IEEE Power and Energy Society
STANDARDS
IEEE Standard Requirements for PadMounted, Compartmental-Type, SelfCooled, Three-Phase Distribution
Transformers, 10 MVA and Smaller;
High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV
Nominal System Voltage and Below
Developed by the
Transformers Committee
IEEE Std C57.12.34™-2022
(Revision of IEEE Std C57.12.34-2015)
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IEEE Std C57.12.34™-2022
(Revision of IEEE Std C57.12.34-2015)
IEEE Standard Requirements for PadMounted, Compartmental-Type, SelfCooled, Three-Phase Distribution
Transformers, 10 MVA and Smaller;
High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV
Nominal System Voltage and Below
Developed by the
Transformers Committee
of the
IEEE Power and Energy Society
Approved 23 September 2022
IEEE SA Standards Board
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Abstract: Certain electrical, dimensional, and mechanical characteristics are set forth, as well
as certain safety features of three-phase, 60 Hz, liquid-immersed, self-cooled, pad-mounted,
compartmental-type distribution transformers. These transformers are rated 10 MVA and smaller,
with the high-voltage limit of 34.5 kV nominal system voltage and below, and with low-voltage limit
of 15 kV nominal system voltage and below. This standard covers the connector, bushing, and
terminal arrangements for radial or loop-feed systems. This standard does not cover the electrical
and mechanical requirements of any accessory devices that may be supplied with the transformer.
Keywords: compartmental, connector arrangements, IEEE C57.12.34™, loop, pad-mounted, padmount distribution transformers, radial, three phase, transformer
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Participants
At the time this IEEE standard was completed, the C57.12.34 Working Group had the following membership:
Stephen Shull, Chair
Michael Dahlke, Vice Chair/Secretary
Jerry Allen
Irving Antweiler
Israel Barrientos
David Blew
Thoma Callsen
John Chisholm
Rhett Chrysler
John Crotty
Craig DeRouen
Matthew Enders
Jose Gamboa
Benjamin Garcia
Carlos Gaytan
Ali Ghafourian
Jose Antonio Gonzalez
Ceballos
Said Hachichi
Kenneth Hampton
Michael Hardin
Gael Kennedy
Gary King
Brian Klaponski
Weijun Li
Parry Lively
Alejandro Macias
Terence Martin
Rhea Montpool
Charles Morgan
Michael Morgan
Daniel Mulkey
Jerry Murphy
Gerald Paiva
Dwight Parkinson
Jarrod Prince
Juan Ramirez
Martin Rave
Steven Schappell
Jeffrey Schneider
Jaber Shalabi
Audrey Siebert-Timmer
Igor Simonov
Edward Smith
James Spaulding
Christopher Sullivan
Liz Sullivan
Babanna Suresh
Eric Theisen
Michael Thibault
Alan Traut
Donnie Trivitt
Reinaldo Valentin
Jeremy Van Horn
John Vartanian
Pragnesh Vyas
Bruce Webb
Alan Wilks
Joshua Yun
The following members of the individual balloting committee voted on this standard. Balloters may have
voted for approval, disapproval, or abstention.
Samuel Aguirre
Donald Ayers
Robert Ballard
Peter Balma
Thomas Barnes
Frank Basciano
Barry Beaster
Jeffrey Benach
Steven Bezner
Kevin Biggie
Wallace Binder
Thomas Blackburn
Jon Brasher
Koti Reddy Butukuri
William Byrd
Paul Cardinal
Juan Castellanos
Ritwik Chowdhury
John Crouse
Thomas Dauzat
Huan Dinh
Gary Donner
Donald Dunn
Michael Faulkenberry
Jorge Fernandez Daher
Bruce Forsyth
Carl Fredericks
Shubhanker Garg
Saurabh Ghosh
Jalal Gohari
Said Hachichi
John Harley
Roger Hayes
Kyle Heiden
Steven Hensley
Werner Hoelzl
Robert Hoerauf
Richard Jackson
Jack Jester
John John
Thomas Keels
Peter Kelly
Gael Kennedy
Sheldon Kennedy
Gary King
Axel Kraemer
Jim Kulchisky
Chung-Yiu Lam
Moonhee Lee
Aleksandr Levin
Weijun Li
Mario Locarno
Martine-Denise Long
Richard Marek
7
Lee Matthews
Jeffrey McElray
Mark Mcnally
Daniel Mulkey
Jerry Murphy
Ali Naderian Jahromi
K.R. M. Nair
Arthur Neubauer
James Niemira
Sivaraman P
Lorraine Padden
Dwight Parkinson
Dhiru Patel
Howard Penrose
Jim Phillips
Alvaro Portillo
Allan Powers
Jarrod Prince
Ion Radu
Diego Robalino
Charles Rogers
Rodrigo Ronchi
Ryandi Ryandi
Daniel Sauer
Bartien Sayogo
Alan Sbravati
Stephen Shull
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Jerry Smith
Gary Smullin
Steve Snyder
Sanjib Som
David Stankes
David Tepen
Alan Traut
James Van De Ligt
Jason Varnell
John Vartanian
John Vergis
David Walker
David Wallace
Bruce Webb
Drew Welton
Kenneth White
Alan Wilks
Peter Zhao
Kris Zibert
When the IEEE SA Standards Board approved this standard on 23 September 2022, it had the following
membership:
David J. Law, Chair
Ted Burse, Vice Chair
Gary Hoffman, Past Chair
Konstantinos Karachalios, Secretary
Edward A. Addy
Ramy Ahmed Fathy
J.Travis Griffith
Guido R. Hiertz
Yousef Kimiagar
Joseph L. Koepfinger*
Thomas Koshy
John D. Kulick
Johnny Daozhuang Lin
Kevin Lu
Daleep C. Mohla
Andrew Myles
Damir Novosel
Annette D. Reilly
Robby Robson
Jon Walter Rosdahl
Mark Siira
Dorothy V. Stanley
Lei Wang
F.Keith Waters
Karl Weber
Sha Wei
Philip B. Winston
Daidi Zhong
*Member Emeritus
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Introduction
This introduction is not part of IEEE Std C57.12.34–2022, IEEE Standard Requirements for Pad-Mounted,
Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5
kV Nominal System Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below.
This standard was first published in 2004 as a revision and combination of ANSI Std C57.12.22™-1993
“American National Standard for Transformers—Pad-Mounted, Compartmental-Type, Self-Cooled
Three-Phase Distribution Transformers with High-Voltage Bushings, 2500 kVA and Smaller: High
Voltage, 34 500 Grd Y/19 920 Volts and Below; Low Voltage, 480 Volts and Below—Requirements” and
IEEE Std C57.12.26™-1992 “IEEE Standard for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers for Use with Separable Insulated High-Voltage Connectors (34 500 Grd
Y/19 920 V and Below; 2500 kVA and Smaller).” This resulted in a single standard that brought together padmounted transformers with either high-voltage bushing or separable connectors.
In 2009, the standard was revised and two significant changes to the scope were made. First, the size of the
units was increased to 5 MVA, and second, the low-voltage rating was changed from 480 V and below to 15
kV and below. This facilitated the development of standard requirements for pad-mounted transformers that
could be used for step-down as well as distribution service and required substantial updating of most figures
and tables.
