Observer-based state-space current control for a three-phase grid-connected converter equipped with an LCL filter

This paper presents a state-space current control method for active damping of the resonance frequency of the LCL filter and setting the dominant dynamics of the converter current through the direct pole placement. A state observer is used, whereupon additional sensors are not needed compared to the conventional L filter design. The relationship between the system delay and instability caused by the resonance phenomenon is considered. Nyquist diagrams are used to examine the parameter sensitivity of the proposed method, and the method is validated with simulations and experiments.