摘要:

The electric strength of polythene when subjected to 50-c/s alternating electric stress has been measured for a range of ambient temperatures from −196° C to 17° C, and that of polystyrene for temperatures between −196° C and 70° C. The a.c. breakdown strengths are found to be lower than the corresponding intrinsic electric strengths at room temperature and above, particularly with polythene, but no difference is observed at low temperatures. In measurements in which this lowering occurred, the results were dependent on the duration of the stress application.It is concluded that an increase in the internal temperature of the test specimens can explain these results.

DOI:10.1049/pi-2a.1953.0025

出版商:IEE    

年代:1953

数据来源: IET

摘要:

Published information on the impulse breakdown of high-voltage cables of the solid and gas-pressure types is summarized with particular reference to the effect of the testing variables on the impulse strength.An impulse generator and its associated measuring equipment for performing tests on cables are briefly described.Details of measurements of the impulse strength of solid-type and gas-cushion cables are given, and reference is made to impulse-strength experiments on impregnating compound and impregnated paper, and on cable models.The impulse breakdown process in high-voltage cables of these types is discussed in relation to the properties of their constituent dielectric materials.

DOI:10.1049/pi-2a.1953.0026

出版商:IEE    

年代:1953

数据来源: IET

摘要:

A research programme has been commenced which is designed to determine the mechanisms of electric breakdown in insulating liquids under various conditions. The paper describes experimental equipment and techniques, and some results which have been obtained on transformer oil.Special equipment includes a transparent pressure chamber with an internal glass test-vessel, which enables the liquid under test to be subjected to hydrostatic pressures up to 500 lb/in2and voltages up to 100 kV peak. A protective circuit hasbeen developed, incorporating triggered spark-gap, which limits the duration of breakdown currents to a few microseconds.A wide range of breakdown strengths has been obtained, varying from 5 000 kVpeak/cm with 1-microsec impulses, to below 100 kV(r.m.s.)/cm for a fibre contaminated oil under a.c. test conditions. The removal of fibres raises the a.c. strength of otherwise untreated oil to 400 kV(r.m.s.)/cm.It is concluded that a breakdown mechanism initiated by cavitation is operative in certain circumstances.

DOI:10.1049/pi-2a.1953.0027

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The effect of various conditions maintained for considerable periods on the properties of some of the more recently developed laminated-sheet insulating materials is investigated. The tests are not yet complete, but in most cases they have been carried on for a sufficient time to indicate trends which it is hoped to confirm at some future date.Some other properties of the test sheets, measured in accordance with established procedure, are also included.

DOI:10.1049/pi-2a.1953.0030

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The objects and aims of impregnation are discussed, and methods of test for impregnating varnish are suggested in order to provide simple practical methods designed to relate the properties of a varnish with its particular application.

DOI:10.1049/pi-2a.1953.0031

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The paper discusses the use, in rotating machines, of new or improved insulation materials which have become available during the last 25 years, with the object of indicating the advantages obtained by using such materials and also the disadvantages and difficulties attendant with some of them. Practical experience over many years with some of the materials has revealed some serious drawbacks; on the other hand, some materials have proved to be even more useful than was thought at first.

DOI:10.1049/pi-2a.1953.0032

出版商:IEE    

年代:1953

数据来源: IET

摘要:

Thermosetting varnishes based on oil-modified phenolic resins appear to have certain advantages over the conventional oil varnishes for the treatment of coils which operate under oil. These anticipated advantages are associated with their heat-hardening characteristics and the absence of metallic driers. Oxidizing oil varnishes dry and harden by oxidation; hence, when applied as a thin film they will harden satisfactorily. However, if they are applied thickly only the surface hardens, and the varnish underneath remains fluid indefinitely. These varnishes also contain ingredients which may adversely affect the insulating oil.Laboratory tests were therefore made to compare the behaviour in oil of small coils (a) untreated, (b) treated with thermosetting varnishes and (c) treated with conventional oil varnishes. The criterion adopted in these comparisons was the ultimate effect of the specimen coil on the sludging and general properties of the mineral oil in which it had been immersed. Other factors likely to influence the effect of the coils, such as the conductor, conductor covering, the presence of air and susceptibility of the oil to deterioration, were also taken into consideration. The tests were made at temperatures between 75° and 80° C and the normal period of heating was about 10 weeks.The results of the experiments indicate that thermosetting varnishes are superior to oxidizing oil varnishes for use under oil. Their greater oil resistance eliminates direct contamination of the oil, and the permanency of the varnish coating effectively suppresses the potential catalytic activity of the copper in causing oxidation of the oil. There are also indications that the thermosetting varnish inhibits the oiloxidation process. On the other hand, there were equally definite indications that, unless adequately dried, conventional oil varnishes were liable to accelerate the formation of sludge and the development of acidity in the oil. Nevertheless, it was evident that treatment of a coil in one of the more oil-resisting grades of such oxidizing varnish was preferable to leaving the coil in an unvarnished condition.The results of the subsidiary tests confirm the effectiveness of a coating of tin or enamel on the copper as a barrier to the catalytic activity of the metal. They also demonstrate the marked influence of the inherent characteristics of an oil upon the manner and progress of its subsequent deterioration and of the predominating effect of oxidation in the oil-deteriorative process.

DOI:10.1049/pi-2a.1953.0033

出版商:IEE    

年代:1953

数据来源: IET

摘要:

A review of the available ceramic dielectrics shows that they fall into two groups: non-ferro-electric materials, which have low losses and high stability, and ferro-electric materials, which have very high permittivities and higher losses and are not so stable. The latter behave in a manner analogous to ferromagnetic materials; their properties, besides being highly field-dependent, also vary considerably with temperature.The most suitable shapes for capacitors are derived from a consideration of the process of manufacture of ceramic materials. Capacitors in electronic circuits are classified according to their functions. Ceramic capacitors fulfil admirably the requirements for low- or controlled-temperature-coefficient capacitors in tuned circuits as by-pass capacitors at higher frequencies, because of their small size and low self-inductance. Ceramic capacitors can be used in equipment of all power levels, from hearing-aids to high-power transmitters.

DOI:10.1049/pi-2a.1953.0034

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The properties and behaviour of some silicone materials of interest to power engineers have been examined, using mainly British Standard test methods, or modifications of them, so that comparison with conventional materials can readily be made. These, together with the results of accelerated life-tests on motors employing silicone resins as bonding and impregnating media, show that such materials are capable of working at temperatures at least 50° C higher than those permissible for conventional insulants.Many of the materials also possess properties, other than that of thermal endurance, which make them eminently suitable for specific applications. Some of these are described, together with some of the difficulties encountered during limited experience in using the materials.

DOI:10.1049/pi-2a.1953.0035

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The materials surveyed include porcelain, modified porcelains, cordierite ceramics, normal and low-loss steatite, alumina and zircon porcelains, sintered alumina and other oxides, the semi-conducting ceramics and the dielectric materials based on titania and other oxides.Ranges of composition and/or the general nature of the materials used, methods of forming insulators or capacitors from such mixes, limitations of manufacturing methods and the limits of firing temperatureare detailed.The relationship between the type and amount of glassy phase in the insulating ceramics and their physical properties (particularly their electrical properties) is discussed. The limitations or outstanding qualities of each material are considered.The theory of semi-conductors, the principles underlying their production and certain of their important uses are surveyed.A broad outline of the dielectric ceramics is given, and the most important properties of several of the best known groups of ceramic materials are tabulated.

DOI:10.1049/pi-2a.1953.0036

出版商:IEE    

年代:1953

数据来源: IET