Active damping of LCL filter at low switching to resonance frequency ratio

Use of LCL filter along with a voltage source inverter (VSI) requires damping of resonance. Active damping (AD) methods are preferable compared with passive damping methods from efficiency point of view. Recent researches show that achieving stable operation of a grid connected VSI with LCL filter at a low switching frequency to resonance frequency ratio can cause instability. In this study a resonant integrator (RI) based second order filter (RISF) has been proposed for AD of LCL filter resonance for stable operation at low switching frequency to resonance frequency ratio. The method extracts resonance frequency capacitor current from sensed capacitor voltage in a per phase manner and does not need any additional sensors. The modelling and analysis of the closed loop system with PR current controller and proposed AD has been carried out in discrete domain including all delay effects. Experimental results have been included in support of the theoretical analysis.