IEEE
C57.ll5-1991
(Redesignation of IEEE Std 754,
issued for trial UIW in May 1984)
IEEE Guide for Loading Mineral-Oil-Immersed
Power Transformers Rated in Excess of 100 MVA
(65 "C Winding Rise)
Sponsor
Transformers Committee
of the
IEEE Power E n g i " g Society
Approved March 21,1991
IEm3:dardSM
Abstract: This guide covers modern power transformers rated above 100 MVA, three-phase equivalent, and 65 "C rise. The general approach is tutorial. Four different types of loading, that is, normal life expectancy loading, planned loading beyond nameplate rating, long-time emergency
loading, and short-time emergency loading, are considered. The various effects of loading a
transformer in excess of its nameplate rating are discussed. Temperature and maximum loading
limitations are suggested.
Keywords: emergency loading, loading, mineral-oil-immersed power transformers, nameplate
rating, power transformers, temperature, transformers
The Institute of Electrical and Electronics Engineers, Inc.
345 East 47th Street, New York, NY 10017-2394, USA
Copyright 0 1991 by the
Institute of Electrical and Electronics Engineers, Inc.
All rights reserved. Published 1991
Printed in the United States of America
ISBN 1-55937-131-5
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documents.
A
Foreword
(This Foreword is not a part of IEEE C57.115-1991, IEEE Guide for Loading Mineral-Oil-ImmersedPower Transformers
Rated in Excess of 100 MVA (65 "C Winding Rise.)
This guide for loading mineral-oil-immersed power transformers rated in excess of 100 MVA
has been developed to cover modern power transformers rated above 100 MVA, three-phase equivalent, and 65 "C rise. I t was prepared t o make available the best engineering knowledge on subjects
of concern to the industry.
The general approach in this guide is tutorial. Four different types of loading
(1 Normal Life Expectancy Loading
(2) Planned Loading Beyond Nameplate Rating
(3) Long-Time Emergency Loading
(4) Short-Time Emergency Loading
are considered. The various effects of loading a transformer in excess of its nameplate rating are
discussed. Temperature and maximum loading limitations are suggested.
The equations given in Section 3 are from IEEE C57.92-1981and have been in general industry
use for many years. However, questions have now been raised on the accuracy of these equations,
and a task force has been set up within the IEEE Transformers Committee t o review the state of the
art in this area.
One of the major concerns in this guide is based on the evidence that a reduction in dielectric
strength may occur due to the formation of free gas bubbles a t temperatures above 140 "C in the insulated conductors. A potential risk could occur in the operation of the transformer above 140 "C.This
risk is a function of the amount of free gas generated because it can cause dielectric failure during
an overvoltage condition and possibly a t rated power frequency voltage. In the event of generation
of free gas within the insulation, dielectric strength will be degraded while the gas is present.
However, this fact does not necessarily indicate that any permanent dielectric degradation would
occur.
Tutorial appendixes covering gas evolution, ancillary parts, methods of calculating acceptable
loading levels, and philosophy applicable t o older transformers are included.
Comments on this guide are welcomed and should be addressed t o the
Secretary
IEEE Standards Board
445 Hoes Lane
P.O. Box 1331
Piscataway, NJ 08855-1331,
USA
At the time that this guide was approved, the members of the Task Force to Develop a Loading
Guide for Transformers Above 100 MVA of the Working Group on Guides for Loading within the
Insulation Life Subcommittee were as follows:
C . E. Mitchell, Secretary
D. H. Douglas, Chair
J. Aubin
L. Burnett
0.0. Chew
0. R. Compton
C. Crichton
J. C. Dutton
D. J. Fallon
J. A. Forster
C. M. Gardam
R. L. Grubb
C. C. Honey
R. C. Kiercn
J. J. Kunes
W. Lampe
C. Lindsay
J. W. Matthews
L. S. McCormick
W. J. McNutt
R. E. Minkwitz
P. Q. Nelson
R. A. Olsson
J. H. Ottevangers
B. K. Pate1
D. D. Perco
D. H. Ryder
T. Singh
F. W. Thomason
T. P. Traub
R. A. Veitch
R. J. Whearty
E.J. Yasuda
The following persons were members of the balloting group that approved this document for
submission to the IEEE Standards Board as a trial-use standard:
L. C. Aicher
J. Alacchi
D. J. Allen
B. F. Allen
R. Allustiarti
R. I. Alton
S. J. Antalis
E. H. Aieski
J. C. Arnold
R. Bancroft
G. M. Bell
P. L. Bellaschi
S. Bennon
J. J. Bergeron
J. V. Bonucchi
J. D. Borst
G. H. Bowers
F. J. Brutt
D. F. Buchanon
D. J. Cash
E. E. Chartier
0. R. Compton
F. W. Cook
J. Cockran
M. G. Daniels
R. C. Degeneff
J. E. Dind
D.H. Douglas
P. P. Falkowski
W. R. Farber
H. G. Fisher
J. A. Forster
S.L. Foster
M. Frydman
H. E. Gabel, Jr.
J. D. Douglass
D. A. Duckett
J. C. Dutton
J. K. Easley
E. C. Edwards
R. L. Ensign
C. G. Evans
C. M. Gardarn
D. A. Gillies
A. W. Goldman
J. C. Gorub
W. F. Grifford
R. L. Grubb
G. Gunnels
G. Hall
J. L. Harbell
J. H. Harlaw
T. K. Hawkins
F. W. Heinrichs
J. J. Herrera
K. R. Highton
P. J. Hoefler
C. C. Honey
E. L. Hook
E. J. Huber
C. Hurty
G. W. Iliff
R. G. Jacobsen
E. T. Jauch
D. C. Johnson
C. P. Kappeler
0. Keller
J. J. Kelly
A. D.Kline
T. S. Lauber
R. E. Liebich
H. F. Light
C. Lindsay
T. G. Lipscomb
L. W. Long
R. I. Lowe
M. L. Manning
H. B. Margolis
J. W.Matthews
L. S.McCormick
G. G. McCrae
C. J. McMillen
W.J. McNutt
S.P. Mehta
N. J. Melton
C. K. Miller
C. Millian
R. E. Minkwitz, Sr.
C. E. Mitchell
H. P. Moser
R. J. Musil
W. H. Mutschler
E. T. Norton
R. A. Olsson
J. H. Ottevangers
H. A. Pearce
D. A. Roach
L. J. Savio
R. L. Schmid
B. E. Smith
L. R. Smith
I. W. W. Stein
L. R. Stensland
R. B. Stetson
F. R. Stockum
D. Takach
A. L. Tanton
R. C. Thomas
F. W. Thomaeon
T. P. Traub
E. F. Troy
D. E. Trusx
R. E. Uptegraff, Jr.
S. G. Vargo
R. A. Veitch
J. P. Vora
J. W. Walton
S. A. Weinck
R. J. Whearty
A. Wilks
J. R. Woodall
W. E. Wrenn
A. C. Wurdack
D. A. Yannucci
At the time this recommended practice was published, it was under consideration for approval as
an American National Standard. The Accredited Standards Committee on Transformers, Regulators, and Reactors, C57, had the following members at the time this document was sent to letter
ballot :
Leo J. Savio, Chair
Organization Represented
Electric Light and Power Group
John G Gauthier,Secretary
Name of Representative
...................................................................................
P. E. Orehek
S.M. A. Rizvi
F. Stevens
J. Sullivan
J. C. Thompson
M. C. Mingoia (Alt.)
Institute of Electrical and Electronics Engineers ..................................................................
J. D.Borst
J. Davis
J. H. Harlow
L. Savio
H. D. Smith
R. A. Veitch
f l
National Electrical Manufacturers Association.. ..............................................................
G. D. Coulter
P. Dewever
J. D. Douglas
A. A. Ghafourian
K. R. Linsley
R. L. Plaster
H. Robin
R. E. Uptegraff, Jr.
P. J. Hopkinson (Alt.)
J. Nay (Alt.)
......................................................................................... F. A. Lewis
Underwriters Laboratories, Inc. ...............................................................................
W. T. O'Grady
US Department of Agriculture, REA ...................................................................................
J. Bohlk
US Department of Energy, Western Area Power Administration ........................................
D. R. Torgerson
US Department of the Interior, Bureau of Reclamation.. ....................................................
F. W. Cook, Sr.
US Department of the Navy, Civil Engineering Corps.. ......................................................
.H. P. Stickley
Tennessee Valley Authority
When the IEEE Standards Board approved this standard on March 21,1991, it had the following
membership:
Marco W.Migliaro, Chairman
Donald C. Loughry, Vice Chairman
Andrew G. Salem, Secretary
Dennis Bodson
Paul L. Borrill
Clyde Camp
James M. Daly
Donald C. Fleckenstein
Jay Forster"
David F. Franklin
Ingrid Fromm
*Member Emeritus
Thomas L. Hannan
Donald N. Heirman
Kenneth D. Hendrix
John W. Horch
Ben C. Johnson
Ivor N. Knight
Joseph L. Koepfinger*
Irving Kolodny
Michael A. Lawler
John E. May, Jr.
Lawrence V. McCall
Donald T. Michael*
Stig L. Nilsson
John L. Rankine
Ronald H. Reimer
Gary S. Robinson
Terrance R. Whittemore
Contents
r
SECTION
PAGE
1. Scope ......................................................................
7
2. General.....................................................................
2.1 References ..............................................................
2.2 Effect of Loading Beyond Nameplate Rating ....................................
2.3 Types of Loading and Their Interrelationship ....................................
2.3.1 Normal Life Expectancy Loading .......................................
2.3.2 Planned Loading Beyond Nameplate Rating ................................
2.3.3 Long-Time Emergency Loading..........................................
2.3.4 Short-Time Emergency Loading .........................................
2.3.5 Interrelationship of Loading Types ......................................
2.3.6 Risk Considerations..................................................
2.4 Temperature Limitations...................................................
2.5 Maximum Loading Limitations ..............................................
2.6 Information for User Calculations............................................
2.7 Voltage and Frequency Considerations........................................
7
7
7
8
-8
8
9
9
11
11
12
12
12
13
3. Calculation of Temperature .................................................... 13
3.1 List of Symbols.......................................................... 13
3.2 Temperature Determination Equations ........................................ 14
3.3 Equation Corrections ..................................................... 14
3.4 OilTimeConstant .........................................................
14
3.5 LoadLoss .............................................................. 15
3.6 ViscosityofOil .......................................................... 15
3.7 Aging of Insulation .......................................................
15
3.8 Approximating Ambient Temperature for Air-cooled Transformers.................. 18
3.8.1 Average Temperature ................................................ 18
3.8.2 Average of Maximum Daily Temperatures.................................
18
3.9 Basic Logic Diagram of Computer Program .....................................
18
4. Operation with Part or All of the Cooling Out of Service .............................. 18
4.1 For OA/FA and OA/FA/FA Transformers .....................................
18
4.2 For OA/FA/FOA and OA/FOA/FOA Transformers ..............................
18
4.3 For FOA and FOW Transformers ............................................
19
A
FIGURES
Fig 1 (a) Normal Life Expectancy Loading (b) Planned Loading Beyond
Nameplate Rating (c) Long-Time Emergency Rating (d) Short-Time
Emergency Rating .......................................................
Fig 2 Reciprocal of Absolute Temperature Scale.....................................
Fig 3 Logic Diagram for a Computer Program that Meets the Requirements
OutlinedintheGuide ....................................................
10
16
17
TABLES
Table 1 Coordination of Suggested Loading Type, Duration, and Temperature Range .........9
Table 2 Suggested Maximum Temperature Limits for the Four Types of Loading ............9
Table 3 Suggested Design Limits for New Transformers when Loading Information
isnotsupplied ........................................................
12
APPENDIXES
Appendix A Thermal Evolution of Gas from Transformer Insulation.(References) ...........21
Appendix B Effect of Loading Transformers Above Nameplate Rating on Bushings.
Tap Changers. and Auxiliary Components ...............................
24
0
IEEE Guide for Loading Mineral-Oil-Immersed
Power Transformers Rated in Excess of 100 MVA
(65 "C Winding Rise)
-
1.Scope
This guide covers general recommendations for loading mineral-oil immersed power
transformers manufactured in accordance
with IEEE C57.12.00-1987Ill' and having a
maximum nameplate rating in excess of 100
MVA (three phase) or 33 1/3 MVA (single
phase). Such transformers have an average
winding rise of 65 "C maximum and a hottestspot rise of 80 "C maximum a t rated load.
[31 IEEE C57.92-1981,IEEE Guide for Loading
Mineral-Oil-Immersed Power Transformers
Up to and Including 100 MVA With 55 "C or
65 "C Average Winding Rise.
