On the stability of discrete-time active damping methods for VSI converters with a LCL input filter

The use of LCL filters with VSI converters is interesting since they present good attenuation of current ripple at high frequencies. Nevertheless, they also present a high resonance peak which can cause undesired oscillations, and even instability problems. Passive and Active Damping are methods which try to reduce those resonance effects. In this paper, Passive Damping is briefly reviewed and Active Damping is formally analyzed in continuous-time and, specially, in discrete-time. It will be shown that the one period delay included in sampled-data control systems and the grid impedance play a very important role in stability conditions. The presented results are useful to design discrete-time Active Damping controllers. Moreover, important implications for higher level controls can be deduced since the Active Damping is the first control loop in the power converter's controller.