Modeling of Capacitor Parameters Related to the Metal Film Layer With Partial Edge Disconnection

Portions of the metallized surface in metallized polypropylene capacitors, can lose direct connection with the schooping variously by sparking, mechanical forces, and corrosion. This results in lengthened connection pathways which in turn leads to rapidly increased dissipation factors and increased localized current density. Such outcomes threaten the endurance of the capacitor and may be a key factor in catastrophic failures in the field. In this paper, concentrating on symmetrical edge-disconnection whereby both layers are disconnected for the same lengths, the characteristics of edge-disconnected wound metal strip are modeled using distributed circuit analysis. It is shown that equivalent series resistance (R_ES ) and equivalent series capacitance (C_ES ) ultimately reduce with the square root of frequency. Empirically-derived formulae for these and for the alternate capacitor model of equivalent parallel resistance (R_EP ) and equivalent parallel capacitance (C_EP ) are demonstrated. It is shown that for symmetrical edge-disconnection, impedance represented by series distributed inductance is negligible for most considerations compared to ohmic resistance. Lead and package inductance are external to this analysis and are not included.