2.2 Effect of Loading Beyond Nameplate
Rating. Application of loads in excess of nameplate rating involve some degree of risk. It is
the purpose of this guide to identify these
risks and to establish limitations and rules the
application of which will minimize the risks
to the fullest extent that present state-of-theart knowledge permits. While aging and longtime mechanical deterioration of winding
insulation have been the basis for the loading
of power transformers for many years, it is
recognized that there are additional factors
which may involve greater risk for transformers
of higher megavoltampere and voltage ratings.
The risk areas which should be considered
when loading large transformers beyond nameplate rating are broadly defined as follows:
(1)Evolution of free gas from insulation of
winding and lead conductors (insulated conductors) heated by load and eddy currents
(circulating currents between or within insulated conductor strands) may jeopardize
dielectric integrity. Since the free gas is generated in the insulation near the conductor,
where dielectric stresses are high, the risk of
electrical failure is increased. This is principally a temporary risk which lasts for the
period that the gas bubbles exist in the insulation. Following the period of overheating
the gas bubbles will dissolve in the oil and full
NOTE: When this guide is to be used for transformers
purchased prior to publication of the guide, the applicability of 2.5 and 4.3 should be checked with the
manufacturer. Also see Appendix D.
2. General
2.1 References
[ll IEEE C57.12.00-1987,IEEE Standard
General Requirements for Liquid-Immersed
Distribution, Power, a n d Regulating
Transformers.2
[21IEEE (257.91-1981,
IEEE Guide for Loading
Mineral-Oil-Immersed Overhead and PadMounted Distribution Transformers Rated
500 kVA and Less With 65 "C or 55 "C Average
Winding Rise.
The numbers in brackets correspond to the references listed in 2.1.
zIEEE publications are available from the Institute of
Electrical and Electronics Engineers, Service Center, 446
Hoes Lane, P.O. Box 1331, Piscataway, NJ 08856-1331,
USA.
7
IEEE
CS7115-1991
IEEE GUIDE FOR LOADING MINERAL-OILIMMERSED POWER
(6) Pressure build-up with bushings for currents above rating could result in leaking
gaskets, loss of oil, and ultimate dielectric
failure. See Appendix B for further discussion.
(7) Increased resistance in the contacts of tap
changers can result from a build-up of oil decomposition products in a very localized high
temperature region at the contact point when
the tap changer is loaded beyond its rating.
In the extreme, this could result in a thermal
runaway condition with contact arcing and
violent gas evolution. See Appendix B for
further discussion.
(8) Auxiliary equipment internal to the
transformer, such as reactors and current transformers, may also be subject to some of the
risk identified above. See Appendix B for
further discussion.
(9)When the temperature of the top oil exceeds 105 "C (65 "C rise over 40 "C ambient according to IEEE C57.12.00-1987[ll),there is a
possibility that oil expansion will result in an
oil loss through the pressure relief device. The
overflow may also create problems with the oil
preservation system.
dielectric integrity will retum unless partial
insulation breakdown, which may produce
carbonized tracking, occurs while the free gas
is present. See Appendix A for further discussion.
(2) Evolution of free gas from insulation adjacent to metallic structural parts linked by
electromagnetic flux produced by winding or
lead currents may also reduce dielectric
strength. Since these parts are not located in
the regions of high dielectric stress, evolved
free gas has to migrate to areas of the insulation system where dielectric stresses are at
more critical levels before they can have
significant impact on dielectric integrity. In
the course of migration, the free gas bubbles
tend to be dissolved into the oil. As a consequence, slightly higher hottest-spot temperatures may be tolerated for other metallic
parts than for insulated conductors. Any risk
of reduced dielectric integrity lasts essentially
for the period of free-gas evolution.
(3) Aging or deterioration of conductor insulation as a function of time and temperature is
the most widely recognized permanent effect
of power transformer loading. Aging is fully
described in IEEE C57.92-1981131. Over a relatively long period of time, there will be a cumulative degradation of the electrical and
mechanical properties of the conductor insulation, which could result in failure. Loss of life
calculations may be made as described in
IEEE C57.92-1981 131. One should recognize
that the calculated results may not be a s
conservative for transformers rated above
100 MVA as they are for smaller units, since
the calculation does not consider mechanical
wear effects, which increase with megavoltampere rating.
(4) Structural insulation materials also suffer
cumulative permanent degradation of mechanical properties as a function of timetemperature history. In addition, there is some
degree of temporary reduction of mechanical
capability at elevated temperatures. These effects are expected to be of major concern during periods of transient over-current (shortcircuit faults) when mechanical forces reach
their highest levels.
(5)Thermal expansion of conductors, insulation materials, or structural parts at high
temperatures may result in permanent defonnations which could contribute to mechanical or
dielectric failures.
/-
2.3 Types of Loading and Their Interrelationship. Loads in excess of nameplate rating
may be applied to transformers on four different time-temperature bases. An increased
risk is probable for each successive loading
with its attendant increased temperature and
duration. The applicable times and temperature ranges are shown in Table 1, maxi"
temperatures in Table 2, and examples of loads
which fall within these categories are illustrated
in Fig 1 and described in 2.3.5. The four load
types are described in 2.3.1 through 2.3.4.
2.3.1 Normal Life Expectancy Loading.
Normal life expectancy loading is loading
that may exceed nameplate rating provided the
maximum conductor hottest-spot and maximum top-oil temperatures permitted in IEEE
C57.12.00-1987C13 are not exceeded. Other
metallic temperatures reached under this
loading will not be of concern. This loading
may be continued indefinitely and is considered risk-free with respect to the risks outlined
in 2.2.
2.3;2 Planned Loading Beyond Nameplate
Rating. Planned loading beyond nameplate
rating results in either the conductor hottestspot or topoil temperature exceeding those
suggested in Table 2 for normal life expectancy
8
A
IEEE
TRANSFORMERSRATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
C57.115-1991
Table 1
Coordination of Suggested Loading Type, Duration, and Temperature Range
Type of Loading
Daily Duration in the
Specified Hottest-Spot
Temperature Range
(hour)
Insulated** Conductor
Hottest-Spot
Temperature Range
("C)
*
110-120
4
4
6
4
120-130
120-130
130-140
120-1 30
130-180
130-140
Normal life expectancy loading
Planned loading beyond nameplate rating
Long-time emergency loading
Short-time emergency loading
1
G
*Since loss-of-life is cumulative, transformers may be operated above 110 "C hottest-spot temperature (not to exceed 1 2 0 "C) for short periods provided they are operated for much longer periods at
temperatures below 110 "C.
**See 2.3.5 for explanations.
Table 2
Suggested Maximum Temperature Limits for the Four Types of Loading
Normal
Life
Expectancy
Loading
("C)
Planned
Loading
Beyond
Nameplate
Rating
("C)
Long-Time
Emergency
Loading
("C)
120*
130
140
180**
140
105
150
160
110
200
110
Short-Time
Emergency
Loading
("C)
~
Insulated conductor hottest-spot temperature
Other metallic hot-spot temperature (in contact and not in contact with insulation)
Top-oil temperature
110
*110 "C on a continuous 24 h basis.
**Operation at insulated conductor hottest-spot temperatures above 140 "C may cause gassing in
the solid insulation and oil. Gassing may produce a potential risk to the dielectric strength of the
transformer. This risk should be considered when the guide is applied.
loading, and is accepted by the user as a
normal, planned-for repetitive load.
Suggested conductor hottest-spot temperatures coordinated with daily durations are
presented in Table 1.
This loading occurs with no system outages
and some risk is associated with it as outlined
in 2.2.
2.3.3 LongTime Emergency Loading. Longtime emergency loading results from the pro
longed outage of some system element and
causes either the conductor hottest-spot or
the top-oil temperature to exceed those suggested for planned loading beyond nameplate
rating. This is not a normal operating condition, but may persist for some time. It is expected that such occurrences will be rare.
Suggested conductor hottest-spot temperatures coordinated with daily durations are
presented in Table 1. Top-oil temperature
should not exceed 110 "C at any time.
This loading results from a system outage
and is more severe than planned loading beyond nameplate rating. Both loss of insulation
life and risk are greater than under planned
loading beyond nameplate rating. It is expected
that there will be only two or three such occurrences in the life of a transformer.
2.3.4 Short-Time Emergency Loading. Shorttime emergency loading is an unusually heavy
loading brought about by the occurrence of
one or more unlikely events which seriously
disturb normal system loading and cause either
the conductor hottest-spot or top-oil tempera-
lEEE
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSED POWER
CS7116-1991
r'
('C)
40
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Fig 1
(a) Normal Life Expectancy Loading (b) Planned Loading Beyond
Nameplate Rating (c) Long-Time Emergency Loading (d) Short-Time Emergency Loading
10
I
i
1 0 1 2
OF THE D A Y
IEEE
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65"C WINDING RISE)
-
~
C57115-1991
again in the temperature range of 120 "C130 "C for approximately 2 h resulting in a
total of 4 h in the temperature range of
12O"C-13O0C for the 24 h period. The
hottest-spot temperature is allowed to exceed
120 "Cfor a maximum of 10 h.
Short-time emergency loading defines a loading condition wherein a transformer is so
loaded that its hottest-spot temperature is
in the temperature range of 12O0C-130"C
for a maximum of 4 h, in the temperature
range of 13O0C-180"C and back to 140°C
for 1 h, and in the temperature range of
13O"C-14O0C for a maximum of 6 h. The
characteristics of this type of loading are a
series of unlikely conditions on the transmission system (2nd or 3rd contingency), one
or two occurrences over the normal lifetime
of the transformer, each occurrence is 1 h
duration or less, and the risk is greater than
for long-time emergency loading. Figure l ( d )
illustrates a short-time emergency loading profile. This figure presents a temperature curve
that has leveled off for the day until about
4 pm when a system condition occurs which
loads the transformer so that its hottest-spot
temperature rises rapidly to 163 "C in 1 h.
Note that Fig l(d) presents a loading that is
within the spirit of the guide. On the rising
side of the curve the hottest-spot temperature
is in the range of 120 "C-130 "C for approximately 15 min. The hottest-spQt temperature
continues rising from 130°C toward 180°C
and back to 140 "C in approximately 1 h while
peaking at approximately 163°C. On the
descending portions of the curve, the hottestspot temperature is in the 13O"C-14O0C
range for approximately one-half hour and
once again in the temperature range of 120 "C130 "C for 2 h, resulting in a total of 2 h 1 5 min
in the temperature range of 120 "C-130 "C for
the 24 h period. The total time above 120 "C
is 3 h 30 min for this example as opposed to
the 11 h maximum permitted.
2.3.6 Risk Considerations. Normal life expectancy loading is considered to be risk free;
however, the remaining three types of loading
have associated with them some indefinite
level of risk. Specifically, the level of risk is
based on the quantity of free gas and voltage.
The presence of free gas as outlined in 2.2 can
cause dielectric failure during an overvoltage
condition and possibly at rated power frequency voltage. The Task Force has selected
ture to exceed the temperature limits suggested for planned loading beyond nameplate
rating. Acceptance of these conditions for a
short time may be preferrable to other altematives. It is expected that this type of loading
will be reduced to long-time emergency loading
in not more than 1 h.
Suggested conductor hottest-spot temperatures coordinated with daily durations are presented in Table 1. Top-oil temperature should
not exceed 110 "C at any time.
This type of loading, with its greater risk,
is expected to occur rarely.
2.3.5 Interrelationship of Loading Types.
Examples of hottest-spot temperature profiles
for the four types of loading defined in 2.3.1
to 2.3.4 are illustrated in Fig l ( a ) through l(d).
Note that for each higher temperature, a higher
risk loading condition can be assumed to be
additive to any lower risk condition accepted
by the user except for the short-time emergency loading. For further clarification a brief
description of the last three loading types are
described below.
Planned loading beyond nameplate rating
defines a condition wherein a transformer is
so loaded that its hottest-spot temperature is
in the temperature range of 12OoC-130 "C
for a maximum of 4 h daily. The characteristics
of this type of loading are no system outages,
regular and comparatively frequent occurrences, and there is some risk. Figure l ( b ) illustrates a planned loading beyond nameplate
rating profile.
Long-time emergency loading defines a condition wherein a transformer is so loaded that
its hottest-spot temperature is in the temperature range of 120 "C-130 "C for a maximum
of 4 h and in the temperature range of 130 "C140 "C for a maximum of 6 h daily. The characteristics of this type of loading are one long
time outage of a transmission system element,
two or three occurrences over the normal lifetime of the transformer, each occurrence may
last several months, and the risk is greater than
planned loading beyond nameplate rating.
