A phase-lead compensation strategy on enhancing robustness against grid impedance for LCL-type grid-tied inverters

In current-controlled LCL-type grid-tied inverters, the capacitor-current-feedback active-damping method can suppress the LCL-filter resonance effectively. However, due to the digital-control-delay, LCL-filter resonance can hardly be suppressed when the resonant frequency (f
r
) is equal to one-sixth of the sampling frequency (f
s
/6). Thus, under weak grid conditions, the variable grid impedance may compromise the system stability. For weakening the adverse effect of the digital-control-delay, this paper proposes a phase-lead compensation method, which can ensure the system stability in various resonance frequency scenarios. Next, analyze stability margin requirements for the modified system, and give the design method of optimal capacitor-current-feedback coefficient (H
il
_op). With this select coefficient, stable operations can be maintained under severe grid impedance variations. Finally, the validity of the proposed theoretical analysis has been proved by testing a 1-kW prototype.