摘要:

During an over-voltage withstand test, equipment in which the conductors are misplaced or the insulation is contaminated or undercured may fail, or it may suffer only undetected deterioration by discharges, e.g. a subsequent increase in dielectric losses in oil-impregnated paper or the propagation of discharge channels partly through or over the surface of the insulation. It is shown that such damage is quite probable during a.c. tests, whereas d.c. and impulse tests are more likely to cause either negligible damage or complete breakdown. If impulse tests can be applied for routine quality control, they are preferable to d.c. tests as they more closely simulate surge conditions which may occur in service. If neither impulse nor d.c. tests are appropriate to the particular equipment, a test with a few cycles of low-frequency (e.g. 0.1 c/s) voltage would be preferable to the usual power-frequency test. To detect very local defects which might cause thermal breakdown in service it is proposed that the impulse (or equivalent d.c. or 0.1 c/s) test should be made with insulation at the normal operating temperature.Over-voltage withstand tests of short duration give no assurance against deterioration and eventual breakdown in service by electrical discharges or electrochemical processes. It is therefore recommended that design tests should include time-to-breakdown tests and appropriate non-destructive tests in order to determine the magnitude of discharges and degree of ionic contamination which can be tolerated for a required service performance. Tests to assess resistance to breakdown by discharges should be made at the maximum operating temperature; often such tests can be accelerated by applying a higher frequency than that used in service. Life tests should be made at at least three stress levels to permit extrapolation to the service stress. Life tests to assess resistance to electrochemical deterioration may be similarly accelerated by raising the temperature.

DOI:10.1049/pi-a.1962.0012

出版商:IEE    

年代:1962

数据来源: IET

摘要:

The need is indicated for informative non-destructive test methods to assess the quality of insulation, both as manufactured and during its service life, and the limitations of high-voltage proof-testing are reviewed. The types of insulation system used for high-voltage stator bars are described briefly, and the nature and cause of defects which can give rise to trouble in service are analysed.Low-voltage test methods available are discussed, with particular reference to low-voltage variable-frequency tests, and some results obtained using this technique are reported. Published work describing high-voltage measurements using d.c. and low-frequency sources are reviewed, and the need is emphasized for a better understanding of fundamental dielectric behaviour under varying frequency and stress conditions.Power-frequency measurements at voltages up to line potential are dealt with at greater length. The aim of such measurements is primarily the detection of voids. They can be of great value in controlling insulation quality and can also be used to monitor the condition of the insulation during maintenance checks in service. Tests dealt with include loss-tangent/voltage and corona discharge measurements, and special bridge tests aimed at separating solid-dielectric and discharge losses. Bridge circuits are described which, combined with oscillograph cyclogram displays, permit the discharge energy to be determined, and also enable solid-dielectric losses to be measured. Test results obtained using a novel bridge circuit of this type are reported.

DOI:10.1049/pi-a.1962.0013

出版商:IEE    

年代:1962

数据来源: IET

摘要:

Power transformers are likely to break down at service voltage due to excessive water in the solid insulation. It is shown that insulation measurements between the transformer terminals and tank do not give a reliable indication of the condition of the solid insulation of a transformer in service. It is suggested that a paper probe or image would give the required information. A design of probe is described and some test results are given.A simple method of measuring the degree of water saturation of oil is described in an Appendix.

DOI:10.1049/pi-a.1962.0014

出版商:IEE    

年代:1962

数据来源: IET

摘要:

The deterioration of the insulation in high-voltage turbo-generator stators can be followed by means of non-destructive tests. The relative merits of the available methods of testing stator insulation are given, but at present a single test will not reliably indicate the insulation condition. The detection of discharges with an electronic instrument most closely approaches this ideal and has the unique ability of indicating the existence of preferential deterioration near the line terminals of a high-voltage stator. A dependable guide to the insulation condition can be obtained only from considering the results of several tests, employing both alternating and direct current. This allows the variation in the insulation of a turbo-generator stator undergoing normal usage to be determined. Departure from the normal variation indicates those machines that have suffered abnormal deterioration.The presence of deleterious discharges in high-voltage bushings at the time of manufacture has been ascertained with the E.R.A. discharge detector. The discharge magnitude is measured directly and has been related to the indirect methods normally used, such as loss angle and audible hiss. For synthetic-resin-bonded-paper bushings of 33-132kV rating, some discharges are permissible near the working voltage. Discharges in oil-impregnated-paper bushings of 132, 220 and 330 kV rating, however, are undesirable at the maximum working voltage. Recommendations are made of the permissible discharge magnitude for the two types of bushings.

