This paper presents the first results of a procedure which allows the determination of the threshold for space charge build-up in polymeric insulation subjected to ac electrical field. Thresholds for space charge accumulation under dc were determined poling insulation until space charge appeared steadily. In ac the amount of charge accumulated is much smaller than in dc, and the time needed to reach steady conditions can be very long at fields close to threshold. Hencefiom, ac poling tests were made at different fields and for different times until a chosen value of accumulated charge was reached. Decreasing the test voltage, a limit field was determined below which charge accumulates much more slowly with respect to the linear relation obtained at higher field levels, and its value constitutes an approximate estimate of the searched threshold field. This procedure is applied to specimens of cross-linked polyethylene. A threshold field lower than, but close to, 20 kV/mm is found.
Trends of several applications like down-hole drilling, commercial aviation (e.g. jet engines), heavy industrial and automotive are challenging the capabilities of capacitors and other electronic components. The growing harsh-environment conditions for these applications are: high temperature, high voltage and high current. At the capacitor component level, required features are: very high reliability under mechanical shock, rapid changes in temperature, low leakage current (high insulation resistance), small dimensions, good stability with time and humidity, and high peak withstanding voltage. Capacitors for power-conversion circuitry must maintain a low AC loss and DC leakage at high temperatures. KEMET has recently designed film capacitor series using PEN to address the needs of the above mentioned circuits, in particular regarding the working temperature, voltage and current. This paper will cover technological advances in film capacitor technology to address harsh environment conditions needs, providing test results on temperature, voltage and thermal shock acceleration factor.
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