4.5.1 High-voltage winding taps for de-energized operation
If specified, the de-energized tap changer (DETC), the following four high-voltage rated kilovoltampere
taps shall be provided: 2.5% and 5.0% above rated voltage, and 2.5% and 5% below rated voltage.
Voltages and currents should be listed in accordance with 5.4.
When a load tap changer (LTC) is furnished per 4.5.2, the high-voltage DETC may not be required.
4.5.2 Taps for LTC transformers
When an LTC transformer is specified, LTC equipment shall be furnished in the low-voltage winding to
provide approximately ± 10% automatic regulation of the low-voltage winding voltage in approximately
0.625% steps, with 16 steps above and 16 steps below rated low voltage. The transformer shall be capable
of delivering rated kilovoltamperes at the rated low-voltage position and on all positions above rated low
voltage. The transformer shall be capable of delivering low-voltage current corresponding to rated low
voltage at all positions below rated low voltage.
When agreed on by the user, the LTC may be located in an alternate winding to regulate the high- or lowvoltage winding. This application may make the transformer operate with variable flux voltage operation
when the tap positions are changed. Annex A indicates the effect in the transformer operation during this
condition and other variations.
5
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IEEE Standard Requirements for Liquid-Immersed Power Transformers
When required by the user, the transformer may be designed to deliver rated kilovoltampere output on all
tap positions.
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4.6 Impedance voltage
4.6.1 Percent impedance voltage
The percent impedance voltage at the self-cooled rating as measured on the rated voltage connection shall
be as listed in Table 3 if the user does not specify another value. For cases not covered in Table 3, the
percent impedance voltage value shall be agreed between user and manufacturer, and the user should
perform a system study to determine the proper value of impedance.
For autotransformers, the percent impedance voltage shall be as specified by the user, or it should be the
lower of the value from Table 3 and the value obtained according to the following equation:
Autotransformer impedance voltage = (Value from Table 3) × (Autotransformer co-ratio) × 1.5
where
Autotransformer co-ratio = (High-Voltage – Low-Voltage)/(High-Voltage)
This impedance voltage is the autotransformer impedance and not the equivalent autotransformer
impedance.
Table 3 — Percent impedance at self-cooled (ONAN) rating
High-voltage BIL
(kV)
Without LTC
With LTC
≤ 110
5.5
—
150
6.5
7.0
200
7.0
7.5
250
7.5
8.0
350
8.0
8.5
450
8.5
9.0
550
9.0
9.5
650
9.5
10.0
750
10.0
10.5
4.6.2 Tolerance on impedance voltage
The tolerance shall be as specified in IEEE Std C57.12.00.
6
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IEEE Standard Requirements for Liquid-Immersed Power Transformers
4.6.3 Percent departure of impedance voltage on taps for de-energized operation
The percent variation of tested impedance voltage on any tap from the tested impedance voltage at rated
voltage shall not be greater than the value of the total tap voltage range when expressed as a percentage of
the rated voltage.
NOTE— This requirement does not apply to LTC taps.
4.7 Top-liquid temperature-range limits
The transformer shall be suitable for operation over a range of top-liquid temperatures from –20 °C to
105 °C, provided the liquid level was established by following the manufacturer’s filling procedure.
NOTE—Operation at these temperatures may cause the mechanical pressure-vacuum bleeder device (5.1.6), if
provided, to function to relieve excessive positive or negative pressures.
4.8 Routine tests
4.8.1 General
Routine tests shall be made in accordance with IEEE Std C57.12.00.
4.8.2 LTC transformers
Additional routine tests for LTC transformers listed in IEEE Std C57.12.00 shall be made.
5. Construction
5.1 Accessories
Accessories as required and identified in Table 4 shall be located as shown in Figure 1.
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
CL
Accessories
HIGH-VOLTAGE COVER
BUSHINGS
CL
SEGMENT 4
SEGMENT 2
SEGMENT 3
SEGMENT 1
LOW-VOLTAGE COVER
BUSHINGS
Clause
ref.
Table 4
Locations
DETC operating handle
S1, S4, see
Clause ref.
Liquid level indicator
S1
5.1.2
Liquid temperature indicator
S1
5.1.3
Winding temperature indicator
S1
5.1.4
Pressure-vacuum gauge
S1 or S4
5.1.5
Pressure-vacuum bleeder valve
S1
5.1.6
Pressure relief device
Cover
5.1.7
Drain and filter valves
S1
5.1.8
Jacking facilities
See ref.
5.3.4
Nameplate
S1
5.4
Ground pad(s)
See ref.
5.5
†Auxiliary cooling control
S1 or S2
5.9
†LTC equipment
S1 or S2
6
†When furnished.
NOTE—Some designs include accessories and wiring connections as part of the LTC equipment assembly. In such cases,
accessories may be located in the same segment as the LTC and may be viewed parallel to the segment centerline.
Figure 1 — Accessories
See Table 4 for information on accessories and construction features to be provided on transformers.
Clause
Items
Without LTC
With LTC
DETC
A
A
5.1.2
Liquid Level Indicator
S
S
5.1.3
Liquid Temperature Indicator
S
S
5.1.4
Winding Temperature Indictor
S
S
5.1.5
Pressure-Vacuum Gauge
A
A
5.1.6
Pressure-Vacuum Bleeder Valve
A
A
5.1.7
Pressure Relief Device
S
S
5.1.8
Drain and Filter Valves
S
S
5.1.9
Sudden Pressure Relay
A
A
5.1.10
Alarm Contacts
S
S
5.1.11
Contact Wiring and Wire Color Coding
S
S
5.2
Bushings
S
S
5.2.1
Neutral Terminations
S
S
5.2.1.1
Y-Connected High-Voltage Windings
A
A
5.2.1.2
Y-Connected Low-Voltage Windings
A
A
5.1
Accessories
Table 4
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Table 4 — “Basic standard” construction features
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
Clause
Items
Without LTC
With LTC
5.2.1.3
Constructions for Neutral Terminations
A
A
5.3
Lifting, Moving, and Jacking Facilities
S
S
5.3.3.2
Other Moving Facilities (Wheels)
A
A
5.3.4
Jacking Facilities
S
S
5.4
Nameplate
S
S
5.5
Ground Pads
S
S
5.6
Polarity, Angular Displacement, and Terminal Markings
S
S
5.7
Liquid Preservation System
S
S
5.8
5.8.3.2
Tanks
Bolted Cover
S
A
S
A
5.9
5.9.1
Auxiliary Cooling Equipment
Controls for Auxiliary Cooling Equipment
A
A
A
A
5.9.2
5.9.2.2
Fans
Future Forced-Air Cooling
A
A
A
A
5.9.3
5.10
Pumps
Auxiliary Equipment Power Supply
A
A
A
A
5.11
5.12
Terminal Board
Junction Box
A
A
A
A
5.12.1
5.12.2
High Voltage
Low Voltage
A
A
A
A
5.13
Disconnecting Switches
A
A
5.13.1
High-Voltage Terminal Chamber
A
A
5.13.2
5.14
Low-Voltage Terminal Chamber
Throat Connection
A
A
A
A
5.14.1
5.14.2
High-Voltage Throat
Low-Voltage Throat
A
A
A
A
A
A
5.15
Current Transformers
5.15.1
Bushing Type Current Transformer
0
Terminal Blocks
A
A
5.16
Surge Arresters
A
A
5.17
Other Insulating Liquid
A
A
6
6.1
LTC Equipment
LTC
−
−
S
6.2
6.3
6.4
Tap Selection Switch
Motor and Drive Mechanism
Position Indicator
−
−
−
S
S
S
6.5
A.2
Control Equipment and Accessories
Transformer Paralleling
−
−
S
A
NOTE: “S” indicates “standard”, “A” indicates “available when specified.”
9
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Table 4 — “Basic standard” construction features (continued)
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
5.1.1 De-energized tap changer (DETC)
When a DETC is provided, its operating handle shall be brought out through the side of the tank in
Segment 1 or 4 at a height convenient for operators to safely change the taps. If the user requires operation
from ground level, then the height should not exceed 2 m (79 in). If for design reasons it cannot be located
in Segment 1 or 4, it may be located in the sidewall of one of the other segments.
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 (0.375 in) shall be provided for the
padlock. The plate indicating tap changer position 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.
5.1.2 Liquid level indicator
A magnetic level gauge with vertical face shall be mounted on the side of the tank in Segment 1 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 as follows:
a)
82.6 mm (3.25 in) ± 6.4 mm (0.25 in) minimum when the 25 °C liquid level is 2.44 m (96 in) or
less above the bottom of the base
b)
140 mm (50.5 in) ± 12.7 mm (0.5 in) minimum when the 25 °C liquid level is more than 2.44 m
(96 in) above the bottom of the base
Dial markings shall show the 25 °C level and the maximum and minimum levels with the letters HI-LO or
MAX-MIN.
The words “Liquid Level” shall be on the dial or on a suitable nameplate adjacent to the gauge.
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 handhole or manhole
flange surface.
//^:^^#^~^^"#@::"~^$:~@""#:$@"^~^:$~"^~~$^~:~^~^~:^@:\\
The change in liquid level per 10 °C change in temperature shall be indicated on the nameplate.
Nonadjustable alarm contacts shall be provided and shall be set to close at the minimum safe operating
level of the liquid. The contacts shall be in accordance to 5.1.10 and 5.1.11.
5.1.3 Liquid temperature indicator
A thermometer that measures top-liquid temperature shall be mounted on the side of the tank and shall be
readable to a person standing at the level of the base. Gauges, when required to have operating controls on
their cases, shall be mounted between 1.22 m (4 ft) and 1.83 m (6 ft) above the base. The minimum scale
range shall be 0 to 120 °C.
