Adaptability Enhancement of LCL-Type Grid-Connected Inverter with Single-Loop Inverter-Side Current Feedback to Complex Power Grid

Grid-connected Voltage Source Inverters (VSI) equipped with LCL filters are attractive in utility applications for their better power quality features. The single-loop inverter-side current feedback (ICF) control is often used due to its low cost in practice. A stable single-loop ICF-controlled grid-connected system designed under ideal power grid (i.e., grid impedance is 0) can still maintain stability under inductive power grid (i.e., grid impedance only contains inductance), but may be unstable under complex power grid (i.e., grid impedance contains inductance and capacitance simultaneously) when frequency responses of inverter output admittance and equivalent grid admittance intersect at non-passive frequency region of inverter output admittance. Literatures have revealed that non-passive region of inverter output admittance occurs above critical frequency fcri due to digital control delay, and that fcri is fs/6 considering typical delay of 1.5 times Ts. Various phase compensators proposed in literatures can help to improve fcri for enhancing the adaptability of single-loop ICF-controlled grid-connected inverter to complex power grid by inserting them in series or parallel with current regulator. In this paper, a biquadratic compensator is inserted in series with current regulator and parameter design of biquadratic compensator is presented. The fcri is improved to 0.407fs with the biquadratic compensator designed in this paper, which is higher than fcri in literatures.