Wide Damping Region for LCL-Type Grid-Connected Inverter With an Improved Capacitor-Current-Feedback Method

This paper has presented a stability analysis of a LCLtype grid-connected inverter in the discrete-time domain. It has been found that even though the system is stable when the resonance frequency fr is higher than one-sixth of the sampling frequency (f(s)/6), an effective damping scheme is still required due to the potential influence of the grid impedance. With a conventional proportional capacitor-current-feedback active damping (AD), the valid damping region is only up to f(s)/6. This however is not sufficient in the design process for obtaining a high quality output current and the system can easily become unstable due to the resonance frequency shifting. Considering the resonance frequency design rules of the LCL filter, this paper proposes an improved capacitor-current-feedback AD method. With a detailed analysis and proper parameter design, the upper limit of the damping region is extended to f(s)/4, which can cover all the possible resonance frequencies. Then, the damping performance of the proposed AD method is studied. It shows that the optimal damping is obtained when the actual resonance frequency is (f(r) + f(s)/4)/2. Moreover, an approximate calculation for the optimal damping coefficient R is given. Finally, the experimental results have validated the effectiveness of the proposed AD method.