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 so that it is covered by at least 2.5 cm (1 in) of fluid at the lowest
permissible fluid level. For dimensions of the well, see IEEE Std C57.12.00.
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The gauge’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 “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 minimum dial diameter (inside the bezel) shall be 114 mm (4.5 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.
For digital indicators, the measurement title may be shown on an alphanumeric display separately from the
measured value or marked on the dial or a plate mounted adjacent to the gauge. A method of displaying the
peak temperature, using externally operated controls, shall be provided. Display colors may be black, red,
green, or amber.
The contacts on the gauge when required shall be in accordance with 5.1.10 and 5.1.11 and have
independent field-adjustable set-point values. Each of the three relay contacts shall provide the ability to
turn on a cooling stage, alarm, or actuate another relay or contactor. See 5.9.1.1 and 5.9.1.2 for switches
and relays or contactors that allow for redundant manual control of cooling equipment. The alarm contacts
shall be adjustable over a minimum range of 40 °C to 120 °C.
5.1.4 Winding temperature indicator
The winding temperature indicator shall use direct-measurement, simulated or calculated methods to
determine winding hottest spot temperature. Depending on the type of technology, the gauge may require
that the transformer be equipped with a heated thermowell, a load current signal from a bushing current
transformer, 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 thermowell is required, the
tank wall shall be ported to accept the specified heater. The well shall be positioned so that it is covered by
at least 2.5 cm (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 minimum 114 mm (4.5 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.
For digital indicators, the measurement title may be shown on an alphanumeric display separately from the
measured value or marked on the dial or a nameplate mounted adjacent to the gauge. A method of
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A thermometer that indicates winding temperature shall be mounted on the side of the tank and shall be
readable to a person standing at the level of the base. Gauges, when required to have operating controls on
their cases, shall be mounted between 1.22 m (4 ft) and 1.83 m (6 ft) above the base. The minimum scale
range shall be 0 to 180 °C.
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
displaying the historical highest recorded temperature, through externally operated controls, shall be
provided. Display colors may be black, red, green, or amber.
The gauge shall have a minimum of three sets of contacts in accordance with 5.1.10 and 5.1.11 and have
independent field-adjustable set-point values. Each of the three relay contacts shall provide the ability to
turn on a cooling stage, alarm, or actuate another relay or contactor. The alarm contacts shall be adjustable
over a minimum range of 40 °C to 140 °C.
5.1.5 Pressure-vacuum gauge
A pressure-vacuum gauge shall be provided for power transformers without a conservator.
The diameter of the dial (inside bezel) shall be 89 mm (3.5 in) ± 6.4 mm (0.25 in). The gauge shall have a
dark-face dial with light-colored markings and a light-colored pointer, and it shall be located either in
Segment 1 or in the half of Segment 4 that is adjacent to Segment 1.
The scale range for the pressure-vacuum gauge shall be between 69 kPa (10 lb/in²), 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) indicted on the nameplate shall be furnished for power transformers without a conservator.
5.1.7 Pressure relief device
A pressure relief device shall be provided on the cover of the transformer, with a minimum pressure relief
rating of 142 m3/min (5000 CFM) at 69 kPa (10 lb/in²). This relief rating (rate of release) applies for all
pressure relief devices regardless of pressure setting.
The pressure relief device shall be supplied with an alarm contact in accordance with 5.1.10 and 5.1.11.
5.1.8 Drain and filter valves
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 mm (1 in) of the bottom of the tank.
The drain valve shall have a built-in 0.375 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-per-inch (5/16-32 in) male thread for the
user’s connection and shall be equipped with a cap.
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The size of the drain valve shall be 2 in National Pipe Thread (NPT) and shall have tapered pipe threads
(in accordance with ASME B1.20.1) with a pipe plug in the open end.
The upper filter valve shall be provided and located below the 25 °C liquid level in Segment 1. The size of
the upper filter valve shall be 2 in NPT, and the upper filter valve shall have 51 mm (2 in) NPT
(in accordance with ASME B1.20.1) with a pipe plug in the open end.
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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 mounted relay or a under fluid relay. Under fluid
relays shall actuate under rapidly changing pressures of 10 kPa/s to 38 kPa/s (1.5 lb/in²/s to 5.5 lb/in²/s).
The gas space mounted relays shall actuate with a pressure change of 3.5 kPa/s to 21 kPa/s (0.5 lb/in²/s to
3.0 lb/in²/s). The relay shall actuate within 3 cycles of the rated power frequency.
The sudden pressure relay shall be able to withstand full vacuum or positive pressure of 103 kPa (15 lb/in²)
without damage.
The relay shall as a minimum be supplied with an alarm contact and a trip contact in accordance with
5.1.10 and 5.1.11.
5.1.10 Alarm contacts
Nongrounded alarm contacts shall be suitable for interrupting the following:
⎯
0.02 A dc inductive load
⎯
0.20 A dc noninductive load
⎯
2.5 A ac noninductive or inductive load
⎯
250 V maximum in all classes
5.1.11 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.
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 low-voltage neutral bushing having a grounded Y-connected low-voltage
winding shall be the same as that of the low-voltage line bushings for windings 25 kV and below. For
windings above 25 kV, a 25 kV neutral bushing with 150 kV BIL shall be provided.
Unless otherwise specified, bushings shall be mounted on the cover and located as shown in Figure 3.
Electrical characteristics and dimension of outdoor transformer bushings shall be as listed in
IEEE Std C57.19.00 and IEEE Std C57.19.01 where applicable.
13
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
Figure 2 — Contact wiring and wire color coding
H 0 OR X0 WHEN REQUIRED
H1
H2
H3
X1
X2
X3
NOTE—For single-phase transformers, omit H3, X3, and neutral bushings.
Figure 3 — Bushing arrangement
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
5.2.1 Neutral terminations
Four cover bushings shall be provided for each permanently connected Y-winding on three-phase
transformers.
When specified, other neutral terminations shall be provided as listed in 5.2.1.1, 5.2.1.2, and 5.2.1.3.
5.2.1.1 Neutral termination of Y-connected high-voltage windings
When specified, designated neutral terminations of Y-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 in Segment 2.
c)
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.
d)
High-voltage windings of transformers with a Y-Δ terminal board supplied in accordance with 5.11
item a) shall be available in one of the following constructions:
1)
Neutral ungrounded and not accessible
2)
Neutral brought through the cover in Segment 2
5.2.1.2 Neutral termination of Y-connected low-voltage windings
When specified, one of the following neutral terminations of Y-connected low-voltage windings shall be
provided:
a)
Permanently Y-connected low-voltage windings shall have the low-voltage neutral bushing
furnished as provided for in 5.2.
b)
Low-voltage windings of transformers with a Y-Δ terminal board supplied in accordance with 5.11
item b) 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
5.2.1.3 Constructions for neutral terminations
Neutral terminations, when furnished, shall be provided on the cover or in the junction box, terminal
chamber, or throat as necessary.
5.3.1 Safety factor
Lifting, moving, and jacking facilities shall be designed to provide a safety factor of 5. This safety factor is
the ratio of the ultimate stress of the material used to the working stress. The working stress is the
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5.3 Lifting, moving, and jacking facilities
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
maximum combined stress developed in the lifting facilities by the static load of the component being
lifted. This factor does not apply to pulling facilities since the unit is not suspended. For pulling, a safety
factor of 2 is acceptable.
5.3.2 Lifting facilities
Lifting facilities shall be provided for lifting the cover separately and also for lifting the core and coil
assembly from the tank using four lifting cables.
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Facilities for lifting the complete transformer (with the cover securely fastened in place) shall be provided.
Lifting facilities shall be designed for lifting with four vertical slings. (For large transformers, the use of
spreaders or a lifting beam may be involved.) The bearing surfaces of the lifting facilities shall be free from
sharp edges and shall be provided with a hole having a minimum diameter of 21 mm (0.8125 in) for guying
purposes.
5.3.3 Moving facilities
5.3.3.1 General
The base of the transformer shall be of heavy plate or have members forming a rectangle that shall permit
rolling or skidding in the directions of the centerlines of the segments.
The points of support shall be located so that the center of gravity of the transformer as prepared for
shipment does not fall outside these points of support when the base is tilted 15° or less from the horizontal,
with or without oil in the transformer.
Provision shall be made on or adjacent to the base for pulling the transformer parallel to the centerline of
Segments 1 and 3 and to the centerline of Segments 2 and 4.
The base shall be constructed so that the external edges on all sides are rounded or slope upward at an angle
of approximately 45°. A flat bottom base with material thickness of less than 12.7 mm (0.5 in) does not
require rounded or upward sloping edges.
5.3.3.2 Other moving facilities
When specified, flanged wheels for a 1.435 m (56.5 in) rail gauge for motion parallel to the centerline of
Segments 1 and 3 shall be available.
5.3.4 Jacking facilities
Jacking facilities shall be located near the extreme ends of the junctions of the segments.
Dimensions and clearances for jacking provisions shall be as shown in Figure 4.
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
CL
OF JACK
CLEARANC
TANK
SIDEWALL
F
E
JACKING
POINT
F
TUBES,
ETC
TANK
BASEPLATE
A
FLOOR
LINE
B
Weight
15,900 kg (35,000 lb) or less
Dim.
(mm)
(in)
A
88.9
3.5
B
63.5
2.5
E
686.0
27.0
F
127.0
5.0
G
76.2
3.0
H
127.0
5.0
G
H
G
H
Weight 15,900–29,500 kg
(35,000–65,000 lb)
Dim.
(mm)
(in)
A
127.0
5.0
B
63.5
2.5
E
686.0
27.0
F
127.0
5.0
G
76.2
3.0
H
127.0
5.0
Weight over
29,500 kg (65,000 lb)
Dim.
