Grid current control methods by using compensation voltages under distorted and unbalanced grid voltages

This paper proposes the control method for reducing a total harmonic distortion (THD) of the grid current at the three-phase grid-connected inverter systems when the grid voltages are distorted and unbalanced. The THD of the grid current caused by grid voltage harmonics is derived by considering the phase delay and magnitude attenuation due to the hardware low-pass filter (LPF). The Cauchy-Schwarz inequality theory is used in order to search more easily for a minimum point of THD. Both the gain and angle of a compensation voltage at the minimum point of THD of the grid current are derived with a variation of cut-off frequencies of the hardware LPF. The unbalanced grid currents due to the unbalanced grid voltages are compensated by using a negative-sequence component of the grid voltage. The simulation and experimental results show the validity of the proposed current control methods.