Figure l ( c ) illustrates a long-time emergency
loading profile. Note that while the hottestspot temperature is increasing with time, it is in
the temperature range of 12O0C-130"C for
approximately 2 h. The hottest-spot temperature is in the temperature range of 130°C140 "C for 6 h. On the descending part of the
curve the hottest-spot temperature is once
11
lEEE
IEEE GUIDE FOR LOADING m R A L - O I L - m R S E D POWER
C67116-1991
the temperature levels and time durations shown
in Table 1for each of the defined types of loading which they subjectively believe will result
in an acceptable degree of risk for the special
circumstances which require loading beyond
nameplate rating. A scientific basis for the
users' evaluation of the degree of risk is not
available at this time. Current research in the
area of model testing has not established sufficient quantitative data on the relationship
between conductor temperature, length of
time at that temperature, and reduction in
winding dielectric strength. Additionally, there
are other important factors which may affect
any reduction, such as moisture content of the
winding insulation and rate of rise of conductor temperature.
2.4 Temperature Limitations. It is suggested
that the temperature limitations in Table 2
should not be exceeded during any overload
event.
2.5 Maximum Loading Limitations. M a x i "
load capacity which a transformer user plans
to utilize on a planned or emergency basis, or
both, should be included in the specifications
at the time of purchase. The following information should be given:
(1)Prior steady-state load, percent of maximum rating
(2) Ambient temperature, "C
(3)Maximum load, percent of maximum rating
(4) Duration of maximum load, hours
( 5 ) Type of loading, planned or emergency,
long-time or short-time, or acceptable limiting
temperatures
( 6 ) Acceptable loss-of-life, percent of normal
life, per occurrence
More than one set of loading conditions may
be defined or a 24 h load cycle (with associated
ambient temperatures) may be given.
In the absence of specified loading capabilities, the maximum load and duration limits
shown in Table 3 are suggested for transformers purchased after the publication of this
guide. These suggestions apply to loading beyond nameplate rating on a long-time emergency basis for generator step-up transformers
and a short-time emergency basis for system-tie
or substation transformers.
Other combinations of load, duration, prior
load, and ambient temperature may be used,
if the maximum loads shown in Table 3 and
the insulated condutor hottest-spot temperature limits of 2.4 are not exceeded. Tap
changers, bushings, leads, and other ancillary
equipment will not normally restrict loading
to levels below those given in Table 3, but
the user may wish to specify that ancillary
equipment not restrict loading to levels below
those permitted by the insulated conductor
and other metallic part hot-spots.
P
/--.
2.6 Information for User Calculations. If the
user intends to perform calculations to help
determine the loading capability of a transformer, the user should request information
of the following type in his specification:
(1)Top-oil temperature rise over ambient
temperature
(2) Bottom-oil temperature rise over ambient temperature
(3) Average conductor temperature rise over
ambient temperature
Table 3
Suggested Design Limits for
New Transformers when Loading Information Is Not Supplied
Maximum Load
(% of Maximum Rating)
ADDlication
(%I
Generation step-up
System tie
Substation
110
150
Duration*
(hours)
8
1
1
150
Insulated Conductor Maximum
Hottest-Spot Temperature
Not t o Exceed
("C)
140
180**
180**
*Based on prior load of 100%of rated and 30 "C ambient.
**This limit is based upon consideration of thermal aging and does not take into account the risk
of insulation breakdown because of bubble formation above 140 ',C. (See 2.2(1).)
12
,---.
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65"C WINDING RISE)
-
(4) Hottest-spot conductor temperature rise
over ambient temperature
(5) Load loss at rated load
(6) No-load loss
(7)Total loss in watts at rated load
(8) Confirmation of oil flow design (that is,
directed or nondirected)
For all of the information in (1)through (8)
state the conditions under which the measurements were made - load, ambient temperature, tap, etc.
2.7 Voltage and Frequency Considerations.
Voltage and frequency influences should be
recognized when determining limitations for
loading a transformer beyond its nameplate.
This is true even though in all probability there
may be little control of these parameters during a loading beyond nameplate event.
IEEE C57.12.00-1987113,4.1.6defines the capability of a transformer to operate above rated
voltage and below rated frequency.
The user of this guide should recognize that,
during conditions of loading beyond nameplate, the voltage regulation through the transformer may increase significantly (depending
on the transformer impedance) due t o the increased kilovoltampere loading and possibly
dropping power factor.
A conservative guideline, t o prevent excessive
core heating due to saturation, is to reduce the
transformer output volts per hertz limit by 1%
for every 1%increase in voltage regulation during the loading beyond nameplate event. For
example, if the voltage regulation at rated conditions is 6% and increases t o 9%at some load
above nameplate, the output volts per hertz
limit might be reduced from 105% to 102%.
3. Calculation of Temperature
Top-oil temperature and winding conductor
hottest-spot temperature may be calculated by
the user employing methods defined in IEEE
C57.92-1981131 if he or she wishes to assess the
acceptability of any given load cycle.
Before basing load capabilities on temperatures determined from temperature rise tests,
it is wise to consider the following information:
(1)Coolers for FOA and FOW transformers
are designed to have a safety margin when
tested in a new condition. This margin is called
Em
C57.115-1991
a fouling factor and allows for the fact that
cooler efficiency will decrease as the coolers
operate in normal service. For example, forced
air coolers can become partially blocked with
airborne dirt. If the coolers are not kept clean,
their efficiency decreases and oil temperature
rises. If load capability is based on test results
with new coolers, the transformer temperatures may be exceeded if the coolers are not
operating at their new condition efficiency.
( 2 ) The average winding rise is a temperature
calculated from the resistance value of the total
circuit. Within this circuit there will be bushings, insulated cables, and winding conductors
connected in series. Hottest-spot temperatures
of insulated cables may increase at a greater
rate than hottest-spot temperatures of the
windings. If load calculations are made to the
maximum winding hottest-spot temperatures,
heavily insulated cable temperatures may exceed winding temperatures by a considerable
amount.
(3) The load capability of ancillary equipment, such as tap changers, has little or no
bearing on temperature rise test results for
average winding rise. The proper operation
of such equipment is much more dependent
on current than temperature. Thus a lower
than guaranteed average winding rise does
not necessarily permit loading to greater
than designed into the ancillary equipment.
(4) Stray flux shielding, such as provided
by magnetic shunts, it designed to prevent
saturation under maximum load conditions
required by the purchaser. Greater loads
will cause the shunts to saturate allowing
hot-spots to form in cores and structural
members.
Hot-spot temperatures for other metallic
parts will not exceed the limiting values given
in 2.4 provided that:
(a) The maximum load values of 2.5 are
not exceeded and
(b) The top-oil temperature resulting from
the maximum load and duration condition of
2.5 is not exceeded.
3.1 List of Symbols. Unless otherwise expressed, all temperatures are in "C and all
times are in hours.
8, = ambient temperature
8 = hottest-spot conductor rise over top-
oil temperature
13
IEEE
C67116-1991
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSEDPOWER
Ultimate hottest-spot conductor rise over topoil for load L :
= hottest-spot conductor rise over top-
oil temperature at rated load
8 h E = hottest-spot winding temperature
0 = top-oil rise over ambient temperature
0 I = top-oil rise over ambient temperature
at rated load
ei = initial top-oil rise for t = 0
8 , = ultimate top-oil rise for load L
A P = change in total loss in watts because
of a change in kilovoltampere load
K = ratio of load L to rated load
L = load under consideration, in kilovoltampere
R = ratio of load loss at rated load to noload loss
e = 2.71828 (base of natural logarithm)
t = duration of load in hours
7, = oil thermal time constant (in hours)
of transformer for any load L and
for any specific temperature differential between the ultimate topoil rise and the initial top-oil rise, or
0
average conductor
rise over top oil 4 + 15 "C5
(Eq 5)
For nondirected flow transformers:
C = 0.06
-
(weight of core and coil assembly
in pounds) + 0.04 (weight of tank and
fittings in pounds) + 1.33 (gallons of oil)
-
For directed flow transformers:
n
C = 0.06
(weight of core and coil assembly
in pounds) + 0.06 (weight of tank and
fittings in pounds) + 1.93 (gallons of
Oil)
3.3 Equation Corrections. Theoretically, several corrections should be made when using the
foregoing equations in calculating transient
oil rises such as corrections for change in:
(1)Time constant for loads above rating
(2)Ultimate load loss at end of load period
(3)Oil viscosity
3.2 Temperature Determination Equations
(See Foreword, third paragraph.)
Hottest-spot temperature :
(Eq
3.4 Oil Time Constant. The oil time constant is
the length of time which is required for the
temperature of the oil to change from initial
value to the ultimate value if the initial rate of
change is continued until the ultimate temperature is reached.
Transient heating equation for top-oil rise over
ambient temperature:
(Eq 2)
~
At rated kilovoltampere from manufacturer's test.
In the absence of known values, the difference between the average conductor rise and the hottest-spot
conductor temperature rise can be assumed to be approximately 15 "C. However, one should recognize
that the actual temperature different may be higher
than 15 "C for some designs and lower for others.
Ultimate top-oil rise for load L:
e,=ef,(
I) =
Time constant a t rated kilovoltampere:
- Bi)/AP
7, = time constant (in hours) for rated
load beginning with initial temperature rise of O "C
P f I= total loss in watts at rated load
C = thermal capacity of transformer
(Wh/"C)
n = exponential power of total loss
versus top-oil temperature rise above
ambient
m = exponential power of winding loss
versus winding temperature rise
above oil
KzR+l
R+1
(
m = 0 . 8 for OA, OA/FA, OA/FA/FA and
nondirected FOA, FOW, OA/FA/FOA,
OA/FOA/FOA operation
= 1.0 for directed flow FOA, FOW operation
n = 0.8 for OA operation
= 0.9 for OA/FA, OA/FA/FA operation
= 1.0 for FOA, FOW, OA/FA/FOA,OA/
FOA/FOA operation
T, = C(0,
ehs=e,+eo +eg
(Eq 4)
0 , = O s ( f l )K2"
(Eq 3,
14
r'
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
If n (the exponent used in Eq 3) is equal to
1.0, 63% of the temperature change occurs
in a length of time equal to the time constant
regardless of the relationship of initial temperature rise and ultimate temperature rise. If n is
not unity, the temperature change in a similar
time interval will be different, depending on
both initial temperature rise and ultimate
temperature rise. For example, if n = 0.8, the
change is 67% if the initial temperature rise
B i = 0 "C and the applied load L = rated load.
If the initial temperature rise > 0, the change
< 67% and decreases as the initial temperature
rise approaches a given ultimate temperature
rise. If the initial temperature rise is approximately equal to the final temperature rise,
whether just above or just below it, the change
= 63%. If the initial temperature rise is greater
than the ultimate temperature rise, the change
< 63%.
Since evaluation of the exact change for
cases where n is not unity and where the initial
temperature rise is not zero becomes very
laborious, it is frequently advisable to use
the value of 63% as an approximation. In the
more frequently encountered cases where n
x 0.8, the error resulting from this procedure
is not large compared to the expected error
in the input data.
If n = 1.0, Eq 6 is correct for any load and
any starting temperature. If n or m < 1.0,
Eq 6 holds only for full-rating load L starting
cold. The time constant for any load and for
any starting temperature for either a heating
cycle or cooling cycle is given in Eqs 7 and 8
as follows:
(2)-($)
7,
= 7,
1
1
-
-
(2) ($)
-
If starting cold, as B i
=
0, Eq 7 reduces to:
3.5 Load Loss. Load loss in a transformer is
greater for loads above rating than K 2 in (Eqs 3)
IEEE
C57.115-1991
and (4) indicate due to the increased temperature of the winding conductors and their consequent increase in resistance. Some of the increased loss in the conductors due to temperature increase is offset by a relative decrease
in the conductor eddy current losses and losses
in other metallic parts of the transformer due
to stray magnetic flux. Therefore, if corrections to Eqs (3) and (4) are applied for increased conductor resistance, the exponents
m and n will usually be different and lower
than the recommended values. The manufacturer should be consulted when such a correction is considered since the correction is influenced by the proportion of the eddy and
stray losses in the load loss. The correction
factor for resistance change is a multiplier
applied to the K 2 expressed in each equation.
3.6 Viscosity of Oil. The ultimate temperature
rise of oil for a constant heat input decreases
slightly as the temperature of the oil increases.
This is due to a decrease in the viscosity of
the oil. The viscosity change tends to offset
the effect of increased ohmic resistance of
conductors.
