Ieee c57-15-1999

Recognized as an IEEE Std C57.15-1999
American National Standard (ANSI) (Revision of
IEEE Std C57.15-1986)

IEEE Standard Requirements,
Terminology, and Test Code
for Step-Voltage Regulators

Sponsor
Transformers Committee
of the
IEEE Power Engineering Society

Approved 16 September 1999
IEEE-SA Standards Board

Abstract: Electrical, mechanical, and safety requirements of oil-ﬁlled, single- and three-phase
voltage regulators not exceeding regulation of 2500 kVA (for three-phase units) or 833 kVA (for
single-phase units) are covered.
Keywords: electrical, mechanical, safety, step-voltage regulators, test code

The Institute of Electrical and Electronics Engineers, Inc.
3 Park Avenue, New York, NY 10016-5997, USA

Copyright © 2000 by the Institute of Electrical and Electronics Engineers, Inc.
All rights reserved. Published 13 April 2000. Printed in the United States of America.

Print: ISBN 0-7381-1833-8 SH94799
PDF: ISBN 0-7381-1834-6 SS94799

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written permission of the publisher.

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Introduction
(This introduction is not part of IEEE Std C57.15-1999, IEEE Standard Requirements, Terminology, and Test Code for
Step-Voltage Regulators.)

The working group has undertaken the task to update this standard to
a) Reﬂect the latest revisions of IEEE Std C57.12.00 and IEEE Std C57.12.90.
b) Permit the winding temperature rise to increase from 55 °C to 65 °C for selected regulator (sealed)
designs.
c) Adapt the new IEEE approved format to ensure compatibility with the latest ISO and IEC standards.
d) Eliminate references to induction voltage and dry-type regulators.
e) Revise the list of standard sizes.

The working group membership for the preparation of this standard was as follows:

Craig A. Colopy, Co-Chair
Tom Diamantis, Co-Chair
Sam Aguirre Robert Grunert Sam Michael
Glenn Andersen Ken Hanus Tom Pekarek
Wallace Binder James Harlow Jerry Rowe
Jerry H. Bishop Ron Jordan Ron Stahara
Kevin Edwards Mark Loveless Ralph D. Wakeam

The following members of the balloting committee voted on this standard:
Sam Aguirre Keith R. Highton Larry C. Pearson
David Aho Philip J. Hopkinson Linden W. Pierce
Glenn Andersen Bert R. Hughes Paul. Pillitteri
Donald E. Ballard Rowland I. James R. Leon Plaster
Ron L. Barker Gene Kallaur John R. Rossetti
Mike Barnes Egon Koenig Subhas Sarkar
A. Bartek Donald L. Lowe Vic Shenoy
Martin Baur William A. Maguire Hyeong Jin Sim
Edward A. Bertolini K. T. Massouda Kenneth R. Skinger
Wallace Binder John W. Matthews Leonard R. Smith
Jerry H. Bishop Nigel P. McQuin James E. Smith
Alain Bolliger Charles Patrick McShane James E. Smith
Alvaro Cancino Joe Melanson Steven L. Snyder
Craig A. Colopy Sam Michael Craig L. Stiegemeier
Jerry L. Corkran C. Kent Miller Ron W. Stoner
Tom Diamantis R. E. Minkwitz Sr. Richard E. Sullivan
Dieter Dohnal Daniel H. Mulkey James A. Thompson
Carlos Gaytan Paul E. Orehek Thomas P. Traub
Harry D. Gianakouros Gerald A. Paiva Subhash C. Tuli
Robert Grunert B. K. Patel Joseph J. Vaschak
N. Wayne Hansen Sanjay Y. Patel Robert A. Veitch
James Harlow Wesley F. Patterson Barry H. Ward
Theodore J. Haupert Paulette A. Payne Christopher Wickersham
William R. Henning B. Scott Wilson

Copyright © 2000 IEEE. All rights reserved. iii

When the IEEE-SA Standards Board approved this standard on 16 September 1999, it had the following
membership:

Richard J. Holleman, Chair
Donald N. Heirman, Vice Chair
Judith Gorman, Secretary
Satish K. Aggarwal James H. Gurney Louis-François Pau
Dennis Bodson Lowell G. Johnson Ronald C. Petersen
Mark D. Bowman Robert J. Kennelly Gerald H. Peterson
James T. Carlo E. G. “Al” Kiener John B. Posey
Gary R. Engmann Joseph L. Koepﬁnger* Gary S. Robinson
Harold E. Epstein L. Bruce McClung Akio Tojo
Jay Forster* Daleep C. Mohla Hans E. Weinrich
Ruben D. Garzon Robert F. Munzner Donald W. Zipse

*Member Emeritus

Also included is the following nonvoting IEEE-SA Standards Board liaison:

Robert E. Hebner

Greg Kohn
IEEE Standards Project Editor

iv Copyright © 2000 IEEE. All rights reserved.

Contents
1. Overview.............................................................................................................................................. 1

1.1 Scope............................................................................................................................................ 1
1.2 Purpose......................................................................................................................................... 1
1.3 Word usage .................................................................................................................................. 1

2. References............................................................................................................................................ 1

3. Definitions............................................................................................................................................ 2

4. Service conditions................................................................................................................................ 7

4.1 Usual service conditions .............................................................................................................. 7
4.2 Loading at other than rated conditions......................................................................................... 7
4.3 Unusual service conditions .......................................................................................................... 8
4.4 Frequency..................................................................................................................................... 9

5. Rating data ......................................................................................................................................... 10

5.1 Cooling classes of regulators ..................................................................................................... 10
5.2 Ratings in kVA .......................................................................................................................... 10
5.3 Operating voltage limits............................................................................................................. 11
5.4 Supplementary continuous-current ratings ................................................................................ 11
5.5 Voltage supply ratios ................................................................................................................. 11
5.6 Insulation levels ......................................................................................................................... 14
5.7 Losses......................................................................................................................................... 14
5.8 Short-circuit requirements ......................................................................................................... 17
5.9 Tests ........................................................................................................................................... 18

6. Construction....................................................................................................................................... 19

6.1 Bushings..................................................................................................................................... 19
6.2 Terminal markings ..................................................................................................................... 19
6.3 Diagram of connections ............................................................................................................. 21
6.4 Nameplates................................................................................................................................. 21
6.5 Tank construction....................................................................................................................... 22
6.6 Tank grounding provisions ........................................................................................................ 22
6.7 Components and accessories...................................................................................................... 22

7. Other requirements............................................................................................................................. 23

7.1 Other supplementary continuous-current ratings....................................................................... 23
7.2 Other short-circuit capability ..................................................................................................... 24
7.3 Other components and accessories ............................................................................................ 24

8. Test code ............................................................................................................................................ 25

8.1 Resistance measurements........................................................................................................... 25
8.2 Polarity test ................................................................................................................................ 28
8.3 Ratio tests................................................................................................................................... 29
8.4 No-load losses and excitation current ........................................................................................ 31

Copyright © 2000 IEEE. All rights reserved. v

8.5 Impedance and load losses......................................................................................................... 36
8.6 Dielectric tests............................................................................................................................ 41
8.7 Temperature rise ........................................................................................................................ 52
8.8 Short-circuit tests ....................................................................................................................... 58
8.9 Data ............................................................................................................................................ 62

9. Control systems.................................................................................................................................. 65

9.1 General....................................................................................................................................... 65
9.2 Control device construction ....................................................................................................... 65
9.3 Control system requirements ..................................................................................................... 66
9.4 Tests ........................................................................................................................................... 68

Annex A (informative) Unusual temperature and altitude conditions........................................................... 71

Annex B (informative) Bushing and field dielectric tests ............................................................................. 73

Annex C (informative) Bibliography............................................................................................................. 74

Copyright © 2000 IEEE. All rights reserved. vi

IEEE Standard Requirements,
Terminology, and Test Code for
Step-Voltage Regulators

1. Overview

1.1 Scope

This standard describes electrical, mechanical, and safety requirements of oil-ﬁlled, single- and three-phase
step-voltage regulators not exceeding a regulation of 2500 kVA (for three-phase units) or 833 kVA (for
single-phase units). This standard does not apply to load tap-changing transformers.

1.2 Purpose

This standard is intended as a basis for the establishment of performance, limited electrical and mechanical
interchangeability, and safety requirements of equipment described. It also assists in the proper selection of
such equipment.

1.3 Word usage

When this document is used on a mandatory basis, the word shall indicates mandatory requirements; and the
words should or may refer to matters that are recommended or permissive, but not mandatory.

2. References

This standard shall be used in conjunction with the following publications. When the following standards are
superseded by an approved revision, the revision shall apply.

ANSI C57.12.20-1997, American National Standard for Transformers—Standard for Overhead Type
Distribution Transformers, 500 kVA and Smaller: High Voltage, 34 500 Volts and Below; Low Voltage,
7970/13 800 Y Volts and Below.1

1ANSI C57.12.20-1997 is available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, P.O. Box 1331, Piscat-
away, NJ 08855-1331, USA (http://standards.ieee.org/).

Copyright © 2000 IEEE. All rights reserved. 1

IEEE
Std C57.15-1999 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND

ANSI C84.1-1995, American National Standard Voltage Ratings (60 Hz) for Electric Power Systems and
Equipment.2

IEEE Std 4-1995, IEEE Standard Techniques for High-Voltage Testing.3

IEEE Std 315-1975 (Reaff 1993), IEEE Standard Graphic Symbols for Electrical and Electronics Diagrams
(Including Reference Designation Letters).

IEEE Std C37.90.1-1989 (Reaff 1994), IEEE Standard Surge Withstand Capability (SWC) Tests for
Protective Relays and Relay Systems.

IEEE Std C57.12.00-1993, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power,
and Regulating Transformers.

IEEE Std C57.12.80-1978 (Reaff 1992), IEEE Standard Terminology for Power and Distribution
Transformers.

IEEE Std C57.12.90-1999, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and
Regulating Transformers.

IEEE Std C57.13-1993, IEEE Standard Requirements for Instrument Transformers.

IEEE Std C57.19.00-1991 (Reaff 1997), IEEE Standard General Requirements and Test Procedure for
Outdoor Power Apparatus Bushings.

IEEE Std C57.95-1984 (Reaff 1991), IEEE Guide for Loading Liquid-Immersed, Step-Voltage and Induc-
tion-Voltage Regulators (withdrawn).4

IEEE Std C57.98-1993, IEEE Guide for Transformer Impulse Tests.

IEEE Std C57.106-1991, IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment
(withdrawn).5

3. Definitions

For the purposes of this standard, the following terms and definitions apply. IEEE Std C57.12.80-19786 and
The IEEE Standards Dictionary of Electrical and Electronics Terms [B7]7 should be referenced for terms not
defined in this clause.

3.1 ambient temperature: The temperature of the medium, such as air or water, into which the heat
generated in the equipment is dissipated.

3.2 angular displacement of polyphase regulator: (A) The time angle, expressed in degrees, between the
line-to-neutral voltage of the reference identified source voltage terminal S1 and the line-to-neutral voltage

2ANSI publications are available from the Sales Department, American National Standards Institute, 11 West 42nd Street, 13th Floor,
New York, NY 10036, USA (http://www.ansi.org/).
3IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, P.O. Box 1331, Piscataway,
NJ 08855-1331, USA (http://standards.ieee.org/).
4IEEE Std C57.95-1984 has been withdrawn; however, copies can be obtained from Global Engineering, 15 Inverness Way East,
Englewood, CO 80112-5704, USA, tel. (303) 792-2181 (http://www.global.ihs.com/).
5IEEE Std C57.106-1991 has been withdrawn; however, copies can be obtained from Global Engineering.
6Information on references can be found in Clause 2.
7The numbers in brackets correspond to those of the bibliography in Annex C.

2 Copyright © 2000 IEEE. All rights reserved.

IEEE
TEST CODE FOR STEP-VOLTAGE REGULATORS Std C57.15-1999

of the corresponding identified load voltage terminal L1. (B) The connection and arrangement of terminal
markings for three-phase regulators in a wye connection has an angular displacement of zero degrees. (C)
The connection and arrangement of terminal markings for three-phase regulators in a delta connection has
an angular displacement of zero degrees when the regulator is on the neutral tap position. When the regulator
is on a tap position other than neutral, the angular displacement will be other than zero degrees. The angular
displacement with the regulator connected in delta will be less than ±5° for a ±10% range of regulation.

3.3 autotransformer: A transformer in which part of one winding is common to both the primary and the
secondary circuits associated with that winding.

3.4 common winding: That part of the autotransformer winding that is common to both the primary and
secondary circuits. Syn: shunt winding.

3.5 conservator system: An oil preservation system in which the oil in the main tank is sealed from the
atmosphere, over the temperature range specified, by means of an ancillary tank partly filled with oil and
connected to the completely filled main tank. Syn: expansion tank system.

3.6 dielectric tests: Tests that consist of the application of a voltage higher than the rated voltage for a
specified time for the purpose of determining the adequacy of insulating materials against breakdown, and
for spacing under normal conditions.

3.7 excitation current: The current that maintains the excitation of the regulator. It may be expressed in
amperes, per unit, or percent of the rated current of the regulator.

3.8 excitation losses: See: no-load losses.

3.9 expansion tank system: See: conservator system.

3.10 gas-oil sealed system: An oil preservation system in which the interior of the tank is sealed from the
atmosphere, over the temperature range specified, by means of an ancillary tank or tanks to form a gas-oil
seal operating on the manometer principle.

3.11 impedance voltage drop: The phasor sum of the resistance voltage drop and the reactance voltage
drop. For regulators, the resistance drop, the reactance drop, and the impedance drop are, respectively, the
sum of the primary and secondary drops reduced to the same terms. They are usually expressed in per unit or
percent of the rated voltage of the regulator. Since they differ at different operating positions of the regulator,
two values of impedance shall be considered, in practice, to be the tap positions that result in the minimum
and the maximum impedance. Neutral position has the minimum amount of impedance.

3.12 impedance voltage of a regulator: The voltage required to circulate rated current through one winding
of the regulator when another winding is short-circuited, with the respective windings connected as for a
rated voltage operation. Impedance voltage is usually referred to the series winding, and then that voltage is
expressed in per unit, or percent, of the rated voltage of the regulator.

3.13 indoor regulator: A regulator that, because of its construction, must be protected from the weather.

3.14 line-drop compensator: A device that causes the voltage regulating device to vary the output voltage
an amount that compensates for the impedance voltage drop in the circuit between the regulator and a prede-
termined location on the circuit (sometimes referred to as the load center).

3.15 liquid: Refers to both synthetic fluids and mineral transformer oil.

NOTE—Some synthetic fluids may be unsuitable for use in the arcing environment of a step-voltage regulator.


IEEE

3.16 liquid-immersed regulator: A regulator in which the core and coils are immersed in an insulating
liquid.

3.17 liquid-immersed self-cooled (Class ONAN): A regulator having its core and coil immersed in a liquid
and cooled by the natural circulation of air over the cooling surfaces.

3.18 liquid-immersed self-cooled/forced-air-cooled (Classes ONAN/ONAF and ONAN/ONAF/ONAF):
A regulator having its core and coils immersed in liquid and having a self-cooled rating with cooling
obtained by the natural circulation of air over the cooling surface and a forced-air-cooled rating with cooling
obtained by the forced circulation of air over this same cooling surface.

3.19 liquid-immersed self-cooled/forced-air-cooled/forced-liquid-cooled (Class ONAN/ONAF/OFAF):
A regulator having its core and coils immersed in liquid and having a self-cooled rating with cooling
obtained by the natural circulation of air over the cooling surface; a forced-air-cooled rating with cooling
obtained by the forced circulation of air over this same air cooling surface; and a forced-liquid-cooled rating
with cooling obtained by the forced circulation of liquid over the core and coils and adjacent to this same
cooling surface over which the cooling air is being forced-circulated.

3.20 liquid-immersed water-cooled (Class ONWF): A regulator having its core and coils immersed in a
liquid and cooled by the natural circulation of the liquid over the water-cooled surface.

3.21 liquid-immersed water-cooled/self-cooled (Class ONWF/ONAN): A regulator having its core and
coils immersed in liquid and having a water-cooled rating with cooling obtained by the natural circulation of
liquid over the water-cooled surface, and a self-cooled rating with cooling obtained by the natural circulation
of air over the air-cooled surface.

3.22 load losses of a regulator: Those losses that are incident to the carrying of the load. Load losses
include I2R loss in the windings due to load current, stray loss due to stray ﬂuxes in the windings, core
clamps, and so forth.

3.23 no-load losses: Those losses that are incident to the excitation of the regulator. No-load losses include
core loss, dielectric loss, conductor loss in the winding due to exciting current, and conductor loss due to
circulating current in parallel windings. These losses change with the excitation voltage. Syn: excitation
losses.

3.24 nominal system voltage: A nominal value assigned to a system or circuit of a given voltage for the
purpose of convenient designation. The term nominal voltage designates the line-to-line voltage, as distin-
guished from the line-to-neutral voltage. It applies to all parts of the system or circuit.

3.25 nonsealed system: A system in which a tank or compartment is vented to the atmosphere usually with
two breather openings to permit circulation of air across the gas space above the oil. Circulation can be made
unidirectional when a pipe is extended up through the oil and is heated by the oil to induce movement of air
drawn from the outside into the gas space and across the oil to a breather on top of the tank.

3.26 outdoor regulator: A regulator designed for use outside of buildings.

3.27 polarity: (A) The polarity of a regulator is intrinsic in its design. Polarity is correct if the regulator
boosts the voltage in the “raise” range and bucks the voltage in the “lower” range. The relative polarity of the
shunt winding and the series windings of a step-voltage regulator will differ in the boost and buck modes
between Type A and Type B regulators. (B) The relative instantaneous polarity of the main transformer
windings, instrument transformer(s), and utility winding(s), as applicable, will be designated by an appropri-
ate polarity mark on the diagram of connection on the nameplate, in accordance with 6.3 of IEEE Std
C57.15-1999.


IEEE

3.28 pole-type regulator: A regulator that is designed for mounting on a pole or similar structure.

3.29 primary circuit: The circuit on the input side of the regulator.

3.30 rated range of regulation of a voltage regulator: The amount that the regulator will raise or lower its
rated voltage. The rated range may be expressed in per unit, or in percent, of rated voltage; or it may be
expressed in kilovolts.

