Power Control of a Three-phase Grid-connected Inverter using a PI Controller under Unbalanced Conditions

Under balanced three-phase system conditions, various conventional control methods were applied for controlling a grid-connected three-phase inverter, such as proportional-integral (PI) controller and proportional-resonant (PR) controller. The grid can become imbalanced for a variety of causes, including single-phase loading and single-phase renewable energy sources, impacting inverter operations and other grid-connected load. The PI control method functions poor in an unbalanced three-phase system, as illustrated by a current oscillation. This paper describes a method for converting an unbalanced three-phase system into three balanced components: positive, negative, and zero sequence, utilizing a time-domain symmetrical component extraction method. The symmetrical components currents are controlled by the PI controller. This system is simpler to use than other ways reported in the literature because it only requires one phase-locked loop.