Complete Time-Delay Compensation Method for LCL-Type Grid-Connected Inverter with Capacitor-Current Active Damping

Capacitor-current proportional feedback active damping is often used to suppress LCL inherent resonance of inverters, but the stability can be threatened by time delay in digital control system. In a grid-side current controlled inverter, it has been proved that, because of the digital time delay, the virtual resistance paralleled with the LCL filter capacitor is negative if the resonance frequency is higher than one-sixth of the sampling frequency ($f_{\mathrm{s}}$/6) and positive when the LCL resonance frequency is lower than $f_{\mathrm{s}}$/6. So, the system will be possibly unstable when resonance frequency crosses f s /6 due to the grid impedance variation, and the robustness is poor in the case of the negative virtual resistance. To address this issue, this study proposes a method to completely compensate the time delay by introducing a second-order compensator reshaped from the conventional Biquad filter in capacitor-current-feedback control loop. Theoretical analysis shows that this method can expand the positive damping region up to Nyquist frequency. Hence, high robustness is achieved against the grid impedance variation. Finally, the effectiveness of the proposed method is verified by experimental results.