In 2015, the scope of the standard was expanded to include pad-mounted transformers up to 10 MVA. To reflect
this change, a new figure was added and many tables and figures were updated to include the characteristics and
dimensions of the larger units. In addition, many figures were updated to include optional high-voltage neutral
bushings, and the standard was generally revised. The standard was also updated to current style requirements.
In this version, a new detailed informative Annex A was added to reflect the various internal accessories that
could be specified. Annex B was added to provide clarification of the horizontal angle of loading.
This standard was prepared by the Working Group of the Subcommittee on Distribution Transformers, for
Three-Phase, Pad-Mounted Transformers.
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Contents
1. Overview��������������������������������������������������������������������������������������������������������������������������������������������������� 11
1.1 Scope�������������������������������������������������������������������������������������������������������������������������������������������������� 11
1.2 Purpose����������������������������������������������������������������������������������������������������������������������������������������������� 11
1.3 Word usage����������������������������������������������������������������������������������������������������������������������������������������� 12
2. Normative references�������������������������������������������������������������������������������������������������������������������������������� 12
3. Definitions������������������������������������������������������������������������������������������������������������������������������������������������� 13
4. Ratings������������������������������������������������������������������������������������������������������������������������������������������������������ 13
4.1 Kilovolt-ampere ratings���������������������������������������������������������������������������������������������������������������������� 13
4.2 Voltage ratings������������������������������������������������������������������������������������������������������������������������������������ 13
4.3 Tap ratings������������������������������������������������������������������������������������������������������������������������������������������ 13
5. Impedance voltage������������������������������������������������������������������������������������������������������������������������������������ 14
5.1 Nominal percent impedance voltage�������������������������������������������������������������������������������������������������� 14
5.2 Tolerance on impedance voltage��������������������������������������������������������������������������������������������������������� 14
5.3 Tolerance on impedance voltage on a tap�������������������������������������������������������������������������������������������� 14
6. Basic lightning impulse insulation levels�������������������������������������������������������������������������������������������������� 14
7. Tests���������������������������������������������������������������������������������������������������������������������������������������������������������� 16
7.1 General����������������������������������������������������������������������������������������������������������������������������������������������� 16
7.2 Dielectric tests������������������������������������������������������������������������������������������������������������������������������������ 16
7.3 Dielectric test levels��������������������������������������������������������������������������������������������������������������������������� 16
8. Construction���������������������������������������������������������������������������������������������������������������������������������������������� 16
8.1 General����������������������������������������������������������������������������������������������������������������������������������������������� 16
8.2 Compartment configuration���������������������������������������������������������������������������������������������������������������� 16
8.3 Access������������������������������������������������������������������������������������������������������������������������������������������������� 16
8.4 Enclosure security and coating system����������������������������������������������������������������������������������������������� 17
8.5 Pad attachment����������������������������������������������������������������������������������������������������������������������������������� 17
8.6 Lifting������������������������������������������������������������������������������������������������������������������������������������������������� 17
8.7 Connectors, bushings, and terminals�������������������������������������������������������������������������������������������������� 17
8.8 Instruction nameplate������������������������������������������������������������������������������������������������������������������������� 20
8.9 Oil preservation���������������������������������������������������������������������������������������������������������������������������������� 20
8.10 Tanks������������������������������������������������������������������������������������������������������������������������������������������������ 20
8.11 Grounding provisions����������������������������������������������������������������������������������������������������������������������� 20
8.12 Components for loop primary cable systems������������������������������������������������������������������������������������ 21
9. Storage������������������������������������������������������������������������������������������������������������������������������������������������������ 21
10. Installation����������������������������������������������������������������������������������������������������������������������������������������������� 21
11. Other requirements that may be specified for some applications������������������������������������������������������������� 21
Annex A (informative) Accessories for three-phase pad-mounted transformers�������������������������������������������� 43
Annex B (informative) Clarification of the horizontal angle of loading��������������������������������������������������������� 53
Annex C (informative) Bibliography������������������������������������������������������������������������������������������������������������� 54
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IEEE Standard Requirements for PadMounted, Compartmental-Type, SelfCooled, Three-Phase Distribution
Transformers, 10 MVA and Smaller;
High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV
Nominal System Voltage and Below
1. Overview
1.1 Scope
This standard covers certain electrical, dimensional, and mechanical characteristics and takes into consideration
certain safety features of three-phase, 60 Hz, liquid-immersed, self-cooled, pad-mounted, compartmentaltype distribution transformers. These transformers are rated 10 MVA and smaller, with the high-voltage limit
of 34.5 kV system nominal voltage and below, and with low-voltage limit of 15 kV system nominal voltage
and below. These transformers are generally used for step-down purposes from an underground primary
cable supply. This standard covers the connector, bushing, and terminal arrangements for radial or loop-feed
systems. Either certain minimum dimensions or certain specific dimensions shall be specified. This standard
does not cover the electrical and mechanical requirements of any accessory devices that may be supplied with
the transformer.
1.2 Purpose
This standard is intended for use as a basis for determining performance, interchangeability, and safety of the
equipment covered, and to assist in the proper selection of such equipment.
1.3 Word usage
The word shall indicates mandatory requirements strictly to be followed in order to conform to the standard
and from which no deviation is permitted (shall equals is required to).1, 2
1
The use of the word must is deprecated and cannot be used when stating mandatory requirements; must is used only to describe
unavoidable situations.
2
The use of will is deprecated and cannot be used when stating mandatory requirements; will is only used in statements of fact.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
The word should indicates that among several possibilities one is recommended as particularly suitable,
without mentioning or excluding others; or that a certain course of action is preferred but not necessarily
required (should equals is recommended that).
The word may is used to indicate a course of action permissible within the limits of the standard (may equals
is permitted to).
The word can is used for statements of possibility and capability, whether material, physical, or causal (can
equals is able to).
2. Normative references
The following referenced documents are indispensable for the application of this document (i.e., they must
be understood and used, so each referenced document is cited in text and its relationship to this document is
explained). For dated references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments or corrigenda) applies.
ASME B30.9, Slings: Safety Standard for Cableways, Cranes, Derricks, Hoists, Hooks, Jacks, and Slings.
IEEE Std 386-2006™, IEEE Standard for Separable Insulated Connector Systems for Power Distribution
Systems above 600 V.3,4
IEEE Std C57.12.00™-2021, IEEE Standard for General Requirements for Liquid-Immersed Distribution,
Power, and Regulating Transformers.
IEEE Std C57.12.28™, IEEE Standard for Pad-Mounted Equipment—Enclosure Integrity.
IEEE Std C57.12.29™, IEEE Standard for Pad-Mounted Equipment—Enclosure Integrity for Coastal
Environments.
IEEE Std C57.12.39™, IEEE Standard for Requirements for Distribution Transformer Tank Pressure
Coordination
IEEE Std C57.12.70™, IEEE Standard for Standard Terminal Markings and Connections for Distribution and
Power Transformers.
IEEE Std C57.12.90™, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating
Transformers.
IEEE Std C57.91™, IEEE Guide for Loading Mineral-Oil-Immersed Transformers.
3. Definitions
For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary
Online should be consulted for terms not defined in this clause.5
safety factor: The ratio of the ultimate stress of the material used in the lifting, moving, and jacking provisions
to the working stress.
IEEE publications are available from The Institute of Electrical and Electronics Engineers (https://​standards​.ieee​.org/​).
The IEEE standards or products referred to in Clause 2 are trademarks owned by The Institute of Electrical and Electronics Engineers,
Incorporated.
