Active Damping Techniques for Suppressing the LCL Filter Resonance in Distributed Generators

In grid connected DC-AC systems, low pass filters are utilized to reduce injected current harmonics. LCL filters have recently drawn attention for system grid interfaces due to its small size and have shown better attenuation to switching harmonics than simple L filters. However, the LCL filter causes resonance resulting in oscillation and instability of the current controller. In order to stabilize the current controller, either a passive damping method i.e. a resistor within the filter or an active damping method i.e. modification of inverter control has to be incorporated. Active damping techniques are preferred to avoid additional losses with passive techniques and thereby increasing the inverter efficiency. This paper discusses the LCL problem and classifies the active damping methods that have been proposed to date. A simulation study is conducted to validate the discussions.