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

Abstract

Digital-controlled <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> 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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -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.