5
IEEE Standards Dictionary Online is available at: http://​dictionary​.ieee​.org. An IEEE Account is required for access to the dictionary,
and one can be created at no charge on the dictionary sign-in page.
3
4
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
specific dimensions: The concept of a universal face plate is established where the size and location of certain
key product features are enumerated. The purpose is to provide a consistent front panel or face plate design
from manufacturer to manufacturer so that operating practices of the purchaser can be maintained, and a
transformer built under IEEE Std C57.12.34™ can work as a replacement in the field for another transformer
built under this standard.
working stress: The maximum combined stress developed in the lifting provisions by the static load of the
completely assembled transformer.
4. Ratings
4.1 Kilovolt-ampere ratings
Kilovolt-ampere ratings are for continuous balanced, three-phase operation and based on not exceeding either
a 65 °C average winding temperature rise or an 80 °C hot-spot conductor temperature rise. The temperature rise
of the insulating liquid shall not exceed 65 °C when measured near the top of the tank. These kilovolt-ampere
ratings are based on the usual temperature and altitude service conditions specified in IEEE Std C57.12.002021.6 The kilovolt-ampere ratings shall be as in Table 1.
Table 1—Kilovolt-ampere ratings
45
1000
75
1500
112.5
2000
150
2500
225
3750
300
5000
500
7500
750
10 000
4.2 Voltage ratings
Voltage ratings shall be in accordance with Table 3.
4.3 Tap ratings
When specified, high-voltage taps shall be composed of two 2.5% above the high-voltage rating and two
2.5% below the high-voltage rating. Transformers with a 208Y/120 low-voltage rating may be specified with
four 2.5% taps below the high-voltage rating. The tap changer handle or the area next to the handle in the
terminating compartment shall be marked indicating suitability for de-energized operation only.
5. Impedance voltage
5.1 Nominal percent impedance voltage
The percent impedance voltage, as measured on the rated voltage connection, shall be as shown in Table 2.
6
Information on references can be found in Clause 2.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Table 2—Nominal percent impedance voltage
For low-voltage rating
6900 Δ
through
13 800 Grd Y/7970 or
13 800 Δ
Rating
(kVA)
For low-voltage rating
600 V and below
For low-voltage rating
2400 Δ
through
4800 Δ
45
2.70–5.75 a
2.70–5.75 a
2.70–5.75 a
75
2.70–5.75
2.70–5.75
a
2.70–5.75 a
112.5
3.10–5.75 a
3.10–5.75 a
3.10–5.75 a
150
3.10–5.75
3.10–5.75
a
3.10–5.75 a
225
3.10–5.75 a
3.10–5.75 a
3.10–5.75 a
300
3.10–5.75 a
3.10–5.75 a
3.10–5.75 a
500
4.35–5.75
4.35–5.75
4.35–5.75 a
750
5.75
5.75
5.75
1000
5.75
5.75
5.75
1500
5.75
5.75
5.75
2000
5.75
5.75
5.75
2500
5.75
5.75
5.75
3750
5.75
5.75
6.00
5000
6.00
6.50
7500
6.00
6.50
10 000
6.00
6.50
a
a
a
a
a
These impedance values were designed to meet clause 7.1.4.1 and 7.1.4.2 of IEEE Std C57.12.00-2021, which defines the
maximum per-unit short-circuit withstand capability. Distribution transformers are designed as Category I and Category II
transformers even though the kVA rating is higher than specified in these clauses. It is recommended that the end user
evaluate other requirements such as voltage regulation, system impedance, or available fault current when using these
values.
5.2 Tolerance on impedance voltage
The tolerance on nominal impedance voltages shall be as specified in 9.2 of IEEE Std C57.12.00-2021.
5.3 Tolerance on impedance voltage on a tap
The percent departure of the tested impedance voltage on any tap from the tested impedance voltage at rated
voltage shall not be greater than the total tap voltage range and shall be expressed as a percentage of the rated
voltage.
6. Basic lightning impulse insulation levels
Minimum basic lightning impulse insulation levels (BILs) shall be in accordance with Table 3.
7. Tests
7.1 General
Except as specified in 7.2 and 7.3, tests shall be performed as specified in IEEE Std C57.12.00-2021 and
IEEE Std C57.12.90.
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15
45–1000
45–1000
75
95
95
95
12 000 Grd Y/6930 or 12 000 Δ
12 470 Grd Y/7200c
c
45–1000
45–1000
45–1000
45–1000
125
125
125
125
125
20 780 Grd Y/12 000c
22 860 Grd Y/13 200c
c
23 000 Δ
23 900 Grd Y/13 800c
24 940 Grd Y/14 400
34 500 Δ
b
34 500 Grd Y/19 920c, d
45–1000
75–1000
75–1000
150
150
45–1000
45–1000
95
95
c
16 340 Δ
45–1000
13 800 Grd Y/7970 or 13 800 Δ
13 200 Grd Y/7620
c
45–1000
75
6900 Δ
8300 Grd Y/4800c
45–1000
45–1000
75–3750
75–3750
45–3750
45–3750
45–3750
45–3750
45–3750
45–3750
45–3750
45–3750
45–3750
45–3750
45–3750
45–2000
45–2000
45–1500
45–750
For low-voltage rating
480Y/277c, 480,
600Y/347c, or 600 V
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
For standard low-voltage
rating 4800 Δ through
2400 Δe as specified in
the high-voltage rating
column of this table
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
750–10 000
For standard low-voltage
rating 13 800 Grd Y/7970 or
13 800 Δ through 6900 Δe as
specified in the high-voltage
rating column of this table
b
Kilovolt-ampere ratings which are separated by a dash indicate that all ratings covered in this range as shown in 4.1 are included.
The highest BIL level for separable insulated connectors is 150 kV BIL. If 200 kV BIL level is required, bushings shall be used.
c
Unsymmetrical excitation or loading of Y-Y connected units may cause heating of their tanks in excess of that which would be produced by balanced conditions. To reduce the probability
of tank heating, such units shall be provided with a core construction that will not saturate when 33% zero-sequence voltage is applied.
d
When specifying 125 kV BIL, adequate grounding and surge protection studies should be made.
e
Care should be taken in that some of these kVA ranges result in providing over 200 A at the rated voltage, thus requiring a 600 A connector. Consult IEEE Std 386-2006 for complete
connector ratings.
a
45–1000
60
60
4800 Δ
4160 Grd Y/2400 or 4160 Δ
45–750
45
c
2400 Δ
For low-voltage
rating
208Y/120c or 240 V
Minimum
BIL
(kV)
High-voltage rating
(V)
Ratinga
(kVA)
Table 3—Range of kVA and voltage ratings
IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
7.2 Dielectric tests
For wye-wye connected transformers, no applied voltage test is required on a winding greater than 600 V. An
induced voltage test shall be performed as specified in IEEE Std C57.12.90. The voltage induced between the
line terminals of a winding with a voltage greater than 600 V and ground shall be the lower of the following
values:
a)
3.46 times the rated winding voltage plus 1000 V
b)
The applied voltage test values provided in Table 3 of IEEE Std C57.12.00-2021
When high- or low-voltage terminations are used that fall under IEEE Std 386, they may not be suitable for
chopped wave testing; therefore, when terminations that conform to IEEE Std 386 are used (bushing wells,
600 A integral bushings, etc.) chopped wave tests are not required.
7.3 Dielectric test levels
Dielectric test levels shall be in accordance with the distribution levels in Table 3 of IEEE Std C57.12.00-2021.
