Direct impedance control of grid-connected inverter

Grid-connected inverter are commonly utilized to interface renewable energy sources into the utility, especially for micro-grid and distributed generation system applications. Different from available direct power, direct voltage, or direct current based control strategies of grid-connected inverters for renewable energy integration, to achieve flexible operation, the direct impedance control of grid-connected inverters is presented in this paper. Instantaneous power theory is used to estimate the output impedance of the inverter. An external impedance loop with PI controller is applied to directly control the inverter impedance, and an inner current loop with PR controller is employed to improve the transient features. By means of the proposed approach based on the equivalent circuit of control scheme, mathematical models of PI and PR controllers for direct impedance control of inverters are changed into electric circuits to confirm the performance of impedance-tracking, which have been confirmed by detailed analysis and discussions. Finally, simulation results by using PLECS are presented to verify the validation of proposed impedance-based control of the grid-connected inverter.