3.31 rated voltage (of equipment): The voltage to which operating and performance characteristics are
referred.

3.32 rated voltage of a step-voltage regulator: The voltage for which the regulator is designed and on
which performance characteristics are based.

3.33 rated voltage of a winding: The rated voltage of a winding is the voltage to which operating and
performance characteristics are referred.

3.34 rated voltage of the series winding of a step-voltage regulator: The voltage between terminals of the
series winding, with rated voltage applied to the regulator, when the regulator is in the position that results in
maximum voltage change and is delivering rated output at 80% lagging power factor.

3.35 rating in kVA of a voltage regulator: (A) The rating that is the product of the rated load amperes and
the rated “raise” or “lower” range of regulation in kilovolts (kV). If the rated raise and lower range of regula-
tion are unequal, the larger shall be used in determining the rating in kVA. (B) The rating in kVA of a three-
phase voltage regulator is the product of the rated load amperes and the rated range of regulation in kilovolts
multiplied by 1.732.

3.36 reactance voltage drop: The component of the impedance voltage in quadrature with the current.

3.37 regulated circuit: The circuit on the output side of the regulator, where it is desired to control the volt-
age, or the phase relation, or both. The voltage may be held constant at any selected point on the regulated
circuit.

3.38 resistance method of temperature determination: The determination of the temperature by
comparison of the resistance of a winding at the temperature to be determined with the resistance at a known
temperature.

3.39 resistance voltage drop: The component of the impedance voltage in phase with the current.

3.40 sealed tank system: A method of oil preservation in which the interior of the tank is sealed from the
atmosphere and in which the gas volume plus the oil volume remains constant. The regulator shall remain
effectively sealed for top oil temperature range of –5 °C to 105 °C, continuous, and under the operating
conditions described in IEEE Std C57.95-1984.

3.41 series winding: That portion of the autotransformer winding that is not common to both the primary
and secondary circuits, but is connected in series between the input and output circuits.

3.42 shunt winding: See: common winding.

3.43 station-type regulator: A regulator designed for ground-type installations in stations or substations.

3.44 step-voltage regulator (transformer type): An induction device having one or more windings in shunt
with, and excited from, the primary circuit, and having one or more windings in series between the primary


IEEE

circuit and the regulated circuit, all suitably adapted and arranged for the control of the voltage, or of the
phase angle, or of both, of the regulated circuit in steps by means of taps without interrupting the load.

3.45 tap: A connection brought out of a winding at some point between its extremities to permit the chang-
ing of the voltage ratio.

3.46 thermometer method of temperature determination: This method consists of the determination of
the temperature by thermocouple or suitable thermometer, with either being applied to the hottest accessible
part of the equipment.

3.47 total losses: Those losses that are the sum of the no-load losses and the load losses. Power required for
cooling fans, oil pumps, space heaters, and other ancillary equipment is not included in the total loss. When
speciﬁed, loss data on such ancillary equipment shall be furnished.

3.48 Type A step-voltage regulator: A step-voltage regulator in which the primary circuit is connected
directly to the shunt winding of the regulator. The series winding is connected to the shunt winding and, in
turn, via taps, to the regulated circuit, per Figure 1. In a Type A step-voltage regulator, the core excitation
varies because the shunt winding is connected across the primary circuit.

Figure 1—Schematic diagram of single-phase, Type A step-voltage regulator

3.49 Type B step-voltage regulator: A step-voltage regulator in which the primary circuit is connected, via
taps, to the series winding of the regulator. The series winding is connected to the shunt winding, which is
connected directly to the regulated circuit, per Figure 2. In a Type B step-voltage regulator, the core excita-
tion is constant because the shunt winding is connected across the regulated circuit.

Figure 2—Schematic diagram of single-phase, Type B step-voltage regulator

3.50 voltage regulating device: A voltage sensitive device that is used on an automatically operated voltage
regulator to control the voltage of the regulated circuit.


IEEE

4. Service conditions

4.1 Usual service conditions

Apparatus conforming to this standard shall be suitable for operation at rated kilovolt-amperes under the
service conditions given in 4.1.1 through 4.1.6.

4.1.1 Temperature

a) If air-cooled, the temperature of the cooling air (ambient temperature) does not exceed 40 °C, and
the average temperature of the cooling air for any 24 h period does not exceed 30 °C.
b) If water-cooled, the temperature of the cooling water (ambient temperature) does not exceed 30 °C,
and the average temperature of the cooling water for any 24 h period does not exceed 25 °C.
(Minimum water temperature shall not be lower than 1 °C, unless the cooling water includes anti-
freeze suitable for –20 °C operation.)
c) The top-liquid temperature of the regulator (when operating) shall not be lower than –20 °C.
(Starting temperatures below –20 °C are not considered as usual service conditions.)

4.1.2 Altitude

The altitude does not exceed 1000 m (3300 ft).

4.1.3 Supply voltage

The supply voltage wave shape shall be approximately sinusoidal, and the phase voltages supplying a poly-
phase regulator shall be approximately equal in magnitude and time displacement.

4.1.4 Load current

The load current shall be approximately sinusoidal. The harmonic factor shall not exceed 0.05 per unit.
Harmonic factor is defined IEEE Std C57.12.80-1978.

4.1.5 Outdoor operation

Unless otherwise specified, regulators shall be suitable for outdoor operation.

4.1.6 Tank or enclosure finish

Temperature limits and tests shall be based on the use of a nonmetallic pigment surface paint finish.

NOTE—Metallic flake paints, such as aluminum, zinc, and so forth, have properties that increase the temperature rise of
regulators, except in direct sunlight.

4.2 Loading at other than rated conditions

IEEE Std C57.95-1984 provides guidance for loading at other than rated conditions including

a) Ambient temperatures higher or lower than the basis of rating.
b) Short-time loading in excess of nameplate kVA with normal life expectancy.
c) Loading that results in reduced life expectancy.


IEEE

NOTE—IEEE Std C57.95-1984 is a guide rather than a standard. It provides the best known general information for the
loading of regulators under various conditions based on typical winding insulation systems, and is based upon the best
engineering information available at the time of preparation. The guide discusses limitations of ancillary components
other than windings that may limit the capability of regulators. When specified, ancillary components and other
construction features (cables, bushings, tap changers, oil expansion space, etc.) shall be supplied such that they in them-
selves will not limit the loading to less than the capability of the windings.

4.3 Unusual service conditions

Conditions other than those described in 4.1 are considered unusual service and, when present, should be
brought to the attention of those responsible for the design and application of the apparatus. Examples of
some of these conditions are discussed in 4.3.1 through 4.3.3.

4.3.1 Unusual temperature and altitude conditions

Regulators may be used at higher or lower ambient temperatures or at higher altitudes than specified in 4.1,
but special consideration must be given to these applications. Annex A and IEEE Std C57.95-1984 provide
information on recommended practices.

4.3.2 Insulation at high altitude

The dielectric strength of regulators that depend in whole or in part upon air for insulation decreases as the
altitude increases due to the effect of decreased air density. When specified, regulators shall be designed with
a larger air spacing, using the correction factors of Table 1, to obtain adequate air dielectric strength at
altitudes above 1000 m (3300 ft).

a) The insulation level at 1000 m (3300 ft) multiplied by the correction factor from Table 1 must not be
less than the required insulation level at the required altitude.
b) Bushings with additional length of creep distance shall be furnished where necessary for operation
above 1000 m (3300 ft).

Table 1—Dielectric strength correction factors for altitudes
greater than 1000 m (3300 ft)

Altitude Altitude correction
factor for dielectric
(m) (ft) strength

1000 3300 1.00

1200 4000 0.98

1500 5000 0.95

1800 6000 0.92

2100 7000 0.89

2400 8000 0.86

2700 9000 0.83

3000 10 000 0.80

3600 12 000 0.75

4200 14 000 0.70

4500 15 000 0.67


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4.3.3 Other unusual service conditions

Other unusual service conditions include the following:

a) Damaging fumes or vapors, excessive or abrasive dust, explosive mixtures of dust or gases, steam,
salt spray, excessive moisture or dripping water, etc.
b) Abnormal vibration, tilting, shock, or seismic conditions.
c) Ambient temperatures outside of normal range.
d) Unusual transportation or storage conditions.
e) Unusual space limitations.
f) Unusual maintenance problems.
g) Unusual duty or frequency of operation, impact loading.
h) Unbalanced alternating-current (ac) voltages or departure of ac systems voltages from a substan-
tially sinusoidal wave form.
i) Loads involving abnormal harmonic currents, such as those that may result where appreciable load
currents are controlled by solid-state or similar devices; such harmonic currents may cause excessive
losses and abnormal heating.
j) Specified loading conditions (kVA outputs and power factors) associated with multiwinding trans-
formers or autotransformers.
k) Excitation exceeding either 110% rated voltage or 110% rated V/Hz.
l) Planned short circuits as a part of regulator operating or relaying practice.
m) Unusual short-circuit application conditions differing from those described as usual in 5.8.
n) Unusual voltage conditions, including transient overvoltages, resonance, switching surges, etc., that
may require special consideration in insulation design.
o) Unusually strong magnetic fields.
NOTE—Solar magnetic disturbances may result in the flow of telluric currents in regulator neutrals.
p) Unusually high nuclear radiation.
q) Parallel operation.
NOTE—While parallel operation is not unusual, it is desirable that users advise the manufacturer if paralleling
with other regulators is planned, and the characteristics of the transformers or reactors so involved.
r) Direct current circulating within the regulator.

4.3.3.1 Voltage regulating device

The voltage regulating device, depending on its construction, may be sensitive to altitude considerations.
The manufacturer should be consulted where applications exceed 2000 m (6600 ft). Altitude of 4500 m
(15 000 ft) is considered a maximum for standard regulators. Dielectric strength correction factors for alti-
tudes greater than 1000 m (3300 ft) are shown in Table 1.

4.4 Frequency

Unless otherwise specified, regulators shall be designed for operation at a frequency of 60 Hz.


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5. Rating data

5.1 Cooling classes of regulators

5.1.1 Liquid-immersed air-cooled

a) Liquid-immersed self-cooled (Class ONAN)
b) Liquid-immersed self-cooled/forced-air-cooled (Class ONAN/ONAF)

5.1.2 Liquid-immersed water-cooled

a) Liquid-immersed water-cooled (Class ONWF)
b) Liquid-immersed water-cooled/self-cooled (Class ONWF/ONAN)

5.2 Ratings in kVA

Ratings (in kVA) for step-voltage regulators are continuous and based on not exceeding the temperature lim-
its covered in Table 2.

Ratings (in kVA) covered by this standard shall be expressed in the terms given in 5.2.1 and as speciﬁed in
5.2.2.

Table 2—Limits of temperature rise

Winding temperature rise Hottest-spot winding
Item Type of apparatusa
by resistance (°C) temperature rise (°C)
55 °C rise liquid immersed
55 65
(vented or sealed tank)
1
65 °C rise liquid immersed
65 80
(sealed tank)
Metallic parts in contact with or adjacent to the insulation shall not attain a temperature in excess of that
2
allowed for the hottest spot of the windings adjacent to that insulation.
3 Metallic parts other than those covered in item 2 shall not attain excessive temperature rises.
Where a regulator is provided with a sealed-tank, conservator, or gas-oil-seal system, the temperature
rise of the insulating oil shall not exceed 55 °C (55 °C rise unit) or 65 °C (65 °C rise unit) when
4 measured near the surface of the oil. The temperature rise of insulating oil in regulator not provided
with the oil preservation systems listed above shall not exceed 50 °C when measured near the exposed
surface of the oil.
aApparatus with specified temperature rise shall have an insulation system that has been proven by experience,
general acceptance, or an accepted test.

5.2.1 Terms in which rating is expressed

The rating of a step-voltage regulator shall be expressed in the following terms:

a) kVA
b) Number of phases
c) Frequency
d) Voltage
e) Current
f) Voltage range in percent (raise and lower)


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Regulators shall be approximately compensated for their internal regulation to provide the specified voltage
range at rating in kVA and with an 80% lagging power factor load.

5.2.2 Preferred kVA ratings

Preferred ratings of step-voltage regulators shall be based on operation at a frequency of 60 Hz and a voltage
range of 10% raise and 10% lower, as given in Table 3 and Table 4. .

5.2.3 Supplementary kVA ratings

In addition to their normal ratings, as defined in 5.2.2, regulators shall deliver rated kVA output without
exceeding the specified temperature rise at the operating voltage given in Table 5. Voltage regulators with
multitapped voltage transformers may be operated at nominal system voltages below the maximum rated
voltage, and may deliver rated line amperes without exceeding the temperature limits of Table 2.

5.3 Operating voltage limits

Regulators, including their controls, shall be suitable for operation within the following limits of voltage
provided that the rated load current is not exceeded:

a) A minimum input voltage of 97.75 V times the ratio of voltage transformer.
b) A maximum input voltage at rated load amperes of 1.05 times the rated input voltage of the regulator
or 137.5 V times the ratio of voltage transformer, whichever is less.
c) A maximum input voltage at no load of 1.10 times the rated input voltage of the regulator or 137.5 V
times the ratio of voltage transformer, whichever is less.
d) A minimum output voltage of 103.5 V times the ratio of voltage transformer.
e) A maximum output voltage of 1.1 times the rated voltage of the regulator or 137.5 V times the ratio
of voltage transformer, whichever is less.
f) The output voltage obtainable with a given input voltage is limited also by the regulator voltage
range.

Typical examples of the application of these rules to some common ratings of regulators are given in Table 8.

5.4 Supplementary continuous-current ratings

Single-phase step-voltage regulators up to 19.9 kV, inclusive, rated 668 A and below shall have the
continuous-current ratings or 668 A, whichever is less, on intermediate ranges of steps as shown in
Table 6.

Three-phase step-voltage regulators up to 13.8 kV, inclusive, rated 668 A and below, have the following
continuous-current ratings or 668 A, whichever is less, on intermediate ranges of steps as shown in Table 7.

5.5 Voltage supply ratios

Values of voltage supply ratios are given in Table 9. When a voltage supply ratio is specified that is not a
preferred value shown in Table 9, an ancillary transformer may be furnished in the unit or control to modify
the preferred ratio.


IEEE

Table 3—Preferred ratings for oil-immersed step-voltage regulators (single phase)

Nominal system BIL
kVA Line amperes
voltage (kV)
50 200
75 300
100 400
125 500
2400/4160Y 60
167 668
250 1000
333 1332
416 1665
50 100
75 150
100 200
125 250
4800/8320Y 75
167 334
250 500
333 668
416 833
38.1 50
57.2 75
76.2 100
114.3 150
167 219
7620/13 200Y 95 250 328
333 438
416 546
500 656
667 875
833 1093
69 50
138 100
207 150
13 800 95
276 200
414 300
552 400
72 50
144 100
216 150
288 200
14 400/24 940Y 150a 333 231
432 300
576 400
667 463
833 578
100 50
200 100
333 167
19 920/34 500Y 150
400 201
667 334
833 418
aLow-frequency test voltage 50 kV by induced test with neutral grounded.


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Table 4—Preferred ratings for oil-immersed step-voltage regulators (three phase)

Self-cooled Self-cooled/forced-cooled
Nominal system BIL
voltage (kV) kVA Line amperes kVA Line amperes
500 1155 625 1443
2400 45 750 1732 937 2165
1000 2309 1250 2887
500 667 625 833
2400/4160Y 60 750 1000 937 1250
1000 1334 1250 1667
500 577 625 721
4800 60 750 866 937 1082
1000 1155 1250 1443
500 219 625 274
750 328 938 410
7620/13 200Y 95 1000 437 1250 546
1500 656 2000 874
2000 874 2667 1166
500 209 625 261
750 313 937 391
1000 418 1250 523
7970/13 800Y 95
1500 628 2000 837
2000 837 2667 1116
2500 1046 3333 1394

500 125.5 625 156.8
750 188.3 937 235.4
1000 251 1250 314
14 400/24 940Y 150
1500 377 2000 502
2000 502 2667 669
2500 628 3333 837
500 83.7 625 104.5
750 125.5 937 156.8
1000 167 1250 209
19 920/34 500Y 150
1500 251 2000 335
2000 335 2667 447
2500 418 3333 557
500 62.8 625 78.5
750 94.1 937 117.6
1000 126 1250 157
26 560/46 000Y 250
1500 188 2000 251
2000 251 2667 335
2500 314 3333 419
500 41.8 625 52.5
750 62.8 937 78.5
1000 83.7 1250 105
39 840/69 000Y 350
1500 126 2000 167
2000 167 2667 223
2500 209 3333 278


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Table 5—Supplementary voltage ratings for regulators

Number of phases Rated voltage Operating voltage
Single-phase 7620 7200
2500 2400
4330 4160
5000 4800
Three-phase
8660 8320
13 200 12 470
13 800 13 200

Table 6—Supplementary continuous-current ratings for single-phase regulators

Range of voltage Continuous-current
regulation (%) rating (%)
10.0 100
8.75 110
7.5 120
6.25 135
5.0 160

Table 7—Supplementary continuous-current ratings for three-phase regulators

Range of voltage Continuous-current
regulation (%) rating (%)
10.0 100
8.75 108
7.5 115
6.25 120
5.0 130

5.6 Insulation levels

Regulators shall be designed to provide coordinated low-frequency and lightning impulse insulation levels
on line terminals, and low-frequency insulation levels on neutral terminals. The identity of a set of
coordinated levels shall be its basic impulse insulation level (BIL), as shown in Table 10.

NOTE—When single-phase regulators are connected in wye, the neutral of the regulator bank shall be connected to the
neutral of the system. A delta connection of the regulators is commonly recommended when the system is three-wire
ungrounded.

5.7 Losses

The losses speciﬁed by the manufacturer shall be the no-load (excitation) and total losses, as deﬁned in 3.23
and 3.47, respectively.


