Enhanced stability of capacitor-current feedback active damping for LCL-filtered grid converters

The proportional capacitor-current feedback active damping method has been widely used to suppress the LCL-filter resonance. However, the time delay in the damping control loop may lead to non-minimum phase or even unstable responses when the resonance frequency varies in a wide range. To improve the robustness of damping, this paper proposes an improved damping controller with the capacitor current feedback loop, which is based on the second-order generalized integrator, instead of a proportional gain, which can effectively mitigate the detrimental effect of the time delay. Robustness of the proposed method against grid inductance variation is proved by z-domain and s-domain analyses. Finally, experimental results validate the effectiveness of the proposed method on a three-phase grid-connected converter.