8. Construction
8.1 General
A pad-mounted, compartmental-type transformer shall consist of a tank with high-voltage and low-voltage
cable terminating compartments, as shown in this standard. The compartment shall be separated by a barrier of
metal or other rigid material.
8.2 Compartment configuration
The high-voltage and low-voltage compartments shall be located side-by-side on one side of the transformer
tank. When viewed from the front, the lower-voltage compartment shall be on the right.
8.3 Access
Each compartment shall have a door so constructed as to provide access to the higher-voltage compartment
only after the door to the lower-voltage compartment has been opened. There shall be one or more additional
captive fastening devices that must be disengaged before the higher-voltage door can be opened. If the lowervoltage compartment has exposed live parts that are over 600 V, a non-hygroscopic barrier shall be placed so
as to require its removal or opening before access to the lower-voltage compartment can be attained. Where
the lower-voltage compartment door is of a flat panel design, the door shall have three-point latching with a
handle provided for a locking device. The compartment doors shall be of sufficient size to provide adequate
operating and working space when removed or open. The doors shall either be equipped for latching in the
open position or designed for manual removal.
8.4 Enclosure security and coating system
The transformer tank and compartment shall conform to IEEE Std C57.12.28 or IEEE Std C57.12.29 as
appropriate and be so constructed as to limit disassembly, breakage, and prying open of any doors, panels, and
sills when the doors are in the closed and locked position.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
8.5 Pad attachment
The bottom edges of the compartments shall be so constructed as to provide for the use of hold-down devices
that are accessible only from the inside of the compartments.
8.6 Lifting
The construction of the unit shall be such that it can be lifted, skidded, or slid, or any combination of these, into
place on the mounting surface without disturbing the high-voltage or low-voltage cables.
a)
Jack bosses or jacking facilities shall be provided on the tank for transformers rated 500 kVA or larger.
The vertical clearance for a jack shall be 38 mm (1.5 in) minimum, 165 mm (6.5 in) maximum.
b)
The transformer shall be arranged for rolling in two directions—parallel to and at right angles to one
side of the transformer.
c)
The transformer shall be provided with lifting provisions permanently attached and arranged on the
tank to provide a distributed balanced lift for the completely assembled transformer. It shall be in a
vertical direction and designed to provide a safety factor of five when the horizontal angle of loading is
not less than 30°.7
8.7 Connectors, bushings, and terminals
8.7.1 Electrical characteristics
The electrical characteristics of the completely assembled high-voltage connectors, high-voltage bushings,
and low-voltage terminals shall be as shown in Table 4 and Table 5.8,9
Table 4—Electrical characteristics and minimum electrical clearances of high-voltage
bushings and low-voltage terminals
Transformer voltage
BIL
(kV)
60-Hz
dry-1-min
withstandb, c
(kV)
208Y/120, 240
30
10
b, c
Minimum clearance—live partsa
Phase to
groundc
Phase to
phasec
25 (1.0)
25 (1.0)
480Y/277, 480
30
10
25 (1.0)
25 (1.0)
600Y/347, 600
30
10
25 (1.0)
25 (1.0)
Phase to nonhygroscopic
insulating barrier
2400
45
15
51 (2.0)
51 (2.0)
4160 to 4800
60
21
64 (2.5)
64 (2.5)
7200
75
27
89 (3.5)
102 (4.0)
76 (3.0)
12 000 to 13 800
95
35
127 (5.0)
140 (5.5)
76 (3.0)
22 860 Grd Y to 24 940 Grd Y
125
42
146 (5.75)
159 (6.25)
76 (3.0)
34 500 Grd Y
150
70
203 (8.0)
229 (9.0)
102 (4.0)
b
NOTE—These dimensions should be increased wherever possible to allow for ease in making connections by the user.
All dimensions are in millimeters (inches).
The use of barriers shall not reduce these electrical characteristics.
c
Where clearances are less than those shown, an adequate non-hygroscopic insulating barrier shall be provided.
a
b
Refer to Annex B for further clarification of the horizontal angle of loading.
Footnotes to tables and figures contain mandatory requirements.
9
Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this
standard.
7
8
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Table 5—Electrical characteristics and ratings of high-voltage connectors
Electrical characteristics of completely
assembled high-voltage connectorsc
Transformer
High-voltage ratings
Phase to ground
(kV)
Phase to ground/
Phase to phase
(kV)
BIL
(kV)
60-Hz Dry-1min withstand
(kV)
45
8.3
8.3/14.4
95
34
4160 to 4800
60
8.3
8.3/14.4
95
34
7200
75
8.3
8.3/14.4
95
34
12 000 to 13 800a
95
8.3 or
8.3/14.4 or
95 or
34 or
15.2
15.2/26.3
125
40
High-voltage ratings (V)
BIL
(kV)
2400
22 860 Grd Y to 24 940 Grd Y
125
15.2
15.2/26.3
125
40
34 500 Grd Yb
150
21.1
21.1/36.6
150
50
The required connector rating should be specified.
When specifying 125 kV BIL, adequate grounding and surge protection studies should be made.
c
IEEE Std 386 should be consulted for complete connector ratings.
a
b
8.7.2 Terminals greater than 600 V
8.7.2.1 General
The number, location, and arrangement of the terminals described by this subclause shall be as shown in
Figure 1, Figure 2, Figure 3, Figure 5, Figure 6, Figure 7, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13,
Figure 14, Figure 15, Figure 16, Figure 17, or Figure 18.
8.7.2.2 Bushing replacement
The terminals described by this subclause, whether bushing wells and bushing inserts, integral bushings,
or bushings shall be externally replaceable. The inside terminal connections shall be externally removable
through the connector’s opening in the transformer tank, or accessible through a handhole to permit removal
and replacement.
8.7.2.3 Voltage terminals using separable insulated high-voltage connectors
The high-voltage connectors shall consist of either 200 A bushing wells and bushing inserts, or 200 A, 600 A,
or 900 A integral bushings, as specified. Cable accessory parking stands shall be provided. For specific details
concerning high-voltage separable connectors and cable accessory parking stands, refer to IEEE Std 386.
Separable 200 A insulated high-voltage connectors that are designed for operation after the transformer is in
place shall be located so that they can be operated with hotline tools.
8.7.2.4 Voltage terminals using high-voltage bushings
Bushings shall have either tin-plated copper alloy clamp-type connectors or a two-hole spade terminal, both
being arranged for a vertical take-off. The clamp connectors shall accommodate No. 6 AWG solid to 250 kcmil
stranded conductors. The holes in the spade terminal shall be on the same plane, of a diameter of 14.3 ± 0.8 mm
(0.56 ± 0.03 in) typical and be 44.5 ± 0.8 mm (1.75 ± 0.03 in) center to center.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
8.7.3 600 V and below terminals
8.7.3.1 General
The phase and neutral terminals shall be in accordance with Figure 4 or Figure 8 of this standard and arranged
for vertical take-off. Terminal dimensions shall be as shown in Figure 19 of this standard or, if specified,
Figure 20 of this standard.
8.7.3.2 Support
Terminals equipped with 10 or more hole spades shall be furnished with additional support. A manufacturerdesigned support shall be attached at the end that is the farthest from the tank wall and mounted in such a way
that it will not interfere with the use of any of the holes of the spade or any spade extenders/connectors that the
user may want to install.
