BS 7803:1995 pdf free download.Insulators of ceramic material or glass for overhead-lines with a nominal voltage greater than 1000 V, puncture testing.
Introduction
A power frequency puncture test on insulators in oil was specified in IEC 383: Insulators for overhead lines with a nominal voltage above 1 000 V. An impulse overvoltage test is given as an alternative, without specification of method or acceptance criteria. Sub-committee 36B has considered questions related to puncture testing already for a long time. The opinions and experience of National Committees have also been requested and collated. In time, interest in an impulse voltage puncture test on insulators in air has increased considerably. This report contains a proposal for a standard impulse voltage puncture test. This test is intended to be the impulse overvoltage test given in 15.2 of
IEC 383-1.
The main problems in introducing an impulse voltage puncture test appeared to be relevant stress in the test and, secondly, the doubt about reproducibility of this test in general and its feasibility, especially in insulator manufacturers’ laboratories. In studying these questions, considerable support has been received from several working groups of CIGRE Study Committee 33, Insulation co-ordination. Annex A to Annex E summarize the findings of the sub-committee and the studies behind the conclusions. For more detailed information, reference is made to Electra, Report by Aro, M. and at.: Puncture testing of ceramic and glass insulators. Final report of CIGRE task force 33.07.01 published in Electra 136, June 1991. This is shown in the text by [El.
The impulse voltage puncture test on insulators in air is reproducible if voltage amplitude of the test impulse is specified instead of steepness. In addition to amplitude, the test procedure has to be specified in detail. Special attention has to be paid to the measurement.
The choice of specification of amplitude instead of steepness derives mainly from the following two facts. It is easier and more accurate to measure peak value than steepness. The stress does not depend on the test circuit and the other test conditions when referring to the peak value.
This report assumes that the manufacturer specifies the impulse puncture withstand voltage as a characteristic of the insulator unit. This voltage is defined as peak value of the measured voltage across the insulator in the actual test. For more general specification of test voltage of different types of insulator units, it is useful to apply per unit (p.u.) values (see Annex A). It is recommended to obtain this information, together with the puncture test results for further consideration of the specification of the test voltage.
The purchaser may adopt the amplitude specification for an insulator by taking into consideration the overvoltage calculations on one hand, as well as test and service experience, on the other hand. This experience, especially and most clearly in Canada, supports the introduction of an impulse voltage puncture test. For consideration of the test specification, results of a large series of measurements in many laboratories have been used for transforming the steepness criteria to amplitude criteria [El. It is considered that, for the moment, the specification for the amplitude has to be kept within the range of the stress applied in tests of current practice.
As to the test procedure, polarity of the impulse related to the pin, starting polarity, possible polarity reversals and time interval between consecutive impulses may affect the severity of the test and hence have to be specified.
It is realized that measurenient of these short impulse voltages needs special attention to achieve an acceptable accuracy. Even the revision of IEC 60 will not fully support this topic. Consequently, special requirements and guidance for measurements are necessary. In this, it is recommended to keep a little on the safe side, because of new considerations. Annex B gives some guidance.
In limited conditions, it is possible to use fixed circuits in tests without measurements. However, this requires considerable calibration work from time to time. Nevertheless, some conditions lead to high dispersion. Thus, this seems not to be a very practical method.
The puncture strength of insulators depends on both design and quality of manufacturing and a sample test is needed to confirm the type test result. It is recognized that this test may be expensive and time-consuming. Annex D gives some guidance. Annex E indicates the problems found in the reproducibility of power frequency puncture tests on insulators in oil. During the test series, the reproducibility of this test was very poor, because initially the importance of the oil characteristics was not fully appreciated.
The results are mainly influenced by the characteristics of the oil, particularly by the resistivity, which varied over a large range during the tests, and possibly by the dielectric strength. The tests, however, indicated that the reproducibility could, in principle, be improved, e.g. by specifying a narrower range for the resistivity and dielectric strength of the oil. Modifying the oil used in the test by specifying tighter tolerances for the oil characteristics is very difficult. It may even be necessary to specify different characteristics of oils for different dielectric materials of the insulators. The manufacturer is in the best position to define the necessary characteristics of oil for his insulators.
