Detection of Capacitor Degradation in Filters for AC Drives

Adjustable-speed drives with regenerative pulsewidth modulation rectifiers-both voltage source rectifiers and current source rectifiers-have LC filters on their input to attenuate the high-frequency components of current and achieve reduced levels of total harmonic distortion. The capacitors used in these filters are predominantly of the metalized film construction and are prone to catastrophic failure. In this paper, a method is proposed for the detection of capacitor degradation and location of the faulted phase. The method uses measurements of voltages and currents-which may already be available for control purposes, for line synchronization and active damping-to detect impedance asymmetry and is impervious to input voltage unbalance.