8.7.3.3 Bushing replacement
These bushings shall be externally replaceable. The inside terminal connections shall be externally removable
through the connector’s opening in the transformer tank, or accessible through a handhole to permit removal
and replacement.
8.7.4 Neutral terminal
8.7.4.1 Neutral terminal—600 V and below
The neutral terminal shall be either a blade connected directly to the tank or a fully insulated terminal. If
a fully insulated terminal is used, a ground pad shall be provided on the outer surface of the tank. One or
more removable ground straps suitably sized for the short-circuit rating of the transformer as defined in
IEEE Std C57.12.00-2021 shall be provided and connected between the neutral terminal and the ground pad.
For wye-wye connected units, the high-voltage neutral shall be connected to the low-voltage neutral internally
with provisions for opening this connection for testing.
8.7.4.2 Neutral terminal—greater than 600 V
When provided and effectively grounded, the neutral bushing may be two insulation classes below that of
the phase bushings or connectors. Otherwise, the neutral bushing shall be the insulation class of the phase
bushings or connectors.
8.7.4.3 Designations
Connector, bushing, and terminal designations shall be as defined in IEEE Std C57.12.70. Typical
configurations are shown in Figure 21.
8.8 Instruction nameplate
8.8.1 Location
The instruction nameplate shall be located in the low-voltage compartment and shall be readable with the
cables in place. When the nameplate is mounted on a removable part, the manufacturer’s name and transformer
serial number shall be permanently affixed to a non-removable part.
8.8.2 Information
The nameplate information shall conform to IEEE Std C57.12.00-2021, nameplate A for 500 kVA or below,
nameplate B for all other ratings. The high-voltage BIL, angular displacement as shown in Figure 21, and
identification of the bushing and terminal connections as shown in the applicable figures of this standard shall
be displayed on the nameplate.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
8.9 Oil preservation
8.9.1 Tank construction
The transformer shall be of sealed-tank construction. The transformer shall remain effectively sealed
for a top-oil temperature of −5 °C to +105 °C continuous and under operating conditions as described in
IEEE Std C57.91.
8.9.2 Pressure relief
The pressure relief for the transformer tank shall be as specified in IEEE Std C57.12.39.
8.10 Tanks
8.10.1 Strength
The tank shall be of sufficient strength to withstand a range of gauge pressures between −35 kPa (−5 psig) to
49 kPa (7 psig) without permanent deformation, and 103 kPa (15 psig) without rupturing or affecting cabinet
security as described in IEEE Std C57.12.28.
8.10.2 Pressure testing and oil access
A 1-in NPT upper plug (or cap) for filling and pressure testing shall be provided in the low-voltage
compartment. A 1-in NPT drain plug (or cap) for transformers rated 75 kVA through 500 kVA and a 1-in
NPT drain valve with built-in sampling device for transformers rated 750 kVA through 10 000 kVA shall be
provided in the low-voltage compartment. Suitable means for indicating the correct liquid level at 25 °C shall
be provided.
8.10.3 Removable covers
If a removable cover or handhole covers are exposed, they shall be secured in such a way as to conform to
IEEE Std C57.12.28.
8.11 Grounding provisions
8.11.1 500 kVA and below
Two steel pads, each with a 1/2-in 13 UNC tapped hole and a minimum thread depth of 11 mm (0.44 in) shall
be provided.
8.11.2 Above 500 kVA
Two unpainted, copper-faced steel or stainless-steel pads, 51 × 89 mm (2.0 × 3.5 in) each with two holes spaced
on 44 mm (1.75 in) centers and tapped for 1/2-in 13 UNC thread shall be provided. The minimum thickness of
the copper facing shall be 0.5 mm (0.02 in). Minimum thread depth of the holes shall be 13 mm (0.5 in).
8.11.3 Location
The ground pads described in 8.11.1 and 8.11.2 shall be welded on or near the transformer base, one in the
high-voltage compartment and one in the low-voltage compartment. In cases where the transformer tank and
compartments are separate, these pads shall be electrically bonded.
8.11.4 Lightning arrester attachment
When designed in accordance with Figure 2, Figure 3, Figure 6, or Figure 7 of this standard, mounting
provisions for lightning arresters shall consist of six steel pads with 1/2-in 13 UNC tapped holes 11 mm (0.44
in) deep, or 1/2-in 13 UNC studs, 25 mm (1.0 in) long, located in the high-voltage compartment.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
8.12 Components for loop primary cable systems
The minimum current-carrying capabilities of components for looped primary systems shall be 200 A
(continuous current rating) and 10 000 A rms symmetrical for 0.17 s (short-time current rating) for transformers
with or without high-voltage switching.
9. Storage
The transformer shall be stored in a vertical position on its base and shall remain essentially in that position at
all times, including transport to the site and during installations.
10. Installation
Equipment manufactured to this specification may be installed in areas where environmental and climatic
conditions make operation at varying angles of tilt from the horizontal an important consideration. Under
these circumstances, the users may wish to make a particular “angle of tilt” part of their specifications.
11. Other requirements that may be specified for some applications
Certain specific applications call for transformer requirements not covered in Clause 4 through Clause 10.
They shall be met only when specified in conjunction with the requirements of Clause 4 through Clause 10.
These specific requirements may change the dimensions in the figures of this standard and are not included in
this standard in order to avoid the implication of great or lesser availability by listing some and omitting others.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 1—Compartment designations and minimum dimensions for radial-feed
transformers with high-voltage bushings
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 2—Minimum dimensions for radial-feed transformers with high-voltage bushings
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 3—Low-voltage terminal arrangements greater than 600 V with minimum dimensions
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 4—600-V and below terminal arrangements with minimum dimensions
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 5—Compartment designations and specific dimensions for radial-feed transformers
with high-voltage bushings
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 6—Specific dimensions for radial-feed transformers with high-voltage bushings
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 7—Low-voltage terminal arrangements greater than 600 V with specific dimensions
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 8—600 V and below terminal arrangements with specific dimensions
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 9—Compartment designations and minimum dimensions for loop-feed or radial-feed
transformers with high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 10—Minimum dimensions for radial-feed transformers with high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 11—Minimum dimensions for loop-feed transformers with high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 12—Compartment designations and specific dimensions for loop-feed or radial-feed
transformers with 200 A high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 13—Compartment designations and specific dimensions for loop-feed or radial-feed
transformers with 600 A or 900 A high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 14—Specific dimensions for radial-feed transformers with high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 15—Specific dimensions for radial-feed transformers with high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 16—Specific dimensions for loop-feed transformers with high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 17—Specific dimensions for loop-feed transformers with high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 18—Specific dimensions for loop-feed transformers with high-voltage connectors
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 19—600 V and below terminals
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 20—600 V and below terminals
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure 21—Angular displacement
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Annex A
(informative)
Accessories for three-phase pad-mounted transformers
A.1 General
This annex highlights a group of accessories that may be added to a distribution three-phase pad-mounted
transformer by a user and provides further explanation of items described in this standard. Caution should be
exercised as the improper combination of some of these accessories could be detrimental to the function of
the transformer, or without a combination of them, a safety hazard could be presented to personnel. Interlocks
may be required when some of these items are specified. Please consult with the manufacturer to understand
these safety concerns.
A.2 Bail
This term describes a wire containment or other mechanical restriction that alerts operating personnel that
a connector should not be removed or a handle should not be operated before special operational or safety
instructions are followed. This is required to prevent injury to operating personnel and/or transformer damage.
