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Wilcarani Lamas. Missing page numbers correspond to the French- language pages. Consolidated editions The IEC is now publishing consolidated versions of its publications. For example, edition numbers 1. Information relating to this publication, including its validity, is available in the IEC Catalogue of publications see below in addition to new editions, amendments and corrigenda. On-line information is also available on recently issued publications, withdrawn and replaced publications, as well as corrigenda.
Please contact the Customer Service Centre see below for further information. The object of the IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. 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.
Any divergence between the IEC Standard and the corresponding national or regional standard shall be clearly indicated in the latter. The IEC shall not be held responsible for identifying any or all such patent rights. This second edition cancels and replaces the first edition published in , amendment 1 and IEC Annex D forms an integral part of this standard.
The committee has decided that the contents of this publication will remain unchanged until The contents of the corrigendum of December have been included in this copy.
It also recommends clearances in air between live parts of bushings on oil-immersed power transformers and to objects at earth potential clause Guidance can be obtained from IEC The insulation levels and dielectric tests which are specified in clauses 4, 5, 6 and 7 in this standard apply to the internal insulation only. Whilst it is reasonable that the rated withstand voltage values which are specified for the internal insulation of the transformer should also be taken as a reference for its external insulation, this may not be true in all cases.
A failure of the non-self-restoring internal insulation is catastrophic and normally leads to the transformer being out of service for a long period, while an external flashover may involve only a short interruption of service without causing lasting damage.
Therefore, it may be that, for increased safety, higher test voltages are specified by the purchaser for the internal insulation of the transformer than for the external insulation of other components in the system. When such a distinction is made, the external clearances must be adjusted to fully cover the internal insulation test requirements. It identifies transformer windings to their highest voltage for equipment U m associated with their corresponding rated insulation levels and details the relevant applicable dielectric tests and minimum external clearances in air between live parts of bushings and to objects at earth potential.
For categories of power transformers and reactors which have their own IEC standards, this standard is applicable only to the extent in which it is specifically called up by cross reference in the other standards.
For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this part of IEC are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies. For oil-immersed transformers, the requirements apply to the internal insulation only.
Any additional requirements or tests regarding external insulation which are deemed necessary shall be subject to agreement between supplier and purchaser, including type tests on a suitable model of the configuration. When an oil-immersed transformer is specified for operation at an altitude higher than 1 m, clearances shall be designed accordingly. It may then be necessary to select bushings designed for higher insulation levels than those specified for the internal insulation of the transformer windings, see clause 16 of this standard and 4.
Bushings are subject to separate type and routine tests according to IEC , which verify their phase-to-earth insulation, external as well as internal. It is presupposed that bushings and tap-changers are specified, designed and tested in accordance with relevant IEC standards. The insulation tests on the complete transformer, however, constitute a check on the correct application and installation of these components. The transformer shall be completely assembled as in service including supervisory equipment.
It is not necessary, however, to fit elements which do not influence the dielectric strength of the internal insulation, for example, the external cooling equipment. If a transformer fails to meet its test requirements and the fault is in a bushing, it is permissible to replace this bushing temporarily with another bushing and continue the test on the transformer to completion without delay.
A particular case arises for tests with partial discharge measurements, where certain types of commonly used high-voltage bushings create difficulties because of their relatively high level of partial discharge in the dielectric. When such bushings are specified by the purchaser, it is permitted to exchange them for bushings of a partial discharge free type during the testing of the transformer, see annex A.
Transformers for cable box connection or direct connection to metal-enclosed SF 6 installations should be designed so that temporary connections can be made for insulation tests, using temporary bushings, if necessary. When the supplier intends to use non-linear elements or surge arresters, built into the transformer or externally fitted, for the limitation of transferred overvoltage transients, this shall be brought to the purchaser's attention at the tender and order stage and it is recommended that it be indicated on the transformer rating plate circuit diagram.
The rules for coordination of transformer insulation with respect to transient overvoltages are formulated differently depending on the value of U m. Rules for a number of special classes of transformers are given in clause 6. Standardized values of U m are listed in tables 2 to 4. The value to be used for a transformer winding is the one equal to, or nearest above, the rated value of the winding. NOTE 2 It may happen that certain tapping voltages are chosen slightly higher than a standardized value of U m, but that the system to which the winding will be connected has a system highest voltage which stays within the standard value.
The insulation requirements are to be coordinated with actual conditions, and therefore this standard value should be accepted as U m for the transformer, and not the nearest higher value. NOTE 3 In certain applications with very special conditions the specification of other combinations of withstand voltages may be justified.
In such cases, general guidance should be obtained from IEC NOTE 4 In certain applications, delta-connected windings are earthed through one of the external terminals. In those applications, a higher withstand voltage with respect to the highest voltage for equipment U m may be required for this winding and should be agreed between supplier and purchaser.