(mm)
(in)
A
457.0
18
B
102.0
4
E
508.0
20
F
127.0
5
G
76.2
3
H
127.0
5
NOTE 1—Dimensions E, F, G, and H are minimum free clearances.
NOTE 2—Where required in manufacturer’s standard designs, any dimensions may be in excess of those shown.
NOTE 3—Dimension E applies to nonremovable coolers only.
NOTE 4—Weight includes completely assembled transformer and fluid.
NOTE 5—Dimension A clarifies minimum jacking clearance.
Figure 4 — Provision for jacking
5.4 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. It may be located in Segment 2 when
LTC equipment is located in Segment 2.
For LTC transformers, the phrases “LTC transformer” or “LTC autotransformer” shall be used instead of
the word “transformer.”
Voltage and current ratings shall be given as follows:
0
100
1 000
10 000
100 000
--`,,```,,,,````-`-`,,`,,`,`,,`---
to
to
to
to
and
99.9
999
9 999
99 999
greater
to nearest 0.1
to nearest 1
to nearest 5
to nearest 10
to nearest 25
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
5.5 Ground pads
A tank-grounding pad shall consist of a copper-faced steel pad or a stainless-steel pad without copper
facing, 50.8 mm × 88.9 mm (2 in × 3.5 in) with two holes horizontally spaced on 44.5 mm ± 0.8 mm
(1.75 in ± 0.03 in) centers and drilled and tapped for 0.5 in - 13 Unified National Coarse (UNC) thread (as
defined in ASME B1.1). Minimum thickness of the copper facing shall be 0.4 mm (0.015 in). Minimum
threaded depth of the holes shall be 13 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, the
ground pad shall be located on the tank wall.
Ground pads shall be located toward the extreme left of Segment 1 and diagonally opposite in Segment 3
and located so that they do not interfere with the jacking facilities.
5.6 Polarity, angular displacement, and terminal markings
5.6.1 Polarity
All single-phase transformers shall have subtractive polarity.
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5.6.2 Angular displacement
The angular displacement between high-voltage and low-voltage terminal voltages of three-phase
transformers with Δ-Δ connections shall be 0°. The angular displacement between high-voltage and lowvoltage terminal voltages of three-phase transformers with Y-Δ or Δ-Y connections shall be 30°, with the
low voltage lagging the high voltage as shown in Figure 5. Phasor relations shall be as shown in Figure 5.
5.6.3 Terminal markings
External terminals shall be marked in accordance with IEEE Std C57.12.70. The high-voltage and lowvoltage bushing arrangements shall be as shown in Figure 3.
--`,,```,,,,````-`-`,,`,,`,`,,`---
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IEEE Standard Requirements for Liquid-Immersed Power Transformers
H2
X2
H1
H3
X3
X1
H2
X2
H1
X0
H3
X3
H2
X2
H0
H1
X1
H3
X3
Figure 5 — Angular displacement
5.7 Liquid preservation system
One of the preservation systems in 5.7.1 through 5.7.4 shall be provided. In these systems, the interior of
the transformer shall be sealed from the atmosphere at a top-liquid temperature of 105 °C.
5.7.1 Sealed-tank system
A sealed-tank system is one in which the gas plus liquid volume remains constant. It shall be designed so
that the internal gas pressure does not exceed 69 kPa (10 lb/in²) gauge positive or 55 kPa (8 lb/in²) gauge
negative.
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--`,,```,,,,````-`-`,,`,,`,`,,`---
X1
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
The system shall include a pressure-vacuum gage indicated in 5.1.5 and a pressure-vacuum bleeder valve
indicated in 5.1.6. The use of this system may result in the introduction of oxygen and moisture into the
transformer due to the operation of the pressure-vacuum bleeder valve.
5.7.2 Inert-gas pressure system
An inert-gas pressure system is a system in which a positive pressure of inert gas is maintained from a
separate inert-gas source and reducing valve system so that the interior of the transformer shall be sealed
from the atmosphere. The internal gas pressure shall not exceed 55 kPa (8 lb/in²) gauge.
5.7.3 Conservator-tank system without diaphragm
A conservator-tank system without diaphragm is a system that, by means of an auxiliary tank partly filled
with liquid and connected to the completely filled main tank, seals the oil in the main tank from the
atmosphere. The internal top oil pressure in the main tank shall not exceed 34 kPa (5 lb/in²) gauge. The
system shall include the devices described in 5.7.3.1 through 5.7.3.5.
5.7.3.1 Shut-off valve
A combination of valves shall be provided in the conservator tank and the main tank to close the flow of
liquid between both tanks. The size of the valves shall be at the manufacturer’s option.
5.7.3.2 Drain valve
A drain valve shall be located on the conservator tank side as near the bottom as possible. The size of the
drain valve shall be 2 in and shall have tapered pipe threads (NPT, in accordance with ASME B1.20.1),
with a pipe plug in the open end.
5.7.3.3 Liquid level indicator
The liquid level indicator indicated on 5.1.2 shall be installed in the conservator tank.
5.7.3.4 Dehydrating breather
5.7.3.5 Gas accumulation relay
The gas accumulation relay shall be located in the liquid connection between the main tank and the
conservator tank in order to monitor the gas and liquid movements. During normal operation, the relay is
completely filled with liquid to keep its internal float in their top limit or rest position. The accumulation of
gases in the float gas chamber causes the float to actuate an electrical contact system. The inside
mechanism shall comprise upper and lower contact systems for alarms and tripping positions.
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A dehydrating breather to prevent the normal moisture in the air from coming in contact with the liquid in
the conservator tank shall be provided. The dehydrating breather shall be filled with silica gel that absorbs
20% of its own weight in moisture and is provided with an oil trap to prevent the continuous contact
between the moist air and the silica gel.
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
5.7.4 Conservator-tank system with diaphragm
The conservator tank shall have the same characteristics of operation and accessories as in 5.7.3, but with
the additional following accessories:
5.7.4.1 Diaphragm
The interior of the conservator tank shall have a rubber air cell to isolate the transformer liquid and the air
and to prevent contamination, oxygen, and/or moisture from coming in contact with the transformer liquid.
The size of the rubber air cell shall be selected to assure the internal operating pressures indicated in 5.7.3
are not exceeded and to compensate the liquid volume displacement due to the temperature variations
specified.
5.7.4.2 Vent valve
A vent valve shall be provided at the top of the conservator tank in order to release any air trapped in the
liquid side. The size of the valve shall be determined by the manufacturer.
5.7.4.3 Vacuum equalizing valve
When the conservator tank is designed for full vacuum filling, a valve between the liquid side and the air
side of the conservator tank shall be provided so liquid filling of the conservator tank can occur under
vacuum while the pressures between both sides are equalized. The size of the valve shall be determined by
the manufacturer.
5.8 Tanks
5.8.1 Pressure design
Maximum operating pressures (positive and negative) for which the transformer is designed shall be
indicated on the nameplate. The completely assembled transformer shall be designed to withstand, without
permanent deformation, a pressure 25% greater than the maximum operating pressure.
5.8.2 Vacuum filling
Tanks shall be designed for vacuum filling (external pressure of one atmosphere, essentially full vacuum)
in the field.
5.8.3 Cover
5.8.3.1 General
A welded main cover shall be provided. 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 be at least 114 mm
(4.5 in) wide and shall have an area of at least 419 cm² (65 in²). Manholes, if circular, shall be a minimum
21
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
of 381 mm (15 in) in diameter. If rectangular or oval, they shall have minimum dimensions of 254 mm ×
406 mm (10 in × 16 in).
5.8.3.2 Bolted cover
When specified by the user, a bolted main cover shall be provided.
5.8.4 LTC compartment
In LTC transformers, if the arcing tap switch has components that involve direct arcing in liquid, these
components shall be located in a compartment sealed so it prevents transfer of liquid to any other
compartment or to the main tank.
5.9 Auxiliary cooling equipment
5.9.1 Control of auxiliary cooling equipment
When auxiliary cooling equipment is provided or future provisions are provided, a suitably sized relay shall
be provided for control from the winding temperature indicator if supplied, the liquid temperature indicator
if the winding temperature is not supplied, or both if specified. The relay shall be mounted inside the
cabinet.
5.9.1.1 Control by the liquid temperature indicator
The equipment for automatic control of auxiliary cooling equipment controlled from the liquid temperature
indicator shall consist of the following:
a)
A liquid temperature indicator defined in 5.1.3.
b)
A manually operable switch connected in parallel with the automatic control contacts and enclosed
in a weatherproof cabinet located on the side of the tank of Segment 1 at a height not greater than
1.52 m (60 in) above the base.
a)
A winding temperature indicator defined in 5.1.4, with alarm contacts as follows:
Contact
b)
Function
1
Supply power to first-bank cooling
2
Supply power to second-bank cooling
3
Initiate alarm or actuate relay
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 (60 in) above the base.
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When specified, or for transformers with forced-cooled ratings of 133% or greater of the self-cooled
ONAN rating, the equipment for automatic control of auxiliary cooling equipment for transformers
controlled from the winding temperature indicator shall consist of the following:
--`,,```,,,,````-`-`,,`,,`,`,,`---
5.9.1.2 Control by the winding temperature indicator
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
5.9.2 Fans
5.9.2.1 General
When specified, fan motors shall be 240 V, 60 Hz or 400 V, 50 Hz, single phase, without centrifugal
switch, and shall be individually fused or otherwise thermally protected.