A.3 Overcurrent protection
This describes a device or devices that allows the transformer to be disconnected from its source in the event of
a fault or overload. The transformer may be equipped with one of the common protection schemes, which are
shown in Table A.1. It is recommended that the user consult the manufacturer to discuss protection options if
the user is unsure about the proper transformer protection needed for their application. The order of listing in
Table A.1 does not represent any preference nor is this a complete list.
Table A.1—Common protection
Subclause
Description
A3.4
Full-Range, Current-Limiting Fuse
A3.5
Bayonet-Type or Cartridge Expulsion Weak Link Fuse
A3.5 and
A3.6
Bayonet Expulsion-Type Weak Link Fuse in Series With Isolation Link
A3.7
Bayonet Expulsion-Type Weak Link Fuse in Series
With Partial Range Under Insulating Liquid Current-Limiting Fuse
NOTE—Certain natural events can lead to an increase in dissolved acetylene and other hightemperature gases, complicating the interpretation of insulating liquid analysis. Accessories can
cause an otherwise healthy liquid-filled transformer to show signs of explainable acetylene gas
that was not present when shipped from the factory: for example, operation of an open element,
internal expulsion fuse such as the bayonet or a block-mounted cartridge fuse. Melting of the
fuse element and the subsequent extinguishing of the arc are in direct contact with the insulating
liquid causing the creation of hot metal and arcing gases.
A.3.1 Location
These fuse protection schemes are connected to the high-voltage side of the transformer. They may be fully
or partially accessible from the high-voltage side of the transformer compartment. Externally accessible fuses
are typically located in the area above the bushings or separable connectors.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
A.3.2 Labeling
Externally accessible fuse access points should be clearly marked as F1, F2, and F3 by either using decals or a
painted enunciation on the tank.
A.3.3 Accessibility and operational considerations
The devices used to access the fuses are designed to be operable with standard hotline tools. Internal fuses
designed to be replaced may be accessible through a handhole or by removing the bolted transformer cover if
the transformer is so equipped.
A.3.4 Full-range, current-limiting fuse
A.3.4.1 Dry well
A full-range, current-limiting fuse located in a dry-well canister should be of the nongassing type and explicitly
designed for use in a dry-well canister. The canister can be provided as non-loadbreak. It is recommended that
when using this type of fuse installation, a disconnecting means capable of breaking the full load current of the
transformer be provided either in the fuse canister or external to the canister utilizing an interlocking loadbreak
switch to prevent the fuse from being removed while the transformer is energized. For those applications
where an interlocked loadbreak switch is not used in conjunction with the non-loadbreak fuseholder, a warning
nameplate must be used as a precaution against the energized operation of the fuseholder and a mechanism of
securing the fuse canisters mechanically to prevent inadvertent operation.
A.3.4.2 Submersible
The submersible full-range, current-limiting fuse is similar to a dry-well fuse canister fuse except it is located
under the insulating liquid of the transformer. This makes it useable with smaller clearances, shorter creep
paths, and simpler load break mechanisms. Submersible installations also eliminate damage from erosion and
chemical changes due to weathering. This is normally accomplished by using a type of Bayonet fuse assembly
that has been designed to accommodate a full-range current-limiting fuse.
A.3.5 Weak link fuse assembly (bayonet or internal cartridge)
The weak link fuse is an assembly designed to be located under insulating liquid to allow for the breaking of
fault or load current. This is accomplished by utilizing a cartridge-type fuse. This fuse is designed to protect
the distribution system from excessive overloads and fault conditions in the transformer.
A.3.5.1 Mounting options
In the case of weak link fuses, use one of the following two installation methods:
a)
Bayonet type fuse canister with cartridge holder
b)
Internal cartridge in an insulated holder
Bayonet-style configurations provide external access to fusing using a hotline tool accessible holder. These are
typically mounted on the transformer front plate in such a way as to locate the fuse under the insulating liquid.
An internal cartridge style fuse is mounted inside the transformer tank under the insulating liquid. To replace
this fuse requires the removal of either a handhole cover or the transformer tank cover if the transformer is so
equipped.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
A.3.5.2 Spill prevention
Each bayonet fuse assembly should be equipped with a means of external liquid retention to accommodate any
liquid spilled that might occur upon removal of the fuse assembly. To reduce the amount of liquid that may
spill, internal pressure should be relieved by activating the pressure relief valve to relieve any pressure inside
the tank prior to removing the bayonet fuse.
A.3.5.3 Fuse link options
There are two main types of weak link fuse elements—current-sensing and dual-sensing. These fuses are
commonly used in series with a partial-range current-limiting fuse as a part of a full-range protection scheme.
A.3.5.3.1 Current-sensing fuse
The current-sensing weak link fuse is a single-element fuse that senses faults or overloads. Current-sensing
fuses have higher melting temperature elements, which tend to be insensitive to high liquid temperatures, and
only open from excessive resistive heating caused by a high current.
A.3.5.3.2 Dual-sensing fuse
The dual-sensing weak link fuse comprises a low melt temperature material. These fuses provide protection
against both faults or overloads and high liquid temperature. Because of its ability to sense and operate when
there is higher liquid temperature, this link aids in preserving the life of a transformer as the life expectancy of
a transformer is reduced when the transformer is exposed to long-term elevated temperatures.
A.3.6 Isolation link
The isolation link is located inside the tank. It is not intended to be externally accessible nor field replaceable.
It may be installed on an internal block or as an accessory part of the bayonet fuse assembly. The isolation
link provides extra protection during refusing and switching operations and must be used in series with a
bayonet-type fuse if a partial-range, current-limiting fuse is not used. Isolation links are not fuses and do not
have an interrupting rating but will melt during most transformer failures preventing operating personnel from
reenergizing the high-voltage circuit.
A.3.7 Weak link fuse in series with partial-range, underinsulating liquid, currentlimiting fuse
A partial-range, current-limiting fuse is located inside the tank. This fuse is not intended to be externally
accessible nor field replaceable. This fuse is defined by IEEE Std C37.41 [B6].10 Because it is only a partialrange fuse, it is required that the fuse be paired with a low-current series weak link fuse to achieve full-range
protection for the distribution transformer and isolation from the distribution system.
A.4 Underinsulating liquid loadbreak switches
Loadbreak switches are transformer components that allow the connection and disconnection of a loaded
transformer in radial feed units or provide a variety of connection options for loop-feed transformers.
These devices are selected by their load-breaking current, continuous current rating, rated operational voltage,
and by a combination of minimum closing fault current and latching current ratings. These devices can
withstand certain electrical test values and mechanical operational life criteria. For further explanation of
these devices, the user should consult IEEE Std C37.74 [B7] standard for specific requirements. The safe
10
The numbers in brackets correspond to those of the bibliography in Annex C.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
operation of these devices is dependent on being fully immersed in an insulating liquid. To that extent, a means
should be provided to verify these are sufficiently immersed.
NOTE—Certain natural events can lead to an increase in dissolved acetylene and other high-temperature gases,
complicating the interpretation of insulating liquid analysis. Any operation of a sectionalizing or load-break switch, when
a transformer is energized, will result in underliquid arcing and the creation of acetylene. Although arcing from the making
or breaking is only a few cycles in duration, it is enough to create acetylene gas, and it can cause an otherwise healthy
liquid-filled transformer to show signs of explainable acetylene gas that was not present when shipped from the factory.
