Interpretation of ratings for plastic film capacitors

Plastic film capacitors have always played an important role in power electronic circuits. In the earlier days of thyristors with forced turn-off, large capacitors of tens or even hundreds of microfarads were used for commutation purposes. Much smaller capacitors are used for snubber networks with transistors and other controlled switching devices, and more recently capacitors play an important role in resonant-turn off circuits and soft switching. In all cases these capacitors handle large currents and experience high dv/dt on a low duty cycle basis. A small snubber capacitor may handle several hundred amps with a dv/dt of 1000V/us. Due to the low duty cycle the rms current may be quite modest. Nevertheless very high grade capacitors are necessary with a very low loss dielectric. For this reason polypropylene film is generally used. This paper examines the various ratings which must not be exceeded for safe operation and in particular discusses how the effective VAr may be estimated for short duration current pulses in repetitive waveforms and how VAr is related to the power dissipation in a capacitor.