Flexible Control Strategies with Current Limitation of VSI Under Unbalanced Grid Voltage Sags

The power injection from a distributed power generation system (DPGS) into the grid is commonly performed through a voltage-source inverter (VSI). However, under unbalanced grid faults, the VSI may disconnect from the grid, which leads to stability problems due to the lack of active power after the fault extinction. To enhance the inverter's fault ride through (FRT) capability i.e., it remains connected to the grid as requested by the grid codes, two flexible control strategies have been developed in the last years. They are named flexible oscillating-power control (FOPC) and voltage-support-oriented control (VSOC). This paper proposes generalized expressions of active and reactive powers' references under faulty conditions to satisfy the grid codes' requirements and limit the current injected by the inverter to the rated value. Numerical simulations are performed to evaluate the performances of the two flexible control strategies along with the proposed current limitation method.