Table 8—Typical examples of operating voltage limits including all operating voltage tolerances

Regulator voltage rating (V) Input voltage (V) Output voltage (V)

Nominal system Voltage Maximum at
voltage supply ratioa Maximum at
Maximum at rated-load
Single-phase Three-phase Minimum rated-load Minimum
no-load amperes or at
amperes
no-load

2400 2500 2500 20 1955 2625 2750 2070 2750
2400 / 4160Y 2500 — 20 1955 2625 2750 2070 2750
2400 / 4160Y — 4330 34.6 3380 4550 4760 3580 4760

4800 5000 5000 40 3910 5250 5500 4140 5500

Copyright © 2000 IEEE. All rights reserved.
7200 7620 — 60 5870 8000 8250 6210 8250
7200 — 8660 60 5870 8250 8250 6210 8250
4800 / 8320Y 5000 — 40 3910 5250 5500 4140 5500
TEST CODE FOR STEP-VOLTAGE REGULATORS

8320 — 8660 69.3 6775 9090 9525 7170 9525
12 470 13 800 13 800 104 10 170 14 300 14 300 10 760 14 300
7200 / 12 470Y 7620 — 60 5870 8000 8250 6210 8250
7200 / 12 470Y — 13 800 104 10 170 14 300 14 300 10 760 14 300

7620 / 13 200Y 7620 — 63.5 6210 8000 8380 6570 8380
7620 / 13 200Y 7620 — 66.3 6480 8000 8380 6860 8380
7620 / 13 200Y — 13 200 110 10 750 13 860 14 520 11 400 14 520
7960 / 13 800Y — 13 800 115 11 240 14 490 15 180 11 900 15 180
7960 / 13 800Y 7690 — 66.3 6480 8360 8760 6860 8760

13 200 13 800 13 800 110 10 750 14 490 15 125 11 400 15 125
14 400 13 800 12 800 120 11 730 14 490 15 180 12 420 15 180

14 400 / 24 940Y 14 400 — 120 11 730 15 120 15 840 12 420 15 840
19 920 / 34 500Y 19 920 — 166 16 230 20 920 21 910 17 180 21 910

14 400 / 24 940Y — 24 940 208 20 330 26 190 27 435 21 530 27 435
19 920 / 34 500Y — 34 500 287.5 28 100 36 225 37 950 29 760 37 950

26 560 / 46 000Y — 46 000 373.3 36 490 48 300 50 600 38 640 50 600
39 840 / 69 000Y — 69 000 575 56 210 72 450 75 900 59 510 75 900
aWhere
the listed potential ratio is provided, an additional ancillary transformer may be required.
NOTE—Example values are derived using procedures of 5.3.

15
Std C57.15-1999
IEEE

IEEE

Table 9—Values of voltage supply ratios

Voltage rating of regulator Values of voltage supply
ratios
Single-phase Three-phase
2500 2500 20, 20.8
— 4330 34.6, 36.1
5000 5000 40, 41.7

7620 — 60, 63.5

7690 — 66.3

— 8660 69.3, 72.2

— 13 200 110, 104

13 800 13 800 115, 110

14 400 — 120

19 920 — 166

— 24 940 208

— 34 500 287.5

— 46 000 383.3

— 69 000 575

Table 10—Interrelationships of dielectric insulation levels for regulators
used on systems with BIL ratings of 350 kV and below

Impulse levels
Low-frequency
voltage Full wave Chopped wave
BIL kV
insulation level
(kV rms) Minimum time
(kV crest) (kV crest)
to ﬂashover (µs)
45 15 45 54 1.5
60 19 60 69 1.5
75 26 75 88 1.6
95 34 95 110 1.8
150 50 150 175 3.0
200 70 200 230 3.0
250 95 250 290 3.0
350 140 350 400 3.0


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5.7.1 Total losses

The total losses of a regulator shall be the sum of the no-load (excitation) and load losses.

5.7.2 Tolerance for losses

Unless otherwise specified, the losses represented by a test of a regulator shall be subject to the following
tolerances: the no-load losses of a regulator shall not exceed the specified no-load losses by more than 10%,
and the total losses of a regulator shall not exceed the specified total losses by more than 6%. Failure to meet
the loss tolerances shall not warrant immediate rejection but lead to consultation between purchaser and
manufacturer about further investigation of possible causes and the consequences of the higher losses.

NOTE—Since losses will differ at different operating positions of the regulator, care must be exercised in the consider-
ation of tap position with losses. Some styles of step-voltage regulators will exhibit appreciable change in load loss when
boosting versus bucking, or will exhibit appreciable change in no-load loss on alternate tap positions. See 5.7.3.

5.7.3 Determination of losses and excitation current

No-load (excitation) losses and exciting current shall be determined for the rated voltage and frequency on a
sine-wave basis, unless a different form is inherent in the operation of the apparatus.

Load losses shall be determined for rated voltage, current, and frequency and shall be corrected to a refer-
ence temperature equal to the sum of the limiting (rated) winding temperature rise by resistance from
Table 2 plus 20 °C.

Since losses may be very different at different operating positions and with various design options, losses
shall be considered in practice as the sum of no-load and load losses where

a) No-load loss is the average of no-load loss in the neutral and next adjacent boost position with rated
voltage applied to the shunt or series winding for regulators that do not include a series transformer.
NOTE—It will be apparent, in the case of a Type B step-voltage regulator that is on the next adjacent boost
position, that the excitation voltage applied at the source terminal will be higher at the shunt winding. Care
must be exercised to assure that rated excitation is present on the shunt winding; this may be accomplished by
exciting the regulator from the load terminal.
b) No-load loss is reported for neutral position, maximum boost position, and position adjacent to
maximum boost position for regulators that include a series transformer.
c) Load loss is the average load loss in both the maximum and adjacent-to-maximum buck positions,
and the maximum and adjacent-to-maximum boost positions (that is, four positions) with rated
current in the windings.

5.8 Short-circuit requirements

5.8.1 General

Step-voltage regulators shall be designed and constructed to withstand the mechanical and thermal stresses
produced by external short circuits of 25 times the base rms symmetrical rated load current.

a) The short-circuit current shall be assumed to be displaced from zero insofar as determining the
mechanical stresses. The maximum peak value of the short-circuit current that the regulator is
required to withstand is equal to 2.26 times the required rms symmetrical short-circuit current.
b) The short-circuit current shall be assumed to be a duration of 2 s to determine the thermal stresses.

It is recognized that short-circuit withstand capability can be adversely affected by the cumulative effects of
repeated mechanical and thermal over-stressing, as produced by short-circuits and loads above the name-


IEEE

plate rating. Since means are not available to continuously monitor and quantitatively evaluate the degrading
effects of such duty, short-circuit tests, when required, should be performed prior to placing the regulator in
service. It is recommended that current-limiting reactors be installed, when necessary, to limit the short-
circuit current to a maximum of 25 times the normal full-load current.

5.8.2 Mechanical capability demonstration

It is not the intent of this subclause that every regulator design be short-circuit tested to demonstrate
adequate construction. When specified, tests of short-circuit mechanical capability shall be performed as
described in 8.8.

5.8.3 Thermal capability of regulators for short-circuit conditions

The temperature of the conductor material in the windings of regulators under the short-circuit conditions
specified in 5.8.1 item b), as calculated by methods described in 8.9.4, shall not exceed 250 °C for a copper
conductor or 200 °C for an EC aluminum conductor. A maximum temperature of 250 °C shall be allowed for
aluminum alloys that have resistance to annealing properties at 250 °C, equivalent to EC aluminum at
200 °C, or for application of EC aluminum where the characteristics of the fully annealed material satisfy
the mechanical requirements. In setting these temperature limits, the following factors were considered:

a) Gas generation from oil or solid insulation
b) Conductor annealing
c) Insulation aging

5.9 Tests

Except as specified in 5.9.1, 5.9.2, and Clause 8, tests shall be performed as specified in IEEE Std
C57.12.00-1993 and in IEEE Std C57.12.90-1999. Tests are divided into two categories: routine and design.
Routine tests are made for quality control by the manufacturer to verify during production that the product
meets the design specifications. Design tests are made to determine the adequacy of the design of a particu-
lar type, style, or model of equipment or its component parts to meet its assigned rating under normal service
conditions or under special conditions if specified.

5.9.1 Routine tests

The routine tests given in the following list shall be made on all regulators. The order of listing does not nec-
essarily indicate the sequence in which the tests shall be made.

a) Resistance measurements of all windings
b) Ratio tests on all tap connections
c) Polarity test
d) No-load (excitation) loss at rated voltage and rated frequency
e) Excitation current at rated voltage and rated frequency
f) Impedance and load loss at rated current and rated frequency
g) Lightning impulse tests specified in 8.6.3
h) Applied-voltage tests
i) Induced-voltage tests
j) Insulation power factor tests
k) Insulation resistance tests


IEEE

5.9.2 Design tests

The following design tests are made only on representative apparatus to substantiate the ratings assigned to
all other apparatus of basically the same design. These tests are not intended to be used as a part of normal
production. The applicable portion of these design tests may also be used to evaluate modifications of a
previous design and to assure that performance has not been adversely affected. Test data from previous sim-
ilar designs may be used for current designs, where appropriate. Once made, the tests need not be repeated
unless the design is changed so as to modify performance.

5.9.2.1 Thermal tests

Temperature design tests shall be made on one unit of a given rating produced by a manufacturer as a record
that this design meets the temperature rise requirement for a 55 °C or 65 °C rise unit. Temperature tests shall
be made at the positions that produce the highest total losses at rated load current for the normal 100% rat-
ing, supplementary kVA rating (see 5.2.3) and the 160% or 668 A rating (see 5.4). When a regulator is sup-
plied with ancillary cooling equipment to provide higher kVA ratings, temperature tests shall be made at
those ratings also. Temperature tests shall be made for all kVA ratings given on the nameplate. Tests shall be
made in accordance with 8.7.

5.9.2.2 Lightning impulse tests

Design lightning impulse tests shall be made on one unit of a given rating produced by a manufacturer for
the purpose of demonstrating the adequacy of insulating materials breakdown and spacing under normal
conditions. Tests shall be made in accordance with 8.6.2. Impulse tests are to be followed by the application
of the applied and induced voltage tests.

5.9.2.3 Short-circuit tests

Short-circuit tests shall be made on one unit of a rating produced by a manufacturer for the purpose of
demonstrating that the unit meets the thermal and mechanical requirements of 5.8. Where lower kVA and
voltage ratings have the same design configuration, core and coil framing, and clamping as the unit tested,
short-circuit tests are not required, and it is adequate to show by calculation that the mechanical forces are
equal or less than the unit tested and the temperature rise of the conductor meets the criteria. Tests are to be
made in accordance with 8.8.

6. Construction

6.1 Bushings

Regulators shall be equipped with bushings with an insulation level not less than that of the winding terminal
to which they are connected, unless otherwise specified.

Bushings for use in regulators shall have impulse and low-frequency insulation levels as listed in Table 11.

6.2 Terminal markings

6.2.1 Terminal markings for step-voltage regulators

Regulator terminals that are connected to the load shall be designated by an L, and those that are connected
to the source shall be designated by an S. For example, in the case of a single-phase regulator, the terminals


IEEE

Table 11—Electrical characteristics of regulator bushings (kV)

Outdoor bushings Indoor bushingsa

Impulse-full 60 Hz Impulse-full
Regulator
60 Hz withstand wave dry withstand wave dry
BIL
withstand 1 min dry withstand
(kV)
1 min dry 10 s wetb kV crest kV crest
kV crest
(kV rms) (kV rms) (1.2 × 50 µm) (1.2 × 50 µs)
45 15 13 45 20 45
60 21 20 60 24 60
75 27 24 75 30 75
95 35 30 95 50c 110c
150 60 50 150 60 150
200 80 75 200 80 200
250 105 95 250 — —
350 160 140 350 — —
aWet withstand values are based on water resistivity of 180 Ω·m (7000 Ω·in) and precipitation rate
of 0.10 mm/s (0.2 in/min).
bIndoor bushings are those intended for use in indoor regulators. Indoor bushing test values do not
apply to bushings used primarily for mechanical protection of insulated cable leads. A wet test value
is not assigned to indoor bushings.
cSmall indoor regulators may be supplied with bushings for a dry test of 38 kV and impulse test of
95 kV.

shall be identiﬁed by S, L, and SL. In the case of a three-phase regulator, the terminals shall be identiﬁed S1,
S2, S3, L1, L2, L3, and, if a neutral is provided, S0L0.

Single-phase regulators, when viewed from the top, shall have the S terminal on the left, followed in
sequence in a clockwise direction by the L terminal and the neutral terminal SL, as shown in Figure 3.

Figure 3—Single-phase regulators

For three-phase regulators, when facing the regulator on the source side, the S1 terminal shall be in front on
the right, and the L1 terminal shall be directly behind the S1 terminal, as shown in Figure 4(a), or the S1 ter-
minal shall be in front on the right, and the L1 terminal shall be directly to the left of the S1 terminal, as
shown in Figure 4(b). The other terminals shall be located as shown in Figure 4.


IEEE

NOTE—The dotted line shows the location of the control compartment and the tap-changing under load equipment.
The dotted circle shows alternate location of neutral bushing.

Figure 4—Three-phase regulators

6.3 Diagram of connections

The manufacturer shall furnish, with each voltage regulator, complete diagrams showing the leads and
internal connections and their markings, including polarity markings, and the voltages obtainable with the
various connections. These diagrams shall be inscribed on and be part of the nameplate.

6.4 Nameplates

Two durable metal nameplates shall be furnished with each voltage regulator and shall be affixed to the main
tank and on the front of the control cabinet. Unless otherwise specified, they shall be of corrosion-resistant
material. The nameplates shall show, at a minimum, the ratings and other essential operating data as
specified as follows:

a) Manufacturer’s name
b) Type and form designation or the equivalent
c) Class
d) Serial number
e) Month and year of manufacturer (not coded)
f) Number of phases
g) Rated kVA
h) Rated current
i) Supplementary continuous-current ratings
j) Rated voltage
k) Voltage transformer ratio
l) Rated range of regulation


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m) Rated frequency
n) Impulse level, full wave in kilovolts (kV)
o) Untanking weight
p) Total weight
q) Insulating fluid type
r) Volume of insulating fluid
s) Conductor material
t) Average winding rise in degrees Celsius (°C)
u) Diagrams as specified in 6.3
v) Installation and operating instructions reference

6.5 Tank construction

Accepted techniques for tank construction as it relates to oil-gas space systems are the conservator
(expansion tank), gas-oil sealed, nonsealed, and sealed tank systems.

6.6 Tank grounding provisions

6.6.1 Maximum continuous rating less than 300 A

Tank grounding provisions shall consist, at a minimum, of one steel pad with a 0.5 in – 13 NC tapped hole,
11 mm (0.44 in) deep and located near the bottom of the tank.

6.6.2 Maximum continuous rating 300 A or greater

Tank grounding provision shall consist, at a minimum, of one unpainted copper-faced steel or stainless steel
pad, 50 mm (2 in) × 90 mm (3.5 in), with two holes horizontally spaced on 44.5 mm (1.75 in) centers, tapped
for 0.5 in – 13 NC thread and located near the bottom of the tank. Minimum thread depth of each hole shall
be 13 mm (0.5 in). Minimum thickness of the copper facing, when used, shall be 0.4 mm (0.015 in).

6.7 Components and accessories

6.7.1 Single-phase and three-phase step-voltage regulators

6.7.1.1 Components for full automatic control and operation

a) Control system and cabinet
b) Current and voltage transformers or the equivalent for supplying the control system
c) Tap-changer drive motor
d) Internal power supply for drive motor
e) Provision for disconnecting control power supply
f) Tap-changer position indicator

6.7.1.2 Accessories for single-phase step-voltage regulators

a) Nameplate.
b) Lifting lugs.
c) Provision for oil drainage and sampling.
d) Tank grounding provision.


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e) Bushing terminals shall be either clamp-type or threaded stud, depending on the nameplate line cur-
rent ratings as shown in Table 12.
The clamp-type terminals shall have at least the conductor range stated and shall be capable of
accepting an aluminum or copper conductor. Threaded stud sizes shown are minimum. The user has
the responsibility of selecting the proper conductor size for use in the clamp-type terminals. When
selecting the conductor size, the user should consider factors such as additional current carrying
capability with reduced regulation (see 5.4), supplementary kVA ratings (see 5.2.3) and loading at
other than rated conditions (see 4.2).
f) Support lugs for pole mounting, when provided, shall conform to one of the alternatives required by
ANSI C57.12.20-1997.
g) Liquid level indicator.

Table 12—Bushing terminal applications

Nameplate line current rating Conductor size range or
(A) threaded stud
150 or less #8–4/0
151–300 #2– 477 kCM
301–668 #2– 800 kCM
669–1200 1-1/8–12 UNF-2A
1201–2000 1-1/2–12 UNF-2A

6.7.1.3 Accessories for three-phase step-voltage regulators

a) Nameplate
b) Liquid level indicator
c) Provision for oil draining and sampling
d) Provision for oil filtering
e) Provision for thermometer
f) Lifting lugs
g) Clamp-type terminals in accord with single-phase criteria [see 6.7.1.2(e)]
h) Handholes or openings to permit inspection of core and coil and load tap-changer
i) Tank grounding provision

7. Other requirements

Certain specific applications have regulator requirements not covered in Clause 4, Clause 5, or Clause 6.
Clause 7 comprises descriptions of the most frequently used requirements for such regulators. They shall be
provided only when specified in conjunction with the requirements of Clause 4 through Clause 6.
Information in the following subclauses may be specified for some applications.

7.1 Other supplementary continuous-current ratings

When specified, other supplementary continuous-current ratings, 668 A maximum, for three-phase regula-
tors rated 8660 V and 13 200 V shall be provided as shown in Table 13 (see 5.4).


IEEE

Table 13—Other supplementary continuous-current ratings for three-phase regulators

Range of voltage regulation Continuous-current ratings
(%) (%)
10.0 100
8.75 110
7.5 120
6.25 135
5.0 160

7.2 Other short-circuit capability

When specified, either of the following short-circuit capabilities for regulators shall be provided:

a) Three-phase regulators rated 1500 kVA and below, capable of withstanding rms symmetrical short-
circuit current of 40 times base rated load current or 20 000 A, whichever is less, for 0.8 s without
damage.
b) Single-phase regulators rated 500 kVA and below, capable of withstanding rms symmetrical short-
circuit current of 40 times the base rated load current or 20 000 A, whichever is less, for 0.8 s
without damage.

