Zero-Sequence Current Controller for a Four-Leg PV Inverter Under Unbalanced Grid Faults

During grid fault conditions, a photovoltaic (PV) power plant must stay connected to the power system, and injects reactive power to support the grid voltage. In this condition, the compensation of active/reactive power oscillations is extremely important to achieve a higher reliability and stability in the power system. This study proposes an enhanced zero-sequence current control approach for a PV inverter under unbalanced grid faults. The controller is implemented using the combination of proportional-integral (PI) and proportional resonant (PR) regulators using symmetrical components. Despite of the conventional methods, which compensate for either active or reactive power using the positive and negative sequence currents, the proposed scheme has the ability to cancel the oscillation of both the active and reactive powers at the same time with a fast dynamic response.