Assessment of the dielectric behaviour of some liquid impregnated systems

Abstract

This thesis reports the results of an extensive investigation carried out to develop a possible alternative to polychlorobiphenyls (PCBs) which are being banned in most countries due to their toxicity and poor biodegradability. It is also intended to acquire a database for suggesting a possible low-loss dielectric system for high-voltage capacitor application. The study includes, among other things, a detailed analysis of the dielectric behaviour of the impregnate itself, impregnated polypropylene and sandwiched paper-polypropylene systems at different frequencies, temperatures and electrical stresses. A detailed study of the partial discharge behaviour of the system has shown that the discharge inception stress depends upon the permittivity of the impregnate, the thickness of the dielectric and operating temperature. The compatibility study of castor oil with polypropylene has been made on the basis of its loss of mechanical and dielectric strength when aged in the presence of oil at elevated temperatures. The deterioration of the oil has been assessed on the basis of variation in its dissipation factor and structural change as shown by its IR spectrum. Gassing tendency of oil-polypropylene system has been assessed by gas chromatographic and mass spectrographic methods. Ageing studies have proved the compatibility of the proposed impregnate with polypropylene. The aspect of life of any insulation system in the proposed system in particular is studied under the condition of synergy. A tangent-delta model for twin dielectrics has been put forward. An equation has been derived on the basis of temperature