The initial current shall be assumed to be displaced from zero insofar as determining the mechanical
stresses. The maximum crest value of the short-circuit current that the regulator is required to withstand is
equal to 2.26 times the required rms symmetrical short-circuited current.

7.3 Other components and accessories

When specified, the other components and accessories listed in 7.3.1 and 7.3.2 may be provided.

7.3.1 For all regulators

a) Control cabinet removable for remote control operation [to 9 m (30 ft) from the regulator]
b) Voltage limit control
c) Reverse power flow relay
d) Remote voltage reduction control

7.3.2 For three-phase regulators

a) Hand operation crank
b) Load tap-changing mechanisms in separate compartment
c) 5 A secondary rating for current transformer
d) Remote position indicator

NOTE—For Selsyn-type systems, care shall be exercised to assure that the conductor size is commensurate with the
distance used.


IEEE

8. Test code

This clause prescribes methods for performing tests specified in 5.9. The test methods covered are as
follows:

a) Resistance measurements (see 8.1)
b) Polarity test (see 8.2)
c) Ratio tests (see 8.3)
d) No-load losses and excitation current (see 8.4)
e) Impedance and load losses (see 8.5)
f) Dielectric tests (see 8.6)
g) Temperature rise (see 8.7)
h) Short-circuit tests (see 8.8)
i) Data (see 8.9)

The same general principles apply to regulator tests as apply to transformers. The following material has,
therefore, been taken from IEEE Std C57.12.90-1993 for uniformity with those provisions. Test system
accuracies and tolerances specified in IEEE Std C57.12.00-1993 shall apply.

8.1 Resistance measurements

Resistance measurements are of fundamental importance for the following purposes:

a) Calculation of the I2R component of conductor losses.
b) Calculation of winding temperatures at the end of a temperature test.
c) As a base for assessing possible damage in the field.

8.1.1 Determination of cold temperature

The cold temperature of the winding shall be determined as accurately as possible when measuring the cold
resistance. The precautions in 8.1.1.1 through 8.1.1.3 shall be observed.

8.1.1.1 General

Cold resistance measurements shall not be made on a regulator when it is located in drafts or when it is
located in a room in which the temperature is fluctuating rapidly.

8.1.1.2 Regulator windings immersed in insulating liquid

The temperature of the windings shall be assumed to be the same as the temperature of the insulating liquid,
provided:

a) The windings have been under insulating liquid with no excitation and with no current in the
windings from 3 h to 8 h (depending upon the size of the regulator) before the cold resistance is
measured.
b) The temperature of the insulating liquid has stabilized, and the difference between top and bottom
temperatures does not exceed 5 °C.


IEEE

8.1.1.3 Regulator windings out of insulating liquid

The temperature of the windings shall be recorded as the average of several thermometers or thermocouples
inserted between the coils, with care taken to see that their measuring points are as nearly as possible in
actual contact with the winding conductors. It should not be assumed that the windings are at the same
temperature as the surrounding air.

8.1.2 Conversion of resistance measurements

Cold winding resistance measurements are normally converted to a standard reference temperature equal to
the rated average winding temperature rise plus 20 °C. In addition, it may be necessary to convert the
resistance measurements to the temperature at which the impedance loss measurements were made. The
conversions are accomplished by the following formula:

(T s + T k)
R s = R m -----------------------
- (1)
(T m + T k)

where

Rs is the resistance at desired temperature Ts ,
Rm is the measured resistance,
Ts is the desired reference temperature (°C),
Tm is the temperature at which resistance was measured (°C),
Tk is 234.5 °C (copper),
Tk is 225 °C (aluminum).

NOTE—225 °C applies for pure or EC aluminum. Tk may be as high as 230 °C for alloyed aluminum. Where copper and
aluminum windings are employed in the same regulator, a value for Tk of 229 °C should be applied for the correction of
losses.

8.1.3 Resistance measurement methods

8.1.3.1 Bridge method

Bridge methods or high-accuracy digital instrumentation are generally preferred because of their accuracy
and convenience, since they may be employed for the measurement of resistances up to 10 000 Ω. They
should be used in cases where the rated current of the regulator winding to be measured is less than 1 A.

NOTE—For resistance values of 1 Ω or more, a Wheatstone bridge (or equivalent) is commonly used; for values less
than 1 Ω, a Kelvin bridge (or equivalent) is commonly used. Some modern resistance bridges have capability in both
ranges.

8.1.3.2 Voltmeter-ammeter method

The voltmeter-ammeter method is sometimes more convenient than the bridge method. It should be
employed only if the rated current of the regulator winding is 1 A or greater. Digital voltmeters and digital
ammeters of appropriate accuracy are commonly used in connection with temperature-rise determinations.
To use this method, perform the following steps:

a) Measurement is made with direct current, and simultaneous readings of current and voltage are
taken using the connections of Figure 5. The required resistance is calculated from the readings in
accordance with Ohm’s law. A battery or ﬁltered rectiﬁer will generally be found to be more satis-
factory as a dc source than will a commutating machine. The latter may cause the voltmeter pointer
to vibrate because of voltage ripple.


IEEE

Figure 5—Connections for the voltmeter-ammeter method of resistance measurement

b) To minimize errors of observation
1) The measuring instruments shall have such ranges as will give reasonably large deflection.
2) The polarity of the core magnetization shall be kept constant during all resistance readings.

NOTE—A reversal in magnetization of the core can change the time constant and result in erroneous readings.

c) The voltmeter leads shall be independent of the current leads and shall be connected as closely as
possible to the terminals of the winding to be measured. This is to avoid including in the reading the
resistances of current-carrying leads and their contacts, and of extra lengths of leads.
To protect the voltmeter from damage by off-scale deflections, the voltmeter should be disconnected
from the circuit before switching the current on or off. To protect test personnel from inductive kick,
the current should be switched off by a suitably insulated switch.
If the drop of voltage is less than 1 V, a potentiometer or millivoltmeter shall be used.
d) Readings shall not be taken until after the current and voltage have reached steady-state values.
When measuring the cold resistance prior to making a heat run, note the time required for the read-
ings to become constant. The period thereby determined should be allowed to elapse before taking
the first reading when final winding hot resistance measurements are being made.
In general, the winding will exhibit a long dc time constant. To reduce the time required for the
current to reach its steady-state value, a noninductive external resistor should be added in series with
the dc source. The resistance should be large compared to the inductance of the winding. It will then
be necessary to increase the source voltage to compensate for the voltage drop in the series resistor.
The time will also be reduced by operating all other regulator windings open-circuited during these
tests.
e) Readings shall be taken with not less than four values of current when deflecting instruments are
used. The average of the resistances calculated from these measurements shall be considered to be
the resistance of the circuit.
The current used shall not exceed 15% of the rated current of the winding whose resistance is to be
measured. Larger values may cause inaccuracy by heating the winding and thereby changing its
temperature and resistance.
When the current is too low to be read on a deflecting ammeter, a shunt and digital millivoltmeter or
potentiometer shall be used.


IEEE

8.2 Polarity test

Polarity testing of a regulator is to ensure correct polarity of the instrument transformers, if supplied, that
may be used in conjunction with the line drop compensation circuit of the control panel. The test method of
inductive kick with direct current is a common technique for this test.

NOTE—Testing for additive or subtractive polarity of the main winding, as commonly required for transformers, is not
required for regulators. See 3.27.

8.2.1 Polarity by inductive kick

The following list details one procedure that may be used to check polarity by means of inductive kick with
direct current. Various acceptable variations of this technique are also in common use. The test is structured
to ensure that instrument transformers display polarity correctly as per the nameplate.

a) Connect the regulator as shown in Figure 6. The example shown is for a Type A regulator with volt-
age transformer, current transformer, and a utility winding on the main core.
b) Impress a direct voltage of known polarity S to SL, with positive polarity at S. Wait several seconds
while the current stabilizes.
c) Connect a zero-center-reading dc voltmeter to the voltage transformer secondary winding, point 1 to
point 0 on Figure 6.
d) Open the switch. A negative kick response on the voltmeter indicates the polarity is correct as
marked.
e) Repeat the test for the current transformer (point 2) and the utility winding (point 3), if supplied.

NOTE—It may be necessary to place a shunt from L to SL when testing the current transformer polarity.

Figure 6—Type A step-voltage regulator connected for polarity testing;
regulator in neutral position


IEEE

8.3 Ratio tests

8.3.1 General

The turn ratio of a regulator is the ratio of the number of turns in the shunt winding to that in the series
winding.

8.3.1.1 Taps

The turn ratio shall be determined for all taps as well as for the full winding.

8.3.1.2 Voltage and frequency

The ratio test shall be made at rated or lower voltage, and at rated or higher frequency.

8.3.1.3 Three-phase regulators

In the case of three-phase regulators, when each phase is independent and accessible, single-phase power
should be used; however, when convenient, three-phase power may be used.

8.3.2 Ratio test methods

8.3.2.1 Voltmeter method

Two voltmeters shall be used with voltage transformers when necessary: one to read the voltage of the shunt
winding, and the other to read the voltage of the series winding. The two voltmeters shall be read simulta-
neously. A second set of readings shall be taken with the instruments interchanged. The average of the two
sets of readings is then calculated to compensate for instrument errors.

Voltage transformer ratios should be such as to yield approximately the same readings on the two voltmeters.
Compensation for instrument errors by an interchange of instruments will otherwise not be satisfactory, and
it will be necessary to apply appropriate corrections to the voltmeter readings.

Tests shall be made at not less than four voltages in approximately 10% steps, and the average result shall be
taken as the true value. These values should fall within 1%. The tests shall otherwise be repeated with other
voltmeters.

When appropriate corrections are applied to the voltmeter readings, tests may be made at only one voltage.

When several regulators of duplicate rating are to be tested, work may be expedited by applying the fore-
going tests to only one unit and then comparing the other units with this one as a standard, in accordance
with the comparison method discussed in 8.3.2.2.

8.3.2.2 Comparison method

A convenient method of measuring the ratio of a regulator is to compare it with a regulator of known ratio.

The regulator to be tested is excited in parallel with a regulator of the same nominal ratio, and the two output
sides are connected in parallel, but with a voltmeter or detector in the connection between two terminals of
similar polarity (see Figure 7). This is the more accurate method because the voltmeter or detector indicates
the difference in voltage.


IEEE

Figure 7—Voltmeter arranged to read the difference between the two output side voltages

For an alternate method, the regulator to be tested is excited in parallel with a regulator of known ratio, and
the voltmeters are arranged to measure the two series winding voltages (see Figure 8).

Figure 8—Voltmeter arranged to read the two series winding voltages

The voltmeters shall be interchanged and the test repeated. The averages of the results are the correct
voltages.

NOTE—Readings are repeated after interchanging voltmeters.

8.3.2.3 Ratio bridge

A bridge using the basic circuit of Figure 9 may be used to measure ratio, as shown for a Type A regulator.

When detector (DET) is in balance, the regulator ratio is equal to R/R1.


IEEE

Figure 9—Basic circuit of ratio bridge

NOTES
1—Measurement of ratio using circuits of this type has in the past also been described as ratio by resistance
potentiometer.
2—More accurate results can be obtained using a ratio bridge that provides phase angle correction.
3—The ratio bridge can also be used to test polarity, phase-relation, and phase-sequence.

8.4 No-load losses and excitation current

8.4.1 General

No-load (excitation) losses are those losses that are incident to the excitation of the regulator. No-load
(excitation) losses include core loss, dielectric loss, and conductor loss in the windings due to excitation
current, and conductor loss due to circulating current in parallel windings. These losses change with the
excitation voltage.

The excitation current (no-load current) includes current that flows in any winding, used to excite the
regulator when all other windings are open-circuited, and the circulating current in parallel windings. The
excitation current referred to the shunt winding is generally expressed in percent of the rated load current.

The no-load loss of a regulator consists primarily of the iron loss in the regulator cores and the circulating
current in parallel windings, both of which are a function of the magnitude, frequency, and waveform of the
impressed voltage.

The no-load loss and current are particularly sensitive to differences in wave shape; therefore, no-load loss
measurements will vary markedly with the waveform of the test voltage.

The exciting kVA is the product of the rated voltage across the energized winding in kV multiplied by the
exciting current in amperes. The ratio of the no-load losses (in kW) to the exciting kVA is the no-load loss
power factor of the regulator during the test, and is used in correction for phase-angle error as specified in
8.4.6.

In addition, several other factors affect the no-load losses and excitation current of a regulator. The design-
related factors include the type and thickness of core steel, the core configuration, the geometry of core
joints, and the core flux density.


IEEE

Factors that cause differences in the no-load losses of regulators of the same design include variability in
characteristics of the core steel, mechanical stresses induced in manufacturing, variation in gap structure,
core joints, variability of reactor (preventive autotransformer) core gaps, and so forth.

8.4.2 No-load loss test

The purpose of the no-load loss test is to measure no-load losses at a specified excitation voltage, frequency
and tap position. The no-load loss determination shall be based on a sine-wave voltage, unless a different
waveform is inherent in the operation of the regulator. The average-voltage voltmeter method is the most
accurate method for correcting the measured no-load losses to a sine-wave basis, and is recommended. This
method employs two parallel-connected voltmeters: one is an average-responding (but rms calibrated) volt-
meter; the other is a true rms-responding voltmeter. The test voltage is adjusted to the specified value as read
by the average-responding voltmeter. The readings of both voltmeters are employed to correct the no-load
losses to a sine-wave basis, using Equation (2) in accordance with 8.4.3.

8.4.2.1 Connection diagrams

Tests for the no-load loss determination of a single-phase regulator are carried out using the schemes
depicted in Figure 10 and Figure 11. Figure 10 shows the necessary equipment and connections for the case
where instrument transformers are not required. When instrument transformers are required, which is the
general case, the equipment and connections shown in Figure 11 apply. If necessary, correction for losses in
connected measurement instruments may be made by disconnecting the regulator under test and noting the
wattmeter reading at the specified test circuit voltage. These losses represent the losses of the connected
instruments (and voltage transformer, if used). They may be subtracted from the earlier wattmeter reading to
obtain the no-load loss of the regulator under test.

Figure 10—Connection for no-load loss test of single-phase regulator
without instrument transformers

W

Figure 11—Connections for no-load loss test of a single-phase regulator
with instrument transformers


IEEE

8.4.2.2 Energized windings

Either the shunt winding or the series winding of the regulator under test may be energized, but it is gener-
ally more convenient to perform this test using the shunt winding. The voltage to be maintained during test
should result in rated voltage being applied to, or induced into, the shunt winding. In any case, the full
winding (not merely a portion of the winding) should be used whenever possible. If, for some unusual rea-
son, only a portion of a winding is excited, this portion shall not be less than 25% of the winding.

8.4.2.3 Voltage and frequency

The operating and performance characteristics of a regulator are based upon rated voltage and rated fre-
quency, unless otherwise specified. Therefore, the no-load loss test is conducted with rated voltage
impressed across the regulator terminals, using a voltage source at a frequency equal to the rated frequency
of the regulator under test, unless otherwise specified.

For the determination of the no-load losses of a single-phase or a three-phase regulator, the frequency of the
test source should be within ±0.5% of the rated frequency of the regulator under test. The voltage shall be
adjusted to the value indicated by the average-voltage voltmeter. Simultaneous values of rms voltage, rms
current, electrical power, and the average-voltage voltmeter readings shall be recorded. For a three-phase
regulator, the average of the three voltmeter readings shall be the desired nominal value.

8.4.3 Waveform correction of no-load losses

The eddy-current component of the no-load loss varies with the square of the rms value of excitation voltage
and is substantially independent of the voltage waveform. When the test voltage is held at the specified
value, as read on the average-voltage voltmeter, the actual rms value of the test voltage may not be equal to
the specified value. The no-load losses of the regulator corrected to a sine-wave basis shall be determined
from the measured value by means of the following equation:

Pm
P = -------------------------
- (2)
( P 1 + kP 2 )

where

P is the no-load loss (W) at voltage Ea corrected to a sine-wave basis,
Pm is the no-load loss measured in test,
P1 is the per unit hysteresis loss, referred toPm ,
P2 is the per unit eddy-current loss, referred to Pm .

Er 2
k =  -----
- (3)
 E a

where

Er is the test voltage measured by rms voltage,
Ea is the test voltage measured by average-voltage voltmeter.

The actual percentage values of hysteresis and eddy-current losses should be used, if available. If actual
values are not available, it is suggested that these two loss components be assumed equal in value, assigning
each a value of 0.5 per unit.


IEEE

Equation (2) is valid only for test voltages with moderate waveform distortion. If waveform distortion in the
test voltage causes the magnitude of the correction to be greater than 5%, then the test voltage waveform
shall be improved for an adequate determination of the no-load losses and currents.

8.4.4 Test methods for three-phase regulators

Tests for the no-load loss determination of a three-phase regulator shall be carried out by using the three
wattmeter method. Figure 12 is a representation of the equipment and connections necessary for conducting
no-load loss measurements of a three-phase regulator.

8.4.5 Determination of excitation (no-load) current

The excitation (no-load) current of a regulator consists of the current that maintains the rated magnetic ﬂux
excitation in the cores of the regulator and the circulating current between parallel windings. The excitation
current is usually expressed in per unit or in percent of the rated load current of the regulator. (Where the
cooling class of the regulator involves more than one kVA rating, the lowest kVA rating is used to determine
the base current.) Measurement of excitation current is usually carried out in conjunction with the tests for
no-load losses. The rms current is recorded simultaneously during the test for no-load losses using the aver-
age-voltage voltmeter method. This value is used in calculating the per unit or percent excitation current. For
a three-phase regulator, the excitation current is calculated by taking the average of the magnitudes of the
three line currents.

W1

W2

W3

Figure 12—Three-phase regulator connections for no-load loss and excitation current test
using three-wattmeter method

8.4.6 Correction of loss measurement due to metering phase-angle errors

After proper consideration of magnitude-related errors such as instrument transformer ratio errors, meter
calibration, and so forth, correction of loss measurement due to phase-angle errors in the wattmeters, voltage
measuring circuit, and current measuring circuit shall be applied in accordance with Table 14 and by using
the following correction formula:


IEEE

Pc = Pm – V m Am [ –W d – ( V d + C d ) ] (4)

where

Pc is the wattmeter reading, corrected for phase-angle error (W),
Pm is the actual wattmeter reading (W),
Vm is the voltmeter reading across wattmeter voltage element (V),
Am is the ammeter reading in wattmeter current element (A),
Wd is the phase-angle error of wattmeter where applicable (rad),
Vd is the phase-angle error of voltage transformer (rad),
Cd is the phase-angle error of current transformer (rad).