If the power supply for 240 V, 60 Hz or 400 V, 50 Hz single-phase motors is not available, provision shall
be made to accommodate another single-phase motor supply voltage in accordance with ANSI C84.1 for
60 Hz operation or IEC 60038 for 50 Hz operation, not in excess of 600 V.
5.9.2.2 Provisions for future forced-air cooling
When cooling 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 indicator, the following equipment shall be
provided:
⎯
The necessary mechanical arrangement
⎯
A thermally operated liquid temperature indicator per 5.1.3
⎯
Provision for mounting the control cabinet
⎯
Provision for mounting the fans
When cooling 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 indicator, the following equipment shall be
provided:
⎯
The necessary mechanical arrangement
⎯
A thermally operated winding temperature indicator per 5.1.4
⎯
Provision for mounting the control cabinet
⎯
Provision for mounting the fans
5.9.3 Pumps
When specified, pump motors shall be 240 V, 60 Hz or 400 V, 50 Hz, single-phase, without centrifugal
switch, and shall be individually fused or otherwise thermally protected.
Pump facilities shall include valves to allow removal of the pump with minimum loss of insulating oil.
5.10 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.
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The voltage rating for auxiliary equipment and controls supplied with the transformer should also be in
accordance with ANSI C84.1.
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
5.11 Terminal board
When specified, 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 for transformers listed in the
appropriate rating table.
b)
A Y-Δ 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 Δ-connected.
5.12 Junction boxes
When specified, junction boxes shall be provided for the cable entrance for windings of 110 kV BIL (15 kV
nominal system voltage) or less. (See 5.2.1.3 when neutral termination is required.)
NOTE—Certain kilovoltampere and voltage ratings may impose design limitations on the availability or location of
these items.
5.12.1 High-voltage junction box
The high-voltage junction box shall be mounted either
a)
On the side of the tank in Segment 2, or
b)
On the cover in Segment 3.
5.12.2 Low-voltage junction box
The low-voltage junction box shall be mounted either
a)
On the side of the tank in Segment 4, or
b)
On the cover in Segment 1, provided no high-voltage junction box is on the cover.
5.13 Disconnecting switches with interlocks and terminal chambers
When specified, disconnecting switches with interlocks and terminal chambers shall be provided for the
cable connection to the windings. (See 5.2.1.3 when neutral termination is required.)
5.13.1 High-voltage terminal chamber
The high-voltage terminal chamber shall be mounted on the side of the tank in Segment 2.
5.13.2 Low-voltage terminal chamber
The low-voltage terminal chamber shall be mounted on the side of the tank in Segment 4.
--`,,```,,,,````-`-`,,`,,`,`,,`---
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NOTE—Certain kilovoltampere and voltage ratings may impose design limitations on the availability or location of
these items.
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
5.14 Throat connection
When specified, a throat connection or connections shall be provided. (See 5.2.1.3 when neutral
termination is required.)
NOTE—Certain kilovoltampere and voltage ratings may impose design limitations on the availability or location of
these items.
5.14.1 High-voltage throat
The high-voltage throat shall be located either
a)
On the side of the tank in Segment 2, or
b)
On the cover in Segment 3, provided a low-voltage throat is not on the cover.
5.14.2 Low-voltage throat
The low-voltage throat shall be located either
a)
On the side of the tank in Segment 1 or 4, or
b)
On the cover in Segment 1, provided a high-voltage throat is not on the cover.
See 5.2.1.3 when neutral termination is required.
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5.15 Current transformers
Bushing current transformers shall be provided as specified by the user in accordance with
IEEE Std C57.13 and with accuracy classifications (full winding) as listed in Table 5 of this standard.
Provisions shall be made for a maximum of two current transformers per bushing, not including current
transformers for winding temperature indicators or line drop compensation.
All secondary leads shall be brought to an outlet box. Provision shall be made for short-circuiting the
current transformer secondary windings.
Provisions shall be made for removing bushing-type current transformers without removing the tank cover.
When revenue metering current transformers are provided, a certified test report shall be provided. In
addition to this information, the manufacture of these current transformers shall specify the accuracy at
specified burdens at all available taps as specified by IEEE Std C57.13.
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5.15.1 Bushing-type current transformers (or provision for their addition in the future)
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
Bushing current
transformer ratio
Revenue 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
.3B-0.9
.3B-1.8
.3B-1.8
C200
C400
C800
69
600:5
1200:5
2000:5 and higher
.3B-0.9
.3B-1.8
.3B-1.8
C200
C400
C800
Above 69
600:5
1200:5 and higher
.3B-1.8
.3B-1.8
C400
C800
Bushing
insulation class
(kV)
5.15.2 Terminal blocks
A nonsplit terminal block shall be provided in a weatherproof enclosure, located near the transformer base
in Segment 1, for termination of all current transformer secondary leads.
5.16 Surge arresters
When specified, one or more of the following types of construction for surge protection shall be provided:
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 5.5) that is
mounted near the top of the tank and that may be specified for each set of arresters—except that
individual ground pads may be supplied where the separation of the arrester stacks is such that
individual pads for grounding each phase arrester represent better design.
NOTE—Material for connecting surge arresters to live parts and to ground pads is not included in item a) through
item c).
5.17 Other insulating liquid
When specified, another suitable insulating liquid shall be furnished instead of mineral oil.
--`,,```,,,,````-`-`,,`,,`,`,,`---
NOTE—When alternate insulating liquids are specified instead of conventional mineral oil, it is the responsibility of
the manufacturer to factor the specified fluid properties in meeting this standard.
5.18 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 (e.g., cables, bushings, tap changers, liquid expansion
space) shall be supplied in such a way that they in themselves do not limit the loading to less than the
capability of the windings.
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Table 5 — Recommended accuracy classification of bushing current transformers
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
NOTE—IEEE Std C57.91 provides the best known general information for loading transformers under various
conditions based on typical winding insulation systems and is based upon the best engineering information available at
the time of preparation. It discusses “limitations” of ancillary components other than windings that may limit the
capability of transformers to meet its guidelines.
5.19 “Other” tests
When specified, “other” tests as described in IEEE Std C57.12.00 shall be performed.
6. LTC equipment—Basic construction features
6.1 Load tap changer (LTC)
NOTE—The LTC equipment is considered for use in the low-voltage winding of a voltage step-down application.
Other applications are considered in Annex A.
6.2 Tap selector switch
6.2.1 General
The tap selector switch equipment shall be liquid immersed and described by one of the following
technologies:
a)
Arcing tap switch
b)
Tap selector with arcing switch
c)
Tap selector with vacuum interrupter
6.2.2 Tap selection switch features
Tap selection switch technologies shall incorporate the following features:
a)
Components located in one or more liquid-filled compartments with removable bolted cover(s) for
access to such components. Access shall be accomplished without exposing or draining any liquid
in the transformer main tank. All covers shall have handles and be removable; covers weighing
more than 20 kg (44 lb) shall additionally be hinged.
b)
A drain valve located in each liquid-filled compartment to provide maximal drainage. The valve
shall be 1 in NPT in accordance with ASME B1.20.1, with a pipe plug in the open end. The drain
valve shall have a built-in 0.375 in sampling device located on the side of the valve between the
main valve seat and the pipe plug. The device shall be supplied with a 5/16-32 in male thread for
the user’s connection and shall be equipped with a cap.
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The LTC equipment, when supplied, shall consist of a liquid-immersed tap selector switch, a diverter
switch with an arcing tap switch or a diverter switch with vacuum interrupter, motor drive mechanism, tap
position display apparatus, and control devices. The equipment shall be located in Segment 1 or 2 of the
transformer. The equipment shall meet the requirements of IEEE Std C57.131.
c)
A filling plug located in the top of each liquid-filled compartment. The plug shall be 1 in NPT.
d)
A magnetic liquid-level gauge with a vertical face mounted on the side of each oil-filled
compartment. For details, see 5.1.2.
e)
For arcing tap switch and tap selector with arcing switch technologies only: provision for venting to
the atmosphere of gases produced by the arcing.
f)
Provisions for liquid temperature, pressure relief device, oil filtration for arcing-in-oil LTCs, online
dissolved-gas-in-oil monitoring, and dehydrating breathers for nonsealed LTCs.
6.3 Motor and drive mechanism
The motor and drive mechanism assembly shall have the following features:
a)
A single-phase motor without centrifugal switch suitable for operation from a 240/120 V, 60 Hz or
230/400 V, 50 Hz, three-wire source. When specified, a lightning surge arrester shall be provided
for surge protection of the motor and power supply.
The power source for the motor shall be 240/120 V, 60 Hz or 230/400 V, 50 Hz, three-wire, singlephase, 60 Hz, with maximum voltage to ground at 60 Hz not to exceed 150 V. This power source
shall be provided by the user and shall be separate from the transformer. In some cases, the user
may additionally use this source for powering forced-air-cooling fans.
b)
A hand crank or similar apparatus for manual operation of the driving mechanism. An electrical
interlock shall be provided to prevent LTC operation by the motor drive while the manual means is
engaged. A place for storing of the manual drive means, if detachable, shall be provided.
WARNING
Hand crank operation of the LTC may not be designed for operation under load.
Consult the transformer supplier’s instructions.
c)
Mechanically actuated electric limit switches and mechanical stops on the LTC drive mechanism to
prevent travel beyond the maximum raise and lower positions.
6.4 Position indicator
6.4.1 General
An indicator of the operating position of the LTC shall be supplied. The indicator shall include the means
for displaying the past maximum and minimum operating tap position of the LTC. An operator with access
to the control shall have the means to reset the past maximum and minimum display function to the then
present operating position. The indicator shall be located so that it can be read while the LTC is operated by
hand.
6.4.2 Position indicator markings
The position indicator shall be marked in accordance with the following (see Figure 6).
a)
The nominal (rated low-voltage) tap position shall be located on the centerline at the top of a
circular dial and shall be indicated by the letter “N.”