3.7 Aging of Insulation. Aging or deterioration
of insulation is a function of temperature and
the time it is at this temperature. Since, in
most apparatus, the temperature distribution
is not uniform, that part which is operating
at the highest temperature will ordinarily
undergo the greatest deterioration. Therefore,
in aging studies, it is usual to consider the
aging effects produced by the highest (hottestspot) temperature.
3.7.1 Much of the fundamental data on aging of insulation at different temperatures has
been based on laboratory and model tests in
which the decrease in mechanical and electrical strength has been measured. Under the
auspices of the IEEE, a series of life expectancy
tests were made under controlled conditions
on production-type distribution transformers.
Data from these tests were used as the basis for
the distribution transformer life expectancy
curves in IEEE C57.91-1981C21. While these
are the best insulation life data available at
this time, they are not directly applicable to
power transformers. Because the end point
strength of insulation (the final strength of
aged insulation) must be greater in power
transformers and because model tests of power
15
IEEE
IEEE GUIDE FOR LOADING M I N E R A L - O I L - m m D POWER
CS7116-1991
106
1 o3
,-
1o'3
HOTTEST-SPOT TEMPERATURE IN *C
RECIPROCAL OF ABSOLUTE TEMPERATURE SCALE
Fig 2
Reciprocal of Absolute Temperature Scale
transformers have not yet been reported, the
power transformer insulation life expectancy
curves were selected to be more conservative
than those used for distribution transformers.
The transformer life expectancy predictions
contained herein are based on the insulation
life expectancy curves shown in Fig 2 and
do not take into account deterioration of
gaskets, rusting of tanks, etc which result
from factors other than winding temperatures.
While the life expectancy curves are be-
lieved to be conservative, any estimates of
transformer life derived therefrom are intended only as a guide.
3.7.2 Because the cumulative effects of
temperature and time in causing deterioration of transformer insulation are not
thoroughly established, it is not possible
to predict with any great degree of accuracy
the life span of a transformer even under
constant or closely controlled conditions,
much less under widely varying service conditions.
16
P
IEEE
C57.115-1991
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
READ IN SYSTEM VOLTAGE,
SPECIFIED MAXIMUM AGING.
SPECIFIED TEMPERATURE LIMITS
DAILY AMBIENT TEMPERATURE
INFORMATION,
TRANSFORMER CHARACTERISTICS
CALCULATE STEP-BY-STEP FOR ALL INTERVALS:
TOP OIL TEMPERATURE.
WINDING HOTTEST-SPOT TEMPERATURE
AND WINDING AGING FOR EACH OF THE
WINDINGS
ESTABLISH FOR EACH WINDING:
TOTAL AGING
HOTTEST-SPOT TEMPERATURE
TIME ABOVE TEMPERATURE LIMITS
TRANSFORMER
OF THE ABOVE VALUES
[F:Gkl&
UCE LOAD MULTIPLIER
44
EDUCE LOAD MULTIPLIER
*specify aging within tolerance criteria
Fig 3
Logic Diagram for a Computer Program
that Meets the Requirements Outlined in the Guide
17
IEEE
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSED
POWER
C67115-1991
recommended that these temperatures be increased by 5 "C since aging at higher than
average temperature is not fully compensated
by decreased aging at lower than average
temperature. With this margin the approximated temperature will not be exceeded on
more than few days per month and, where
it is exceeded, the additional loss of life will
not be serious.
3.7.3 This guide assumes the insulation deterioration relationship with respect to temperature and time follows an adaption of the Arrhenius reaction rate theory which states that
the logarithm of insulation life is a function
of the reciprocal of absolute temperature:
Log,
life (hours of life) =
-
13.391 +&
,( 6972 15
3.9 Basic Logic Diagram of Computer Program. Calculating a load level for a transformer,
which is at all times constrained by time/
temperature and maximum temperature limitations and that will yield a prescribed amount
of aging is an iterative procedure. A basic logic
diagram of such an iterative procedure for a
computer program incorporating the fundamental ideas in this guide is shown in Fig 3.
where
T = absolute temperature in "K
=
ehs + 273
where
O h s = temperature in "C at the hottest-spot at
the end of time interval t
100 ( t )
7% loss-of-life =
4. Operation with Part or
All of the Cooling Out of Service
10
Where auxiliary equipment, such as pumps
or fans, or both, is used to increase the cooling
efficiency, the transformer may be required
to operate for some time without this equipment functioning. The permissible loading
under such conditions is given in the following
paragraphs.
for the time interval t hours.
3.8 Approximating Ambient Temperature for
Air-cooled Transformers. It is often necessary
to predict the load which a transformer can
safely carry at some future time in an unknown ambient.
The probable ambient temperature for any
month may be approximated as follows from
reports prepared by the National Weather
Service of the US Department of Commerce
for various sections of the country:
3.8.1 Average Temperature. Use average daily
temperature for the month involved - averaged over a number of years.
3.8.2 Average of Maximum Daily Temperatures. Use average of the maximum daily
temperatures for month involved - averaged
over several years.
These ambients should be used as follows:
for loads with normal life expectancy, use
3.8.1 as the ambient for the month involved;
for short-time loads with moderate sacrifice
of life expectancy, use 3.8.2 for the month
involved.
During any one day the average of maximum
temperatures may exceed the value derived
from 3.8.1 or 3.8.2. To be conservative, it is
4.1 For OA/FA and OA/FA/FA Transformers.
For dual rated forced-air-cooled transformers
(OA/FA), if one or more fans are inoperative,
use the self-cooled (OA) rating.
For triple rated force-air-cooled transformers
with part of the fans inoperative, use the nameplate rating based on the full stage of cooling
remaining in operation, or if less than a full
stage of fan cooling is operative, use the selfcooled (OA) rating.
4.2 For OA/FA/FOA and OA/FOA/FOA
Transformers. For triple rated forced-air,
forced-oil-cooled transformers with all or part
of the cooling inoperative use the nameplate
rating based on the full stage of cooling remaining in operation, or if less than a full stage of
fan and pump cooling is operative, use the selfcooled (OA) rating. For loss of either fans or
pumps on a stage of cooling, use the rating
which pertains to total loss of that stage of
cooling.
18
--.-.
IEEE
C57.115-1991
TRANSFORMERSRATED IN EXCESS OF 100 MVA (66"C WINDING RISE)
-
-
eau = ultimate rise of average oil in "C
4.3 For FOA and FOW Transformers. In general, the heat exchangers used to cool FOA
and FOW type transformers will dissipate only
an insignificant amount of heat when either the
forced-oil circulation or the forced cooling
medium (air or water) is inoperative. If only
part of the coolers are inoperative, then refer
to 4.3.3 for load capability. If all of the coolers
are inoperative, loading amounts and durations
can be calculated as in 4.3.1.
The amount of load carried, the duration of
the load, the previous loading condition, the
ambient temperature, and the physical parameters of the transformer determine its hottestspot temperature and the loss-of-life experienced during the period of loss of all cooling.
The user should calculate in accordance with
the method below and refer to other sections
of this guide to determine the effects of the
operating condition.
During the period of loss of all cooling the
only significant amount of heat dissipated
by the transformer will depend on tank radiation and its convection characteristics which,
in tum, are dependent on tank dimensions.
Heat dissipation characteristics may be calculated from measurements obtained by measuring the actual unit or from estimations
based on the transformer outline drawings.
4.3.1 An approximation of the effect of
loading and time upon the oil and hottestspot temperature can be determined from
4.3.2. (More accurate data may be obtained
from the manufacturer.)
4.3.2 List of Symbols
A = sum of surface areas of tank walls
and cover neglecting braces, appurtenances, etc, square inches
C = thermal capacity as defined in 3.1
and 3.2 for nondirected flow mode
K = ratio of load to be carried to 100%
FOA nameplate rating
P = total losses at load to be maintained
minus losses that will be dissipated
by tank at f3a ,,watts
F = 2 for directed oil flow and 1 for
nondirected oil flow
T L = total losses in watts, at load to be
maintained
LD = losses dissipated by the tank at reference temperature of e a
eafl = average oil rise at maximum nameplate rating obtained from factory
test data
e,,
average oil rise of unit at instant of
loss of all cooling in "C
0 a t = average oil rise at time t
O g = hottest-spot rise above top-oil rise at
load to be maintained
m = exponential as defined in 3.2
0 g ( f l ) = hottest-spot conductor rise over topoil temperature at rated load
8 a = ambient temperature
e o = top-oil temperature rise "C
0 = hottest-spot temperature ,"C
=
(1)Estimate the losses in watts that will be
dissipated by the tank at the 100% FOA oil
rise after loss of all cooling.
LD
=
(0.00365) (A) ( B a f 1 ) 1 . 2 1 Watts (Eq 11)
(2) Estimate the ultimate rise of average oil
for the load that is to be maintained.
(3)The time constant corresponding to this
loading condition should be calculated as follows:
(4) The average oil rise at any time t for the
transformer in this operating mode can be
estimated from the following formula:
_ -t
eat = (eau -
eatl)
(I- e
7 L )+ o a f 1
(Eq 14)
(5) During the time period of t h L = 0 to
0.1 5, the difference between top-oil temperature and average oil temperature can be estimated as follows:
Ao, = 7t + 6
(t is in hours)
(Eq 15)
The estimated top-oil rise can then be determined as follows:
It is recommended that 0 ,
110 "C.
+
8, not exceed
NOTE: Estimates of top-oil rises at t/rL greater than
0.15 will have to be obtained from the manufacturer.
4.3.2.1 The hottest-spot rise above top-oil
temperature, for directed oil flow units, will
increase substantially when the forced-oil flow
is stopped. An estimate of this rise can be obtained from the manufacturer. On the premise
that some reasonable oil circulation will con19
IEEE
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSED POWER
(267115-1991
tinue by natural convection, a rough estimate
can be made as follows:
O g ( f l ) = (avg wdg rise
eg = eg(fl)~
2
"c
m
- Bail)
F+5
(Eq 17)
(Eq 18)
NOTE: The average winding rise and average oil rise
should be obtained from the certified test reports
for the maximum nameplate rating.
4.3.2.2 The hottest-spot temperature at the
load to be maintained can be estimated as follows:
eas = ea + e, + eg
(Eq19)
It is recommended that O h s not exceed 140 "C.
CAUTION: In using the above guidelines,
the following factors should be considered
during a loss o f cooling situation.
( 1 ) Much of the normal overload protection (overcurrent relay, etc) installed on a
transformer will be inadequate for this operating condition.
(2) The hottest-spot relay (for alarm and
in many cases trip), using the two input
parameters of phase current and top-oil
temperature, is calibrated to a hottest-spot
rise over oil with forced oil circulation in
the windings. It will indicate a temperature
many degrees lower than actual hottestspot temperature if there is no forced-oil
flow in the windings.
(3)If the transformer is o f directed-flow
design and pumps have been lost, it may be
necessary to hold top-oil temperature well
below normal to keep the hottest-spot
temperature within its limitation, since,
with drastically reduced oil flow, the
hottest-spot gradient is greatly increased.
Hence the top-oil temperatures must be
kept lower to stay within design hottestspot limitation.
,
4.3.3 For forced-oil cooled (FOA or FOW)
transformer ratings with part of the coolers in
operation, use the following reductions in permissible loading:
% of
Total Coolers
in Operation
100
80
60
50
40
33
Permissible
Load in 5%
of Nameplate
Rating
100
90
78
70
60
50
A
These permissible loads will give approximately the same temperature rise as full load
with all cooling in operation.
n
IEEE
C57115-1991
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
Appendixes
[Thee Appendixes are not a part of IEEE C67.116-1991, IEEE Guide for Loading Mineral-Oil-Immersed Power
Transformem Rated in Excess of 100 MVA (66 "C Winding Rise), but are included for information only.]
Appendix C provides assistance in calculating
loading capabilities for both existing transformers and for new transformers being specified; the methods described include hand calculations and computer programs.
Appendix D provides a philosophy in regard
to loading older transformers.
The Appendixes are intended for general and
tutorial information and should not necessarily
be applied for specific loading purposes.
Appendixes A and B provide additional reference material for a broader understanding by
the user in employing this guide.
Appendix A
Thermal Evolution of Gas from
Transformer Insulation - (References)
This paper extended the laboratory studies
on hot-spot conductor gassing reported in the
companion paper F-79-170-2 to full sized
coreform pancake coils representative of
550 kV, BIL power transformer construction.
The results of this study were conclusive indicating that the cellulose gassing mechanism
disclosed in F-79-170-2 would produce a
similar reduction in the impulse dielectric
strengths of a transformer as predicted from
th_e low frequency effects. Following are the
results of the impulse tests applied to separate
pancake coils at conductor temperatures representing a transformer before (90 "C) and during
(145 "C) overload.