Table 14—Requirements for phase-angle error correction

Apparent loss power factor
Comments
(PF = Pm/VA)

PF ≤ 0.03 Apply phase-angle error correction

0.03 < PF ≤ 0.10 Apply phase-angle error correction if (–Wd – Vd + Cd) > 290 µrad (1 min)

PF > 0.10 Apply phase-angle error correction if (–Wd – Vd + Cd) > 870 µrad (3 min)

In general, instrument transformer phase-angle errors are a function of burden and excitation. Likewise,
wattmeter phase-angle errors are a function of the scale being used and the circuit power factor. Thus, the
instrumentation phase-angle errors used in the correction formula shall be speciﬁc for the test conditions
involved. Only instrument transformers meeting 0.3 metering accuracy class, or better, are acceptable for
measurements.

Use of Equation (4) is limited to conditions of the apparent power factor less than 0.20 and the total system
phase-angle less than 20 min. If corrections are required for the apparent power factor or system phase error
outside this range, the following exact formula applies:

–1 Pm
φ a = cos --------------
- (5)
V m Am

P c = V m A m cos [ φ a – W d – V d ] + C d (6)

For three-phase measurements, the corrections are applied to the reading of each wattmeter employed. The
regulator loss at temperature Tm is then calculated as follows:

N
P(T m) = ∑ Rv Ra Rci (7)
i=1

where

P(Tm) is the regulator losses, corrected for phase-angle error at temperature, Tm ,
N is the number of phases (wattmeters),
Pci is the corrected wattmeter reading of the ith wattmeter,


IEEE

Rv is the true voltage ratio of voltage measuring circuit,
Ra is the true current ratio of current measuring circuit.

8.5 Impedance and load losses

8.5.1 General

The load losses of a regulator are those losses incident to a specified load carried by the regulator. Load
losses include I2R loss in the windings due to load current and stray losses due to eddy currents induced by
leakage flux in the windings, core clamps, magnetic shields, tank walls, and other conducting parts. Stray
losses may also be caused by circulating currents in parallel windings or strands. Load losses are measured
by applying a short circuit across the series winding and applying sufficient voltage across the shunt winding
to cause a specified current to flow in the windings. The power loss within the regulator under these condi-
tions equals the load losses of the regulator at the temperature of the test for the specified load current and
tap position.

8.5.1.1 Impedance voltage

The impedance voltage of a regulator is the voltage required to circulate rated current through one of two
specified windings when the other winding is short-circuited while in a specified tap position. Impedance
voltage is usually expressed in per unit, or percent, of the rated voltage of the winding across which the
voltage is applied and measured. The impedance voltage comprises a resistive component and a reactive
component. The resistive component of the impedance voltage, called the resistance drop, is in phase with
the current and corresponds to the load losses. The reactive component of the impedance voltage, called the
reactance drop, is in quadrature with the current and corresponds to the leakage-flux linkages of the wind-
ings. The impedance voltage is the phasor sum of the two components. The impedance voltage is measured
during the load loss test by measuring the voltage required to circulate rated current in the windings. The
measured voltage is the impedance voltage at the temperature of the test, and the power loss dissipated
within the regulator is equal to the load losses at the temperature of the test and at rated load. The impedance
voltage and the load losses are corrected to a reference temperature using the formulas specified in 8.5.4.1.

The impedance voltage of a step-voltage regulator generally will be less than 0.5% of the rated voltage,
stated on the circuit kVA base. The impedance voltage will vary with tap position and may be somewhat
higher for a two-core design.

8.5.1.2 Impedance kVA

The impedance kVA is the product of the impedance voltage across the energized winding (in kV) multiplied
by the winding current in amperes. The ratio of the load losses (in kW) at the temperature of test to the
impedance kVA at the temperature of test is the load loss power factor of the regulator during the test, and is
used in correction for phase-angle error as specified in this standard.

8.5.2 Factors affecting the values of load losses and impedance voltage

The magnitudes of the load losses and the impedance voltage will vary depending on the regulator tap
position. These changes are due to the changes in the magnitudes of winding currents and associated leak-
age-flux linkages, as well as changes in stray flux and accompanying stray losses. In addition, several other
factors, which are detailed in the following subclauses, affect the values of load losses and impedance volt-
age of a regulator. Considerations of these factors, in part, explain variations in values of load losses and
impedance voltage for the same regulator under different test conditions, as well as variations between the
values of load losses and impedance voltage of different regulators of the same design.


IEEE

8.5.2.1 Design

The design-related factors include conductor material, conductor dimensions, winding design, winding
arrangement, shielding design, and selection of structural materials.

8.5.2.2 Process

The process-related factors that impact the values of load losses and impedance voltage are the dimensional
tolerances of conductor materials, the final dimensions of completed windings, phase assemblies, metallic
parts exposed to stray flux, and variations in properties of conductor material and other metallic parts.

8.5.2.3 Temperature

Load losses are also a function of temperature. The I2R component of the load losses increases with temper-
ature, while the stray loss component decreases with temperature. Procedures for correcting the load losses
and impedance voltage to the standard reference temperature are described in 8.5.4.

8.5.2.4 Measurements

At low power factors, judicious selection of measurement method and test system components is essential
for accurate and repeatable test results. The phase-angle errors in the instrument transformers, measuring
instruments, bridge networks, and accessories affect the load loss test results. Procedures for correcting the
load losses for metering phase-angle errors are described in 8.4.6.

8.5.3 Tests for measuring load loss and impedance voltage

8.5.3.1 Preparation

The following preparatory requirements shall be satisfied for accurate test results:

a) To determine the temperature of the windings with sufficient accuracy, the following conditions
shall be met (and, except as noted, the following conditions are necessary):
1) The temperature of the insulating liquid has stabilized and the difference between top and bot-
tom oil temperatures does not exceed 5 °C.
2) The temperature of the windings shall be taken immediately before and after the load loss and
impedance voltage test in a manner similar to that described in 8.1.1. (The average shall be
taken as the true temperature.)
3) The difference in winding temperature before and after the test shall not exceed 5 °C.
NOTE—For regulators, where it may not be practical to wait for thermal equilibrium, the method used to
determine the winding temperature shall take into consideration the lack of thermal equilibrium and the
effect of ohmic heating of the winding conductors by load current during the test. The method used can be
verified by staging a repeated measurement of the load losses and impedance voltage at a later time when
above conditions are met.
b) Conductors used for short-circuiting the series winding of the regulator shall have a cross-sectional
area equal to or greater than the corresponding regulator leads. They should be as short as possible
and should be kept away from magnetic masses. Contacts should be clean and tight.
c) The frequency of the test source used for measuring load losses and impedance voltage shall be
within ±0.5% of the nominal value.
d) The maximum value of correction to the measured load losses due to the test system phase-angle is
limited to ±5% of measured losses. If more than 5% correction is required, test methods and/or test
apparatus should be improved for an adequate determination of load losses.


IEEE

8.5.3.2 Load loss and impedance test of a single-phase regulator

A regulator, which basically is an autotransformer, may be tested for load losses and impedance with its
internal connections unchanged and with the unit in a specified tap position. The test is made by shorting the
input (or output) terminals while voltage (at rated frequency) is applied to the other terminals. The voltage is
adjusted to cause rated line current to flow. For the purpose of measuring load losses and impedance voltage,
it is more common that the series and shunt windings of the regulator are treated as separate windings—the
series winding short-circuited and the shunt winding excited. In this situation, where the regulator is con-
nected in the two winding connection for the test, the current held shall be the rated current of the excited
winding. The load loss watts and applied voltamperes will be the same, whether series and shunt windings
are treated as separate windings in the two winding connection or are connected in the autotransformer con-
nections so long as rated winding current is held in the first case and rated line current is held in the second
case. The impedance voltage measurement from the two winding connection will need to be revised to
reflect the autotransformer connection. Simultaneous readings of the ammeter, voltmeter, and wattmeter are
recorded for determinations of load losses and impedance voltage. The regulator under test should then be
disconnected, and readings of losses taken on the wattmeter that represent the losses of the measuring equip-
ment, similar to the procedure in the no-load loss test.

The connections and apparatus needed for the determination of the load loss and impedance voltage of a
single-phase regulator are shown in Figure 13 and Figure 14. Figure 13 applies when instrument transform-
ers are not required. If instrument transformers are required, which is the general case, then Figure 14
applies.

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Figure 13—Single-phase regulator connections for load loss and impedance voltage test
without instrument transformers

W

Figure 14—Single-phase regulator connections for load loss and impedance voltage test
with instrument transformers


IEEE

8.5.3.3 Impedance test of a three-phase regulator

The terminals of the series winding of each phase are short-circuited together, and three-phase voltages (at
rated frequency) at suitable magnitude are applied to the terminals of the shunt windings to cause their rated
winding currents to ﬂow in a speciﬁed tap position. The procedure is similar to that described for single-
phase units except that all connections and measurements are three phase instead of single phase. If the three
line currents cannot be balanced, their average rms value should correspond to the desired value, at which
time simultaneous readings of wattmeters, voltmeters, and ammeters should be recorded.

8.5.3.3.1 Measurement connections

For three-phase regulators, Figure 15 shows the apparatus and connections using the three-wattmeter
method.

W1

W2

W3

Figure 15—Three-phase regulator connections for load loss and impedance voltage test
using the three-wattmeter method

8.5.4 Calculation of impedance voltage and load losses from test data

Load losses and impedance voltage measurements vary with temperature and, in general, shall be corrected
to a reference temperature. In addition, load loss measurement values shall be corrected for metering phase-
angle error (see 8.4.6).

8.5.4.1 Temperature correction of load losses

Both I2R losses and stray losses of a regulator vary with temperature. The I2R losses, Pr (Tm), of a regulator
are calculated from the ohmic resistance measurements (corrected to the temperature, Tm, at which the
measurement of load losses and impedance voltage was done) and the currents that were used in the imped-
ance measurement. These I2R losses subtracted from the measured load loss watts, P(Tm), give the stray
losses, Ps(Tm), of the regulator at the temperature at which the load loss test was made.


IEEE

Ps ( T m ) = P ( T m ) – Pr ( T m ) (8)

where

Ps(Tm) is the calculated stray losses (W) at temperature Tm ,
P(Tm) is the regulator load losses (W), corrected in accordance with 8.4.6, for phase angle error at
temperature Tm ,
Pr(Tm) is the calculated I2R loss (W) at temperature Tm.

The I2R component of the load losses increases with temperature. The stray loss component diminishes with
temperature. Therefore, when it is desirable to convert the load losses from the temperature at which it is
measured, Tm, to another temperature, T, the two components of the load losses are corrected separately.

Thus,

Pr ( T m ) ( T k + T )
P r ( T ) = --------------------------------------
- (9)
(T k + T m)

P rs ( T m ) ( T k + T m )
P rs ( T ) = -------------------------------------------
- (10)
(T k + T )

then

P ( T ) = P r ( T ) + P rs ( T ) (11)

where

Pr(T) is the I2R loss (W) at temperature T (°C),
Ps(T) is the stray losses (W) at temperature T (°C),
P(T) is the regulator load losses (W) corrected to temperature T (°C),

losses.

8.5.4.2 Impedance voltage

The impedance voltage and its resistive and reactive components at the speciﬁed tap position are determined
by the use of the following equations:

P(T m)
E r ( T m ) = --------------- (12)
I

2 2
Ex = Ez(T m) – Er(T m) (13)

P(T )
E r ( T ) = -----------
- (14)
I

2 2
Ez(T ) = Er(T ) + E x (15)


IEEE

where

Er(Tm) is the resistance voltage drop (volts) of in-phase component at temperature Tm ,
Er(T) is the resistance voltage drop (volts) of in-phase component corrected to temperature T,
Ex is the reactance voltage drop (volts) of quadrature component,
Ez(Tm) is the impedance voltage (volts) at temperature Tm ,
Ez(T) is the impedance voltage (volts) at temperature T,
P(T) is the regulator load losses (watts) corrected to temperature T,
P(Tm) is the regulator load losses (watts) measured at temperature Tm ,
I is the current in amperes in the excited winding.

Per unit values of the resistance, reactance, and impedance voltage are obtained by dividing Er(T), Ex, and
Ez(T) by the rated voltage. Percentage values are obtained by multiplying per-unit values by 100.

8.6 Dielectric tests

8.6.1 General

8.6.1.1 Factory dielectric tests

The purpose of dielectric tests in the factory is to demonstrate that the regulator has been designed and
constructed to withstand the specified insulation levels.

8.6.1.2 Test requirements

Test levels and other test parameters shall be as outlined in IEEE Std C57.12.00-1999 or as otherwise
specified.

8.6.1.3 Measurement of test voltages

Unless otherwise specified, the dielectric test voltages shall be measured or applied, or both, in accordance
with IEEE Std 4-1995 with the following exceptions:

a) A protective resistance may be used in series with sphere gaps, on either the live or grounded sphere.
Where necessary to protect the spheres from arc damage, it may be omitted.
b) The bushing-type potential divider method shall be considered a standard method for regulator tests.
c) The rectified capacitor-current method shall be considered a standard method for regulator tests.
d) In conducting low-frequency tests for regulators of 100 kVA and less to be tested at 50 kV or less, it
is permissible to depend on the ratio of the testing transformer to indicate the proper test voltage.

8.6.1.4 Tests on bushings

Separate bushings tests will be performed in accordance with Annex B.

8.6.1.5 Dielectric tests in the field

Field dielectric tests will be performed in accordance with Annex B.


IEEE

8.6.1.6 Factory dielectric tests and conditions

8.6.1.6.1 Test sequence

Lightning impulse voltage tests shall precede the low-frequency tests.

8.6.1.6.2 Temperature

Dielectric tests may be made at temperatures assumed under normal operation or at the temperatures
attained under the conditions of routine test.

8.6.1.6.3 Assembly

Regulators, including bushings and terminal compartments when necessary to verify air clearances, shall be
assembled prior to making dielectric tests, but assembly of items, such as radiators and cabinets, that do not
affect dielectric tests is not necessary. Bushings shall, unless otherwise authorized by the purchaser, be those
to be supplied with the regulator.

8.6.2 Design lightning impulse test procedures

Lightning impulse tests, when required as a design test, shall consist of and be applied in the following
order: one reduced full wave, two chopped waves, and one full wave. The time interval between application
of the last chopped wave and the final full wave should be minimized to avoid recovery of dielectric strength
if a failure were to occur prior to the final full wave.

NOTE—See IEEE Std C57.98-1993 for guide information on impulse testing techniques, interpretation of oscillograms,
and failure detection criteria.

8.6.2.1 General

Impulse tests shall be made without excitation.

8.6.2.1.1 Reduced full-wave test

This wave is the same as a full wave, except that the crest value shall be between 50% and 70% of the full-
wave value given in Table 10.

8.6.2.1.2 Chopped-wave test

This wave is also the same as the full wave, except that the crest value shall be at the required higher level
and the voltage wave shall be chopped at or after the required minimum time to sparkover in accordance
with Table 8. In general, the gap or other equivalent chopping device shall be located as close as possible to
the terminals and the impedance shall be limited to that of the necessary leads to the gap; however, it shall be
permissible for the manufacturer to add resistance to limit the amount of overswing to the opposite polarity
to 30% of the amplitude of the chopped wave.

8.6.2.1.3 Full-wave test

The test wave rises to crest in 1.2 µs and decays to half of crest value in 50 µs from the virtual time zero. The
crest value shall be in accordance with Table 10, subject to a tolerance of ±3%, and no flashover of the bush-
ing or test gap shall occur. The tolerance on time to crest should normally be ±30% and the tolerance on time
to half of crest shall normally be ±20%; however, as a practical matter, the following shall be considered:


IEEE

a) The time to crest shall not exceed 2.5 µs except for windings of large impulse capacitance (low volt-
age, high kVA and some high voltage, high kVA windings). To demonstrate that the large
capacitance of the winding causes the long front, the impulse generator series resistance may be
reduced, which should cause super-imposed oscillations. Only the inherent generator and lead
inductances should be in the circuit.
b) The impedance of some windings may be so low that the desired time to the 50% voltage point on
the tail of the wave cannot be obtained with available equipment. In such cases, shorter waves may
be used. To ensure that an adequate test is obtained, the capacitance of the generator with the
connection used should exceed 0.011 µF.

For convenience in measurement, the time to crest may be considered 1.67 times the actual time between
points on the front of the wave at 30% and 90% of the crest value.

The virtual time zero can be determined by locating points on the front of the wave at which the voltage is,
respectively, 30% and 90% of the crest value, and then drawing a straight line through these points. The
intersection of this line with the time axis (zero-voltage line) is the virtual time zero.

When oscillations exist on the front of the waves, the 30% and 90% points shall be determined from the
average, smooth wave front sketched in through the oscillations. The magnitude of the oscillations prefera-
bly should not exceed 10% of the applied voltage.

When there are high-frequency oscillations on the crest of the wave, the crest value shall be determined from
a smooth wave sketched through the oscillations. If the period of these oscillations is 2 µs or more, the actual
crest value shall be used.

8.6.2.1.4 Wave polarity

The test waves are normally of negative polarity to reduce the risk of erratic external flashover in the test
circuit.

8.6.2.1.5 Impulse oscillograms

All impulses applied to a regulator shall be recorded by an oscilloscope or by a suitable digital transient
recorder, unless their crest voltage is less than 40% of the full-wave level. These oscillograms shall
include voltage oscillograms for all impulses and ground-current oscillograms for all full-wave and reduced
full-wave impulses. Sweep times should be in the order of 5–10 µs for chopped-wave tests, 50–100 µs for
full-wave tests, and 100–600 µs for ground-current measurements.

When reports require oscillograms, those of the first reduced full-wave voltage and current, the last two
chopped-waves, and the last full-wave of voltage and current shall represent a record of the successful appli-
cation of the impulse test to the regulator.

