Exploring inherent damping characteristic of LCL-filters for three-phase grid-connected voltage source inverters

This paper investigates the inherent damping characteristic of LCL-filters for three-phase grid-connected voltage source inverters (VSIs). Specifically, it is found that there is an inherent damping term embedded in the feedback loop when converter current is used for implementing closed-loop control. This additional damping term can indeed neutralize the resonance introduced by LCL-filters, and thus giving rise to a more stable system than that of grid current feedback control. In this case, only one set of current sensors is required to stabilize the system, doing away with passive damping, active damping or complex state observer. Theoretical analysis is then presented and lead to a general design guideline, which suggests a way of choosing the values of grid- and converter-side inductors, so that optimum damping can be naturally achieved by solely using converter current control. In the case when this design criterion is not fulfilled, a simple compensation method is also proposed to tune the damping factor. Both simulation and experimental results are finally provided to validate the theoretical findings developed in this paper.