Al. HEINRICHS, F. W. Bubble Formation in
Power Transformer Windings at Overload
Temperatures. Paper F-79-170-2 presented at
the IEEE Winter Meeting, Feb 4-9, 1979;
IEEE Transactions, vol PAS-98, no 5, Sept/
Oct 1979, pp 1576-1582.
The paper considers known gas evolving
mechanisms in oil, paper, and combinations
of both and their relation to the physics
of bubble formation. Thermal decomposition
is postulated as the most likely gas bubble
generating event during transformer overload.
Laboratory experiments were conducted with
transformer conductor insulation utilizing
various models designed to isolate the separate
effects of heat, power frequency electric stress,
and finally conductor heating combined with
electric stress typical of transformer designs.
The thermal model produced quantities of
visible gas bubbles which issued spontaneously from the paper pores at 130 "C-150 "C
conductor temperatures. Simulated conductor models with both thermal and electrical
stress demonstrated the simultaneous appearance of gas bubbles and partial discharge at
130 "C - 150 "C while at an average stress of
27.7 kV/cm. This work appears to provide
the first tangible evidence for limiting the
maximum allowable hottest-spot temperature
in power transformer loading guides.
Conductor temperature
Oil temperature
Mean breakdown
Sigma
90 "C
90 "C
118kV
15.2 kV
145 "C
90 "c
50kV
4.4 kV
~~
Minute pearl-like bubbles were observed issuing
from the conductor insulation at 145 "C.
A3. McNUTT, W. J., KAUFMANN, G. H.,
VITOLS, A. P. MacDONALD, J. D.Short-Time
Failure Mode Considerations Associated with
Power Transformer Overloading. Paper F79
695-8, IEEE Transactions, vol PAS 99, no
3 May-June 1980, pp 1186-1197.
Model assemblies representative of the
constructions used in power transformer disk
windings and heavily insulated leads were
tested over a temperature range from 25 "C
to 250 "C. Tests at 100 "C and higher involved
immersion oil temperature of 80 "C, with additional conductor temperature rise achieved by
A2. HEINRICHS, F. W., TRUAX, D. E, and
PHILLIPS, J. D. The Effects of Gassing During
Overloads of the Impulse Strength o f Transformer Insulation. Paper A79 434-2, presented
at the IEEE Summer Power Meeting, July
1979.
21
IEEE
C67116-1991
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSEDPOWER
n
0
100
200
300
HOT SPOT TEMPERATURE,'C
Fig A1
Sixty Hertz Breakdown Voltage in
Percent of 25 "C Strength of Dry Paper (< 0.5% H,O)
circulation of current through the conductors.
Dielectric tests of 60 Hz and of 1 min duration
were performed step-wise to breakdown after
the heating current had been flowing for
30 min. The results are summarized in Fig Al.
Results were also reported for a special test
apparatus designed to make visual observations
and quantitative measurements of gas evolved
from heated metal in oil, with or without cellulose in contact with the metal. Visible gas bubbles were observed at metal temperature of
150 "C when pressboard was present, whereas
bubbles were not visible to almost 350 "C with
only oil in contact with the hot metal. An
opinion was offered that "three factors make the
temperature of metallic hot-spot less critical
than conductor hottest-spots: reduced area
of heated insulation, tendency for no gas entrapment, and generally lower level of dielectric stress. As a consequence, it appears that
low level gassing associated with temperatures
of 175 "C-200"C for metallic parts may constitute no greater risk than a temperature
of 150 "C for insulated conductors."
It was observed that "three types of cellulosic insulation at levels of dryness which may
be anticipated in operating power transformers (0.2%-0.5% HzO by weight) evolved visible
gas bubbles at temperatures as low as 140 "C."
A4. KAUFMA", G. H.Gas Bubbles in Distribution Transformers, IEEE Trensaction, vol
PAS-96,no 5 Sept/Oct 1977, pp 1596-1601.
Distribution transformers were aged in a tank
with a viewing port over various time/temperature cycles. Simulated rain was then applied
to the tank of the loaded transformer. Visual
observations were made of the intemal transformer assembly before and after the application of simulated rain, and impulse tests were
conducted during the rain condition. It was
reported that prior to the application of simulated rain, bubbles were not visible at rated
load, but there were occasionaZ bubbles at
185% steady-state. This corresponds to 188 "C
hobspot temperature. Bubbles were also seen
approximately 10-15 min after the current was
increased from 175%-300%.
More detailed descriptions of the tests and
observations are contained in KAUFMANN,
G. H. ERDA Report CONS-2157-1, Sept
1976,Impulse Strength o f Distribution Transformers Under Load. This document reports
A
A
IEEE
C57.115-1991
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 “CWINDING RISE)
PERCENT OF NO-LOAD
IMPULSE BREAKDOWN VOLTAGE
120
%
100
80
60
40
20
0
0
20
40
60
.a0 100 120 140
160
HOT SPOT TEMPERATURE, ‘C
180
200
220
Fig A2
Impulse Breakdown Voltage of
Unaged Shell Type Transformers
As a Function of Hot-Spot Temperature
“The paper describes a new functional test
procedure for evaluating the impulse strength
of distribution transformers under servicerelated conditions, including rated voltage,
different current levels, and the effect of pre?
cipitation. The test procedure was applied
t o a representative sample of 54 commercial
distribution transformers having a 1 5 kV
class insulation system. Specimen transformers used in this project were taken from utility stockpiles. The results reported here
showed a significant decrease of impulse
strength with increasing load current and
with aging under laboratory conditions.”
Figure A2 presents a graph of impulse
breakdown voltage of unaged shell-type dis-
that “bubbles were observed during steadystate aging at 175% of rated current. Activity
was observed only occasionally, usually in the
form of single small bubbles at intervals of five
minutes or more.” 175% of rated current correspond approximately to 160 “C hottest-spot
temperature.
A5. KAUFMANN, G. H. Impulse Testing of
Distribution Transformers Under Load, IEEE
Transactions, vol PAS-96, no 5, Sept/Oct
1977, pp 1583-1595.
This reference reports a different phase of
work under the same ERDA contract as 4
ERDA E (49-181-2157. See also ERDA Report CONS-2157-1,Sept 1976.
23
IEEE
C57.115-1991
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSEDPOWER
a small number of sample coils demonstrated,
paper and enamel conductor insulation retained functionality after aging for time periods in excess of five times the life defined in
IEEE C57.92-1981, IEEE Guide for Loading
Mineral-Oil-Immersed Power Transformers
Up to and Including 100 MVA with 55 "C or
65 "C Winding Rise. Following this degree of
simulated loading, the insulation had suffered approximately a 10% reduction of dielectric strength. In addition, the following facts
related t o evolution of gas within the winding
insulation were demonstrated:
tribution transformers as a function of hot-spot
temperature.
A6. IEC 354-1972, Loading Guide for Oil-Immersed Transformem6
One of the limitations adopted in this guide
is a conductor hottest-spot temperature of
40 "C. This limitation is amplified by the
following note:
NOTE: It has been mentioned by various authors that
above 140 "C the Arrhenius law is not completely applicable, owing to accelerated deterioration effects, either because the formation of deterioration products is too fast for
them to be taken away by the oil, or because a gaseous
phase is started, sufficiently rapid to lead to oversaturation and the formation of bubbles that may endanger the
electric strength.
Temperature and pressure cycling with
a gas blanket oil preservation system
can lead to entrapment of free bubbles of
the blanket gas within winding and
lead conductor insulation. The result is
significantly reduced dielectric
strength of the insulation, which persists for long periods of time (weeks).
With a loading cycle maximum temperature of lSO"C, thermally evolved
free gas within the conductor insulation
degraded the dielectric strength. However, this strength reduction disappeared within one day after temperature
reduction.
A7. McNUTT, W. J., KAUFMANN, G. H.
Evaluation of a Functional Life Test Model for
Power Transformers, Paper 82 SM 343-2, presented at the IEEE Summer Power Meeting,
July 19,1982.
A physical model of a power transformer
winding was evaluated as a tool for establishing functional life characteristics. Test procedures similar to those described in IEEE
(257.100-1986, IEEE Standard Test Procedure
for Thermal Evaluation of Oil-Immersed
Distribution Transformers, were utilized. In
AppendkB
Effect of Loading Transformers Above Nameplate Rating on Bushings, Tap Changers, and
Auxiliary Components
B1. Bushings. Bushings are normally designed to limit the bushing hottest-spot t o a
105 "C total temperature a t rated current of the
transformer, assuming a top-oil temperature
of 95 "C. Operating the transformer beyond
nameplate current can result in bushing temperatures above 105 "C, and the bushing lossof-life will be a function of the actual temperature and time operating a t that temperature.
"The severity of loss-of-life in a bushing
can be lessened or minimized by installing
bushings which have nameplate ratings
greater than the transformer current ratings."
A number of factors that reduce the severity
of overloading bushings compared to transformer winding insulation include:
(1) Top-oil temperature may be well below
95 "C at rated transformer output
(2) Bushings are sealed units, preserving
their insulation and thermal integrity
(3) Bushing insulation is generally drier
(4) Bushing insulation is not significantly
stressed by fault-current forces.
61EC publications are available from IEC Sales
Department, Case Postale 131,3 rue de Varemb6, CH 1211,
Genbve 20, SwitzerlandSuisse. IEC publications are also
available in the United States from the Sales Department,
American National Standards Institute, 11 West 42nd
Street, 13th Floor, New York, NY 10036, USA.
24
-
IEEE
TRANSFORMERS RATED IN EXCESS OF 100 MVA (66% WININ" RISE)
Overload concerns include
(1) Intemal pressure build-ups
(2) Aging of gasket materials
(3) Unusual increases in power factor indicating t h e r m a l deterioration of
insulation
(4) Gassing due to hottest-spots in excess of
140%
(5) Thermal runaway due to increased dielectric losses at higher temperatures
To coordinate bushings and their transformers, the following limits are established:
-
Ambient air, 40 "C max
Transformer top-oil temp, 110 "C max
Max current, 2
rated current of
bushing
Bushing insulation hottest-spot, 150 "C
Bottom lead hottest-spot rise over
ambient air a t rated current, 80 "C
The importance of sizing the air end and oil
end terminals is noted.
B2.Tap Changers
B2.1 Tap Changers for DeEnergized Operation (TCDO) General. The current rating of
the TCDO is generally limited by the heating
of the contacts or the cable connections to the
switch. The ANSI or CSA standards do not
specify details on which a given rating is
based. However, a temperature rise limit of
20 "C for the contacts at rated throughput is used
by some manufacturers. I t is recommended
that for overload conditions the temperature of
the contacts not be allowed to exceed the hottestspot temperature of the winding.
Tap changers for de-energized operation are
also sensitive t o overloading. Although tapchanger contacts may have certain overload
capabilities when new, these capabilities may
decrease due t o a thin film build-up on the
contacts that occurs during normal service.
Once a contact reaches a critical temperature,
a thermal runaway condition can occur. The
contacts overheat and a deposit builds up
around the contacts, increasing contact resistance until it finally reaches a temperature
that will generate gas. As a minimum, this
will produce a gas alarm. As a maximum, the
gas may trigger a dielectric failure of the
transformer.
(37116-1991
B2.2 Load Tap Changers (LTC). The ANSI
standards presently do not cover details on
which a given rating of LTC is based. Some of
the North American manufacturers have been
using IEC standards as a guide or have complied with the requirements of this standard.
The Canadian standard CAN/CSA C88-M90
now refers t o IEC Pub 214, On-Load
Tap Changers. According t o IEC standards, the (current) rating of the LTC is based
on:
(1) Temperature rise limit of 20 "C for any
current carrying contact (in oil).
(2) Temperature rise limit of 200 "C for
transition resistors during half cycle
operations (uninterrupted operation
through the entire regulating range).
(3) Breaking capability test (interrupting
capability) a t twice the rated current and
kilovoltampere.
The overload capability of the LTC is derived from the breaking capacity of the switch.
Most LTC manufacturers would allow solitary
operation of LTC at twice rated current (and
kVA). The frequency of such operations, however, has not been defined, but is considered to
be in the order of a few operations per year,
which would normally be adequate for emergency loading conditions. For more frequent
and specific overload cycles, a switch with a
higher rating is recommended.
The wear of contacts and contamination of
oil increases rapidly with current. Higher
overloads on LTC will generally necessitate
more frequent maintenance of the switch.
IEC Pub 76-1 (1976), Part 1 , Power Transformers.
IEC Pub 214 (1989), On-Load Tap Changers.
IEC Pub 354 (1972), Loading Guide for OilImmersed Transformers.
IEC Pub 542 (19761, Application Guide for
On-Load Tap Changers.