8.6.2.2 Connections and tap positions for impulse tests of line terminals

The series and shunt windings of a regulator are considered as a single winding for the purpose of the
impulse test. The line terminals, S and L, are tied together through a resistor of 450 W ± 10% to limit
induced voltage. Current flowing in this limiting resistor shall not interfere with the ability to detect a staged
single-turn fault. A Type A regulator shall have the test applied to the source (S) terminal while set in the
maximum buck position. A Type B regulator shall have the test applied to the load (L) terminal while set in
the maximum boost position. The value of the induced voltage on the non-impulsed line terminal shall be
in accordance with Table 10, subject to a tolerance of ±10%. Regulators intended for delta connections shall
in addition have impulse voltage applied to the SL line terminal.


IEEE

8.6.2.2.1 Terminals not being tested

Neutral terminals shall be solidly grounded. Line terminals shall be either solidly grounded or grounded
through a resistor with an ohmic value not in excess of 450 Ω. The following factors shall be considered in
the actual choice of grounding for each terminal:

a) The voltage-to-ground on any terminal that is not being tested should not exceed 80% of the full-
wave impulse voltage level for that terminal.
b) When a terminal has been specified to be directly grounded in service, then that terminal shall be
solidly grounded.
c) When a terminal is to be connected to a low-impedance cable connection in service, then that termi-
nal shall either be directly grounded or grounded through a resistor with an ohmic value not in
excess of the surge impedance of the cable.
d) Grounding through a low-impedance shunt for current measurements may be considered the equiva-
lent of a solid ground.

8.6.2.2.2 Windings for series or multiple connections

When either connection is 25 kV nominal system voltage or above, the windings shall be tested on both
series and multiple connections. The test voltage for the two conditions shall correspond to the BIL of the
winding for that connection. For nominal system voltage 15 kV and below, only the series connections shall
be tested unless tests on both connections are specified.

8.6.2.2.3 Windings for delta and wye connections

When either connection is 25 kV nominal system voltage or above, the three-phase regulator shall be tested
on both delta and wye connections. The test voltage for each connection shall be that corresponding to the
BIL of the winding for that connection. For nominal system voltage 15 kV and below, only the wye connec-
tion shall be tested unless tests on both connections are specified.

8.6.2.2.4 Protective devices that are an integral part of the regulator

Regulators may have as an integral part nonlinear protective devices connected across whole or portions of
windings. During impulse testing, operation of these protective devices may cause differences between the
reduced full-wave and the full-wave oscillograms. That these differences are caused by the operation of the
protective devices may be demonstrated by making two or more reduced full-wave impulse tests at different
voltage levels to show the trend in their operation.

Typical oscillograms depicting the operation of protective devices during impulse testing are shown in
IEEE Std C57.98-1993.

8.6.2.2.5 Current transformer grounding

The secondaries of current transformers, either on bushings or permanently connected to the equipment
being tested, shall be short-circuited and grounded.

8.6.2.2.6 Core and tank grounding

The core and tank shall be grounded for all impulse tests.


IEEE

8.6.2.2.7 Grounding of potential transformers and utility windings

The secondaries of potential transformers and utility windings shall be terminated with an impedance not to
exceed 450 Ω to ground. Current flowing in this limiting resistor shall not interfere with the ability to detect
a staged single-turn fault.

8.6.2.3 Impulse tests on regulator neutrals

Impulse tests on the neutral terminal of a regulator or a separate regulator connected in the neutral of a trans-
former require one reduced and two full-waves to be applied directly to the neutral or regulator winding with
an amplitude equal to the insulation level of the neutral. The regulator being tested shall be set on the
maximum buck or boost position. A wave having a front of not more than 10 µs and a tail of 50 µs to half-
crest shall be used except when the inductance of the winding is so low that the desired voltage magnitude
and duration to the 50% point on the tail of the wave cannot be obtained. In that case, a shorter wave-tail
may be used.

8.6.2.4 Detection of failure during impulse test

Given the nature of impulse test failures, one of the most important matters is the detection of such failures.
There are a number of indications of insulation failure.

8.6.2.4.1 Voltage oscillograms

Any unexplained differences between the reduced full wave and final full wave detected by comparison of
the two voltage oscillograms, or any such differences observed by comparing the chopped waves to each
other and to the full wave up to the time of flashover, are indications of failure.

8.6.2.4.2 Smoke and bubbles

Smoke bubbles rising through the oil in the regulator are definite evidence of failure. Clear bubbles may or
may not be evidence of trouble; they may be caused by entrapped air. They should be investigated by repeat-
ing the test, or by reprocessing the regulator and repeating the test to determine if a failure has occurred.

8.6.2.4.3 Failure of gap to sparkover

In making the chopped-wave test, failure of the chopping gap, or any external part to sparkover, is a definite
indication of a failure either within the regulator or in the test circuit, even though the voltage oscillogram
shows a chopped wave.

8.6.2.4.4 Audible noise

Unusual audible noise within the regulator at the instant of applying the impulse is an indication of trouble.
Such noise should be investigated.

8.6.2.4.5 Ground current oscillograms

In this method of failure detection, the impulse current in the grounded end of the winding tested is
measured by means of a oscilloscope or by a suitable digital transient recorder connected across a suitable
shunt inserted between the normally grounded end of the winding and ground. Any differences in the wave
shape between the reduced full wave and final full wave detected by comparing the two current oscillograms
may be indications of failure or deviations due to noninjurious causes. They should be fully investigated and
explained by a new reduced wave and full-wave test. Examples of probable causes of different wave shapes
are operation of protective devices, core saturation, or conditions in the test circuit external to the regulator.


IEEE

The ground current method of detection is not suitable for use with chopped-wave tests.

8.6.3 Routine lightning impulse test procedures

For regulators, the impulse tests specified in 8.6.2 are design tests. This subclause defines a routine quality
control test that is suitable for high-volume, production-line testing.

8.6.3.1 Connections and tap positions for impulse tests of line terminals

The series and shunt windings of a regulator are considered a single winding for the purpose of the impulse
test. The line terminals, S and L, are tied together through a resistor of 450 Ω ± 10% to limit induced voltage.
Current flowing in this limiting resistor shall not interfere with the ability to detect a staged single-turn fault.
A Type A regulator shall have the test applied to the S terminal while set in the maximum buck position. A
Type B regulator shall have the test applied to the L terminal while set in the maximum boost position. The
value of the induced voltage on the non-impulsed line terminal shall be in accordance with Table 10, subject
to a tolerance of ±10%. Regulators intended for delta connections shall in addition have impulse voltage
applied to the SL line terminal.

8.6.3.2 Procedure

The tank and core are grounded, while the windings under test are connected to ground through a low
impedance shunt. This shunt shall consist of either of the following:

a) Ground current method. A suitable resistance shunt or wide-band pulse current transformer is
employed to examine the waveform of the ground current.
b) Neutral impedance method. A low-impedance shunt, consisting of a parallel combination of
resistance and capacitance, R-C, is employed. The voltage across this neutral impedance shunt is
examined.

An impulse voltage with 1.2 × 50 µs wave shape and with specified crest magnitude shall be applied in each
test. The tolerances, polarity, and method of determining the wave shape shall be as specified in 8.6.2.1.3
and 8.6.2.1.4. During each test the waveform of the ground current or the voltage wave across the neutral
impedance shall be examined.

The required impulse tests shall be applied using either of the test series described in 8.6.3.2.1 and 8.6.3.2.2.

8.6.3.2.1 Method 1

One reduced full-wave test is performed, followed by one 100% magnitude full-wave test. The applied volt-
age wave in the first test shall have a crest value of between 50% and 70% of the assigned BIL. The applied
voltage wave in the second test shall have a crest value of 100% of the assigned BIL, subject to a tolerance of
±3%. Failure detection is accomplished by comparing the reduced full-wave test with the 100% magnitude
full-wave test, using either the ground current waveform or the neutral impedance voltage waveform.
A dielectric breakdown will cause a difference in compared waveforms. Observed differences in the wave-
forms may be indications of failure, or they may be due to noninjurious causes. The criteria used to judge the
magnitude of observed differences shall be based upon the ability to detect a staged single-turn fault made
by placing a loop of wire around the core leg and over the coil.

8.6.3.2.2 Method 2

Two full-wave tests, with crest magnitude equal to the assigned BIL, are applied to the regulator under test.
A neutral impedance shunt, using suitable values of R and C, is employed to record waveforms for compari-
son. The waveforms in both tests are compared to pre-established levels. A dielectric breakdown will cause a
significant upturn and increase in magnitude of the voltage wave examined across the neutral impedance.


IEEE

The pre-established levels are based upon a staged single-turn fault test, made by placing a loop of wire
around the core leg and over the coil.

8.6.3.2.3 Failure detection

The failure detection methods for the routine impulse test described in 8.6.3.2.1 (method 1) and 8.6.3.2.2
(method 2) are based on the following two conditions:

a) The regulator connections during the test are such that the series winding is not shorted.
b) Chopped-wave tests are not applied.

In addition to these methods of failure detection, other methods of failure detection, as described in 8.6.2.4,
are also indications of failure and should be investigated.

When the test is complete and the process of failure detection is complete, the waveform records may be
discarded.

The routine impulse test shall be conducted before the low-frequency dielectric tests.

8.6.3.3 Terminals not being tested

Refer to 8.6.2.2.1.

8.6.3.4 Windings for series or multiple connections

Refer to 8.6.2.2.2.

8.6.3.5 Windings for delta and wye connections

Refer to 8.6.2.2.3.

8.6.4 Low-frequency tests

Low-frequency tests shall be performed in accordance with the requirements of 5.6 and Table 10.

The low-frequency tests levels are developed by the applied voltage and induced voltage tests described in
8.6.5 and 8.6.6, or combinations thereof. The induced voltage tests may involve either single- or three-phase
excitation.

8.6.4.1 Failure detection

During either applied or induced low-frequency tests, careful attention should be maintained for evidence of
possible failure. Evidence of failure could include items such as an indication of smoke and bubbles rising in
the oil, an audible sound such as a “thump,” a sudden increase in test-circuit current, an appreciable increase
in partial discharge (corona) level, etc. Any such indication should be carefully investigated by observation,
by repeating the test, or by other tests to determine if a failure has occurred.

8.6.5 Applied voltage tests

8.6.5.1 Duration, frequency, and connections

A normal power frequency, such as 60 Hz, shall be used, and the duration of the test shall be 1 min.


IEEE

The winding being tested shall have all its parts joined together and connected to the line terminal of the test-
ing transformer.

All other terminals and parts (including core and tank) shall be connected to ground and to the other terminal
of the testing transformer.

The ground connections between the apparatus being tested and the testing transformer shall be a substantial
metallic circuit. All connections shall form a good mechanical joint without forming sharp corners or points.

8.6.5.2 Relief gap

A relief gap set at a voltage of 10% or more in excess of the specified test voltage may be connected during
the applied voltage test.

8.6.5.3 Application of test voltage

The voltage should be started at one quarter or less of the full value and be increased gradually to full value
in not more than 15 s. After being held for the time specified, the voltage should be reduced gradually (in not
more than 5 s) to one quarter or less of the maximum value, and the circuit opened.

8.6.6 Induced voltage tests

8.6.6.1 Test duration

The induced voltage test shall be applied for 7200 cycles, or 60 s, whichever is shorter.

8.6.6.2 Test frequency

As this test applies greater than rated volts per turn to the regulator, the frequency of the impressed voltage
shall be high enough to limit the flux density in the core to that permitted by IEEE Std C57.12.00-1993. The
minimum test frequency to meet this condition is

Et
Minimum test frequency = ------------------ × rated frequency
- (16)
1.1 × E r

where

Et is the induced test voltage across winding,
Er is the rated voltage across winding.

8.6.6.3 Application of voltage

The voltage should be started at one quarter or less of the full value and be increased gradually to a full value
in not more than 15 s. After being held for the time specified in 8.6.6.1, the voltage should be reduced grad-
ually (in not more than 5 s) to one quarter or less of the maximum value, and the circuit opened.

8.6.6.3.1 Time during partial discharge measurement concurrent with induced voltage test

The timing involved in reaching test voltage level and reducing voltage may be longer when partial
discharge measurements or tests are being made concurrently with the induced voltage test.


IEEE

8.6.6.4 Need for additional induced test

When the induced test on a winding results in a voltage between terminals of other windings in excess of the
low-frequency test voltage speciﬁed in Table 10, the other winding may be sectionalized and grounded.
Additional induced tests shall then be made to give the required test voltage between the terminals of wind-
ings that were sectionalized.

8.6.6.5 Grounded windings

When regulators have one winding grounded for operation on a grounded-neutral system, special care
should be taken to avoid high electrostatic stresses between the other windings and ground.

8.6.6.6 Single-phase testing of three-phase regulators

Three-phase regulators may be tested with single-phase voltage. The speciﬁed test voltage is induced,
successively, from each line terminal to ground and to adjacent line terminals. The neutrals of the windings
may or may not be held at ground potential during these tests. A separate single-phase test or three-phase test
may be required when the test voltage between adjacent line terminals is higher than the test voltage from
the line terminals to ground.

8.6.7 Insulation power factor tests

Insulation power factor is the ratio of the power dissipated in the insulation, in watts, to the product of the
effective voltage and current in voltamperes when tested under a sinusoidal voltage and prescribed
conditions.

8.6.7.1 Preparation for tests

The test specimen shall have the following:

a) All windings immersed in insulating liquid.
b) All windings short-circuited.
c) All bushings in place.
d) Temperature of windings and insulating liquid near the reference temperature of 20 °C.

8.6.7.2 Instrumentation

Insulation power factor may be measured by special bridge circuits or by the voltampere-watt method. The
accuracy of measurement should be within ±0.25% insulation power factor and the measurement should be
made at or near a frequency of 60 Hz.

8.6.7.3 Voltage to be applied

The voltage to be applied for measuring insulation power factor shall not exceed half of the low-frequency
test voltage given in Table 10 for any part of the winding, or 10 000 V, whichever is lower.

8.6.7.4 Procedure

Insulation power factor tests shall be made from windings to ground and between windings as shown in
Table 15.


IEEE

Table 15—Measurements to be in insulation power factor tests

Method 1—Test without guard circuita Method 2—Test with guard circuita

Regulators with shunt and series windings only Regulator with utility winding
Shunt and series windings to ground
Shunt and series windings to utility winding and ground
Regulators with utility winding Shunt and series windings to ground, guard on utility
Shunt and series windings to utility winding and ground winding
Utility winding to ground Utility winding to shunt and series winding and ground
Shunt and series winding to ground Utility winding to ground, guard on shunt and series
winding
aInthis table, the term guard signifies one or more conducting elements arranged and connected on an electrical instrument
or measuring circuit so as to divert unwanted currents from the measuring means.

NOTES
1—While the real significance that can be attached to the power factor of liquid-immersed regulators is still a matter of
opinion, experience has shown that power factor is helpful in assessing the probable condition of the insulation when
good judgment is used.
2—In interpreting the results of power factor test values, the comparative values of tests taken at periodic intervals are
useful in identifying potential problems rather than an absolute value of power factor.
3—A factory power factor test will be of value for comparison with field power factor measurements to assess the prob-
able condition of the insulation. It has not been feasible to establish standard power factor values for liquid-immersed
regulators for the following reasons:
a) Experience has indicated that little or no relation exists between power factor and the ability of the regulator to
withstand the prescribed dielectric tests.
b) Experience has shown that the variation in power factor with temperature is substantial and erratic so that no
single correction curve will fit all cases.
c) The various liquids and insulating materials used in regulators result in large variations in insulation power-
factor values.

8.6.7.5 Temperature correction factors

Temperature correction factors for the insulation power factor depend upon the insulating materials, their
structure, moisture content, and so forth. Values of correction factor K, given in Table 16, are typical and
satisfactory for practical purposes and for use in the following equation:

F pt
F p20 = -------
- (17)
K

where
Fp20 is the power factor corrected to 20 °C,
Fpt is the power factor measured at T,
T is the test temperature (°C),
K is the correction factor.

Insulation temperature may be considered to be that of the average liquid temperature. When insulation
power factor is measured at a relatively high temperature and the corrected values are unusually high, the
regulator should be allowed to cool and the measurements should be repeated at or near 20 °C.

NOTE—The correction factors listed in Table 16 are based on insulation systems using mineral oil as an insulating liq-
uid. Other insulating liquids may have different corrections factors.


IEEE

Table 16—Temperature correction factors

Test temperature T (°C) Correction factor K

10 0.80

15 0.90

20 1.00

25 1.12

30 1.25

35 1.40

40 1.55

45 1.75

50 1.95

55 2.18

60 2.42

65 2.70

70 3.00

8.6.8 Insulation resistance tests

Insulation resistance tests are made to determine the insulation resistance from individual windings to
ground or between individual windings. The insulation resistance in such tests is commonly measured in
megaohms, or may be calculated from measurements of applied voltage and leakage current.

NOTES
1—The insulation resistance of electrical apparatus is of doubtful signiﬁcance compared with the dielectric strength. It is
subject to wide variation in design, temperature, dryness, and cleanliness of the parts. When the insulation resistance
falls below prescribed values, it can, in most cases of good design and where no defect exists, be brought up to the
required standard by cleaning and drying the apparatus. The insulation resistance, therefore, may afford a useful indica-
tion as to whether the apparatus is in suitable condition for application of dielectric test.
2—The signiﬁcance of values of insulation resistance tests generally requires some interpretation, depending on the
design and the dryness and cleanliness of the insulation involved. When a user decides to make insulation resistance
tests, it is recommended that insulation resistance values be measured periodically (during maintenance shutdown) and
that these periodic values be plotted. Substantial variations in the plotted values of insulation resistance should be inves-
tigated for cause.
3—Insulation resistances may vary with applied voltage and any comparison shall be made with measurements at the
same voltage.
4—Under no conditions should tests be made while the regulator is under vacuum.

8.6.8.1 Preparation for tests

The test specimen shall have

a) All windings immersed in insulating liquid.
b) All windings short-circuited.


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c) All bushings in place.
d) Temperature of windings and insulating liquid near the reference temperature of 20 °C.

8.6.8.2 Instrumentation

Insulation resistance may be measured using the following equipment:

a) A variable voltage dc power supply with means to measure voltage and current (generally in micro-
amperes or milliamperes).
b) A megohmeter.

