Heat generation and temperature distribution in DC metallized polymer film capacitors

It is known that elevated temperature can considerably reduce the life expectancy of metallized polymer film capacitors. For film capacitors in service, both the application of a time-varying external voltage and the extinction of internal breakdown events can deposit significant heat onto the capacitor structure leading to an undesirable temperature rise. Often such heat generation is also localized spatially and the resulting temperature gradient enhances the probability of subsequent breakdowns. To gain an understanding, a numerical model is developed to simulate the dynamics of heat generation and transfer under the switching impact of an external DC voltage. Thermal processes in the solids (polymer layers, electrode coating and zinc-sprayed segment) are modelled with a conduction mechanism, whereas a convection mechanism is considered for silicone oil and air. Heat generation is induced by ohmic losses of the externally applied electric field, which can be calculated from an equivalent circuit model recently developed for metallized film capacitors. Thus the heat transfer model interfaces with the equivalent circuit based field calculation model. Based on this electrothermal model, numerical examples are used to study the temperature distribution within film capacitors and then to examine the likelihood of localized temperature hot-spots that may lead to spatially preferential breakdowns.