All-pass Filers Based Active Damping for LCL Filters with Converter Current Feedback Control

LCL filters is an effective solution to suppress the switching harmonic in PWM converter. However, the LCL filter may introduce resonance pole to the current control loop, and challenge the current control stability. Thus, active damping control is commonly required to stabilize the system. In this paper, an all-pass filter is employed to reshaping the system open-loop phase-frequency curve to stabilized the converter side current controlled single-phase AC/DC converter. By introducing phase-lag with a second-order all-pass filter, the - 180° phase crossing at the resonant frequency is shifted to a designed point with negative gain. By doing so, there no -180° phase crossing with gain above 0 dB, and the Nyquist stability criterion tells that the current control stability promised. Design guide for the all-pass filter is given based on two frequency points phase reshaping to guarantee system stability with certain grid impedance changes and the LCL parameters drifts. Simulations and experiments performed on a 1kW single-phase PWM rectifier to verify the effectiveness of the active damping method.