Interpretation of short-circuit discharge current in film capacitors based on an equivalent circuit

Misfire phenomena in a discharge circuit composed of a capacitor and thyristor were investigated for operating frequencies to 198 Hz. Misfire was found to occur in polyethylene terephthalate (PET) capacitor circuits at low temperatures, whereas polypropylene (PP) capacitor circuits did not misfire under any conditions. Transient currents in the range 1 mA to 40 A were measured using a digital oscilloscope. A PET capacitor was shown to discharge initially with a given time constant, but subsequently discharge at a lower rate, extending the discharge period. It is this delayed gradual decay that causes misfire, and it is associated with residual charges trapped in the bulk of the PET film. The correlation between the transient discharge current and dielectric dispersion in PET and PP capacitors was investigated over a wide frequency range with the aid of the Debye model for dipolar polarization. It was found that the time dependence of the discharge current is related closely to the broad distribution of dipole relaxation time, with a discrepancy between the discharge current and the dielectric response attributed to charge trapping.