SIST EN 60076-2:1997
(Main)Power transformers -- Part 2: Temperature rise
Power transformers -- Part 2: Temperature rise
Includes clauses on identification according to cooling method, temperature-rise limits and type tests of temperature rise.
Leistungstransformatoren -- Teil 2: Übertemperaturen
Transformateurs de puissance -- Partie 2: Echauffement
Comprend des articles sur la désignation suivant le mode de refroidissement, sur les limites d'échauffement, ainsi que les essais de type portant sur l'échauffement.
Power transformers - Part 2: Temperature rise (IEC 76-2:1993 modified)
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN 60076-2:1997
01-oktober-1997
Power transformers - Part 2: Temperature rise (IEC 76-2:1993 modified)
Power transformers -- Part 2: Temperature rise
Leistungstransformatoren -- Teil 2: Übertemperaturen
Transformateurs de puissance -- Partie 2: Echauffement
Ta slovenski standard je istoveten z: EN 60076-2:1997
ICS:
29.180 Transformatorji. Dušilke Transformers. Reactors
SIST EN 60076-2:1997 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN 60076-2:1997
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SIST EN 60076-2:1997
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SIST EN 60076-2:1997
INTERNATIONAL IEC
STANDARD 60076-2
Second edition
1993-04
Power transformers –
Part 2:
Temperature rise
IEC 1993 Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical,
including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch Web: www.iec.ch
PRICE CODE
U
Commission Electrotechnique Internationale
International Electrotechnical Commission
МеждународнаяЭлектротехническаяКомиссия
For price, see current catalogue
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SIST EN 60076-2:1997
I
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SIST EN 60076-2:1997
76-2 © I EC: 1993 – 3 –
CONTENTS
Page
FOREWORD 5
Clause
1 Scope 7
2 Normative references 7
3 Identification symbols according to cooling method 7
4 Temperature-rise limits 11
4.1 General 11
4.2 Normal temperature-rise limits at continuous rated power 11
4.3 Modified requirements because of unusual service conditions 15
4.4 Temperature rise during a specified load cycle 17
5 Test of temperature rise 17
5.1 General 17
5.2 Test methods for temperature-rise determination 19
5.3 Determination of oil temperatures 25
5.4 Determination of average winding temperature 27
5.5 Determination of winding temperature before shutdown 29
5.6 Corrections 29
Annexes
A Note on oil temperature in transformers with forced oil circulation 31
B Transient loading – Mathematical model and testing 35
C Techniques used in temperature-rise testing of oil-immersed
transformers 43
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INTERNATIONAL ELECTROTECHNICAL COMMISSION
POWER TRANSFORMERS
Part 2: Temperature rise
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization
comprising all national electrotechnical committees (IEC National Committees). The object of the IEC is to
promote international cooperation on all questions concerning standardization in the electrical and
electronic fields. To this end and in addition to other activities, the IEC publishes International Standards.
Their preparation is entrusted to technical committees; any IEC National Committee interested in
the subject dealt with may participate in this preparatory work. International, governmental and
non-governmental organizations liaising with the IEC also participate in this preparation. The IEC
collaborates closely with the International Organization for Standardization (ISO) in accordance with
conditions determined by agreement between the two organizations.
2)
The formal decisions or agreements of the IEC on technical matters, prepared by technical committees on
which all the National Committees having a special interest therein are represented, express, as nearly as
possible, an international consensus of opinion on the subjects dealt with.
3)
They have the form of recommendations for international use published in the form of standards, technical
reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
This part of International Standard IEC 76 has been prepared by IEC technical committee
14: Power transformers.
This second edition cancels and replaces the first edition published in 1976.
The text of this standard is based on the following documents:
Six Months' Rule Report on Voting
14(CO)76 14(CO)78
Full information on the voting for the approval of this standard can be found in the repo rt
on voting indicated in the above table.
IEC 76 consists of the following pa rts, under the general title: Power transformers.
