Uncoupling behaviour of current gates in self healing capacitors

Recent high power capacitor technology use thin polypropylene (PP) foils as a dielectric with 15 nm thin patterned electrodes instead of all-over metallized films. The metal electrode consists of individual segments interconnected by narrow current gates. The gates serve as fuses in case of a breakdown in one of the segments. They isolate the segment and therefore the breakdown channel from the rest of the electrode. Therefore the capacity only decreases slightly and the capacitor is protected against complete destruction. This process is called self healing. Capacitors in operation showed that in case of a breakdown not only the defect segment but also the surrounding and the distant segments are often uncoupled, leading to a higher decrease of capacity and consequently of the capacitor lifetime. For traction applications, a capacitor lifetime of more than 20 years is required, whereby the presently accepted capacity loss is 2%. The aim of the study was to understand the mechanism that breaks off distant current gates. Therefore we stressed current gates with low voltages and currents, determined the energy involved in the uncoupling process and investigated the broken gates with light microscopy. Resistance curves gave important information about the influence of structures at the PP foil surface on the uncoupling behaviour. The studies showed that the electrode segments are not separated by evaporation, but mechanical stress leading to adhesion failure due to thermal expansion between the PP foil and the metallization determine the uncoupling of distant current gates.