B3.Bushing Type-CurrentTransformers. In
their normal location, bushing type current
transformers have the transformer top-oil as
their ambient, which is limited to 105 "C total
temperature a t rated output for 65 "C rise
transformers.
Loading the power transformer beyond
nameplate results in an increase in top-oil
IEEE
C57115-1991
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSEDPOWER
It may also be possible t o select higher current transformer ratios to reduce secondary
currents and thus increase the capability of the
current t r a n ~ f o r m e r . ~
temperature and an increase in secondary
current in the current transformer with an associated temperature rise.
A factor in reducing the severity of the current transformer overload is that the top-oil
temperature at rated transformer output maybe
well below 105 "C.
In cases where consideration of the loading
and top-oil temperature rise of the power transformer and the current in the current transformer indicate the possibility of excessive
operating temperatures in the current transformer, the manufacturer should be consulted
on the current transformer capability before
loading beyond nameplates. The capability of
bushing current transformers under operating conditions cannot necessarily be derived
from the rating factor.
-
B4. Insulated Lead Conductors. Within the
transformer, connections to tap changer and
line terminals and other internal connections
are made with insulated leads and cables. The
method of calculating the hottest-spot temperature for these leads is different from that employed for the windings. However, the same
hottest-spot limits apply equally for both windings and leads since similar insulating materials are normally used.
Table 1 of this guide thus refers to insulated
conductors, which are intended to include the
windings and leads.
Appendixc
Calculationsto show Methods ofDeterminingRatingsand SelectingTransformerSize
A transformer application problem usually
needs to answer the question: Is an available
transformer suitable for a given load cycle?
Calculations required to answer this question can be made by hand, or a computer program can be written to automate the
calculation.
This appendix will illustrate the determination of loading limits by a hand calculation
and the selection of a transformer, using a
computer program.
It should be noted that the purpose of the illustration is to show one way that might be used to
approach the problem. As in most engineering
problems, different approaches are possible,
and judgement must be used in interpreting
the results.
an Existing Transformer. For this example, a
65 "C rise triple rated, OA/FOA/FOA cooled
transformer, rated 112 OOOLL49 333/186 666 kVA
will be used. A load profile is given (see Table
C1, normal load in per unit) for a day starting
a t 6:OO A.M.; the hottest-spot winding temperature profile is required. Some simplifying
assumptions will be made t o make the
calculation easier.
The first assumption is that maximum cooling will be used throughout the day, even
though a t the lowest part of the load cycle, the
loading will be less than the intermediate rating. The assumption may be optimistic; on the
o t h e r h a n d , when loading beyond
'IEEE C57.13-1982,
IEEE Standard Requirements for
Instrument Transformers.
C1. Hand Calculation Determining Rating of
26
-
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
I,"
v)
8
@d
8
0
m
m
9
m
c
M
z
a
-
4
0
m
27
EEE
C67115-1991
IEEE
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSED
POWER
CS7116-1991
( -L) +
nameplate rating is applied on purpose, it is
not unreasonable to assume that every measure is taken to assist the transformer, including the use of maximum cooling
throughout the day.
The second assumption is that the oil timeconstant correction will be ignored; refinements of this nature are more appropriate
for a computer program.
The third assumption is that the load is
kept constant during the following hour.
For the rising part of the load curve this
assumption will give loads that are too low,
but on the falling part of the load curve loading values that will be too high. It is possible
to refine the load representation when there
is need.
Lastly, the ambient temperature is considered constant during the day.
The transformer characteristics at 187 MVA
are :
Top-oil rise over ambient at rated load,
epI= 36.0 "c
Hottest-spot conductor rise over top-oil
temperature, e,,,,, = 28.6 "C
Ratio of load loss at rated load to no-load
loss, R = 4.87
Oil thermal time constant for rated load,
7, = 3.5 h
Exponential power of loss versus top-oil
rise, n = 1.0
Exponential power of winding loss versus
winding gradient, m = 1.0
The ultimate top-oil rise over ambient for
load K will be, according to 3.2, Eq 3,
B o = (29.87K2 + 6.13) 1 - e
eo
and for t
eo
K
=
(e,
- e i ) (1 - e
e
(Eq (33)
2
=m 28.6 ~2
(Eq C4)
= .\/[(0.81)2 i(0.68)2 +
+ (0.55)'
(0.61)2 + (0.58)2
+ (0.53)2]/6
= 0.634
Using C4 a load of this magnitude yields
an ultimate top-oil rise of
e,
= 29.87 ~2
+ 6.13
= 29.87(0.34)2 + 6.13
=
18.14 "C
Using B i = 18.14 at 6:OO am, and
K = 0.52
we can determine 0 ,at 7 :00 am as follows:
eo
eo
(EqC1)
After 1 h the top-oil temperature rise will be,
Eq 2.
e,
= 3.98 ~2 + 0.82 + 0.87
One quantity, the initial top-oil rise, is still
missing and we will have to estimate it.
If we assume the load cycle for Normal Load
found in Table C1, we may establish the rms
value of the load curve, as an example, for the
6 h load preceding 6:OO am.
(4.87) 1
4.87+1
= 29.87K2+ 6.13
(Eq C2)
0.5 h
eg = egtn, ~
1.0
=36.0
=
1.53 + 0.75 ei
tl.e-=
I
The winding gradient will be according to 3.2,
Eq 4,
eu=6fl(Rtl)l
[K 2 + ]
K2R'1
= 7.42 K Z +
3.5
- t
G)+ ei
or rewritten
- -t
=
7.42 ~2 + 1.53 + o.75ei
= 17.14
"c
To determine the top-oil temperature rise at
8:OO am set Bi = B o , calculated at 7:OO am. Repeated application of Eq C2 will produce a
top-oil temperature rise profile; however, a
slight discrepancy occurs 24 h later at 6:OO
am. When one continues to apply Eq C2, convergency to true values is soon obtained, as
shown in Table C1, normal load, 8 , columns.
The first column represents the first iteration
and the second column represents the results
after an additional iteration.
The winding gradient 8 , is considered to be
instantaneous. Only where current discontinuities occur will some consideration be given
t o the winding time-constant.
At 6:OO am
e g = (28.6) (0.52)2
When we substitute 7 , = 7,. = 3.5 and the flu
value of Eq C1, we obtain for t = 1 h
=
28
7.73 "C for example.
IEEE
C57115-1991
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
0
ehs P L B N
C
x
130
-
120
-
110
-up:;
- - x-
- ehs
-K-
NORMAL LOADING
LOAD PROFILE
-o---o--
t
100 - 1.0
90 - 0.9
80 - 0.8
70 - 0.7
60
- 0.6
I
I
6
I
8
I
I
10
J
,
12
,
lb
I
16
I
I
18
I
I
20
I
I
22
I
I
24
I
I
2
>
*-
I
I
4
I
I
6
HOURS
Fig C1
Planned Loading Beyond Nameplate
In 3.2, Eq 1,
ohs
= e o + eg + ea,
will be used to establish the hottest-spot winding temperature 8 h s, using for ambient temperature 8, = 30.0 "C.
A complete daily normal load cycle is presented in Table C1 and shown in Fig C1.
C2. Planned Loading Beyond Nameplate
(PLBN). The constraint on PLBN loading is
hottest-spot winding temperatures in the 120130 "C range for a maximum of 4 h; therefore,
0, + Og = 120 "C - 6, = 90 "C.The three highest
temperatures for the normal loading cycle are
just over 90.3 "C; therefore, eo+ 6, = 60.3"C.
To estimate what load multiplier K should
be to produce B o + O s = 90 "C, we proceed
as follows:
K 2 R + l --90
R + 1 60.3
K = 1.26
The top-oil rise is not quite proportional
to the square of the load current (no-load
losses are constant) but the winding gradient
is proportional to the square of the load current. The multiplier may have to be corrected
if it is unsatisfactory. Again we have to estimate an initial top-oil temperature. Following
the same procedures as for the normal load,
we obtain a temperature profile, based on the
load cycle given in Table C1 under PLBN and
shown in Fig C1. The hottest-spot temperature
is in the 120 "(3-130 "C for close to 4 h.
C3. Long-time Emergency Loading (LTE).
A user has to consider carefully the emergency
IEEE
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSED POWER
CS7115-1991
? l 8 O1l 7 0
160
-
150-
140-
130-
'ER
JNIT
t
120- 1.6
1 1 0 - 1.4
100- 1.2
90-
1 .o
8 0 . 0.8
LOAD
70
0.6
60 -
1
6
1
1
8
1
1
10
1
1
12
1
1
'14
1
1
1
16
1
18
1
1
1
20
1
1
22
1
24
1
1
2
1
1
4
1
1
6
+ HOURS
Fig C2
Hottest-Spot Temperature Profile
Repeated application of Eq C2 finally gives
at 19:OO h an equation for 8, and O g in terms
of K2 :
loading conditions that may occur on his system. A maximum period of 6 h is used in our
example. Assume that the long time emergency
begins at 13:OO h, and was preceded by a
PLBN loading. Suppose a load multiplier of
value K2 is applied.
At 13:OO h 8, = 33.90 "C which is equal to
8, in the PLBN loading;
apply Eq C2 to find
A t 1 4 : 0 0 h 8 , = (7.42 0.93 K g ) + 1.53 +
(0.75 34.14)
= 6.42 KE + 27.14
e, = (28.6 0.96 K;) = 26.36 K ;
At. 15:OO h e , = (7.42
0.96 K g ) + 1.53
+ (0.75) (6.42 Kg + 27.14)
e, = 11.66 K; + 21.89
e, = 2 7 . 4 7 ~ ;
e, + eg
= 51.25 K;
+ 11.11
The LTE restraint is 140 O C , thus 51.25 K; +
11.11 + 30.0 = 140.0 and K2 = 1.39.
Table C1 shows the top-oil rise and the winding gradient. At 13:OO h and at 19:OO h there is
a discontinuity in current. The winding timeconstant usually is in the order of 3-5 min.
After 20 min 8, will be according to the new
load. Figure C2 shows the hottest-spot temperature profile. The 140 "C temperature limitation
has been met. The hottest-spot temperature is
less than the permissible 6 h in the 130 "C140 "C range but longer than 4 h in the
30
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65"CWINDING RISE)
-
-
IEEE
C57115-1991
sults of the four load cycles, Tables C3, C4,
C5,and C6 show the following results.
(1)A normal load cycle - meets the load requirement, is below daily loss-of-life of
0.0369%, and temperature at hottest-spot
is not in excess of the 120 "C limit.
(2)Planned loading beyond nameplate - for
the required loading condition, the 250 MVA
transformer's
calculated hottest-spot 165.3 "C, significantly exceeds the limiting
hottest-spot of 130 "C.
(3)Short Time Emergency Loading - for
the required loading condition, calculated
hottest-spot - 188.5 "C exceeds the guide
limit of 180 "C.
(4)Long Time Emergency Loading - for
the required loading condition, calculated
hottest-spot - 169.4 "C exceeds the guide
limit of 140 "C.
120 "C-130"C range, so a value of 1.39 applied to the per unit load from 13:OO-19:OOh
seems t o be in order.
C4. Short Time Emergency Loading (STE).
In our example an STE is assumed to occur
at 13:OO h, following a PLBN loading. After
one half hour, the load is reduced to the LTE
loading which will persist for 5.5 h.
We will use an interval load value K , . The
STE constraint is a maximum hottest-spot
temperature of 180 "C.
At 13:OO h B i = 34.14 "C
Apply Eq C3 (for t = 0.5h)to obtain at 13:30h
e o = 3.98K; + 0.82 + (0.87)(34.14)
eg = 28.6K ;
ehs = e o + eg + ea
= 32.58Kg + 30.52 + 30.0
= 180.0
K , = 1.92
At 13:30 h B o = 45.19,8 , = 105.43,O h s =
180.6,load = 1.29 per unit
At 14:OO h B o = (3.98) (1.29)2 + 0.82 +
(0.87)(45.19)= 46.76
Figure C2 shows the temperature excursion
to be within the limits for the STE loading.
The hottest-spot temperature will be somewhat
longer in the 130 "C-140"C range but will be
well within the 6 h limit.
Conclusion: 250 MVA unsatisfactory for the
required loading conditions.
C5.2 Second Test - 275 MVA. Tables C7,
C8, C9,and C10 show the following results of
the four-load cycles with a 275 MVA transformer:
(1)Normal load cycle - meets load requirement, is below daily loss-of-life requirement,
and maximum hotteat-spot temperature 107.3 "C is below the guide limit of 120 "C.
