PULSE WITHSTAND CAPABILITY OF SELF-HEALING METALIZED POLYPROPYLENE CAPACITORS IN POWER APPLICATIONS - AN EXPERIMENTAL INVESTIGATION

One of the most frequent reasons for failure of metalized polypropylene capacitors is the detaching of the 'sprayed ends' from the electrode edges. This is typical in power electronic applications due to the action of voltage and current pulses which may result in electrical, thermal and mechanical stresses. A study is carried out starting from the hypothesis that, according to previous results obtained, one of these stresses is prevailing and a combined effect is not to be considered. Assuming that the thermal stress is the most significant one, several tests have been carried out in order to verify this hypothesis. This is performed by checking whether current pulses with different wave-shapes, but with the same value of the Joule Integral, lead to the same degradation level of the end-edge contact. The degradation level is ascertained by measuring the tan-delta variation, since degradation of the contact results in an increase of the equivalent series resistance of the capacitor. Contrary to the starting assumption, the results obtained show only a partial dependence of the degradation level on thermal stress, revealing also a contribution from the electrical and mechanical stresses. This has to be taken into account both to improve capacitor design in view of their application, and to develop significant type-test standards.