It is recommended to retain the power frequency puncture test in oil without modification until wider experience of impulse voltage puncture testing has been gained. The puncture test in oil is designed to check the head of the insulator, and applies the power frequency voltage suitably higher than the dry power frequency flashover voltage in air between the pin and the cap. The oil allows this voltage to be applied by increasing the breakdown value above that of the air flashover path. Providing this voltage is applied without puncturing the head of the insulator at the specified voltage, there is a high degree of assurance that the test result and the insulator are satisfactory. It is important, however, that the oil is selected to ensure that pre-discharges, flashover, or puncture of the outer sheds do not occur below the specified withstand voltage.
1 Scope and object
This technical report deals with puncture testing of ceramic and glass insulator units of class B only. Although there is little or no experience of testing class B long-rod and line-post insulators, the experience already gained with cap and pin, and pin insulators, permits their mounting arrangements and test procedures to be specified for use in future tests to gain experience.
This report may be applied also in tests on insulators of other materials (e.g. polymers and composites). However, it may be useful to consider the possible need to adjust the test procedure accordingly.
An impulse voltage puncture test on insulators in air is required and applied in more and more countries, and by a growing number of users. The applied test procedures differ from each other considerably. This report is published with the object of specifying a standard procedure for impulse voltage puncture test on insulators in air, so that experience with this test may be obtained on a common basis.
It is recommended that wider manufacturer and user experience with the impulse test he obtained in order to establish the practicability of including it in a future standard.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this technical report. At the time of publication, the editions indicated were valid. All normative documents are subject to revision, and parties to agreements based on this technical report 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 60-1:1989. High-voltage test techniques — Part 1: General definitions and test requirements. IEC 383-1:1993, Insulators for overhead lines with a nominal voltage above 1 000 V—Part 1: ceramic or glass insulator units for a.c. systems — Definitions, test methods and acceptance criteria.
IEC 790:1984, Oscilloscopes and peak voltmeters for impulse tests.
3 Classification of the puncture test on insulators
The puncture characteristics of insulators depend both on the design, and manufacturing conditions. and on the materials used. IEC 383-1 specifies the puncture test as a sample test, and allows for an impulse overvoltage puncture test as an alternative by agreement. It is recommended to apply this report in the case where this alternative is chosen. It is realized that impulse voltage puncture tests take more time than power frequency tests. Thus, the procedure for the sample test is given for guidance, to be used as desired for both research and specification for supply of insulators, and to gain more information on the costs of the sample test. The costs can be reduced by automation.
4 Impulse voltage puncture test on
insulators in air
4.1 General requirements for the test Except if otherwise specified, IEC 60-1 and IEC 383-1 are applicable. The test specimen shall be clean, dry, and in thermal equilibrium with surroundings before starting the test.
4.2 Mounting arrangements
The mounting arrangement depends on the type of the insulator unit and on its possible integral metallic parts.
Further to the peak amplitude, there are no requirements for the shape of the impulse front. The length of the impulse is determined by flashover of the insulator under test. The test circuit and the charging voltage are adjusted until the flashover voltage of specified peak amplitude is achieved.
NOTE For generation of test voltage, any impulse voltage generator may be used. See Annex C, clause C.4.
4.6 Test procedure
On cap and pin and class B long-rod insulators: 10 positive and 10 negative impulses are applied in this order, with a time interval of 1 mm to 2 mm between consecutive impulses. On pin and class B line-post insulators: 10 negative and 10 positive impulses are applied in this order, with a time interval of 1 mm to 2 mm between consecutive impulses.
4.7 Puncture determination
If the puncture is not already evident, the possible puncture is observed either by absence of further flashovers, or by studying the oscillogram of the last impulse, or by two flashovers at power frequency.
4.8 Acceptance criteria
Either the design, the manufacturing, or both do not comply with this specification if any insulator punctures in the type test.
The lot does not comply with this specification if more than one insulator punctures in the sample test.
In the case of only one puncture in the sample test, the re-test procedure may he applied (see 4.9).
4.9 Re-test procedure
If one of the insulators fails in the sample test, twice the number of insulators are additionally tested with the same procedure. If any of these insulators fail, the lot is considered not to comply with this specification.