Identification of Capacitor Degradation in LCL Filter for Energy Storage Systems Converter

Active damping has been applied to LCL- filtered converter in energy storage systems to ensure a stable current control. However, the aging effect and operational conditions may lead to a significant parameter drifting of the filter capacitance. This may result in a new resonant frequency beyond the designed active damping region. To address the issue, this paper analyzes the resonance characteristic of the LCL filter with all-pass filter based active damping. The analysis reveals that a resonant at 0dB- magnitude crossing point will be detected by utilizing all-pass filter move the -180°-phase crossing point to the 0dB- magnitude crossing point, which is filter inductance and capacitance dependent. To decouple the magnitude crossing points with the LCL parameters, six-order harmonic injection method is used to identify the ratio between capacitance and inductance, then the filter capacitance can be calculated based on identified 0dB-magnitude crossing point by LCL system model. Real-time Simulation tests are performed, and the results verify that the proposed method can accurately obtain the filter capacitance.