NOTE—Megohmeters are commonly available with nominal voltages of 500 V, 1000 V, and 2500 V; dc applied test
equipment is available at higher voltages.

8.6.8.3 Voltage to be applied

The dc voltage applied for measuring insulation resistance to ground shall not exceed a value equal to the
rms low-frequency applied voltage allowed in Table 10.

NOTES
1—Partial discharges should not be present during insulation resistance tests, since they can damage a regulator and may
also result in erroneous values of insulation resistance.
2—When measurements are to be made using dc voltages exceeding the rms operating voltage of the windings involved
(or 1000 V for a solidly grounded wye winding), a relief gap may be employed to protect the insulation.

8.6.8.4 Procedure

a) Insulation resistance tests shall be made with all above-ground circuits of equal voltage connected
together. Circuits or groups of circuits of different voltage above ground shall be tested separately.
b) Voltage should be increased in increments of usually 1–5 kV and held for 1 min while current is
read.
c) The test should be discontinued immediately in the event the current begins to increase without
stabilizing.
d) After the test has been completed, all terminals should be grounded for a period of time sufﬁcient to
allow any trapped charges to decay to a negligible value.

8.7 Temperature rise

See IEEE Std C57.12.00-1993 for conditions under which temperature limits apply. The regulators shall be
tested in the combination of connections and taps that give the highest winding temperature rises as
determined by the manufacturer and reviewed by the purchaser’s representative when available. This will
generally involve those connections and taps resulting in the highest losses.

All temperature rise tests shall be made under normal (or equivalent to normal) conditions of the means of
cooling. These conditions are as follows:

a) Regulators shall be completely assembled and ﬁlled to the proper liquid level.
b) When the regulators are equipped with thermal indicators, bushing-type current transformers, or the
like, such devices shall be assembled with the regulator.
c) The temperature rise test shall be made in a room that is as free from drafts as practicable.


IEEE

8.7.1 Ambient temperature measurement

8.7.1.1 Air-cooled regulators

For air-cooled regulators, the ambient temperature shall be taken as that of the surrounding air, which shall
not be less than 10 °C nor more than 40 °C. For temperatures within this range, no correction factor shall be
applied. Tests may be made at temperatures outside this range when suitable correction factors are available.

The temperature of the surrounding air shall be determined by at least three thermocouples or thermometers
in containers spaced uniformly around the regulator under test. They shall be located at about half the height
of the regulator and at a distance of 1–2 m (3–6 ft) from the regulator. They shall be protected from drafts
and from radiant heat from the regulator under test or other sources.

When the time constant of the regulator as calculated according to IEEE Std C57.95-1984 is 2 h or less, the
time constant of the containers shall be between 50% and 150% of that of the regulator under test. When the
time constant of the regulator under test is more than 2 h, the time constant of the containers shall be within
1 h of that of the regulator under test.

The time constant of the containers shall be taken as the time necessary for its temperature to change 6.3 °C
when the ambient temperature is abruptly changed by 10 °C.

8.7.1.2 Water-cooled regulators

For water-cooled regulators, the ﬂow rate in liters per minute (gallons per minute) and the temperature of the
incoming and outgoing water shall be measured.

The ambient temperature shall be taken as that of the incoming water, which shall not be less than 20 °C nor
more than 30 °C. For temperatures within this range, no correction factor shall be applied. Tests may be
made at temperatures outside this range when suitable correction factors are available.

8.7.2 Liquid rise measurement

a) Liquid temperature rise is the difference between liquid temperature and the ambient temperature.
The ultimate liquid temperature rise above ambient shall be considered reached when the
temperature rise does not vary more than 2.5% or 1 °C, whichever is greater, during a consecutive
3 h period. It is permissible to shorten the time required for the test by the use of initial overloads,
restricted cooling, or the like.
b) The top liquid temperature shall be measured by a thermocouple or suitable thermometer immersed
approximately 50 mm (2 in) below the top liquid surface.
c) The average liquid temperature shall be taken to be equal to the top liquid temperature minus half
the difference in temperature of the moving liquid at the top and the bottom of the cooling means.
Where the bottom liquid temperature cannot be measured directly, the temperature difference may
be taken to be the difference between the surface temperature of the liquid inlet and outlet of the
regulator’s external cooling path.
d) A thermocouple is the preferred method of measuring surface temperature. (See 8.7.4 for method of
measurement.)

8.7.3 Average winding temperature rise measurement

The average temperature rise of a winding shall be the average winding temperature minus the ambient
temperature.


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The average temperature of the winding shall be determined by the resistance method. Where the use of the
resistance method is impossible (for example, with extremely low-resistance windings) other methods may
be used. Readings should be taken as soon as possible after shutdown, allowing sufﬁcient time for the induc-
tive effects to disappear as indicated by the cold-resistance measurement. The time from the instant of shut-
down for each resistance measurement shall be recorded. Fans and cooling water shall be shut off during
shutdown for resistance measurement. Oil pumps may be shut off or left running during shutdown for resis-
tance measurement. The average temperature of a winding shall be determined by the following formula:

R
T =  ----- ( T k + T 0 ) – T k
- (18)
 R 0

where

T is the temperature in °C, corresponding to hot resistance R,
T0 is the temperature in °C at which cold resistance R0 was measured,
R0 is the cold resistance (W), measured according to 8.1,
R is the hot resistance (Ω),
Tk is 225.0 °C (aluminum).

losses.

8.7.3.1 Temperature correction to instant of shutdown

Either of two correction factor procedures shall be used, depending upon the winding load loss density. For
these determinations, the winding load loss density for the winding connection shall be taken as the sum of
the calculated I2R and eddy losses, or the winding at the rated temperature rise plus 20 °C divided by the
calculated conductor weight of the connected winding.

8.7.3.1.1 Empirical method

This method may be used for regulators typical of those built to the speciﬁcation of this standard when the
load loss of the winding does not exceed 66 W/kg (30 W/lb) for copper, or 132 W/kg (60 W/lb) for
aluminum.

One reading of hot resistance shall be taken on each winding, with the time after shutdown recorded and the
corresponding temperature determined.

All readings of hot resistance shall be made within 4 min of shutdown. If all required readings cannot be
made within 4 min, the temperature test shall be resumed for 1 h, after which readings may again be taken.

The temperature correction to the instant of shutdown shall be an added number of degrees equal to the fac-
tor taken from Table 17 multiplied by the windings W/kg (W/lb). Factors for intermediate times may be
obtained by interpolation.

When the load loss of the winding does not exceed 15 W/kg (7 W/lb) for copper, or 31 W/kg (14 W/lb) for
aluminum, a correction of 1 °C/min may be used.


IEEE

Table 17—Winding temperature correction factor

Winding temperature correction factor
Time after
shutdown W/kg W/lb
(min)
Copper Aluminum Copper Aluminum
1 0.19 0.07 0.09 0.032
1.5 0.26 0.10 0.12 0.045
2 0.32 0.13 0.15 0.059
3 0.43 0.17 0.20 0.077
4 0.50 0.21 0.23 0.095

8.7.3.1.2 Cooling curve method

A series of at least four readings of resistance shall be made on one phase of each winding and the time
recorded for each reading.

The first reading of each series shall be made as soon as the inductive effect has subsided and not more than
4 min after shutdown.

After a set of resistance readings has been taken, the run shall be resumed for a period of 1 h, after which
further readings may be taken. This shall be repeated until all necessary readings have been taken.

The resistance/time data shall be plotted on suitable coordinate paper and the resulting curve extrapolated to
obtain the resistance at the instant of shutdown. This resistance shall be used to calculate the average wind-
ing temperature at shutdown.

The resistance/time data obtained on one phase of a winding may be used to determine the correction back to
shutdown for the other phases of the same winding, provided the first reading on each of the other phases has
been taken within 4 min after shutdown.

8.7.4 Other temperature measurements

When measured, the temperature rise of metal parts other than windings shall be determined by use of a
thermocouple or suitable thermometer.

A thermocouple is the preferred method of measuring surface temperature. When used for this purpose, the
thermocouple should be soldered to the surface. When this is not practical, the thermocouple should be
soldered to a thin metal plate or foil approximately 650 mm2 (1 in2). The plate should be held firmly and
snugly against the surface. The thermocouple should be thoroughly insulated thermally from the
surrounding medium.

The surface temperature of metal parts surrounding or adjacent to outlet leads or terminals carrying heavy
current may be measured at intervals or immediately after shutdown.

8.7.5 Test methods

Test shall be made by one of the following methods:

a) Actual loading.
b) Simulated loading.


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1) The short-circuit method, in which appropriate total losses are produced by the effect of short-
circuit current.
2) The loading back (opposition) method, in which rated voltage and current are induced in the
regulator under test.

8.7.5.1 Actual loading

The actual loading method is the most accurate of all methods, but its energy requirements are excessive for
large regulators.

Regulators of small output may be tested under actual load conditions by loading them on a rheostat, a bank
of lamps, a water box, and so forth.

8.7.5.2 Simulated loading

8.7.5.2.1 Short-circuit method

a) Short circuit one or more windings and circulate sufficient current at rated frequency to produce total
losses for the connection and loading used. Total losses shall be those measured in accordance with
8.4 and 8.5 and converted to a temperature equal to the rated average winding temperature rise plus
20 °C.
b) Determine liquid rise as described in 8.7.2.
c) Immediately reduce the currents in the windings to the rated value for the connection and the load-
ing used, hold constant for 1 h, measure liquid temperature, shut down, and measure the average
winding temperature as described in 8.7.3. When test equipment limitations dictate, it is permissible
to operate at a value lower than rated current, but not less than 85% of rated current.
d) Average winding rise shall be calculated by using either top liquid rise or average liquid rise. When
other than rated winding current is used, the average liquid rise method shall be used to determine
winding rises.
1) In the top liquid rise method, the average winding temperature rise is equal to the top liquid
rise, measured during the total loss run, plus the quantity (average winding temperature at shut-
down minus top liquid temperature at shutdown).
2) In the average liquid rise method, the average winding rise is the average liquid rise, measured
during the total loss run, plus the quantity (average winding temperature at shutdown minus
average liquid temperature at shutdown).

When the current held in any of the windings under test differs from the rated current, the observed differ-
ences between the average winding temperature at shutdown and the average liquid temperature at shutdown
shall be corrected to give the average temperature rise of the windings at the rated current by using the
following formula:

2m
rated current
T c = T 0 -----------------------------
- (19)
test current

where

Tc is the corrected difference between average winding temperature, corrected to shutdown, and the
average liquid temperature at shutdown,
T0 is the observed difference between average winding temperature, corrected to shutdown, and the
average liquid temperature at shutdown,
m is 0.8 for Class ONAN and ONAF and nondirected-flow Classes OFAF and OFWF,
m is 1.0 for directed-flow Classes ODAF and ODWF.


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The corrected average winding rise is the average liquid rise, measured during the total loss run, plus Tc.

8.7.5.2.2 Loading back method

Duplicate regulators may be tested by connecting their respective shunt and series windings in parallel
(Figure 16 and Figure 17). Apply rated voltage at rated frequency to one set of windings. Circulate load
current by opening the connections of either pair of windings at one point and impressing a voltage across
the break just sufficient to circulate rated current through the windings. Obtain top fluid rise as described in
8.7.2, then shut down and measure winding rise as in 8.7.3. When load current at other than rated frequency
is used, the frequency may not differ from rated frequency by more than 10%, and liquid rise shall be
corrected using one of the following methods.

a) By calculation. This method may be used when actual loss is within 20% of the required loss.

W n
T d = T b  ----  – 1
- (20)
 w

where

Td is the liquid rise correction in °C,
Tb is the observed liquid rise in °C,
W is the required loss (W),
w is the actual loss (W),
n is 0.8 for Class ONAN,
n is 0.9 for Class ONAF,
n is 1.0 for nondirected-flow Classes OFAF and OFWF and for directed-flow Classes ODAF
and ODWF.

Corrected liquid rise = observed liquid rise + T d (21)

Corrected winding rise = observed winding rise + T b (22)

b) By adjusting the losses. When the top liquid rise approaches a constant condition, adjust the excita-
tion voltage until the sum of the excitation loss and the load loss as measured during the temperature
test equals the required loss. Obtain top fluid rise as described in 8.7.2.

Figure 16—Example of loading back method: single-phase


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Figure 17—Example of loading back method: three-phase

8.7.6 Correction of temperature rises for differences in altitude

When tests are made at an altitude of 1000 m (3000 ft) or less, no altitude correction shall be applied to the
temperature rises.

When a regulator tested at an altitude of less than 1000 m (3300 ft) is to be operated at an altitude above
1000 m (3300 ft), it shall be assumed that the temperature rises will increase in accordance with the follow-
ing formula:

A
T A = T e  ----- – 1 F
- (23)
 A0 

where

TA is the increase in temperature rise at altitude A meters (°C),
Te is the observed temperature rise (°C),
A is altitude (m),
A0 is 1000 m,
F is 0.04 (self-cooled mode),
F is 0.06 (forced-air-cooled mode).

8.8 Short-circuit tests

8.8.1 Scope

This test code applies to liquid-immersed step-voltage regulators, both single- and three-phase.

The code deﬁnes a procedure by which the mechanical capability of a voltage regulator to withstand short-
circuit stresses may be demonstrated. The prescribed tests are not designed to verify thermal performance.
Conformance to short-circuit thermal requirements shall be by calculation in accordance with 8.9.4.


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The short-circuit test procedure described herein is intended principally for application to new regulators for
the purpose of design verification. Tests may be conducted at a manufacturer’s facilities, test laboratories, or
in the field; but it shall be recognized that complete equipment is not usually available in the field for con-
ducting tests and verifying results.

8.8.2 Test connections

8.8.2.1 Fault location

The short circuit should be applied on the voltage regulator regulated circuit terminals since this most
closely represents a system fault condition. The short circuit shall be applied by means of suitable low-
resistance connectors.

The tests may be conducted by either of the following:

a) Closing a breaker at the faulted terminal to apply a short circuit to the previously energized voltage
regulator.
b) Closing a breaker at the source terminal to apply energy to the previously short-circuited voltage
regulator.

8.8.2.2 Fault type

The type of fault to be applied will be dependent on the available energy source. Any of the following may
be used for three-phase regulators:

a) Three-phase source: three-phase short circuit.
b) Three-phase source: single-phase-to-ground short circuit.
c) Single-phase source: simulated three-phase short circuit.

NOTE—Apply the source to one primary circuit terminal. Apply the fault to the other two phases connected
together. (Alternatively, the source may be connected to two phases connected together and the fault be
connected to the output side of the remaining phase.)

d) Single-phase source: single-phase short circuit on one phase at a time. This applies to all single-
phase regulators.

8.8.2.3 Tap connection for test

One test satisfying the asymmetrical current requirement shall be made with the voltage regulator at the
maximum boost position and also at the maximum buck position. Two tests satisfying the symmetrical
current requirement shall be made at the maximum boost position and also at the maximum buck position.

8.8.3 Test requirements

8.8.3.1 Symmetrical current requirements

The rms symmetrical short-circuit current shall have a magnitude equal to 25 times the base current of the
voltage regulator. When specified for three-phase regulators rated 500 kVA per phase and below, the rms
symmetrical short circuit current shall have magnitude equal to 40 times the base rated load current or
20 000 A, whichever is less. When specified for single-phase regulators rated 500 kVA and below, the rms
symmetrical short-circuit shall have a magnitude equal to 40 times the base rated load current or 20 000 A,
whichever is less. The base rated load current is the rated self-cooled load current of the voltage regulator.


IEEE

8.8.3.2 Asymmetrical current requirements

The initial current shall have a 1.6 offset from zero, resulting in a maximum crest value equal to 2.26 times
the required rms symmetrical short-circuit current.

8.8.3.3 Number of tests

Each phase of the voltage regulator shall be subjected to a total of six tests. Four of these tests should satisfy
the symmetrical current requirements. Two additional tests on each phase shall satisfy the asymmetrical
current requirements.

8.8.3.4 Duration of tests

The duration of each test shall be 15 complete cycles of rated frequency current.

8.8.4 Test procedure

8.8.4.1 Fault application

To produce the fully asymmetrical current wave specified in 8.8.3.2, a synchronous switch should be used to
control the timing of fault application.

8.8.4.2 Calibration tests

Calibration tests to establish required source voltage or switch closing times should be made at voltage
levels not greater than 50% of the value that would produce the specified symmetrical short-circuit current.
For field testing, calibration tests should be made at reduced voltage levels, if possible. Tests with voltage
equal to or greater than that required to produce 95% of the specified symmetrical short-circuit current may
be counted toward fulfillment of the required number of tests.

8.8.4.3 Terminal voltage limits

When tests are to be made by applying the short circuit to the energized voltage regulator, the no-load source
voltage shall not exceed 110% of the rated voltage, unless otherwise approved by the manufacturer.

Throughout the course of any test, the voltage at the regulator primary circuit terminals shall be maintained
within a range of 95–105% of that necessary to produce the required symmetrical short-circuit current as
determined in 8.8.3.1.

8.8.4.4 Temperature limits

The top liquid temperature at the start of the test shall be between 0 °C and 40 °C.

8.8.4.5 Current measurements

Current magnitudes shall be measured in the low-resistance connection between the shorted regulated circuit
terminals and on the regulator primary circuit terminals connected to the energy source. The maximum rms
symmetrical current shall be established as half of the peak-to-peak envelope of the current wave, measured
at the midpoint of the second cycle of test current. The first cycle peak asymmetrical currents shall be mea-
sured directly from the oscillograms of the terminal currents.


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8.8.4.6 Tolerances on required currents

The measured currents, symmetrical or asymmetrical, in the tested phase or phases shall not be less than
95% of the required current.

8.8.4.7 Tap-changing switch operation

Upon completion of each of the required short-circuit tests, the tap-changing switch shall be operated from
the test position through the neutral position, and then back to the test position or on to the next test position.

