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
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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.
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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
76-2 © IEC: 1993 - 5 -
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.
76-2 © I EC: 1993
-7 -
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.
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.
76-2 © IEC: 1993 - 11 -
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).
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.
76-2 © IEC: 1993 -13 -
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.
76-2©IEC:1993 - 15 -
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
...
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