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
b)
The raise range (output voltage is greater than the rated low-voltage) shall be located clockwise
from the “N” position. A single letter “R” (Raise) shall be located in the right half with an arrow
indicating the direction of raise. The 16 tap positions in the raise range shall be marked, and a
number shall appear opposite at least every fourth position. Number 16 shall be the highest voltage
position.
c)
The lower range (output voltage is less than the rated low-voltage) shall be located
counterclockwise from the “N” position. A single letter “L” (Lower) shall be located in the left half
with an arrow indicating the direction of lower. The 16 tap positions in the lower range shall be
marked, and a number shall appear opposite at least every fourth position. Number 16 shall be the
lowest voltage position.
N
4
4
L
8
R
12
8
12
16
16
--`,,```,,,,````-`-`,,`,,`,`,,`---
NOTE—This figure is intended to present a schematic rather than
a pictorial illustration of the dial face. See 6.4.2.
Figure 6 — Position indicator for LTC
6.5 Control equipment and accessories
6.5.1 General
Control devices to accommodate manual and automatic control of the LTC equipment shall be provided
unless the user specifies that the LTC transformer be supplied with no control for automatic LTC.
6.5.2 Control equipment enclosure
6.5.2.1 General
A weather-resistant cabinet shall be provided for housing the automatic control and related devices. The
cabinet shall be equipped with breather, hinged doors, and provision for entrance of up to three 1.5 in
conduits in the bottom. The doors shall provide access to the control and accessory devices and shall have
provision for padlocking consisting of matching holes having a minimum diameter of 9.5 mm (0.375 in).
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
Space shall be provided for mounting the control equipment required for parallel operation by the method
specified by the user (see 1.1).
6.5.2.2 Terminal blocks
Terminal blocks shall be provided in the control enclosure for terminating contacts specified in 5.1.10 for
liquid level and temperature indicators and for current transformer secondaries (two leads per current
transformer) specified in 5.15.1.
6.5.3 Control equipment
The LTC control system is composed of the following:
a)
Sensing apparatus to provide signals proportioned to the transformer low-voltage and load current.
b)
A control device to interpret the voltage and current signals of the sensing apparatus, relate this
information to conditions desired by the operator, and automatically command the LTC to hold the
output thereby required.
6.5.3.1 Sensing apparatus
The usual sensing apparatus consists of current transformer(s) and voltage transformer(s).
6.5.3.1.1 Current transformers
a)
For a Y-connected winding, the current transformer(s) shall deliver to the line drop compensator of
the control a current that is nominally in phase with the current at the X1 load terminal of the
transformer.
b)
For a Δ-connected winding, the current transformers shall deliver to the line drop compensator of
the control a current that is nominally in phase with a phasor derived from the relationship of the
current at the X1 load terminal minus the current at the X2 load terminal.
6.5.3.1.2 Voltage transformers
It is the responsibility of the user to install appropriate voltage transformer(s) that match the phasing of the
current transformers provided with the transformer.
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The manufacturer shall furnish current transformer(s) to deliver not less than 0.15 A and not more than
0.2 A to the control circuits when the transformer is operating at the maximum continuous current for
which it is designed, including increases that may be obtained by normal cooling modifications.
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
6.5.4 Control device
6.5.4.1 General
6.5.4.1.1 Environmental
The control device shall withstand –40 °C to 80 °C control enclosure temperature, relative humidity from
zero to 100%, and altitude of up to 3000 m (9840 ft) without loss of control.
6.5.4.1.2 Response time
A step change in applied voltage of 0.75 V from outside the band to within the band shall cancel any raise
or lower signal within 0.3 s.
6.5.4.2 Set-point adjustment ranges
The control device shall permit parameter adjustment as follows:
a)
Voltage level setting adjustable from at least 108 V to 132 V (related to line-voltage-by-voltagesupply ratio).
b)
Bandwidth setting adjustable from at least 1.5 V to 3.0 V (total range).
c)
Actuation time delay setting adjustable from at least 15 s to 90 s. (The time delay applies only to
the first required change if subsequent changes are required to bring the system voltage within the
bandwidth setting.)
d)
Line drop compensation adjustment including independently adjustable resistance and reactance.
The resistance shall be adjustable in the range of at least 0 V to +24 V. The reactance shall be
adjustable in the range of at least –24 V to +24 V. The voltage refers to line drop compensation at
the nominal control base voltage of 120 V and rated base current of 0.2 A.
6.5.4.3 Components and accessories
6.5.4.3.1 General
The following components shall be provided as part of the control device or as accessories to the control
system:
a)
Test terminals for measuring voltage proportional to transformer output voltage. The test terminal
voltage shall not be changed more than ± 1% by connecting a burden of 25 VA at 0.7 power factor
across the test terminals, unless otherwise specified. This voltage change is not included in the
specification of accuracy of the control relays.
b)
Manual-automatic control switch.
c)
Manual raise/lower switch(es).
d)
Operation counter to indicate accumulated number of tap changer operations.
e)
Band limit indication means.
31
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
f)
A screw-base lamp socket with a switch and a ground-fault-protected convenience outlet for a
120 V, single-phase 60 Hz supply.
g)
A heater with a manual switch.
6.5.4.3.2 Control of LTC transformers in parallel
Additional components and accessories shall be required when the LTC transformer is planned for
operation in parallel with another LTC transformer. Several alternative means are available for such
applications. The user shall specify the procedure to be used. The more commonly used procedures are
indicated in 1.1.
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6.5.5 Control system accuracy requirements
The LTC control shall have an overall system error not exceeding ± 1%. The accuracy requirement is based
on the combined performance of the control device and sensing apparatus including instrument current and
voltage transformers, utility windings, transducers, etc., with the voltage and current input signals of a
sinusoidal wave shape.
Since it is not practical to test the overall control system accuracy, it is permissible to individually test the
control system components. The accuracy of individual components is then combined to arrive at the
overall control system accuracy. Accuracy tests are design tests, not made on every unit. For the test,
voltage and current signals should have a sinusoidal wave shape. No analytical correction is permitted to
remove effects of harmonics in the accuracy test results.
6.5.5.1 Sensing apparatus
6.5.5.1.1 Voltage source
The voltage transformer shall be presumed to be of accuracy class 0.3; refer to IEEE Std C57.13.
6.5.5.1.2 Current source
The current source accuracy shall be determined on a nominal 0.2 A secondary current and a burden of
3.5 VA; refer to IEEE Std C57.13.
6.5.5.2 Control device
The accuracy of the control device shall be determined based on testing at an ambient temperature of 25 °C,
rated frequency, a nominal input voltage of 120 V and a base current of 0.2 A at 1.0 power factor.
NOTE—The user should be aware that harmonic distortion of the control device input voltage and/or current can result
in differences in the sensed average or root-mean-square (RMS) magnitude that affects the overall accuracy of the
control device and control system. Such differences may be inherent in the product design and do not constitute an
additional error in the context of control accuracy.
6.5.5.2.1 Control device errors
Each individual error-producing parameter is stated in terms of its effect on the response of the control
device and is determined separately with the other parameters held constant. Errors causing the control
32
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
device to hold a higher voltage level than the reference value are plus errors and those causing a lower
voltage level are minus errors. The overall error of the control device is the sum of the individual errors as
separately determined causing a divergence from the voltage level setting, presuming a bandwidth of zero
volts.
6.5.5.2.2 Factors for accuracy determination of control device
The greater magnitude of the sum of the positive or negative errors of the following three areas shall
constitute the accuracy of the control device:
a)
Variations in ambient temperature of the control environment between –30 °C and 65 °C.
b)
Frequency variation of ± 0.25% in rated frequency (0.15 Hz for 60 Hz application).
c)
Line drop compensation:
1)
Resistance compensation of 12 V and an in-phase base current of 0.2 A with reactance
compensation of zero.
2)
Resistance compensation of 12 V and a 90° lagging base current of 0.2 A with reactance
compensation of zero.
3)
Reactance compensation of 12 V and an in-phase base current of 0.2 A with resistance
compensation of zero.
4)
Reactance compensation of 12 V and a 90° lagging base current of 0.2 A with resistance
compensation of zero.
6.5.6 Tests
6.5.6.1.1 Determination of accuracy of control device
Subclause 6.5.6.1 outlines procedures for determining values of errors contributed by the factors described
in 6.5.5.2.2. The voltage and current sources applied shall be as free of harmonics or other distortions as the
test facility permits.
6.5.6.1.1.1 Tests for errors in voltage level
With the control device set at a voltage level of 120 V and at an ambient temperature of 25 °C, energize the
control device for one hour using a 120 V source of rated frequency. The control is calibrated at this point.
Errors in voltage level in the three tests below determine the control device accuracy:
a)
Tests for error in voltage level due to temperature: The control device shall be tested over a
temperature range of –30 °C to 65 °C in not more than 20 °C temperature increments. The air
temperature surrounding the control device shall be held constant and uniform within ± 1 °C of
each increment for a period of not less than one hour before taking a test reading. Tests are made at
rated frequency with zero current in the line drop compensation circuit.
b)
Tests for error in voltage level due to frequency: The control device shall be tested over a sufficient
range of frequencies to accurately determine the error over the specified range of rated frequency,
± 0.25%. Tests are made at a constant temperature of 25 °C with zero current in the line drop
compensation circuit.
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6.5.6.1 Design tests
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
c)
Tests for errors in voltage level due to line drop compensation: Four tests shall be made at rated
frequency and a constant temperature of 25 °C and a voltage level setting of 120 V. Determine the
voltage level required to balance the control with 0.2 A in the compensator circuit of the control
under the following conditions:
Test
Set LDC-R
(V)
Set LDC-X
(V)
Current phasing
Determine voltage error
relative to expected (V)
1
12
0
in-phase
V = 132.0
2
0
12
in-phase
V = 119.4
3
12
0
90° lagging
V = 119.4
4
0
12
90° lagging
V = 132.0
Use the individual test error (plus or minus) that produces the largest overall error magnitude when
summed in accordance with 6.5.5.2.1.
