Robust Active Damping Control for LCL-Type Shunt Active Power Filters

A dual-loop current control is discussed for LCL-type shunt active power filters (APFs), where inverter-side current feedback and point of common coupling (PCC) voltage feedforward both provide active damping (AD) effects for LCL resonance. However, two ADs are equivalently applied to different elements of LCL filter with narrow damping boundaries, which causes less robustness against grid impedance variation. Hence, this paper has presented a comprehensive approach to strengthen the system robustness against grid impedance variation. On the basis of phase compensated resonant unit, a delay-compensation scheme is firstly proposed to extend the damping boundary of inverter-side current feedback up to almost Nyquist frequency ( $f_{s}/2$ ). Theory analysis considering PCC voltage feedforward indicates that the control system can keep robust to grid impedance when the initial LCL resonance frequency falls between $f_{s}/3$ and the extended damping boundary. Moreover, a typical weighted proportional-resonant feedforward scheme is suggested to prevent high-frequency resonant units from inducing system instability under a large grid impedance. We finally validate the effectiveness of the proposed approach through experiments.