Part 1: 1993, General.
Part 2: 1993, Temperature rise.
Part 3: 1980, Insulation levels and dielectric tests.
Part 5: 1976, Ability to withstand sho
rt circuit.
Annexes A, B and C are for information only.
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POWER TRANSFORMERS
Part 2: Temperature rise
1 Scope
This part of International Standard IEC 76 identifies transformers according to their
cooling methods, defines temperature-rise limits and details the methods of test for
temperature-rise measurements. It applies to transformers as defined in the scope of
IEC 76-1.
2 Normative references
The following normative documents contain provisions which, through reference in this
text, constitute provisions of this part of IEC 76. At the time of publication, the editions
rties to
indicated were valid. All normative documents are subject to revision, and pa
agreements based on this pa rt of IEC 76 are encouraged to investigate the possibility of
applying the most recent edition of the normative documents indicated below. Members
of IEC and ISO maintain registers of currently valid International Standards.
IEC 76-1: 1993, Power transformers - Part 1: General
Thermal evaluation and classification of electrical insulation
IEC 85: 1984,
IEC 279: 1969, Measurement of the winding resistance of an a.c. machine during
operation at alternating voltage
IEC 354: 1991, Loading guide for oil-immersed power transformers
IEC 606: 1978, Application guide for power transformers
IEC 726: 1982, Dry-type power transformers
IEC 905: 1987, Loading guide for dry-type power transformers
ISO 2592: 1973, Petroleum products - Determination of flash and fire points - Cleveland
open-cup method
3 Identification symbols according to cooling method
Transformers shall be identified according to the cooling method employed. For oil-immersed
transformers this identification is expressed by a four-letter code as described below.
Corresponding codes for dry-type transformers are given in IEC 726.
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76-2©IEC:1993 - 9 -
First letter:
Internal cooling medium in contact with the windings:
O
mineral oil or synthetic insulating liquid with fire point* <_ 300 °C;
K
insulating liquid with fire point* > 300 °C;
L insulating liquid with no measurable fire point.
Second letter: Circulation
mechanism for internal cooling medium:
N natura/thermosiphon flow through cooling equipment
and in windings;
F forced circulation through cooling equipment, thermosiphon
flow in windings;
D forced circulation through cooling equipment,
directed from the cooling
equipment into at least the main windings.
Third letter: External cooling medium:
A air;
W water.
Fourth letter: Circulation mechanism for external cooling medium:
N
natural convection;
F forced circulation (fans, pumps).
NOTE - In a transformer designated as having forced directed oil circulation (second code letter D), the
rate of oil flow through the main windings is determined by the pumps and is not, in principle, determined
by the loading. A minor fraction of the flow of oil through the cooling equipment may be directed as a
controlled bypass to provide cooling for core and other parts outside the main windings. Regulating
windings and/or other windings having relatively low power may also have non-directed circulation of
bypass oil.
In a transformer with forced, non-directed cooling, on the other hand (second code letter F), the rates of
flow of oil through all the windings are variable with the loading, and not directly related to the pumped flow
through the cooling equipment.
A transformer may be specified with alternative cooling methods. The specification and the
nameplate shall then carry information about the power figures at which the transformer
fulfils the temperature-rise limitations when these alternatives apply, see 7.1 m) of IEC
76-1. The power figure for the alternative with the highest cooling capacity is the rated
power of the transformer (or of an individual winding of a multi-winding transformer,
see 4.1 of IEC 76-1). The alternatives are conventionally listed in rising order of cooling
capacity.
"Cleveland open-cup" test method, see ISO 2592.
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Examples:
ONAN/ONAF. The transformer has a set of fans which may be put in service as desired
at high loading. The oil circulation is by thermosiphon effect only - in both cases.
ONAN/OFAF. The transformer has cooling equipment with pumps and fans but is also
specified with a reduced power-carrying capacity under natural cooling (for example, in
case of failure of auxiliary power).
