Examination of influence of electrical stress on parameters of PP film capacitors

Properties of polypropylene film capacitors, which are usually used in power electronics equipment, were examined after two-level process of electrical loading. Capacitors with the value of 10 nF and nominal voltage 630 V DC, 300 V AC, were loaded by the AC voltage with the frequency of 50 Hz in following steps: the voltage 450 V was applied for 30 min at first and consequently the voltage 500 V for 60 min. With regard to the values of the voltages applied a self-healing process occurred. The impedance of the capacitors was measured in the frequency range of 15 MHz-20 MHz, the capacitance and loss factor in the frequency range of 300 Hz-1 MHz. The changes of these parameters caused by electrical stress of the capacitors were examined. It was found that the capacitance decreases due to the decrease of the electrodes area caused by the self-healing process of the capacitors. Small changes only of the loss factor and impedance of the capacitors caused by ageing under electrical stress were found.