A.4.1 Labeling
The two-position switch should be labeled “OPEN” or “CLOSED.” The four-position switch should be
labeled as dictated by its operational function. Labels should be distinctly observable from a position directly
in front of the handle at a distance of 2.4 m (8 ft) from the transformer.
NOTE—The transformer is considered energized until proven deenergized by testing for voltage and then applying a
ground. The position of a switch handle is never a guarantee of being deenergized as it may be backfed or could have
an internal short. The switch may be labeled “ON” or “OFF” as long as it is understood that this may not deenergize the
transformer.
A.4.2 Location
The switch operating handle may be located in the transformer’s primary compartment and is designed to be
operable by using standard hotline tools. An optional location may be in a separate compartment as described
in A.7. This cabinet should be located on the high-voltage side of the transformer.
A.4.3 Switch rotation
The two-position switch rotation should be clockwise to close and counterclockwise to open. The four-position
switch may be moved in either direction to accomplish the desired switching as indicated by the labeling of the
switch.
A.4.4 Current carrying
The continuous current-rating capability is typically 200 A, 300 A, or 600 A.
A.4.5 Short-time current rating
The minimum short-term current rating should be 10 kA rms symmetrical for a minimum of 0.17 s.
A.4.6 Triple application of two-position, underinsulating liquid, loadbreak switches
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
A.4.7 Loadbreak four-position switch types
Figure A.1—Standard switch arrangements
Figure A.2—Special switch arrangements
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
A.4.8 Underinsulating liquid, non-loadbreak switches
A.4.8.1 Deenergized underinsulating liquid tap changer switch
Deenergized switches are normally located in and operated from the transformer compartment and not located
in any auxiliary cabinet. These switches are manually operated, multiple-phase, and multiple-position tandemswitches, which are designed to be operated when the transformer is deenergized as these devices have no
interrupting capability. The operating handles should be secured by a bail or other means to guard against
energized operation.
The switches should be clearly and permanently labeled as “Suitable for deenergized operation only.”
A.4.8.2 Deenergized tap changer
Each tap changer position and the associated tap voltage should be clearly identifiable by a unique reference to
the nameplate information. All positions of the tap changer switch should be operative positions. Primary tap
ratings are stated in 4.3.
A.4.8.3 Dual-voltage (series multiple) switch
This switch is used to change deenergized transformer windings between series and parallel to provide
different common transformer voltage ratios, such as 12 470 × 24 940 V. Each voltage position should be
clearly identifiable by a unique reference to the nameplate information.
A.4.8.4 Delta-wye switch
This switch is used to change deenergized transformer windings between delta and wye to provide different
common transformer connections and ratios, such as 14 400 V / 24 940Y / 14 400 V. Each voltage position
should be clearly identifiable by a unique reference to the nameplate information.
A.5 Liquid-level indicating devices
A liquid-level device is provided to indicate the level of insulating liquid in the transformer tank. The indicating
devices provide either a “go” or “no go” indication, or they show a range from minimum to maximum.
A.5.1 Sight-type liquid-level indicators
This device is a plug equipped with a window, high-contrast floating ball, and reflector. It is located on the tank
wall to view the desired liquid level and indicates a “go” or “no go” liquid-level indication.
A.5.2 Visual dial indicator
A.5.2.1 Display
This device is a needle indicator that provides continuous display of the liquid level from maximum to
minimum. This can be a direct-drive or a gear-driven mechanism that provides amplified dial pointer
movement relative to the corresponding float travel in the tank.
A.5.2.2 Auxiliary contacts
Lever-driven gauges can be equipped with integrated switches that allow for limit alarm control functions.
Single-pole double-throw (SPDT ) contacts may be connected to the movement to give a remote annunciation
of a low or high liquid level. Depending on the model and manufacturer, the quantity and rating of these
contacts can vary. Therefore, before the user applies these switches in control circuitry, the ratings must be
obtained from the manufacturer.
48
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
A.6 Liquid temperature indicating devices
This device is furnished to reflect the top liquid temperature in the transformer tank in degrees Centigrade.
A.6.1 Temperature label
Temperature labels are temperature monitors consisting of one or more small heat-sensitive indicators sealed
under transparent and heat-resistant windows. This application requires the use of reversible temperature
labels so that they can change back and forth as needed to provide an indication of the present tank temperature.
It can be inferred from this value the temperature of the liquid in the tank. Nonreversible temperature labels are
also available. These provide indication of past operation at elevated temperatures. These labels are typically
applied in the field when a normal liquid temperature measuring mechanism is not available.
A.6.2 Dial-type visual indicator
A dial-type visual indicator is a surface-mounted thermometer with temperature indication using a dial pointer
and a maximum temperature pointer that may be reset. These dial-type visual indicators can be inserted
directly into the tank for direct liquid contact or into a dry well, which allows them to be removed without
lowering the liquid level.
A.6.3 Auxiliary contacts
Dial indication thermometers can be equipped with integrated switches that allow for limit alarm control
functions. Adjustable switches may be connected to the movement for a high- or low-temperature alarm, or
to energize a fan circuit. Depending on the model and manufacturer, the quantity and rating of these can vary.
Therefore, before the user applies these switches in control circuitry, the ratings need to be obtained from the
manufacturer.
A.6.4 Temperature transmitter
A temperature transmitter is an instrument that takes the input from a temperature sensor and converts the
measurement to an instrument-level voltage or current output, or to a digital value. Another role that the
temperature transmitter may play is to isolate the temperature signal by filtering any electromagnetic
interference, as well as by amplifying and converting the temperature sensor’s signal. This device enables
the temperature measurement to be used directly to a display device at the transformer or interfaced to a
supervisory control and data acquisition (SCADA) system or other distributed control system (DCS).
A.6.5 Temperature sensor
The temperature sensors can be a resistance temperature detector (RTD), thermistor, or thermocouple, which
are immersed in the insulating liquid of the transformer. The location of this sensor needs to be discussed with
the manufacturer as it could affect the insulation arrangement of the transformer.
A.7 Accessory boxes
Accessory weather-resistant boxes can be supplied, when specified by the purchaser, to provide access to
accessories without requiring entry into the cable termination cabinet. The typical location is on the lowvoltage side of the tank, but the high-voltage side of the tank can be used as well depending on the accessory
items to be included. A suggested configuration of external accessory boxes is shown in Figure A.3.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Figure A.3—Side view of transformer
These boxes provide a means for routine monitoring of the transformer without exposing individuals to
potential arc flash hazards present in the standard cable termination cabinet on the front of the transformer.
Common applications include monitoring, liquid sampling, and loadbreak switching. Separate boxes may be
supplied to include various accessories as to their application.
The box(es) should be equipped with provisions for padlocking to prevent unauthorized entry, as well as with
hinged or removable doors or covers. The box(es) does not need to meet all of the security requirements in
IEEE Std C57.12.28 as it does not enclose hazardous voltages.