6.5.6.1.2 Set point marks
Deviation of set point marks for voltage level, bandwidth, line drop compensation, and time delay settings
are not considered as a portion of the errors in determining the accuracy classification.
6.5.6.1.2.1 Bandwidth center marking deviation
The difference between the actual bandwidth center voltage and the marked value at any setting over the
range of 120 V ± 10% shall not exceed ± 1%.
6.5.6.1.2.2 Bandwidth marking deviation
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The difference between the actual bandwidth voltage and the marked value shall not exceed ± 10% of the
marked value set.
6.5.6.1.2.3 Compensator marking deviation
The arithmetic difference between the actual compensation voltage, expressed as a percent of 120 V, and
the marked value of any setting of either the resistance or reactance element of the compensator, expressed
as a percent of 120 V, with 0.2 A in the compensator circuit shall not exceed ± 1%.
6.5.6.1.2.4 Time delay set marking deviation
The difference between the actual time delay and the marked value of any setting shall not exceed ± 10%.
This statement is true in an integrating type circuit when the delay is initiated with no stored delay.
6.5.6.1.3 Surge withstand capability (SWC) test
The SWC test is a design test for the control device in its operating environment. In order to pass this test,
the control device shall continue to operate properly and not have any unintentional tap change during and
after the test. Refer to IEEE Std C37.90.1.
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
6.5.6.2 Routine tests
6.5.6.2.1 Applied voltage
The control device shall withstand a dielectric test voltage of 1000 V, 60 Hz from all terminals to case for
1 min. The test shall be performed with the control totally disconnected from equipment. After the test, it
shall be determined that no change in calibration or performance has occurred.
NOTE—To minimize excessive damage or failure, use of a resistor to limit the current is recommended.
6.5.6.2.2 Operation
All features of the control device and its peripherals shall be operated and checked for verification of
proper functioning. The control is also calibrated at this point.
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35
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
Annex A
(informative)
LTC considerations
The low-voltage winding is the standard location for an LTC. There are applications when an LTC could
better be located in the high-voltage winding for step-up or step-down applications. At present, it is beyond
the scope of this standard to establish the standards for the LTC to be located in the high-voltage winding.
This annex provides information on issues for the transformer user and transformer manufacturer to discuss
to ensure that the transformer design and application are properly coordinated for high-voltage LTC
applications.
--`,,```,,,,````-`-`,,`,,`,`,,`---
a)
Will the LTC function by constant flux voltage variation (CFVV) or variable flux voltage variation
(VFVV)?
b)
Will the LTC be used to control the high-voltage voltage or the low-voltage voltage?
c)
Where will the user install the voltage transformer for monitoring the voltage and controlling the
LTC tap position?
A.1 Constant and variable flux LTC applications
A.1.1 CFVV LTC regulation
CFVV LTC operation regulates the transformer secondary by increasing or decreasing the turns in the
regulated winding (typically the low-voltage winding) while the unregulated winding (typically the highvoltage winding) turns are constant. The high-voltage system voltage is relatively constant; therefore, flux
density of the transformer is also relatively constant, impedance and sound levels are constant, and step
voltage is also constant with step voltage tolerances according to IEEE Std C57.12.00. For a CFVV tap
changer to increase the low voltage, turns are added to the low-voltage winding by operating the LTC in the
raise direction.
A.1.2 VFVV LTC regulation
VFVV LTC operation regulates the transformer winding (typically the low-voltage winding) by increasing
or decreasing the turns in the unregulated winding (typically the high-voltage winding) while the regulated
winding turns are constant. The high-voltage system voltage is relatively constant; therefore, flux density of
the transformer varies as the high-voltage turns are varied, impedance and sound levels also vary, and step
voltage is also variable. For a VFVV tap changer to increase the low voltage, turns are subtracted from the
high-voltage winding. As the volts per turn are increased—and, therefore, the low-voltage winding voltage
is increased, the flux density is also increased, the transformer sound level increases until reaching a
maximum level at highest tap position, and the transformer impedance is decreased.
CFVV results in constant no-load losses and sound level, uniform tap step voltages, and relatively small
variations of the transformer impedance. VFVV results in varying no-load losses and sound level,
nonuniform tap step voltages, and relatively large variations of the transformer impedance as shown in
Table A.1.
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Fundamental questions to answer are the following:
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
In some cases, to regulate low voltage in two- and three-winding transformers, the LTC is installed in the
high-voltage winding to reduce the cost. For low-voltage constant flux designs, the regulating winding is
placed inside the low-voltage winding and increases the length of mean turn of the low-voltage and highvoltage windings. As a result, cost and losses are increased. With VFVV operation, the regulating winding
is typically located over the high-voltage winding, and the mean turns of the low-voltage and high-voltage
windings are smaller and should result in lower losses. A second benefit is that the winding currents are
significantly lower in the high-voltage winding, and this level allows smaller conductors for the regulating
winding and eliminates the necessity of a booster/series transformer if the coil currents are less than the
rating of the LTC.
Table A.1 — VFVV cases
Winding connection
Design
High
voltage
Low
voltage
High-voltage
DETC
Low-voltage
LTC
requirements
Variable flux/
variable voltage
solution
Benefits
Two- and three-winding transformers
--`,,```,,,,````-`-`,,`,,`,`,,`---
1
Δ
Grounded
Y
± 2 at 2.5%
± 10% LTC
± 15% in highvoltage Δ
Reduced losses
Reduced cost
Eliminate high cost
of DETC
2
Grounded
Y
Grounded
Y
± 2 at 2.5%
± 10% LTC
± 10% in highvoltage neutral
Reduced losses
Reduced cost
3
Grounded
Y
Grounded
Y
± 2 at 2.5%
± 10% LTC
± 15% in highvoltage neutral
Reduced losses
Reduced cost
Eliminate high cost
of DETC
1
Grounded
Y
Grounded
Y
Autotransformers
± 2 at 2.5%
± 10% LTC
± 10% in neutral
Reduced losses
Reduced cost
Eliminate high cost
of DETC
2
Grounded
Y
Grounded
Y
± 2 at 2.5%
± 10% in neutral
Reduced losses
Reduced cost
Eliminate high cost
of DETC
± 10% LTC
Generally, the results of a variable flux design are as follows:
//^:^^#^~^^"#@::"~^$:~@""#:$@"^~^:$~"^~~$^~:~^~^~:^@:\\
a)
Nonuniform voltage steps and paralleling concerns.
b)
No-load losses change with LTC tap position and are highest at the highest tap position (highest
voltage level of low voltage).
c)
Total losses change with LTC position.
d)
Impedance varies with LTC position due to the flux variation; impedance varies inversely
proportionally to the square of the volts per turn.
e)
Sound level varies with LTC tap position and is highest at the highest tap position.
f)
LTC affects the voltage of the tertiary winding for three-winding transformers.
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g)
Operating the high-voltage LTC in the raise direction reduces the low voltage; operating the highvoltage LTC in the lower direction increases the low voltage.
h)
Placing the regulating winding outside the high-voltage winding can increase the induced transient
voltages and may require nonlinear resistors to mitigate the induced transients.
For design 1 (in Table A.1), high-voltage Δ connections and low-voltage grounded Y connections, the
DETC and LTC are combined into one, and this design incorporates the DETC range into the high-voltage
LTC. Since the DETC needs to be installed in the center of the high-voltage Δ-connected winding and the
LTC should also be installed in the center of the high-voltage winding, a combination is usually the best
solution for cost and technical considerations.
For design 2, the same benefits generally occur, but it is possible to use another regulating winding to
supply the DETC separately from the LTC. For design 3, the DETC tap range is combined into the range of
the LTC, and this design eliminates the cost of the DETC and its installation on the transformer core and
coils.
Autotransformers are unique. Instead of a separate high-voltage and low-voltage winding, the high voltage
and low voltage share the turns of the common winding, and the high-voltage winding is the only winding
using the turns of the series winding. In an autotransformer connection, the flux density variation depends
on the location of the LTC. In a neutral end LTC application, both the common and series windings share
the turns being varied in the common winding. Therefore, it also affects the high-voltage winding turns, the
highest flux density and sound level occur at the lowest tap position (16L), and the impedance increases as
the LTC position increases toward the highest tap position (16R).
Up to 138 kV low voltages, on-tank tap changers are available to allow CFVV designs. Above 138 kV
secondary voltages, single-phase in-tank LTCs may be required, and the cost will be significantly higher
than variable flux designs.
If the tertiary windings, common in autotransformers, are buried and for harmonic suppression only,
variable flux regulation of the autotransformer will not cause concern in the variations of the tertiary
winding voltage. If the tertiary is brought out for station service, capacitive, or reactance loading, special
LTC designs are required to stabilize the tertiary winding voltage when VFVV LTC connections are chosen
for voltage regulation.
--`,,```,,,,````-`-`,,`,,`,`,,`---
A.2 Transformer paralleling
A.2.1 Fundamental control premises and basic methods
There are three basic requirements for the appropriate control of tap position of multiple LTC transformers
operating in parallel:
a)
The transformers must continue their basic function of controlling the load bus voltage as
prescribed by the basic settings on the control: voltage set point (band center), tolerance bandwidth,
and line drop compensation.
b)
The tap changers must operate to maintain tap position to minimize the current that circulates
between them. Depending on the control method and transformer design, the appropriate tap
positions on the paralleled transformers are not necessarily all the same to achieve this requirement.
c)
Actions a) and b) above must operate correctly in applications with multiple paralleled transformers
regardless of planned system configuration changes or breaker operations that would alter the
parallel connection or operation of the transformers.
