Research on electrode-end contact degradation of metallized polypropylene capacitors

The disconnection of "sprayed ends" from the electrode edges is one of the most frequent reasons for the failures of metallized film capacitors, especially in pulsed power applications. The sprayed end-electrode edge contact is influenced by voltage stress, thermal stress, and mechanical stress. Some new special experimental samples, which are similar to the construction of metallized film capacitor but are exempt of voltage stress, are used to investigate the influences of these stresses. Repetitive voltage pulses are applied to these new samples and normal metallized polypropylene (MPP) capacitors. It was found that electrical stress, as well as the mechanical stress resulting from it, plays a very important role in the degradation mechanism of the end-edge contact. Comparative experiments are also conducted toward the new samples, which have been stressed by current pulses with equal peak value but different Joule integral value. There was not much difference in experiment results for the two groups. These new samples were also tested under sole thermal stress with different temperature. The contact resistance rose sharply after a critical temperature and the samples suffered permanent damage even after withdrawing the thermal stress. The results show that within a certain temperature range, electrode-end contact degradation is more related to the mechanical stress, which depends on the peak value of the current pulse, rather than the energy content of the current pulse.