Power converters and control techniques are presented in which capacitor degradation is detected according to negative sequence current by computing an uncompensated negative sequence current and a negative sequence voltage, compensating the negative sequence current based on the negative sequence voltage, comparing the compensated or uncompensated negative sequence current with an automatically calculated threshold and selectively identifying suspected degradation of one or more capacitors if the compensated negative sequence current exceeds the threshold value. The method can be used for power converters, control devices or protection relays for shunt capacitors or filters used in power systems.