38
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
Three basic methods are employed to control tap changer operation of paralleled transformers:
1)
Direct operation of multiple tap changers from one control source (master/follower).
2)
Biasing or restraining of multiple control set points with interconnections or communications
between parallel transformers (circulating current, volt-ampere reactive (VAR) control, and power
factor).
3)
Biasing or restraining multiple controls without interconnections or communications between
parallel transformers (reverse reactance).
None of these paralleling methods are capable of correcting the imbalance between the active (watts)
components of load currents in parallel transformers with different impedances. The methods above are
typically used to reduce the imbalance between reactive (VARs) components of transformers’ currents.
Some methods in items 2) and 3) above can be used for paralleled transformers connected to separate
primary sources with limited differences between the applied primary voltages (that is, not beyond their
tapping ranges).
All methods, albeit sometimes involving special auxiliary apparatus, can be used for paralleling
transformers with different megavoltampere capacities or impedances. However, in all cases, the more
nearly identical are the transformers, the better will be the overall system performance.
A.2.2 Master/follower
//^:^^#^~^^"#@::"~^$:~@""#:$@"^~^:$~"^~~$^~:~^~^~:^@:\\
The master/follower paralleling method assumes that, under all system operating configurations, the
desired operation objectives are met by maintaining the same turns ratio on all paralleled transformers. This
operation is usually accomplished by maintaining the same physical tap position. The operation consists of
one active master control commanding the tap changes of additional transformers to follow. A tap
operation and position feedback scheme is mandatory to confirm to the master unit that the following unit
has operated properly. If that feedback is not received, the controls usually are set to lock out all further
operations. The use of this method is usually confined to transformers with equivalent design parameters.
This method is not applicable for paralleled transformers connected to separate primary sources.
A.2.3 Circulating current
As it is commonly defined, the circulating current paralleling method assumes that, under all system
operating configurations, any circulating current between transformers is representative of the disparity of
tap positions between the paralleled transformers. Variations in load currents do not affect the correct
operation of the circulating current method in minimizing circulating current. This method derives the
magnitude of the circulating current between transformers using external balancing modules and
communications between controls. The circulating current method causes any circulating current between
the transformers to bias the set points in opposite directions and thus cause subsequent tap operations to be
in the direction to reduce the circulating current. This operation is achieved while maintaining the voltage
set point accuracy. This method should not be used when the transformers are, or may be, with primary
circuit switching connected to separate primary sources.
A.2.4 Circulating VARs
Several methods are based on controlling the VARs in the parallel transformers. Each transformer’s VAR
loading is made up of the following:
a)
Its share of the total load VARs as determined by its relative impedance and megavoltampere
capacity compared to the other transformers.
b)
The circulating VARs due to differences in secondary induced voltages.
39
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IEEE Standard Requirements for Liquid-Immersed Power Transformers
In some methods, the objective is to reduce the circulating VARs to a minimum. In other methods, the
objective is for the transformers’ VARs to be shared in proportion to their megavoltampere capacities. The
VAR control methods are suitable for cases where the primaries are connected to different sources
A.2.5 Power factor
The power factor paralleling method is an implementation of the circulating current method where the basis
of operation is to recognize the disparity of power factor, rather than circulating amperes, as recognized by
each transformer. It assumes that, under all system operating configurations, any difference in power factor
between transformer loads indicates the relative tap positions of the paralleled transformers. With the same
voltage applied, the relative angle of the transformer currents will indicate the relative power factors. The
transformer control with the more lagging relative load current angle is allowed only to lower tap position,
if necessary to maintain bus voltage. The transformer control with the less lagging (or perhaps leading)
relative load current angle is allowed only to raise tap position, if necessary to maintain bus voltage. To the
extent that, from the perspective of each transformer, equalizing the power factor is equivalent to
equalizing the VARs, some of the attributes of the VAR control method are realized with the power factor
method.
The reverse reactance paralleling method uses a biasing voltage with X and R components in a somewhat
similar way to line drop compensation, except that the X component is reversed in direction (polarity).
Thus, whereas line drop compensation bias results in control of the voltage down the line, reverse reactance
bias results in control based on the transformer’s induced voltage upstream from the transformer
impedance. If all parallel transformers have the same induced (no-load) voltage, there will be no circulating
current. The R setting compensates for the transformer impedance drop so that in fact the load bus voltage
is controlled. Thus the voltage will be correct at no load and at full load, with a very small variation of set
point voltage (typically around 0.1% of set point) at all loads in between. More variation will be
experienced, but still typically less than 0.5% of set point, with reasonably anticipated variation of the load
power factor. The control is unaffected by switching other transformers in and out of parallel since each
transformer’s control takes care of itself without interconnections or communications between them.
Reverse reactance control thus meets the three basic requirements for LTC control of parallel transformers.
There are two caveats. First, if line drop compensation is required, each controller needs an input for the
summated load current, and this input is the only interconnection required between transformers in this
case. Thus, the use of line drop compensation obviates a principal advantage, that of no required control
interconnections. Second, the X and R settings are optimized for the average power factor at typical load. If
there are large variations in load power factor, such as might occur with crude power factor correction
systems where large banks of capacitors are switched, then reverse reactance control is not recommended.
However, experience has shown that the method works well in most practical situations feeding domestic,
industrial, or mixed loads.
A.2.7 Conclusion
The paralleling method chosen must be compatible with all the circuit conditions expected to occur during
the life of the paralleling. This compatibility must include all configurations that can occur due to
protective relaying operations, maintenance conditions, and system loading conditions. Each of these
methods has additional considerations for settings, field commissioning, and troubleshooting, depending on
specific equipment or system characteristics.
40
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A.2.6 Reverse reactance
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
A.3 Control of the high-voltage voltage or the low-voltage voltage
An LTC in the high-voltage winding could be used to control either the high voltage or the low voltage.
Under this IEEE standard, the user is expected to install the voltage transformer that monitors the
transformer voltage for controlling the LTC tap position. It is important to address the issues listed in A.3.1
and A.3.2.
A.3.1 Location of the voltage transformer
Under this IEEE standard, the user is expected to install the voltage transformer that monitors the
transformer voltage for controlling the LTC tap position. With a high-voltage LTC, it is possible to get a
situation where the manufacturer believes that the user will have the voltage transformer on the high
voltage to control the LTC position. But if the user locates the voltage transformer on the low voltage in a
step-down application, the LTC lowers the voltage when the opposite is required.
A.3.2 Issues to consider for high-voltage LTC applications
Variable-volts-per-turn transformers have large variations in impedance with changes in the LTC tap
position. Impedance varies inversely with the square of the volts per turn. A typical high-voltage LTC with
variable volts per turn (VFVV) causes the impedance to span over a 40% range about the neutral tap
position impedance. The user needs to consider the impedance range in the system design and impedance
value to specify. The manufacturer must consider the lowest impedance value for the transformer shortcircuit design.
A.3.2.2 Sound
VFVV (variable-volts-per-turn) transformers have increasing sound levels from the neutral tap to the
highest tap (16R). The manufacturer and the user must understand what the sound level limits should be for
all taps. Standard test procedures test sound levels only in the neutral tap position.
A.3.2.3 Ratios
A high-voltage LTC to control the low voltage provides other than the standard 0.625% steps. A 10%
change in the number of high-voltage turns produces something other than a 10% change in voltage of the
low voltage (e.g., 1/1.1 does not equal 0.9). An increase of the number of high-voltage winding turns by
10% causes a reduction in the low-voltage winding voltage of 9.09%. A decrease in the number of highvoltage winding turns by 10% causes a low-voltage winding voltage increase of 11.11%. In such an
application, the manufacturer and the user need to come to an agreement on what the actual output voltages
shall be and what shall be shown on the nameplate.
A.3.2.4 Nameplate
Should the nameplate show the low-voltage voltage changing with tap position, or should the high-voltage
voltage be shown changing with tap position? What voltages should be shown?
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A.3.2.1 Impedance variation
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
A.3.2.5 De-energized tap changer (DETC)
Should the high-voltage winding have a DETC? Typically, there is no need for a DETC in the high-voltage
winding when the LTC is located in the high voltage.
A.3.2.6 Over-excitation capability
The standard application requires that the transformer be able to continuously operate at no-load and 110%
of rated voltage without exceeding temperature limits. A variable-volts-per-turn (VFVV) application needs
to maintain the same 110% rule at the worst-case tap position. Test procedures should also be considered to
verify over-excitation capability.
A.3.2.7 Location of LTC switch and accessories
When the LTC is located in the high voltage, frequently the best location for the LTC switch is on the same
side of the transformer tank as the high-voltage bushings. Generally, that location is Segment 3. Generally,
the control cabinet should also be located near the LTC switch and preferably where the control cabinet
operator has a line of sight to the LTC switch. Also then, the gauges, the nameplate, and other accessories
would usually be located near the control cabinet. The manufacturer and the user need to agree on the
location of these items while considering manufacturing limitations and substation design.
A.3.2.8 Line drop compensation
Normally, a line-drop-compensation current transformer is located on the same bushing(s) as the voltage
transformer. For a low-voltage Y-connection with LTC, that location would require that the line-dropcompensating current transformer be located on the X1 bushing. When the LTC is in the high voltage, that
standard location may not be appropriate. The key issues to determine the proper current transformer
location is where the voltage transformer is located and whether the high-voltage or low-voltage voltage is
being controlled.
A.3.2.9 Kilovoltampere rating in all taps
Should a full kilovoltampere rating be expected in all taps? For a high-voltage LTC controlling the highvoltage voltage, a full kilovoltampere rating for all taps is generally appropriate. When a high-voltage LTC
is used to control the low-voltage voltage, then full capacity is generally unnecessary. The transformer user
should consider the question of kilovoltampere rating in all taps.