4
Temperature-rise limits
4.1 General
Temperature-rise limitations for transformers are specified according to different options.
- A set of requirements apply which refer to continuous rated power. These
requirements are given in 4.2.
-
When explicitly specified, an additional set of requirements is imposed which is
related to a specified loading cycle. This procedure is described in 4.4. It is applicable
mainly to large system transformers for which emergency loading conditions deserve
particular attention, and should not be regularly used for small and medium-size
standardized transformers.
It is assumed throughout this part of IEC 76 that the se rvice temperatures of different
part
s of a transformer can each be described as the sum of a cooling medium temperature
(ambient air or cooling water) and a temperature rise of the transformer part.
The cooling medium temperature and the altitude (with regard to cooling air density) are
characteristic of the installation site. When normal se rv
ice conditions in these respects
prevail, see 2.1 of IEC •76-1, then normal values of temperature rise for the transformer
will result in allowable se rvice temperatures.
The values of temperature rise are characteristics of the transformer which are subject to
guarantees and to tests under specified conditions. Normal temperature-rise limits apply
unless the enquiry and contract indicate `unusual se rvice conditions'. In such cases the
limits of temperature rise shall be modified as indicated in 4.3.
No plus tolerance is permitted on temperature-rise limits.
4.2
Normal temperature-rise limits at continuous rated power
When a transformer has a tapped winding with a tapping range exceeding ±5 % then the
temperature-rise limits shall apply to every tapping at the appropriate tapping power,
tapping voltage and tapping current, see 5.6 of IEC 76-1. The load losses are different for
different tappings and sometimes also the no-load losses, namely within tapping ranges
where variable flux voltage variation is specified.
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If a temperature-rise type test is to be made on such a transformer it will, unless otherwise
specified, be carried out on the 'maximum current tapping', see 5.3 of IEC 76-1.
NOTE - In a separate-winding transformer, the maximum current tapping is normally the tapping with the
highest load loss.
In an auto -transformer with tappings, the choice of tapping for the temperature-rise test will be dependent
on how the tappings are arranged.
For a multi-winding transformer, the temperature-rise requirements refer to rated power in
all windings simultaneously if the rated power of one winding is equal to the sum of the
rated powers of the other windings. If this is not the case, one or more particular loading
combinations have to be selected and specified for the temperature-rise test, see 5.2.3.
In transformers with concentric winding arrangement, two or more separate windings may
be situated one above the other. In this case, the winding temperature limit shall apply to
the average of the readings for the stacked windings, if they are of equal size and rating. If
they are not, the evaluation shall be subject to agreement.
The temperature-rise limits given below are valid for transformers with solid insulation
designated as 'Class A' according to IEC 85, and immersed in mineral oil or synthetic
liquid with fire point not above 300 °C (first code letter: O).
Temperature-rise limits of transformers which have a more temperature-resistant
insulation system and/or are immersed in a less flammable liquid (code letter K or L) are
subject to agreement.
Temperature-rise limits for dry-type transformers with different insulation systems are
given in IEC 726.
The following limits for temperature rise in oil-immersed transformers (code letter O) are
referred to steady state under continuous rated power. They are valid only when normal
service conditions with regard to cooling apply, see 4.3.1 below.
- 60 K
Top oil temperature rise, see 5.3.1
Average winding temperature rise (by resistance measurement, see 5.4)
- For transformers identified as ON . or OF . 65 K
- For transformers identified as OD . 70 K
No numerical limits are specified for the temperature rise of the core, of electrical
connections outside the windings or of structural parts in the tank. It is a self-evident
requirement, however, that such parts shall not reach temperatures which will cause
damage to adjacent parts or undue ageing of the oil. For large transformers this may be
investigated by special testing, see annex B.
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4.3
Modified requirements because of unusual service conditions
If the service conditions at the intended installation site do not fall within the limits of
`normal service conditions', then the limits of temperature rise for the transformer shall be
modified accordingly.
