86_compo_tanaka - IEEE Transactions on Electrical...

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IEEE Transactions on Electrical Insulation Vol. EI-21 No.6, December 1986 CHARACTERISTICS OF COMPOSITE INSULATION: LIQUID-IMPREGNATED INSULATION T. Tanaka Central Research Institute of Electric Power Industry Komae-shi, Tokyo, Japan ABSTRACT A description is given of composite insulation, i.e. swelling of polymers by oil, oil streaming electrification of solid dielec- trics, and V-t characteristics of LN2-impregnated insulation due to partial discharge. Swelling can be accelerated by ultrasonic wave irradiation. This reduces the saturation time. Polypropy- lene-laminated paper would swell less inside its cable structure. Ac dielectric breakdown strength increases with swelling in the early stage and then decreases. Streaming electrification was found to be influenced by the oil flow rate, temperature, ac ap- plied voltage, and the type of solid and liquid dielectrics. Surface roughness of solid dielectrics does affect the electri- fication greatly. Addition of benzotriazol at only ppm is ef- fective in suppressing the electrification. V-t characteristics are divided into two regions for liquid nitrogen (LN2) impreg- nated insulation. The value of n in Vnt=const ranges from 50 to 100 in the first region, while it is around 5 to 10 in the second region. INTRODUCTION Composite insulation may be represented by two typi- cal examples, i.e. oil-impregnated paper insulation for power cables, capacitors, and transformers and resin- impregnated micaceous insulation. Since resin-mica in- slatuion and its multistress aging are described else- where in this issue, oil-impregnated paper insulation is emphasized here. As a special topic, paper and other insulation impregnated with LN2 is touched upon briefly. One of the most important propprties of composite in- sulation is the dielectric breakdown strength in the design of power cables and apparatus. The basic con- cepts of electrical insulation [1] are (1) filled-void type insulation for cables and capac- itors (2) barrier insulation for power transformers. The former was based on the improvement of paper insu- lation, and the latter was developed from oil insula- tion. They are all devised to increase the dielectric breakdown strength. Other design criteria are the dissipation factor and the permittivity. Polymer films have a lower tan6 and permittivity, and higher breakdown strength than cellu- lose paper. Therefore, they are now used as a substi- tute for cellulose paper wholly or partly, to make power apparatus less lossy and more compact. All- polymer capacitors and polymer-laminated paper cabbes have been developed for use. Power transformers have not shared in the benefit of polymers yet, but have been reduced in size by oil circulation cooling. Therefore,
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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86_compo_tanaka - IEEE Transactions on Electrical...

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