A.3.2.10 Paralleling
Paralleling of a high-voltage LTC transformer with other LTC transformers needs to consider the following
issues:
a)
Similarity of impedances of the transformers over the range of the LTC taps
b)
Similarity of the high-voltage/low-voltage ratios of the transformers over the range of the LTC taps
c)
Compatibility of controls to maintain the correct tap positions on all the transformers while
minimizing circulating current
42
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
--`,,```,,,,````-`-`,,`,,`,`,,`---
Transformers operated in parallel share the load based on their impedances when connected as shown in
Figure A.1.
Figure A.1 — Load sharing of transformers connected in parallel
The load of each transformer can be calculated using the following equations based on transformers where
ZA, ZB
IA, IB
IL
is per-unit impedance of transformers A and B @ stated megavoltampere base
is per-unit load current of transformers A and B
is per-unit load current of transformers A and B in parallel
Assuming the voltage drop through both transformers is equal, then
IA × ZA = IB × ZB
and
IL = IA + IB
Solving these equations gives the following load distribution between the two transformers based on the
ratio of their separate impedances calculated at the same base megavoltamperes:
Per-unit loads: IA =
ZB
ZA + ZB
and
IB =
ZA
ZA + Z B
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A.3.2.11 Load sharing of transformers connected in parallel
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
A.3.2.12 Some general paralleling operational basics
a)
Transformers of the same megavoltampere rating and equal impedance share the load equally
(Case I).
b)
Transformers of different ratings share loads based on their ratings as long as the impedances at
their base megavoltamperes are equivalent (Case II).
c)
If transformers of different impedances are paralleled, the total capacity of the transformers
connected in parallel is limited to less than the sum of their capacities (Case III).
d)
If transformers of different impedances are paralleled, the total capacity of the transformers
connected in parallel is limited to less than the sum of their capacities (Case IV).
A.3.2.12.1 Case I, identically rated transformers
Bank A = Bank B: 24/32/40 MVA
Impedance = 8% @ 24 MVA
--`,,```,,,,````-`-`,,`,,`,`,,`---
First, state the per-unit impedances on the same megavoltampere base:
Bank A = Bank B:
ZA = ZB = 0.08 @ 24 MVA base
The transformers share load inversely to the ratio of the impedance of the bank to the sum of the
impedances of the two banks in parallel.
Bank A share = Bank B share =
0.08
0.08
=
= 0.50 per-unit load
0.16
(0.08 + 0.08)
NOTE—When connected in parallel, the total bank rating is 80 MVA; Bank A and Bank B both carry 0.5 per unit
(40 MVA); neither unit is loaded in excess of its nameplate rating; and the bank capacity equals the sum of the
transformer nameplate ratings, 80 MVA.
A.3.2.12.2 Case II, transformers of different ratings
//^:^^#^~^^"#@::"~^$:~@""#:$@"^~^:$~"^~~$^~:~^~^~:^@:\\
Transformer:
Bank A
12/16/20 MVA
Impedance 8% @ 12 MVA
Bank B
24/32/40 MVA
Impedance 8% @ 24 MVA
First, state the per-unit impedances on the same megavoltampere base:
Bank A:
ZA = 0.16 @ 24 MVA base
Bank B:
ZB = 0.08 @ 24 MVA base
The transformers share load inversely to the ratio of the impedance of the bank to the sum of the
impedances of the two banks in parallel.
Bank A share =
0.16
0.08
= 0.33 per-unit load
=
(0.08 + 0.16)
0.24
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
Bank B share =
0.16
0.16
=
= 0.67 per-unit load
0.24
(0.08 + 0.16)
A.3.2.12.3 Case III, transformers of different cooling rating
NOTE—When the transformers are both rated with different cooling ratings (Bank A is ONAN/ONAF and Bank B is
ONAN/ONAF/ONAF) and both have identical impedances on their self-cooled bases, each share load according to its
rating but parallel operation loading is limited.
Bank A:
10/12.5 MVA
Impedance 8% @ 10 MVA
Bank B:
12/16/20 MVA
Impedance 8% @ 12 MVA
First, state the per-unit impedances on the same megavoltampere base:
Bank A:
ZA = 0.16 @ 20 MVA base
Bank B:
ZB = 0.13 @ 20 MVA base
The transformers share load inversely to the ratio of the impedance of the bank to the sum of the
impedances of the two banks in parallel.
Bank A share =
0.13
0.13
= 0.45 per-unit load
=
0.29
(0.16 + 0.13)
Bank B share =
0.16
0.16
= 0.55 per-unit load
=
0.29
(0.16 + 0.13)
//^:^^#^~^^"#@::"~^$:~@""#:$@"^~^:$~"^~~$^~:~^~^~:^@:\\
Maximum total load without exceeding the nameplate rating of Transformer A =
12.5
= 27.8 MVA.
0.45
NOTE—When connected in parallel, the total bank rating is 32.5 MVA. Limiting the loading of Bank A to its
nameplate rating of 12.5 MVA limits the total capacity of the paralleled transformers to a capacity of 27.8 MVA;
Bank A carries 0.45 per unit (12.5 MVA); Bank B carries 0.55 per unit (15.3 MVA); neither unit is loaded in excess of
its nameplate rating; and the bank capacity equals the sum of the transformer nameplate ratings, 60 MVA.
A.3.2.12.4 Case IV, transformers of different cooling rating
NOTE—When the transformers are both rated with different cooling ratings (Bank A is ONAN/ONAF and Bank B is
ONAN/ONAF/ONAF), it is possible to specify an impedance that permits each transformer to share load according to
its rating while allowing loading to the sum of the individual ratings.
For this study, it has been assumed that the 12/16/20 MVA transformer is being added to increase the
substation capacity and the goal is to optimize the parallel operation.
Bank A:
10/12.5 MVA
Impedance 8% @ 10 MVA
Bank B:
12/16/20 MVA
Impedance X% @ 12 MVA
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NOTE—When connected in parallel, the total bank rating is 60 MVA; Bank A carries 0.33 per unit (20 MVA); Bank B
carries 0.67 per unit (40 MVA); neither unit is loaded in excess of its nameplate rating; and the bank capacity equals
the sum of the transformer nameplate ratings, 60 MVA.
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
First, state the per-unit impedances on the same megavoltampere base:
Bank A:
ZA = 0.16 @ 20 MVA base
Bank B:
ZB = X @ 20 MVA base
The goal is to have Bank A carry 12.5 MVA while the total bank is carrying a total of 32.5 MVA.
Bank A share =
12.5MVA
= 0.385 per-unit load
32.5MVA
The transformers share load inversely to the ratio of the impedance of the bank to the sum of the
impedances of the two banks in parallel.
Bank A share =
X
= 0.385 per-unit load
(0.16 + X )
Bank B share =
0.16
= 0.615 per-unit load
(0.16 + X )
Solving the Bank A equation for X:
X = 0.385 (0.16+X)
0.615 X = 0.0616
X = 0.10 per unit
ZB = 0.10 @ 20 MVA base, ZB = 6% @ 12 MVA base
NOTE—When connected in parallel, the total bank rating is 32.5 MVA. Specifying the impedance of the
12/16/20 MVA at 6.0% @ 12 MVA base permits parallel operation to the full capacity of both transformers without
exceeding the ratings of either transformer.
//^:^^#^~^^"#@::"~^$:~@""#:$@"^~^:$~"^~~$^~:~^~^~:^@:\\
A.3.2.13 Autotransformer LTC application considerations
See Figure A.2.
a)
The ± LTC is located in the neutral end of the common winding resulting in a VVFV LTC
application used when the tertiary voltage (TV) (tertiary winding) is buried for harmonic
suppression or there is no tertiary winding to be affected by the variation in flux density. Unlike
two- and three-winding VVFV LTCs, the LTC affects both the common and series windings; the
minimum flux density is at tap position 16R; the maximum flux density is at 16L; and impedance
variation is not as large as for the two-winding VVFV application.
b)
A linear LTC is located between the series and common windings, and the low voltage is regulated
by moving the LTC in the raise and lower direction. This application is CFVV as the turns from the
high-voltage line to the H0X0 at the neutral end of the common winding remain constant regardless
of the LTC position.
c)
The ± LTC is located in the end of the series winding connection to the common winding and lowvoltage line bushing resulting in a VFVV LTC application used when the TV (tertiary winding) is
buried for harmonic suppression or there is no tertiary winding to be affected by the variation in
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flux density. Like two- and three-winding VFVV LTCs, the LTC affects only the series winding;
turns in the low voltage and TV are unchanged; the minimum flux density occurs at 16L; the
maximum flux density is at 16R; maximum sound level is at position 16R; and impedance variation
is as large as for the two-winding VFVV application.
d)
The ± LTC is located in the bushing connection from the common point of the series and common
windings resulting in a CFVV LTC application which does not affect TV (tertiary winding).
However, if the low voltage is greater than 69 kV, an in-tank LTC is required. Such an LTC would
dramatically increase the costs and tank size.
e)
The ± LTC is connected to a separate regulating winding for CFVV application while a series
transformer polarity and output voltage is varied based on the LTC position.
--`,,```,,,,````-`-`,,`,,`,`,,`---
Figure A.2 — Autotransformer LTC application
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IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
IEEE Std C57.12.10-2010
IEEE Standard Requirements for Liquid-Immersed Power Transformers
Annex B
(informative)
Bibliography
[B1] IEEE Std C57.12.36™, IEEE Standard Requirements for Liquid-Immersed Distribution Substation
Transformers. 9, 10
[B2] IEEE Std C57.12.90™, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and
Regulating Transformers.
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