Rules for dry-type transformers are given in 2.2 of IEC 726.
4.3.1
Oil-immersed, air-cooled transformers
Normal ambient temperature limits (-25 °C and +40 °C) for power transformers are given
in 2.1 of IEC 76-1. With regard to cooling of air-cooled transformers the temperature
conditions at the intended installation site should neither exceed:
+ 30 °C monthly average, of the hottest month; nor
+ 20 °C yearly average.
If the temperature conditions at site exceed one of these limits, the specified
temperature-rise limits for the transformer shall all be reduced by the same amount as the
excess. The figures shall be rounded to nearest whole numbers of degrees.
NOTE - The average temperatures are to be derived from meteorological data as follows (IEC 76-1,
definition 3.12).
Monthly average temperature:
half the sum of the average of the daily maxima and the average of the daily minima during a particular
month, over many years;
Yearly average temperature:
one-twelfth of the sum of the monthly average temperatures.
If the installation site is more than 1 000 m above sea-level but the factory is not, then the
allowable temperature rise during the test in the factory shall be reduced as follows.
For a naturally cooled transformer (. AN), the limit of average winding temperature rise
shall be reduced by 1 K for every interval of 400 m by which the installation's altitude
exceeds 1 000 m.
For a forced-cooled transformer (. . AF), the reduction shall be 1 K for every 250 m.
A corresponding reverse correction may be applied in cases where altitude of the factory
is above 1 000 m and the altitude of the installation site is below 1 000 m.
Any altitude correction shall be rounded to the nearest whole number of degree.
When the specified temperature-rise limits of a transformer have been reduced, either
because of high cooling medium temperature or because of high-altitude installation, this
shall be indicated on the rating plate, see 7.2 of IEC 76-1.
NOTE - When standardized transformers are to be applied at high altitudes, a reduced figure of power
may be calculated, which from the point of view of cooling and temperature rise corresponds to service with
rated power under normal ambient conditions.
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4.3.2
Oil-immersed, water-cooled transformers
Normal cooling water temperature is, according to 2.1 of IEC 76-1 not above +25 °C. If the
cooling water temperature exceeds this limit, the specified temperature-rise limits for the
transformer shall all be reduced by the same amount as the excess. The figures shall be
rounded to the nearest whole number of degrees.
The influence of differing ambient temperature or altitude on the air cooling of the tank is
disregarded.
4.4
Temperature rise during a specified load cycle
If guarantees and/or a special test regarding a load cycle are to be specified, this shall
involve the following parameters:
- the initial temperature condition of the transformer, either at ambient temperature or
with steady-state temperature rises corresponding to a specified fraction of rated
current (`preload');
-
the (constant) magnitude of the test current, expressed as a multiple of rated
current, and its duration;
-
the maximum permissible temperature-rise values for top oil and winding average
(by resistance) at the termination of the test. This statement is optional. The test may
be executed for information only, without any limits being agreed on beforehand;
- any special obse
rvations or measurements to be performed, for example direct
hot-spot temperature measurements, thermal imaging of tank-wall heating, and
possible limitations in relation to them.
For further recommendations and discussion regarding load cycle studies - particularly
measurements and evaluation, see clause B.4 of annex B.
5 Test of temperature rise
5.1 General
This clause describes the procedures for determination of temperatures and
temperature-rise values during factory testing and also the methods for substituting
service loading by equivalent test procedures.
The clause gives requirements for the testing of both oil-immersed and d
ry-type
transformers, as applicable.
During the temperature-rise test, the transformer shall be equipped with its protective
devices (for example, Buchholz relay on an oil-immersed transformer). Any indication
during the test shall be noted.