(2)Planned loading beyond nameplate - the
275 MVA transformer maximum hottestspot of 145.2"C exceeds the limit of 130°C.
(3)Short time emergency loading - the
maximum hottest-spot of 164.8"C is within
the guide limit of 180 "C.
(4)Long time emergency load - the maximum hottest-spot of 148.7 "C exceeds the
limit of 140 "C.
Since the guide limits are exceeded for
planned loading beyond nameplate, the
275 MVA size is inadequate. Tests as shown
in Tables C11, C12, C13, and C14 were run
for a 300 MVA transformer which produced
results for conditions C5.2(1), (2), (3) and
(4) which meet the requirements. This, then,
is the transformer required to satisfy load
needs and be within guide limits.
C5. Computer Calculation Method. (Determining required transformer rating to meet a set of
stipulated loads.)
Input Data
(1)Ambient temperatures. See output format
for hourly ambients used - Tables C3 through
C14.
(2)MVA load values for various load conditions. See Table C3 through C14.
(3)Voltages of system in this example are
138 kV and 69 kV (high voltage and low voltage), and typical estimated values of transformer constants required in the calculation
are given in Table C2.
(4)Data is prepared and computer programs
run to determine whether the transformer
selected meets the loading requirements and
limiting conditions of loading within the guide.
C5.1 First Test - 250 MVA. The first test
considers a 250 MVA rated transformer. Re-
31
IEEE
IEZE GUIDE FOR LOADING MINERAL-OIL-IMMERSEDPOWER
C67116-1991
Table C2
Typical Estimated Values of Transformer Constants Required in Calculations
MVA
250
Weight of transformer core and coil - lb
Weight of tank - lb
Volume of oil - gallon
Total transformer loss at full load - watt
Loss ratio at rated load/no load loss - factor
Rise of winding by resistance - "C
Top fluid rise - "C
m value
n value
275
344 000
150 000
22 575
1 160 000
9.0
60.9"
42.7"
0.8
1.0
378
165
24
1280
300
000
000
900
000
9.0
60.9"
42.7"
410 000
180 000
27 000
1 4 0 0 000
9.0
60.9"
42.7"
0.8
1.0
0.8
1.0
Table C3
Transformer 250 MVA 138 - 69 kV
Normal Life Expectancy Loading
Hour
Load
MVA
Top Oil
Rise
Ambient
Temperature
26.00
24.76
26.00
19.89
26.00
16.87
24.00
15.01
23.00
13.86
23.00
14.41
25.00
16.32
27.00
19.24
29.00
23.10
30.00
30.11
31.00
37.10
33.00
41.42
35.00
44.09
35.00
45.75
35.00
46.77
35.00
47.41
35.00
47.80
35.00
48.04
34.00
45.54
33.00
42.75
31.00
41.03
30.00
38.77
31.00
36.25
29.00
31.57
Total loss of life for 24 hours is 0.0288
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
134.00
112.00
112.00
112.00
112.00
134.00
157.00
179.00
202.00
246.00
268.00
268.00
268.00
268.00
268.00
268.00
268.00
268.00
246.00
235.00
235.00
224.00
213.00
179.00
32
Hottest-Spot
cu
63.00
55.08
52.06
48.20
4 6.04
49.65
57.09
65.69
75.70
92.46
105.20
111.52
116.20
117.85
118.88
119.51
119.90
120.15
111.89
105.82
102.10
96.62
92.95
80.02
Loss of Life
%
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0002
0.0008
0.0018
0.0027
0.0033
0.0037
0.0042
0.0042
0.0042
0.0018
0.0009
0.0006
0.0003
0.0002
0.0
Em
C57115-1991
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
Table C4
Transformer 250 MVA 138 - 69 kV
Planned Loading Beyond Nameplate Rating
Hour
Load
MVA
Top Oil
Rise
Ambient
Temperature
134.00
25.95
26.00
2
112.00
20.62
26.00
3
112.00
26.00
17.33
4
112.00
15.29
24.00
5
112.00
14.03
23.00
6
134.00
14.52
23.00
7
157.00
16.39
25.00
8
179.00
19.28
27.00
9
202.00
23.12
29.00
10
246.00
30.12
30.00
11
268.00
37.10
31.00
12
268.00
41.42
33.00
13
268.00
44.10
35.00
14
268.00
45.75
35.00
15
348.00
58.33
35.00
16
348.00
66.11
35.00
17
348.00
70.92
35.00
18
348.00
73.90
35.00
19
246.00
61.54
34.00
20
235.00
52.65
33.00
21
235.00
47.15
31.00
22
224.00
42.56
30.00
23
213.00
38.60
31.00
24
179.00
33.02
29.00
Total loss of life for 24 hours is 0.7632
1
Hottest-Spot
CU
Loss of Life
64.19
55.81
52.52
48.48
46.22
49.76
57.16
65.73
75.73
92.48
105.21
111.53
116.20
117.86
149.69
157.47
162.28
165.26
127.89
115.72
108.22
100.41
95.29
81.47
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0002
0.0008
0.0018
0.0027
0.0033
0.0769
0.1429
0.2353
0.2857
0.0091
0.0027
0.0011
0.0004
0.0002
0.0
%
Table C5
Transformer 250 MVA 138 - 69 kV
ShortrTime Emergency Loading
Hour
Load
MVA
Top Oil
Rise
Ambient
Temperature
134.00
26.26
26.00
2
112.00
20.82
26.00
3
112.00
17.45
26.00
4
112.00
15.36
24.00
5
112.00
14.07
23.00
6
134.00
14.55
23.00
7
157.00
16.41
25.00
8
179.00
19.29
27.00
9
202.00
23.13
29.00
10
246.00
30.13
30.00
11
268.00
37.11
31.00
12
268.00
41.43
33.00
13
268.00
44.10
35.00
14
268.00
45.75
35.00
15
348.00
58.33
35.00
16
348.00
66.11
35.00
17
348.00
70.92
35.00
18
400.00
83.02
35.00
19
246.00
67.18
34.00
20
235.00
56.14
33.00
21
235.00
49.31
31.00
22
224.00
43.90
30.00
23
213.00
39.43
31.00
24
179.00
33.53
29.00
Total loss of life for 24 hours is 2.3053
1
*Denotes temperature has exceeded limit.
33
Hottest-Spot
cu
64.50
56.00
52.64
48.55
46.26
49.79
57.18
65.74
75.74
92.48
105.21
111.53
116.20
117.86
149.69
157.47
162.28
188.45
133.54
119.21
110.38
101.75
96.12
81.99
Loss of Life
%
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0002
0.0008
0.0018
0.0027
0.0033
0.0769
0.1429
0.2353
1.8182*
0.0172
0.0037
0.0014
0.0006
0.0003
0.0
IEEE
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSED POWER
C57115-1991
Table C6
Transformer 250 MVA 138 - 69 kV
Long-Time Emergency Loading
Hour
Load
MVA
~
Top Oil
Rise
~~
~
Ambient
Temperature
26.00
39.34
30.97
26.00
26.00
25.79
22.58
24.00
20.60
23.00
21.47
23.00
24.67
25.00
29.51
27.00
29.00
35.98
30.00
47.90
59.66
31.00
66.93
33.00
71.43
35.00
74.22
35.00
35.00
75.94
77.00
35.00
77.66
35.00
78.07
35.00
73.94
34.00
69.18
33.00
66.24
31.00
62.33
30.00
58.05
31.00
50.27
29.00
Total loss of life for 24 hours is 2.5776
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Hottest-Spot
cu
Loss of Life
83.93
71.01
65.83
60.62
57.64
63.06
73.65
85.97
100.77
127.18
147.01
156.29
162.79
165.57
167.30
168.36
169.02
169.43
157.22
147.82
142.88
134.43
127.95
108.93
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0005
0.0083
0.0588
0.1333
0.2500
0.3077
0.3448*
0.3636*
0.4167*
0.4167*
0.1429
0.0667
0.0400
0.0172
0.0091
0.0012
%
~~
174.00
146.00
146.00
146.00
146.00
174.00
204.00
232.00
262.00
320.00
348.00
348.00
348.00
348.00
348.00
348.00
348.00
348.00
320.00
305.00
305.00
290.00
276.00
233.00
*Denotes temperature has exceeded limit.
Table C7
Transformer 275 MVA 138 - 69 kV
Normal Life Expectancy Loading
Hour
Load
MVA
Top Oil
Rise
Ambient
Temperature
26.00
21.18
17.16
26.00
26.00
14.67
24.00
13.13
23.00
12.18
23.00
12.64
14.22
25.00
27.00
16.63
19.82
29.00
25.62
30.00
31.40
31.00
33.00
34.97
37.18
35.00
35.00
38.55
35.00
39.40
35.00
39.92
35.00
40.24
35.00
40.44
34.QO
38.37
33.00
36.07
31.00
34.64
30.00
32.78
31.00
30.70
29.00
26.82
Total loss of life for 24 hours is 0.0070
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
134.00
112.00
112.00
112.00
112.00
134.00
157.00
179.00
202.00
246.00
268.00
268.00
268.00
268.00
268.00
268.00
268.00
268.00
246.00
235.00
235.00
224.00
213.00
179.00
34
Hottest-Spot
cu
Loss of Life
57.69
51.04
48.56
45.02
43.06
46.15
52.76
60.34
69.09
83.40
94.26
99.83
104.04
105.41
106.25
106.78
107.10
107.30
100.15
94.89
91.46
86.69
83.76
72.53
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0002
0.0004
0.0007
0.0008
0.0009
0.0010
0.0010
0.0010
0.0004
0.0002
0.0001
0.0
0.0
0.0
%
IEEE
C67115-1991
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
Table C8
Transformer 275 MVA 138 - 69 kV
Planned Loading Beyond Nameplate Rating
Hour
Load
MVA
Top Oil
Rise
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
134.00
112.00
112.00
112.00
112.00
134.00
157.00
179.00
202.00
246.00
268.00
268.00
268.00
268.00
348.00
348.00
348.00
348.00
246.00
235.00
235.00
224.00
213.00
179.00
Ambient
Temperature
21.90
26.00
17.60
26.00
14.94
26.00
13.30
24.00
23.00
12.28
12.70
23.00
14.26
25.00
16.66
27.00
19.84
29.00
25.63
30.00
31.41
31.00
34.98
33.00
37.19
35.00
38.55
35.00
48.95
35.00
55.39
35.00
59.37
35.00
61.83
35.00
51.60
34.00
44.25
33.00
39.70
31.00
35.91
30.00
32.63
31.00
28.02
29.00
Total loss of life for 24 hours is 0.1364
Hottest-Spot
cu
Loss of Life
58.41
51.49
48.83
45.19
43.17
46.21
52.80
60.36
69.11
83.41
94.26
99.83
104.04
105.41
132.34
138.77
142.75
145.22
113.38
103.07
96.52
89.82
85.69
73.72
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0002
0.0004
0.0007
0.0008
0.0133
0.0278
0.0400
0.0500
0.0020
0.0006
0.0003
0.0001
0.0
0.0
%
Table C9
Transformer 275 MVA 138 - 69 kV
Short-Time Emergency Loading
Hour
Load
MVA
Top Oil
Rise
Ambient
Temperature
26.00
22.15
26.00
17.76
26.00
15.04
24.00
13.36
23.00
12.32
23.00
12.73
25.00
14.28
27.00
16.67
29.00
19.85
30.00
25.63
31.00
31.41
33.00
34.98
35.00
37.19
35.00
38.55
35.00
48.95
35.00
55.39
35.00
59.37
35.00
18
69.37
34.00
19
56.27
33.00
20
47.14
31.00
41.49
21
22
30.00
37.01
23
31.00
33.32
24
29.00
28.44
Total loss of life for 24 hours is 0.3738
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
134.00
112.00
112.00
112.00
112.00
134.00
157.00
179.00
202.00
246.00
268.00
268.00
268.00
268.00
348.00
348.00
348.00
400.00
246.00
235.00
235.00
224.00
213.00
179.00
35
Hottest-Spot
cu
58.66
51.64
48.93
45.25
43.21
46.24
52.82
60.37
69.11
83.41
94.27
99.84
104.04
105.41
132.34
138.77
142.75
164.83
118.04
105.95
98.30
90.93
86.38
74.15
Loss of Life
%
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0002
0.0004
0.0007
0.0008
0.0133
0.0278
0.0400
0.2857
0.0033
0.0009
0.0003
0.0001
0.0
0.0
ZEEE
C67J.15-1991
IEEE GUIDE FOR LOADING MINERAL-OIL-IMMERSED POWER
Table C10
Transformer 275 MVA 138 - 69 kV
Long-Time Emergency Loading
~
Load
MVA
Hour
~~
Top Oil
Rise
Ambient
Temperature
Hottest-Spot
cu
Loss of Life
%
~
1
174.00
33.24
26.00
2
146.00
26.32
26.00
3
146.00
22.04
26.00
4
146.00
19.39
24.00
17.75
23.00
5
146.00
6
174.00
18.47
23.00
7
204.00
21.12
25.00
8
232.00
25.12
27.00
30.48
29.00
9
262.00
10
320.00
40.33
30.00
11
348.00
50.05
31.00
12
348.00
56.07
33.00
13
348.00
59.79
35.00
14
348.00
62.09
35.00
15
348.00
63.51
35.00
16
348.00
64.39
35.00
17
348.00
64.94
35.00
18
348.00
65.27
35.00
19
320.00
61.85
34.00
20
305.00
57.92
33.00
21
305.00
55.49
31.00
22
290.00
52.25
30.00
23
276.00
48.72
31.00
24
233.00
42.28
29.00
Total loss of life for 24 hours is 0.4385
75.20
64.37
60.09
55.44
52.80
57.44
66.71
77.42
90.20
112.64
129.44
137.45
143.17
145.48
146.90
147.78
148.32
