Three Reference Frame Control Scheme of 4 wire Grid-connected Inverter for Micro Grid Under Unbalanced Grid Voltage Conditions

For 4 wire VSI, Under unbalanced grid voltage conditions, neutral currents that flow through the fourth wire will distort the symmetrical output voltage. And DC link voltage ripple will be produced. We introduced a three reference frame scheme to eliminate the DC link voltage ripple and output voltage distortion. Unbalanced grid voltage and AC link current are decomposed into positive, negative and zero sequence components. Positive sequence components is regulated in a positive synchronous reference frame (SRF), negative sequence components is regulated in a negative SRF, and zero sequence components is regulated in a zero SRF. In positive SRF, positive components appear as dc, while negative components appear as 2omega rad/s. In contrast, in negative SRF, negative components appear as dc, while positive sequence appear as 2omega rad/s. Zero sequence components will not have any impace on dq, it appears as a disturbance in the o-variable at the fundamental frequency. In order to eliminated the dc link voltage ripple and output voltage distortion, positive, negative and zero sequence components should be controlled simultaneously. Simulation and experimental results are presented to verify the validity of the proposed control scheme.