Dual reference frame scheme for distributed generation grid-connected inverter under unbalanced grid voltage conditions

Under unbalanced grid voltage conditions, second harmonics dc link voltage components and negative-sequence ac link current components for distributed generation grid-connected inverter is produced. Through PWM modulation, third harmonics in ac link current is caused by second harmonics dc link voltage components. We introduced a dual reference frame scheme to eliminate the dc link voltage ripple and negative-sequence ac link current components. Unbalanced grid voltage and ac link current are decomposed into positive and negative sequence components. Positive sequence components is regulated in a positive synchronous reference frame (SRF) to flow active power. Negative sequence components is regulated in a negative SRF. In the same time, the negative sequence grid voltages is used as command signals to produce the negative sequence components in the output terminal of distributed generation grid-connected inverter. So negative-sequence ac link current components is reduced. Two sequence components are controlled in it's own rotating reference frames, so the disturbance of dc link voltage and ac link current is eliminated. Computer simulation and experiments results are presented to demonstrate the effectiveness of the proposed control scheme.