Grid-connected converter control during unbalanced grid conditions based on delay signal cancellation

Considering the recent trend in energy sector transformation towards high share of renewable energy sources, it has become very hard to imagine modern power system without the integration of power electronic devices. A grid-connected converter will be on the forefront of future energy trading, while simultaneously striving to offer good dynamic behaviour and operation in full accordance with the relevant grid requirements. The control algorithm of the grid-connected converter has to be capable of achieving the stable steady state operation even during the most severe faults in the system. More importantly, the power quality of the injected currents (and thus the power) needs to be kept at the maximum possible level. This paper presents the control strategy for the grid-connected converter that offers the possibility of symmetrical grid current injection at the point of common coupling even during unbalanced grid conditions. Proposed control strategy uses delay signal cancellation in the negative sequence synchronous rotating reference frame for the mitigation of the respective current components. The negative influence of asymmetrical grid voltages, present at the point of common coupling as a result of unbalanced grid loads or faults, will be shown within the paper. The key features of the improved control method are outlined, with a special reference to basic theoretical background. The proposed method is experimentally verified using sophisticated research and development station for control of grid-connected converter.