A.8 Underinsulating liquid surge arrester
The surge arrester is a highly nonlinear, voltage-dependent resistor (usually a multistage metal-oxide varistor
stack), intended to clamp the incoming line voltage upon a transient overvoltage to a level determined by
its voltage versus discharge current characteristic. These devices are designed to be fully immersed in the
insulating liquid. They are not designed to be field replaceable. For testing to be accomplished, this device
must be disconnected from the circuit. This can be accomplished by using a switch or accessing a disconnect
point through a handhole. The internal mounting of the arrester eliminates the possibility of shortened arrester
life due to high surface contamination, wildlife, vandalism, or moisture ingress. The under-oil arrester may
offer improved transformer protection since internal mounting eliminates the need for long lead lengths to the
arrester. It should be pointed out that if the surge arrester does fail, there is not a good way to test it without
accessing it. Also, its failure could introduce contamination into the insulating liquid.
A.9 Internal current transformer
A.9.1 General
Internal current transformers may be mounted in the tank over the transformer bushing interior extension or on
top of the core coil assembly. This is normally determined by the manufacturer since the transformer’s design
may dictate its placement. These can be used for metering, protection circuitry, or another specific purpose
dictated by the transformer’s intended use.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
A.9.2 Governing standards
The current transformers should meet IEEE Std C57.13 [B11], IEEE Std C57.13.2 [B12], ANSI C12.11 [B1],
IEEE Std C37.235 [B8], IEC 61869-8 [B3], IEC 61869-12 [B4], or IEC 60688 Ed. 3.0 b:2012 [B5], depending
on the intent of its use.
A.9.3 Characteristics
The current transformer should be selected with the following considerations as based on the application:
a)
Voltage—The nominal and maximum voltage level to which the current transformer might be exposed.
For example, a current transformer placed over a bushing might not require a voltage rating. If the
current transformer is placed over the buss of the transformer, it would need to meet the insulation of
the buss. However, it should be noted that an open circuit introduced into the secondary circuit of the
current transformer will cause the secondary terminal voltage to surge to the voltage of the monitoring
circuit. This might be a consideration if the device is used in a circuit where the open-circuit voltage
exceeds 3500 V peak. The manufacturer should be consulted if this is the case.
b)
Frequency—The frequency at which the power system is operated.
c)
Accuracy—Metering or relaying class.
d)
Ampacity—Expected load and fault current to which the current transformer will be exposed.
e)
Burden—Sized for the impedance load of the anticipated instrument or device to which it is to be
paired.
f)
Connections—Wire gauge, lead length, and terminal location on the exterior of the tank. Wire
gauge and lead length will enter into the calculation of the burden of the current transformer circuit
and should be considered. It is strongly recommended that a shorting block be used as the external
termination points for these leads.
g)
Ratio—Primary to secondary ratio.
h)
Support—Mounting plates or nonmagnetic straps might be used. However, the mounting of a current
transformer is dictated by the design of the transformer and the manufacturer should be consulted
in determining the mounting location. Possible future replacement due to failure should be a
consideration in the current transformer placement.
i)
Temperature—The temperature ratings for both the transformer design and normal operating
environment.
A.10 Winding hot-spot monitors
A.10.1 General
This device is rarely used in distribution pad-mounted transformers. Typically, these are used in medium
power transformers. Since this standard can cover transformers up to 10 MVA with primary voltages up to
34.5 kV, it is included as a reference for these very large units.
Hot-spot temperature is used to estimate the life of a transformer as insulation can be damaged by high
temperatures in the insulation system. The hot spots are points in a transformer where the highest temperature
will be encountered during the loading of a transformer. The location of these points is a factor of core-coil
design, and therefore, the manufacturer should perform calculations that will indicate the probable winding
hottest spots.
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
A.10.2 Hot-spot thermal sensor—Resistor temperature detector
A standard RTD tends to have large surface area, which can affect readings and winding insertion. Therefore,
the manufacturer and user must discuss the desired accuracy of these devices. A small surface RTD is most
often required for winding insertion. This fact can affect the cost of the sensing system.
A.10.3 Hot-spot thermal sensor—Winding temperature indicator
The winding temperature indicator (WTI) simulates the winding temperature by using a proportional current
transformer output to power a heating element (resistor) contained in the well located in the top liquid of the
transformer. The winding temperature is simulated by adding the rise due to the heating element (resistor)
output to the liquid temperature. The current transformer is calibrated to increase the temperature of the
resistance to be proportionate to that of the winding. The sensor bulb of the instrument is located in a well
in the hottest liquid of the transformer; therefore, the winding temperature indicates a temperature of hottest
liquid plus the winding temperature above hot liquid, that is, the hot-spot temperature of the winding. The WTI
is provided with a resettable maximum temperature indicator.
Dial-indicating WTIs may be equipped with integrated switches that allow for limit alarm control functions.
Adjustable SPDT switches are connected to the movement for a high- or low-temperature alarm, and/or to
energize a fan circuit. Depending on the model and manufacturer, the quantity and rating of these can vary.
Therefore, before the user applies these switches in control circuitry, the ratings must be obtained from the
manufacturer.
52
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Annex B
(informative)
Clarification of the horizontal angle of loading
B.1 General
The horizontal angle of loading is established by measuring the smallest angle between the horizontal plane of
the top of the transformer and the lifting line. This is illustrated in Figure B.1. For further understanding of this
concept refer to ASME B30.9.
Figure B.1—Horizontal angle of loading
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IEEE Std C57.12.34-2022
IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, ThreePhase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System
Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
Annex C
(informative)
Bibliography
Bibliographical references are resources that provide additional or helpful material but do not need to be
understood or used to implement this standard. Reference to these resources is made for informational use
only.
[B1] ANSI C12.11, Instrument Transformers for Revenue Metering, 10 kV BIL through 350 kV BIL11
[B2] ANSI C84.1, American National Standard for Electric Power Systems and Equipment—Voltage Ratings
(60 Hertz).
[B3] IEC 61869-8, Additional Requirements for Electronic Current Transformers.12
[B4] IEC 61869-12, Additional Requirements for Combined Electronic Instrument Transformers and
Combined Stand Alone Instrument Transformers.
[B5] IEC 60688 Ed. 3.0 b:2012, Electrical measuring transducers for converting A.C. and D.C. electrical
quantities to analogue or digital signals.
[B6] IEEE Std C37.41™, IEEE Standard Design Tests for High-Voltage (>1000 V) Fuses and Accessories.13,14
[B7] IEEE Std C37.74™, IEEE Standard Requirements for Subsurface, Vault, and Padmounted LoadInterrupter Switchgear and Fused Load-Interrupter Switchgear for Alternating Current Systems up to 38 kV.
[B8] IEEE Std C37.235™, IEEE Guide for the Application of Rogowski Coils Used for Protective Relaying
Purposes.
[B9] IEEE Std C57.12.29™, IEEE Standard for Pad-Mounted Equipment—Enclosure Integrity for Coastal
Environments.
[B10] IEEE Std C57.12.80™, IEEE Standard Terminology for Power and Distribution Transformers.
[B11] IEEE Std C57.13™, IEEE Standard Requirements for Instrument Transformers.
[B12] IEEE Std C57.13.2™, IEEE Standard Conformance Test Procedures for Instrument Transformers.
ANSI publications are available from the American National Standards Institute (https://​www​.ansi​.org/​).
IEC publications are available from the International Electrotechnical Commission (https://​www​.iec​.ch) and the American National
Standards Institute (https://​www​.ansi​.org/​).
13
The IEEE standards or products referred to in Annex C are trademarks owned by The Institute of Electrical and Electronics Engineers,
Incorporated.
14
IEEE publications are available from The Institute of Electrical and Electronics Engineers (https://​standards​.ieee​.org/​).
11
12
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