Robust capacitor-current-feedback active damping for the LCL-type grid-connected inverter

This paper explores the effect of the computation and pulsewidth modulation (PWM) delays on the capacitor-current-feedback active damping for the LCL-type grid-connected inverter. It turns out that proportional feedback of the capacitor current is equivalent to a virtual impedance connected in parallel with the filter capacitor in digital control. The LCL-filter resonance frequency is changed by this virtual impedance. And if the actual resonance frequency is higher than one-sixth of the sampling frequency (f_s/6), where the virtual impedance contains a negative resistor component, a pair of open-loop unstable poles will be generated. As a result, the LCL-type grid-connected inverter becomes much easier to be unstable if the resonance frequency is moved closer to f_s/6 due to the variation of the grid impedance. To address this issue, this paper proposes a capacitor-current-feedback active damping with reduced computation delay, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant. With this method, the virtual impedance exhibits more like a resistor in a wider frequency range, and the open-loop unstable poles are removed, thus high robustness against the grid-impedance variation is acquired. Experimental results confirm the theoretical expectations.