DOI:10.1049/pi-a.1962.0015

出版商:IEE    

年代:1962

数据来源: IET

摘要:

Although impulse-voltage acceptance tests are, by assumption, non-destructive when successful, it is not easy in practice to establish working criteria of insulation failure and to co-ordinate these criteria with observations. A tentative definition of impulse failure is used as a basis for considering the significance of indications given by electrical methods of fault detection applied to transformer windings. Exploratory tests on high-voltage motors are described, in which it was found that fault indications were likely to be obscured by disturbances arising from non-cyclic magnetization of the iron and from other causes not determined. Acceptance impulse testing for high-voltage capacitors and cables is briefly discussed. It is concluded that the techniques for acceptance testing of transformer windings are broadly satisfactory, but that, for motor windings, existing methods are inadequate.

DOI:10.1049/pi-a.1962.0016

出版商:IEE    

年代:1962

数据来源: IET

摘要:

The paper describes the design and operation of an apparatus for control of quality of extruded insulation of high-voltage polythene power cables by non-destructive detection and location of harmful electrical discharges.The apparatus scans polythene-insulated cable core at a speed comparable with that of the extrusion process of manufacture. With test voltages of up to 100 kV a.c.(r.m.s.), the sensitivity of detection is more than adequate for discrimination between harmful and harmless discharges. Discharge sites are readily located to within 3 in in cable cores of continuous lengths of more than 5000 yd.The apparatus does not require continual supervision by technical personnel.

DOI:10.1049/pi-a.1962.0018

出版商:IEE    

年代:1962

数据来源: IET

摘要:

The function of the serving and the necessity for clearing any defects are first considered. Field experience shows that insulation tests alone are not sufficient to indicate the presence of certain types of imperfections. The reason is given for selecting 10 kV d.c. as a standard test voltage for new cable.There follows a description of the terminal tests generally used for the localization of faults. Various types of confirmatory tests are then described, these being made at the suspected position indicated by the preliminary tests.Finally, the paper describes in detail new equipment designed specifically for the localization of serving defects.

DOI:10.1049/pi-a.1962.0019

出版商:IEE    

年代:1962

数据来源: IET

摘要:

A brief description is given of the methods that were employed initially for the location of leaks in cables and accessories. These include the use of amyl mercaptan, the oxygen-sandwich method and the use of heat-flow coils. This is followed by some details of the tests now in general use, including fall-of-pressure and direction-of-flow tests and the use of a halogen tracer gas. The last named is of major importance at present and descriptions of the methods of injection and detection are given. The practical problems associated with this method are also discussed.The paper closes with a brief account of some experimental work that has been carried out with radioactive isotopes.

DOI:10.1049/pi-a.1962.0020

出版商:IEE    

年代:1962

数据来源: IET

摘要:

Scanning plastic-insulated cores for discharges is now established as an intermediate test during the manufacture of plastic-insulated high-voltage cables.The paper opens with a brief survey of the existing methods, which are applicable to unscreened cores only; this is followed by a description of a new method for scanning cores provided with a semiconducting screen. Advantages and disadvantages of the various methods are discussed.The recording of discharges is an important factor common to all scanning methods. A device for recording the discharge sites as a function of their location is described.

DOI:10.1049/pi-a.1962.0021

出版商:IEE    

年代:1962

数据来源: IET

摘要:

To measure the diameter of polythene on a cable core, as it emerges from the extrusion nozzle and while it is still hot and sticky, an optical method is useful. In the apparatus described, a beam of light is swept across the cable, whose diameter is measured by the time for which the beam is interrupted. The apparatus is not seriously affected by condensation on the lenses, and the design allows for possible horizontal and vertical movement of the cable; one of these possible movements causes loss of focus in the optical system, and the electronic circuits are designed to deal with the resulting imperfect signals. The apparatus is intended for cable of about ⅝ in diameter and works to an accuracy of about ±0.001 in.

DOI:10.1049/pi-a.1962.0022

出版商:IEE    

年代:1962

数据来源: IET