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5.1.1
Cooling-air temperature
Precautions should be taken to minimize variations of cooling-air temperature, particularly
during the later pa rt
of a test period when a steady state is approached. Rapid variation of
readings due to turbulence should be prevented by appropriate means such as heat sinks
of suitable time constant for the temperature sensors. At least three sensors shall be
used. The average of their readings shall be used for the evaluation of the test. Readings
should be taken at regular intervals, or automatic continuous recording may be used.
The sensors shall be distributed around the tank, 1 m to 2 m away from tank or cooling
su rf
aces, and protected from direct heat radiation. Around a self-cooled transformer, the
sensors shall be placed at a level about halfway up the cooling su rf
aces.
A forced-air-cooled transformer shall have the sensors placed so as to record the true
temperature of the air taken into the coolers. Attention shall be paid to possible
recirculation of hot air. The test object should be placed so as to minimize obstructions to
the air flow and to provide stable ambient conditions.
5.1.2
Cooling-water temperature
Precautions should be taken to minimize variation of cooling-water temperature during the
test period. The temperature is measured at the intake of the cooler. Readings of
temperature and rate of water flow should be taken at regular intervals, or automatic
continuous recording may be used.
5.2 Test methods for temperature-rise determination
5.2.1 General
For practical reasons, the standard method for determination of the steady-state
temperature rise of oil-immersed transformers on the test floor is the equivalent test in
short-circuit connection according to 5.2.2 below.
Alternatively it may be agreed, in special cases, to perform a test with approximately rated
voltage and current by connection to a suitable load. This is mainly applicable to
transformers with low rated power.
A "back-to-back" method may also be agreed. In this method, two transformers, one of
which is the transformer under test, are connected in parallel and excited at the rated
voltage of the transformer under test. By means of different voltage ratios or an injected
voltage, rated current is made to flow in the transformer under test.
Procedures applicable to dry-type transformers are described in lEG 726.
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5.2.2
Test to steady state by short-circuit method
During this test the transformer is not subjected to rated voltage and rated current
simultaneously, but to the calculated total losses, previously obtained by two separate
determinations of losses, namely load loss at reference temperature, and no-load loss,
see 10.4 and 10.5 of IEC 76-1.
The purpose of the test is twofold:
-
to establish the top oil temperature rise in steady-state condition with dissipation of
total losses;
- to establish the average winding temperature rise at rated current and with the top
oil temperature rise as determined above.
This is achieved in two steps:
a) Total loss injection
First the top oil and average oil temperature rises are established when the transformer
is subjected to a test voltage such that the measured active power is equal to the total
losses of the transformer, see 3.6, 10.4 and 10.5 of IEC 76-1. The test current will be
above rated current to the extent necessary for producing an additional amount of loss
equal to the no-load loss, and the winding temperature rise will be correspondingly
elevated.
The oil temperature and cooling medium temperature are monitored, and the test is
continued until a steady-state oil temperature rise is established.
The test may be terminated when the rate of change of top oil temperature rise has fallen
below 1 K per hour and has remained there for a period of 3 h. If discrete readings have
been taken at regular intervals, the mean value of the readings during the last hour is
taken as the result of the test. If continuous automatic recording is applied, the average
value during the last hour is taken.
NOTE - If the time constant of the oil temperature rise is no more than 3 h the truncation error of this
procedure will be negligible. Alternative truncation rules are discussed in annex C.
b) Rated current injection
When the top oil temperature rise has been established, the test shall immediately
continue with the test current reduced to rated current for the winding combination
connected (for a multi-winding transformer see 5.2.3). This condition is maintained for
1 h, with continuous observation of oil and cooling medium temperatures.
At the end of the hour, the resistances of the windings are measured, either after a
rapid disconnection of the supply and sho
rt circuits (see 5.5 and clauses C.2 and C.3 of
annex C) or, without switching off the supply, by means of the superposition method
which consists of injecting into the winding a d.c. measuring current of low value
superimposed on the load current.
NOTE 1 — The use of a superimposed d.c. current for the measurement of winding resistances is described
in IEC 279.