The highest voltage for equipment U m and its assigned withstand voltages, that is, their insulation level, determine the dielectric characteristics of a transformer. They are verified by a set of dielectric tests depending on U m , see clause 7. The value of U m and the insulation level which are assigned to each winding of a transformer are part of the information to be supplied with an enquiry and with an order.
If there is a winding with non-uniform insulation, the assigned U m and the insulation level of the neutral terminal shall also be specified by the purchaser, see 7. The rated withstand voltages for all windings shall appear on the rating plate.
The principles of the standard abbreviated notation are shown in some examples below. The classifications on the insulation design shall independently of the test procedure be derived from the values in table 2, 3 and 4 or from IEC The abbreviations here and in the examples below have the following meaning: SI is the switching impulse withstand voltage for the line terminals of the winding with the highest U m ; LI is the lightning impulse withstand voltage for the line and neutral terminals of each individual winding; AC is the short duration induced and separate source AC withstand voltage for the line and neutral terminals of each individual winding; h.
Insulation levels: h. In transformers which have one or more non-uniformly insulated windings, the test voltages for the induced withstand voltage test, and for the switching impulse test if used, are determined by the winding with the highest U m value, and the windings with lower U m values may not receive their appropriate test voltages. This discrepancy should normally be accepted.
If the ratio between the windings is variable by tappings, this should be used to bring the test voltage for the winding with lower U m voltage as close as possible to the appropriate value.
Rated switching impulse withstand voltages shall only be assigned to the winding with the highest U m. Test stresses in other windings are also proportional to the ratio of numbers of turns and are adjusted by selecting appropriate tappings to come as close as possible to the assigned value in table 4. Series windings in booster regulating transformers, phase shifting transformers, etc. It is often impracticable to test such trans- formers in formal compliance with this standard, and it should be agreed between the supplier and the purchaser as to which tests have to be omitted or modified.
For single-phase transformers intended to be connected between phases, as in the case of railway traction system supplies, higher test values than indicated in this standard may be necessary. Special considerations with respect to test connections and number of tests to be performed on multiple re-connectable transformers shall be agreed at the time of placing the order. This clause details the relevant insulation requirements and applicable dielectric tests. For categories of power transformers and reactors which have their own IEC standards, the requirements are applicable only to the extent in which they are specifically called up by cross reference in the other standards.
Levels of standard withstand voltages, identified by the highest voltage for equipment U m of a winding are given in tables 2, 3 and 4. The choice between the different levels of standard withstand voltages in these tables depends on the severity of overvoltage conditions to be expected in the system and on the importance of the particular installation.
Guidance may be obtained from IEC NOTE 1 Distribution transformers for suburban or rural installation are, in some countries, severely exposed to overvoltages. In such cases, higher test voltages, lightning impulse tests and other tests on individual units may be agreed between supplier and purchaser. They should be clearly stated in the enquiry document. Information about the selected transformer insulation requirements and dielectric tests shall be supplied with an enquiry and with an order, see annex C.
The insulation requirements are specified in 7. The verification of the withstand voltages by dielectric tests is given in 7. The insulation requirements and tests for the neutral terminal of a winding are given in 7. This should be clearly stated in the enquiry document. They shall, where applicable and not otherwise agreed upon, be performed in the sequence as given below.
The test is an essential requirement for transformers subjected to a long-duration induced AC withstand voltage ACLD test. If the lightning impulse test includes impulses chopped on the tail LIC , the impulse test is modified according to clause This test is required if a standard impulse withstand voltage for the neutral is specified. The test shall be performed in accordance with It verifies partial discharge-free operation of the transformer under operating conditions.
Distribution note 1 Class II Distribution and Class II and class I transformers class I transformers transformers note 3 transformers note 2 15 95 34 34 — 40 — 26,4 50 50 36,5 70 70 48,3 95 95 72,5 NOTE 1 Distribution transformers transfer electrical energy from a primary distribution circuit to a secondary distribution circuit.
NOTE 2 For uniformly insulated transformers with extremely low values of rated AC insulation levels, special measures may have to be taken to perform the short-duration AC induced test, see NOTE 3 Not applicable, unless otherwise agreed.
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Do you need a multi-user copy? Our prices are in Swiss francs CHF. We accept all major credit cards American Express, Mastercard and Visa , PayPal and bank transfers as form of payment. It also recommends external clearances in air. It gives details of the applicable dielectric tests and minimum dielectric test levels. Recommended minimum external clearances in air between live parts and between live parts and earth are given for use when these clearances are not specified by the purchaser. For categories of power transformers and reactors which have their own IEC standards, this standard is applicable only to the extent in which it is specifically called up by cross reference in the other standards.
IEC System of Conformity Assessment Schemes