Capacitor Current Feedback With Phase-Lead Compensator to Eliminate Resonant Frequency Forbidden Region for LCL-Type Grid-Connected Inverter in Weak Grid

Digital-controlled LCL-type grid-connected inverter, with capacitor-current-feedback active damping, is widely adopted in the mediate-to-high power application because of low output harmonics and high power quality. Owing to the digital control delay, however, the inverter robustness might be weakened against the grid-impedance variation. In this article, a simple phase-lead compensator is proposed to insert into the capacitor-current feedback-path for compensating the digital control delay. In doing so, the boundary frequency of the virtual-positive-resistance-region (VPRR) can be increased from one-sixth to almost one-fourth of the sampling frequency. In addition, the resonant frequency forbidden region for the LCL filter can be eliminated, such that the inverter robustness can be significantly improved against the grid-impedance variation. To ensure the system stability and attain a good control performance, a design procedure is further presented for the phase-lead compensator as well as the closed-loop parameters. Finally, an LCL-type grid-connected inverter is built and tested in the lab, and experimental results are presented to verify the effectiveness of the proposed control scheme and parameter design procedure.