148.66
138.16
130.10
125.67
118.40
113.11
96.75
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0001
0.0020
0.0105
0.0222
0.0400
0.0500
0.0588
0.0667
0.0667
0.0714
0.0250
0.0118
0.0077
0.0033
0.0020
0.0003
Table C11
Transformer 300 M V A 138 - 69 kV
Normal Life Expectancy Loading
~~~
Hour
Load
MVA
TopOil
Rise
Ambient
Temperature
1
134.00
18.42
26.00
2
112.00
15.04
26.00
3
112.00
12.96
26.00
4
112.00
11.67
24.00
5
112.00
10.88
23.00
6
134.00
11.28
23.00
7
157.00
12.63
25.00
8
179.00
14.67
27.00
9
202.00
17.36
29.00
10
246.00
22.25
3 0.00
11
268.00
27.12
31.00
12
268.00
30.12
33.00
13
268.00
31.97
35.00
14
268.00
33.11
35.00
15
268.00
33.81
35.00
16
268.00
34.24
35.00
17
268.00
34.51
35.00
18
268.00
34.67
35.00
19
246.00
32.92
34.00
20
235.00
30.97
33.00
21
235.00
29.77
31.00
22
224.00
28.21
30.00
23
213.00
26.45
31.00
24
179.00
23.19
29.00
Total loss of life for 24 hours is 0.0019
36
Hottest-Spot
Loss of Life
cu
%
53.56
47.90
45.82
42.54
40.75
43.43
49.41
56.20
63.99
76.42
85.84
90.84
94.69
95.83
96.53
96.96
97.23
97.39
91.09
86.43
83.23
79.01
76.64
66.72
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0001
0.0002
0.0003
0.0003
0.0003
0.0003
0.0003
0.0001
0.0
0.0
0.0
0.0
0.0
IEEE
C57.115-1991
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 OC WINDING RISE)
Table C12
Transformer 300 MVA 138 - 69 kV
Planned Loading Beyond Nameplate Rating
Hour
Load
MVA
Top Oil
Rise
Ambient
Temperature
19.01
26.00
15.40
26.00
13.18
26.00
11.81
24.00
10.97
23.00
11.34
23.00
12.66
25.00
27.00
14.69
17.37
29.00
10
22.26
30.00
11
27.13
31.00
12
30.12
33.00
13
31.97
35.00
14
33.11
35.00
15
41.89
35.00
16
47.30
35.00
17
50.63
35.00
18
52.69
35.00
19
44.02
34.00
20
37.81
33.00
21
33.99
31.00
22
30.80
30.00
23
28.05
31.00
24
24.17
29.00
Total loss of life for 24 hours is 0.0322
1
2
3
4
5
6
7
8
9
134.00
112.00
112.00
112.00
112.00
134.00
157.00
179.00
202.00
246.00
268.00
268.00
268.00
268.00
348.00
348.00
348.00
348.00
246.00
235.00
235.00
224.00
213.00
179.00
Hottest-Spot
cu
54.15
48.26
46.04
42.67
40.83
43.48
49.44
56.22
64.01
76.43
85.84
90.84
94.69
95.83
118.99
124.40
127.73
129.79
102.19
93.27
87.45
81.61
78.25
67.70
Loss of Life
?&
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0001
0.0002
0.0003
0.0037
0.0062
0.0091
0.0118
0.0006
0.0002
0.0
0.0
0.0
0.0
Table C13
Transformer 300 MVA 138 - 69 kV
Short-Time Emergency Loading
Hour
Load
MVA
134.00
112.00
112.00
112.00
112.00
134.00
157.00
179.00
202.00
246.00
268.00
268.00
268.00
268.00
348.00
348.00
348.00
400.00
246.00
235.00
235.00
224.00
213.00
179.00
Top Oil
Rise
Ambient
Temperature
26.00
19.21
26.00
15.53
26.00
13.26
24.00
11.86
23.00
11.00
11.36
23.00
25.00
12.67
14.70
27.00
29.00
17.38
22.26
30.00
31.00
27.13
33.00
30.12
35.00
31.97
35.00
33.11
35.00
41.89
47.30
35.00
35.00
50.63
35.00
59.07
34.00
47.95
33.00
40.23
31.00
35.48
30.00
31.72
31.00
28.62
29.00
24.52
Total loss of life for 24 hours is 0.0798
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Hottest-Spot
cu
54.36
48.39
46.12
42.72
40.86
43.50
49.45
56.23
64.01
76.43
85.85
90.84
94.69
95.83
118.99
124.40
127.73
146.67
106.12
95.70
88.94
82.53
78.81
68.05
Loss of Life
%
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0001
0.0002
0.0003
0.0037
0.0062
0.0091
0.0588
0.0009
0.0003
0.0001
0.0
0.0
0.0
IEEE
B E E GUIDE FOR LOADING MINERAL-OIL-IMMERSEDPOWER
C57116-1991
Table C14
Transformer 300 MVA 138 kV - 69 kV
Long-Time Emergency Loading
Hour
Load
MVA
Top Oil
Rise
Ambient
Temperature
26.00
174.00
28.53
26.00
146.00
22.71
26.00
146.00
19.12
24.00
146.00
16.91
146.00
23.00
15.55
23.00
16.18
174.00
25.00
204.00
18.43
27.00
232.00
21.81
262.00
29.00
26.33
320.00
30.00
34.64
11
348.00
31.00
42.84
33.00
47.88
12
348.00
35.00
50.99
13
348.00
35.00
52.91
14
348.00
54.09
15
348.00
35.00
54.82
16
348.00
35.00
55.27
17
348.00
35.00
55.54
18
348.00
35.00
34.00
52.65
19
320.00
49.32
20
305.00
33.00
47.28
21
305.00
31.00
44.55
22
290.00
30.00
41.58
23
276.00
31.00
24
233.00
36.15
29.00
Total loss of life for 24 hours is 0.0946
1
2
3
4
5
6
7
8
9
10
38
Hottest-Spot
cu
68.41
59.19
55.61
51.40
49.04
53.07
61.34
70.82
82.06
101.46
115.93
122.98
128.09
130.01
131.19
131.92
132.36
132.64
123.46
116.41
112.37
106.00
101.63
87.31
Loss of Life
%
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0005
0.0027
0.0056
0.0091
0.0118
0.0125
0.0133
0.0133
0.0143
0.0056
0.0027
0.0018
0.0009
0.0006
0.0
IEEE
TRANSFORMERS RATED IN EXCESS OF 100 MVA (65 "C WINDING RISE)
Cb7115-1991
Appendix D
Philosophy of Guide Applicable to Older Transformers
I
100 MVA rating rarely causes interruption
of customers. However, loss of one transformer due to its failure or due to the failure
of some other part in the electrical circuit
can result in increased loading of the back-up
transformers. Most utilities do not design for
second contingencies without loss of load. The
adverse consequences are therefore rather great
if the increased loading of the backup transformers results in a failure.
Common sense and good planning is required
to keep the economic gains in balance with the
risks of failure. Because excessive transformer
temperatures weaken the insulation structures
physically and because many of these older
transformers have low impedances, shortcircuit failures should also be considered. The
types of transformer construction are a factor
in making this assessment. Most utilities load
these transformers conservatively.
Gas evolution in power transformers is not
a new insulation contaminant. There are at
least eight causes of gas within the transformer
that have been documented in the literature.
The risk of having a failure due to free gas in
the insulation structure should take into consideration the insulation margins used and the
construction of the insulation structures. The
risk of failure increases when the insulation
levels are reduced three full steps such as use
of 650 kV BIL on 230 kV transformers. The
risk decreases when no insulation collars are
used in highly stressed parts of these transformers with reduced BIL. Knowledgeable
transformer engineers have paid close attention to gas evolution when specifying and designing these transformers.
The loading of transformers having 55 "C
ratings is considered outside of the scope of
this guide and will therefore not be discussed
in this section.
Discussions Concerning the greater than
nameplate load capability of transformers built
before publication of this guide can be very
controversial. There is some hope of agreement
on the loading limits if they have been clearly
specified prior to the design of the transformer.
However, since there has been new knowledge
gained in recent years concerning stray flux
fields and their effects on metallic temperatures, it is desirable to confirm greater than
nameplate load capabilities with the manufacturers of transformers on critical systems.
Some users have considerable experience in
loading power transformers above nameplate
using computer programs in conjunction with
the IEEE C57.92-1981 Appendix and NEMA
TR98-1978, Guide for Loading Oil Immersed
Power Transformers with 65 "C Average
Winding Rise. Since this approach deals with
loss of life due to the effects of thermal aging of
the windings, it should always be accompanied with due consideration given to the load
capabilities of all other components in the
transformer. These components include the
bushings, various tap changers and terminal
boards, and leads. Relay settings should also
be checked so that load is not dumped. Consideration should also be given to oil expansion
and its effect on possible mechanical relief
device operation, subsequent possible operation of the fault-pressure relay, and oil clogging of breathing devices. Forced-oil cooler
fouling should also be a consideration when
determining load capability. This fouling is
particularly found in areas having salt spray
environments or dust and chemical contaminants present. These computer programs
should be modified to reflect this new loading
guide where its use may lead to more conservative loading.
The loss of a single transformer of over
39
IEEE
C57115-1991
Appendix E
Unusual Temperature and Altitude Conditions
El. Unusual Temperatures and Altitude
without exceeding temperature limits, provided the average temperature of the cooling
air does not exceed the values of Table E l for
the respective altitudes.
Service Conditions
Transformers may be applied at higher
ambient temperatures or at higher altitudes
than specified in IEEE C57.12.00-1987111, but
performance may be affected and special
consideration should be given t o these
applications.
NOTES:(1)See Section 4.3.2of IEEE C57.12.00-1987[l]for
transformer insulation capability at altitudes above 3300 R
(1000m).
(2) Operation in low ambient temperature with the top liquid a t a temperature lower than -20"C may reduce dielectric strength between internal energized components
below design levels.
E2. Effect of Altitude on Temperature Rise
The effect of the decreased air density due
to high altitude is to increase the temperature
rise of transformers since they are dependent
upon air for the dissipation of heat losses.
E4. Operation at Less than Rated kVA
Transformers may be operated at altitudes
greater than 3300 f t (1000 m) without exceeding temperature limits, provided the load to be
carried is reduced below rating by the percentages given in Table E2 for each 330 f t
(100 m) that the altitude is above 3300 f t
(1000 m).
E3. Operation at Rated kVA
Transformers may be operated at rated
kVA at altitudes greater than 3300 f t (1000 m)
Table E l
Maximum Allowable Average Temperature* of Cooling Air for Carrying Rated kVA
Method of Cooling Apparatus
Liquid-Immersed Self-Cooled
Liquid-Immersed Forced-Air-cooled
Liquid-Immersed Forced-Oil-Cooled
with Oil-to-Air Cooler
1000 Meters
(3300 Feet)
2000 Meters
(6600 Feet)
30
30
28
26
30
26
3000 Meters
(9900 Feet)
Degrees C
4000 Meters
( 1 3 200 Feet)
25
23
23
20
23
20
*It 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.
Table E2
Rated kVA Correction Factors for Altitudes
Greater than 3300 f t (1000 m)
Derating Factor
Types of Cooling
Liquid-immersed air-cooled
Liquid-immersed water-cooled
Liquid-immersed forced-air-cooled
Liquid-immersed forced-liquidcooled with liquid-to-air cooler
Liquid-immersed forced-liquidcooled with liquid-to-water cooler
40
(%)
0.4
0.0
0.5
0.5
0.0