The values of average temperature of the two windings are determined from the
resistances, according to 5.4.
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During the hour with rated current the oil temperature falls. The measured values of
winding temperature shall therefore be raised by the same amount as the average oil
temperature rise has fallen from the correct value, obtained according to procedure a)
above. The corrected winding temperature value minus the cooling medium temperature at
the end of the total losses injection period is the winding average temperature rise.
NOTE 2 - With regard to calculation of temperatures under variable loading, it is convenient to regard the
winding temperature rise as the sum of two terms: the average oil temperature rise (above cooling medium
temperature) plus the difference between average winding and average oil temperatures (see 5.6 and
clauses B.2 and B.3 of annex B).
By agreement, the two steps of the test may be combined in one single application of
power at a level between load loss and total loss. The temperature-rise figures for the top
oil and for the windings shall then be determined using the correction rules of 5.6. The
power injected during the test shall however be at least 80 % of the total losses figure.
5.2.3 Test modification for particular transformers
Two-winding transformer with tapping range larger than ±5 %
Unless otherwise specified, the temperature-rise test is conducted with the transformer
connected on the `maximum current tapping' (see 5.3 of IEC 76-1) and the tapping current
for that tapping is used during the later pa rt
of the test (see 5.2.2 b)).
The total losses to be injected during the first pa
rt of the test (see 5.2.2 a)), shall be equal
to the highest value of total loss appearing at any tapping (corresponding to its tapping
quantities). This tapping is also often, but not always, the maximum current tapping. This
part of the test determines the maximum top oil temperature rise. For the determination of
winding temperature rise at the maximum current tapping, the figure of oil temperature rise
to be used in the evaluation shall correspond to the total losses of that tapping. The value
from the first pa rt
of the test will be recalculated if obtained with other data.
Multi-winding transformer
For the first part of the test a total loss shall be developed which corresponds to rated
power (or tapping power) in all windings, if the rated power of one winding is equal to the
sum of the rated powers of the other windings.
If this does not apply, there are specified loading cases with different combinations of
individual winding loads. That case which will be associated with the highest total loss
shall determine the test power for the determination of oil temperature rise.
The temperature rise figure for an individual winding above oil shall be obtained with rated
current in the winding.
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In the determination of winding temperature rise above ambient, the oil temperature rise
for the relevant loading case will be recalculated from the total loss injection test,
according to 5.6, and likewise the winding temperature rise above oil for each winding, as
applicable.
Guidance for the recalculation of losses in multi-winding transformers is given in IEC 606.
The injection of total loss for determination of oil temperature rise may be made:
-
either in a manner as near as possible to the actual loading case, by injecting the
current corresponding to the total losses in one winding, the other ones being
simultaneously short-circuited or connected to an impedance;
- or in an approximate manner by not short-circuiting or closing certain windings; for
example if one of the windings has a relatively low rated power and low contribution to
the total loss of the transformer, it may be acceptable to leave it open and raise the
current in the other windings concerned until the correct total loss is obtained.
If none of the methods above can be applied in full, because of limitations of test facilities,
it may be agreed to perform the test with reduced loss, down to 80 % of the correct value.
Then the measured temperature value shall be corrected according to 5.6.
The details of the temperature-rise test for a multi-winding transformer should, as a rule,
be presented and agreed already at the tender stage.
5.3
Determination of oil temperatures
5.3.1 Top oil
The top oil temperature is determined by one or more sensors immersed in the oil in the
top of the tank, in pockets in the cover, or in headers leading from the tank to separate
radiators or coolers. The use of several sensors is particularly impo rtant on large
transformers, and their readings shall be averaged in order to arrive at a representative
temperature value.
NOTE - The temperature of the oil may be different at different places in the top of the tank, depending on
the design. Measurements using a pocket in the cover may be disturbed by eddy current heating of the
cover. In transformers with forced circulation of oil to the cooling equipment there is
...
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