8.8.5 Proof of satisfactory performance

The voltage regulator under test shall be judged to have performed satisfactorily if the visual examination
(8.8.5.1) and dielectric test (8.8.5.2) criteria have been satisfactorily met. Subclauses 8.8.5.3 through 8.8.5.7
list recommended terminal measurements that can be made during the course of the tests, but are not
required to be made unless specified. When the terminal measurements are made and the requirements of
8.8.5.3 through 8.8.5.7 have been met following all tests, it is probable that the voltage regulator has
sustained no mechanical damage during the test series. A composite evaluation of the degree to which all cri-
teria of 8.8.5.3 through 8.8.5.7 have been met may indicate the need for a greater or lesser degree of visual
examination to confirm satisfactory performance. The evidence may be sufficient to permit a judgment of
satisfactory performance to be made without complete dielectric tests. A decision to waive all or part(s) of
the visual inspection or dielectric test criteria shall be based upon discussion and negotiation of all parties
involved in specification and performance of short-circuit tests.

8.8.5.1 Visual inspection

Visual inspection of the core and coils shall give no indication that there has been any change in mechanical
condition that will impair the function of the voltage regulator. The extent of the visual inspection shall be
established on the basis of combined evidence obtained from the terminal measurements described in 8.8.5.3
through 8.8.5.7. When the terminal measurements give no indication of change in condition, external inspec-
tion of the core and coils removed from the tank may suffice. Any evidence of change in condition from
more than one of the terminal measurements warrants disassembly of the windings from the core for a more
detailed inspection.

Visual inspection of the tap-changing switch shall indicate no change that will impair the switch function.
The extent of this examination shall be established on the basis of operation through neutral after each test.
When the tap-changing switch operates successfully through neutral after each test, then an inspection of the
switch as assembled may suffice.

8.8.5.2 Dielectric tests

The regulator shall withstand standard dielectric tests in accordance with 8.6, at the full specification level
following the short-circuit test series. Impulse tests shall be made following the short-circuit test series only
when specified.

8.8.5.3 Wave shape of terminal voltage and current

No abrupt changes shall occur in the terminal voltage or short-circuit current wave shapes during any test.

8.8.5.4 Leakage impedance

Leakage impedance measured on a per-phase basis after the test series, and measured at pre-test tempera-
ture, shall not differ from that measured before the test series by more than 22.5%.


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8.8.5.5 Acceptable conditions

Small changes (less than 5% of amplitude or phase angle) occur following one of the short-circuit tests, but
no of amplitude or phase angle occurs following one of the short-circuit tests, but the trace returns to its
original shape on subsequent test.

8.8.5.6 Conditions requiring further investigations

Small changes (greater than 5% of amplitude or phase angle) occur after the first full amplitude short-circuit
test, and these changes continue to grow with each subsequent test.

8.8.5.7 Excitation current

Excitation current measured after the test series shall not increase above that measured before the test series
by more than 5% for stacked-type cores. For voltage regulators with wound-core construction the increase
shall not exceed 25%. The measuring equipment shall have demonstrated capability of giving reproducible
readings with a minimum accuracy of ±5%.

8.8.5.8 Other diagnostic measurements

Other diagnostic measurements may be made during the course of the tests to evaluate whether there have
been any sudden or progressive changes in the mechanical condition of the voltage regulator. Such results
may be useful to the understanding of the response to short-circuit forces, but they shall not form part of the
proof criteria.

8.9 Data

8.9.1 Reference temperature

The reference temperature for determining total losses, voltage regulation, and efficiency shall be equal to
the sum of the rated average winding temperature rise by resistance plus 20 °C.

8.9.2 Losses and excitation current

8.9.2.1 Determination of no-load losses and exciting current

No-load losses and exciting current shall be determined for the rated voltage and frequency on a sine-wave
basis unless a different form is inherent in the operation of the regulator.

8.9.2.2 Load losses

Load losses shall be determined for rated voltage, current, and frequency and shall be corrected to the refer-
ence temperature.

8.9.2.3 Total losses

Total losses are the sum of the no-load losses and the load losses.

8.9.3 Efficiency

The efficiency of a regulator is the ratio of its useful power output to its total power input, exclusive of
pumps, fans, and other ancillary devices.


IEEE

output ( input – losses ) losses losses
Efficiency =  --------------  = ------------------------------------ = 1 –  -------------  = 1 – -----------------------------------------
- - - - (24)
 input  input  input  ( output + losses )

When specified, efficiency shall be calculated on the basis of the reference temperature for the average wind-
ing temperature rise of the regulator. If pumps or fans are supplied, the power requirements shall be provided
as supplementary data.

8.9.4 Winding temperature during a short-circuit

The final winding temperature, Tf , at the end of a short-circuit of duration, t, shall be calculated on the basis
of all heat stored in the conductor material and its associated turn insulation. Tf shall not exceed the limiting
temperature in 5.8.3. All temperatures are in degrees Celsius.

T f = ( T k + T s )m ( 1 + e + 0.6m ) + T s (25)

where

W st
m = -------------------------------
- (26)
[C(T k + T s)]

Equations (25) is an approximate formula, and its use should be restricted to values of m = 0.6 and less.

For values of m in excess of 0.6, the following more nearly exact formula should be used:

2m 2m
T f = ( T k + T s )[ ε + ( eε – 1 ) – 1] + T s (27)

where


losses.

Ts is the starting temperature equal to
1) 30 °C ambient temperature plus the average winding rise plus the manufacturer’s recommended
hottest spot allowance, or
2) 30 °C ambient temperature plus the limiting winding hottest spot temperature rise specified for
the appropriate type of regulator.
e is the per unit eddy-current loss, based on resistance loss, Ws , at the starting temperature

Tk + Tr 2
e = e r -----------------
- (28)
Tk + Ts

where

er is the per unit eddy-current loss at reference temperature,
e is the base of natural logarithms, approximately = 2.718,
Tr is the reference temperature = 20 °C ambient temperature plus rated average winding rise,
Ws is the short-circuit resistance loss of the winding at the starting temperature, in watts per pound of
conductor material.


IEEE

2
W r N T k + T rs
W s = ------------- -------------------
- - (29)
M T k + T sr

where

Wr is the resistance loss of winding at rated current and reference temperature (W),
N is the symmetric short-circuit magnitude in times normal rated current,
M is the weight of winding conductor (lb),
C is the average thermal capacitance per pound of conductor material and its associated turn insula-
tion (W·s/°C). It shall be determined by iteration from either of the following empirical equations:

Ai
C = 174 + 0.225 ( T s + T f ) + 110 ----- for copper
- (30)
Ac

Ai
C = 405 + 0.1 ( T s + T f ) + 360 ----- for aluminum
- (31)
Ac

where

Ai is the cross-sectional area of turn insulation,
Ac is the cross-sectional area of conductor.

8.9.5 Certified test data

Minimum information to be included in certified test data includes the following:

a) Order data
1) Purchaser
2) Purchaser’s order number
3) Manufacturer’s production order number and serial number
b) Rating data
1) Type
2) Cooling class
3) Number of phases
4) Connections (delta, wye, etc.)*
5) Frequency
6) Insulation medium
7) Temperature rise
8) Winding ratings: voltage, voltampere, BIL, all temperature rise ratings specified, including
future ratings*
9) Harmonic factor if other than standard*
c) Test and calculated data (by individual serial number. If the results are from another regulator that
has been “design” tested, provide serial number, kV and kVA ratings, and date of test.)
1) Date of test
2) Winding resistances*
3) Losses: no-load, load and total
4) Impedances in percent (%)
5) Excitation current in percent (%)


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6) Thermal performance data*
i) Ambient temperature
ii) Tap position, total loss, and line currents for total loss run
iii) Oil flow in winding (directed and non-directed)
iv) Final bottom and top oil temperature rise over ambient for total loss run for each test
v) Average winding temperature rise over ambient for each winding for each test
vi) Calculated winding hottest spot temperature rise over ambient for maximum rating
7) Zero-sequence impedance*
8) Regulation (calculated)*
9) Applied voltage test values for each winding
10) Induced voltage test value
11) Impulse test data per IEEE Std C57.98-1993*
12) Sound level test results*
13) Short-circuit test results*
14) Ratio test results*
15) Polarity test results*
16) Other special test results*
d) Certification statement and approval.

NOTES
1—Items identified with * are not required for regulators unless specified by the user.
2—Number of significant figures of reported data should reflect the level of confidence of the data accuracy.
3—All temperature sensitive data should be reported after correcting to reference temperature (defined in 8.9.1).
4—Other significant information, such as tap position during induced potential test, test connection used, and any partic-
ular method used when alternatives are allowed, should be included.
5—Other drawings, such as nameplate and outline, may be made a part of certified test data in place of duplicating the
same information.

9. Control systems

9.1 General

The control system of a regulator is composed of sensing apparatus to provide signals proportioned to the
system voltage and load current, and a control device that interprets the output of the sensing apparatus,
relates this input to conditions desired by the operator, and automatically commands the regulator to
function to hold the output thereby required.

The total control system is usually furnished as a complete package with the regulator; however, the usual
stand-alone nature of the control device portion of the control system makes it appropriate to consider the
control system in a unified context.

9.2 Control device construction

9.2.1 Setpoint adjustment ranges

The control device shall permit parameter adjustment as follows:


IEEE

a) Voltage level setting adjustable from at least 108–132 V (related to line voltage by voltage supply
ratio as defined in Table 9).
b) Bandwidth setting adjustable from at least 1.5–3.0 V (total range).
c) Actuation time delay setting adjustable from at least 15–90 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
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. It is not required to provide
negative resistance and negative reactance compensation simultaneously.)

9.2.2 Components and accessories

The following components and accessories will be provided as part of the control device:

a) Test terminals for measuring voltage proportional to regulator output voltage. (The test terminal
voltage should 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. Extra burden is not included in the specification
of accuracy of the control relays.)
b) Manual-automatic control switch.
c) Manual raise/lower switch(es).
d) Neutral position indicator independent of the tap-changer position indicator.
e) External source terminals.
f) Internal/external power switch to allow operating the regulator from an external source. (This switch
shall not allow any regulator winding to be energized when in the external position and with an
external source connected to the external source terminals.)
g) Operation counter to indicate accumulated number of tap-changer operations;
h) Ability to withstand –40 °C to 85 °C control enclosure temperature without loss of control.
i) Band limit indication means.

9.3 Control system requirements

9.3.1 Accuracy

The control system of a regulator 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, and so forth, 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 and then add their accuracies together to arrive at the overall control system
accuracy. Accuracy tests are design tests, which are not made on every unit. The test voltage and current
signals should have a sinusoidal wave shape. No analytical correction is permitted to remove effects of har-
monics in the accuracy test results.


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9.3.1.1 Sensing apparatus

9.3.1.1.1 Voltage source

The voltage source accuracy shall be determined on a nominal secondary voltage base of 120 V and a burden
of 10 VA. Refer to IEEE Std C57.13-1993.

9.3.1.1.2 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 C57.13-1993.

9.3.1.2 Control device

The accuracy of the control device shall be determined 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 unity power factor and at zero power factor
lagging.

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 rms magnitude, which will affect the overall accuracy of the control device and
control system. Such differences are inherent in the product design and do not constitute an additional error in the
context of control accuracy.

9.3.1.2.1 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
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; the overall error causes a divergence from the voltage level setting presuming a band-
width of zero volts.

9.3.1.2.2 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 consti-
tute 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 voltage regulator).
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
3) Reactance compensation of 12 V and an in-phase base current of 0.2 A with resistance
4) Reactance compensation of 12 V and a 90° lagging base current of 0.2 A with resistance


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9.4 Tests

9.4.1 Design tests

9.4.1.1 Accuracy

9.4.1.1.1 Procedure for determination of accuracy of control device

This subclause outlines procedures for determining values of errors contributed by the factors described in
9.3.1.2.2. The voltage and current sources applied may be as free of harmonics or other distortions as the test
facility permits.

9.4.1.1.2 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 1 h using a 120 V source of rated frequency. The control is calibrated at this point. Errors
in voltage level in three tests below will determine the control device accuracy.

a) Tests for error in voltage level due to temperature. The control device shall be tested over a tempera-
ture 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 1 h 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 com-
pensation circuit.
c) Tests for errors in voltage level due to line drop compensation. Four tests shall be made at rated fre-
quency and a constant temperature of 25 °C and a voltage level setting of 120 V. Determine the volt-
age level required to balance the control with 0.2 A in the compensator circuit of the control under
the conditions specified in Table 18.

Table 18—Voltage-level values for select line drop compensation settings

Determine
Set Set
voltage error
Test LDC-R LDC-X Current phasing
relative to
(V) (V)
expected (V)
1 12 0 in-phase V = 132.0
2 0 12 in-phase V = 120.6
3 12 0 90° lagging V = 120.6
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
per 9.3.1.2.1.

9.4.1.2 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.


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9.4.1.2.1 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%.

9.4.1.2.2 Bandwidth marking deviation

The difference between the actual bandwidth voltage and the marked value shall not exceed ±10% of the
marked value set.

9.4.1.2.3 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%.

9.4.1.2.4 Time delay set marking deviation

The difference between the actual time delay and the marked value of any setting shall not exceed ±2 s or
±10%, whichever is greater, when initiated with no stored delay in an integrating-type circuit.

9.4.1.3 Surge withstand capability test

9.4.1.3.1 General

The surge withstand capability (SWC) test is a design test for the control device in its operating environ-
ment. In order to pass this test the control device and regulator shall continue to operate properly and not
cause any unintentional tap change during or after the test. Refer to IEEE Std C37.90.1-1989.

9.4.1.3.2 Devices to be tested

The devices that make up the control system vary depending on the requirements of the user. The require-
ments for SWC tests vary with the application. The following examples spell out certain speciﬁcs:

a) A regulator with a control mounted on the tank or on the pole at ground level below the regulator and
with no circuits except the power conductors and a cable from the regulator to the control. (The
control system would be both control and the regulator. Since there are no other control conductors
connected to the system, there is no SWC test required. The power system line conductor connec-
tions are otherwise covered and hence fall beyond this requirement.)
b) A regulator mounted as in item a) but with potential device, control, or metering conductors being
connected into the control. (These conductors would be subject to the SWC test requirements.)
c) A control mounted remotely with potential device, control, or metering connections to the control
device. (Here, all conductors connected to the control, including those to the regulator, would be
subject to the SWC test.)
d) If the foregoing examples do not adequately cover the application, the manufacturer and purchaser
shall agree on the connection scheme and the tests to be applied.


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9.4.2 Routine tests

9.4.2.1 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 prevent excessive damage or failure, use of a resistor to limit the current is suggested

9.4.2.2 Operation

All features of the control device and its peripherals will be operated and checked for veriﬁcation of proper
functioning. The control is also calibrated at this point.


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Annex A
(informative)

Unusual temperature and altitude conditions

A.1 Unusual temperatures and altitude service conditions

Regulators may be applied at higher ambient temperatures or at higher altitudes than speciﬁed in this stan-
dard, but performance may be affected and special consideration should be given to these applications.

A.2 Effect of altitude on temperature rise

The effect of the decreased air density due to high altitude is to increase the temperature rise of regulators
since they are dependent upon air for the dissipation of heat losses.

A.3 Operation at rated kVA

Regulators may be operated at rated kVA at altitudes greater than 1000 m (3300 ft) without exceeding
temperature limits, provided the average temperature of the cooling air does not exceed the values of
Table A.1 for the respective altitudes.

NOTES
1—See 4.3.2 for regulator insulation capability at altitudes above 1000 m (3300 ft).
2—Operation in low ambient temperature with the top liquid at a temperature lower than –20 °C may reduce dielectric
strength between internal energized components below design levels.

Table A.1—Maximum allowable average temperature of cooling air for carrying rated kVAa

1000 m 2000 m 3000 m 4000 m
(3300 ft) (6600 ft) (9900 ft) (13 200 ft)
Method of cooling apparatus
°C

Liquid-immersed self-cooled 30 28 25 23

Liquid-immersed forced-air-cooled 30 26 23 20
aIt is recommended that the average temperature of the cooling air be calculated by averaging 24 consecutive
hourly readings. When the outdoor air is the cooling medium, the average of the maximum and minimum
daily temperatures may be used. The value obtained in this manner is usually slightly higher, by not more than
0.3 °C, than the true daily average.

A.4 Operation at less than rated kVA

Regulators may be operated at altitudes greater than 1000 m (3300 ft) without exceeding temperature limits,
provided the load to be carried is reduced below rating by the percentages given in Table A.2 for each 100 m
(330 ft) that the altitude is above 1000 m (3300 ft).


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TableA. 2—Rated kVA correction factors for altitudes greater than 1000 m (3300 ft)

Types of cooling Derating factors (%)
Liquid-immersed air-cooled 0.4
Liquid-immersed water-cooled 0.0
Liquid-immersed forced-air-cooled 0.5


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Annex B
(informative)

Bushing and field dielectric tests

B.1 Tests on bushings

When tests are required on bushings separately from the regulators, the tests shall be made in accordance
with IEEE Std C57.19.00-1991.

B.2 Dielectric tests in the field

Field dielectric tests may be warranted on the basis of detection of combustible gas or other circumstances.
However, periodic dielectric tests are not recommended because of the severe stress imposed on the insula-
tion.

Where field dielectric tests are required, low-frequency applied voltage and induced voltage tests shall be
used. The line-to-ground or line-to-line voltage stress imposed shall not exceed 150% of normal operating
stress or 85% of full test voltage, whichever is lower. The duration of the tests shall be the same as that spec-
ified in 8.6.5 and 8.6.6 for applied and induced voltage tests, respectively.


IEEE
Std C57.15-1999

Annex C
(informative)

Bibliography
When the following standards are superseded by an approved revision, the revision shall apply.

[B1] Accredited Standards Committee C2-1997, National Electrical Safety Code® (NESC®).

[B2] ASTM D117-1989, Standard Methods of Testing and Speciﬁcations for Electrical Insulating Oils of
Petroleum Origin.

[B3] ASTM D3487-1988, Speciﬁcations for Mineral Insulating Oil Used in Electrical Apparatus.

[B4] IEEE Std 62-1995, IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus—Part 1: Oil
Filled Power Transformers, Regulators, and Reactors.

[B5] IEEE Std C57.104-1991, IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed
Transformers.

[B6] IEEE Std C57.131-1995, IEEE Standard Requirements for Load Tap Changers.

[B7] The IEEE Standard Dictionary of Electrical and Electronics Terms, Sixth Edition.


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