Control Method for Reducing a THD of Grid Current at Three-Phase Grid-Connected Inverters Under Distorted and Unbalanced Grid Voltages

Abstract

This paper proposes the control method for reducing the THD and eliminating an unbalanced component of a grid current under the distorted and unbalanced grid voltages in a three-phase grid-connected inverter system. The unbalance of grid current caused by unbalanced grid voltage is compensated by the magnitude of fundamental negative-sequence voltage from grid voltage. The THD of a grid current due to grid voltage harmonics by considering the phase delay and magnitude attenuation due to the hardware LPF is derived. The THD can be simplified by using the Cauchy-Schwarz inequality theory, the minimum point of THD is searched easily. The variation of both the gain and phase angle of the compensation voltage for minimizing the THD of the current are investigated according to a cut-off frequency of the hardware LPF. Simulated and experimental results will be shown for verification.