Active Damping of LCL-Filter Resonance Using a Digital Resonant-Notch (Biquad) Filter

The LCL filter is a fundamental component which interfaces a voltage-source inverter (VSI) with a power grid, but on the other hand, it introduces a potential resonance risk which may be harmful to the system stability. In this paper, a digital biquad filter, which consists of a resonance and a notch, is proposed to damp the LCL resonance. By carefully tuning the resonance frequency and the notch frequency of the biquad filter, the LCL resonance is equivalently shifted from an unstable region into a stable one. The proposed method possesses a sensor-less benefit, and it can provide a high control bandwidth as well as a strong robustness. Experimental results acquired from a three-phase grid-connected inverter verify the effectiveness of the biquad filter.