An improved capacitor-current-feedback active damping for LCL resonance in grid-connected inverters

Grid-connected inverters equipped with LCL filters are very popular in utility applications for their better power quality features, while the LCL resonance may threaten system stability. To dampen the LCL resonance, the widely used active damping method is based on the proportional feedback of the filter capacitor current. However, it has been revealed that when the LCL resonance frequency is above one-sixth of the system sampling frequency (fs/6), the time delay existed in the digitally controlled system will generate a pair of open-loop unstable poles, which make the system become much easier to be unstable. To address this issue, this paper proposes an improved capacitor-current-feedback active damping, which is realized by inserting a second-order lead-lag compensator into the active damping loop. The added compensator aiming at removing the unstable poles is designed base on the first-order Padé approximation. By deriving the equivalent virtual circuit introduced by the proposed method, it is obtained that the region of paralleled virtual resistor being positive is extended, leading to a more robust system. After that, the modified current control loop gain is derived to compare the damping performance with that of the conventional method. Lastly, the effectiveness of the proposed active damping method is verified by the experimental results.