Control of grid-forming VSC during symmetrical and asymmetrical AC faults

A grid-forming VSC control strategy when connected to a weak grid considering the ability of symmetrical and asymmetrical faults ride-through is studied in this paper. The general control scheme is based on the grid-forming direct voltage control to work as a controlled AC voltage source by regulating the active and reactive power delivering to the AC grid. A voltage balancing control loop is proposed to overcome the limited overcurrent capability of the direct voltage control based grid-forming VSC during AC faults. To provide individual control for positive- and negative-sequence current, the control system is based on the double synchronous reference frames. A dq current distribution control is developed to share the positive-sequence active and reactive current, while a negative-sequence current control is proposed to control the negative-sequence active and reactive current in the event of an asymmetrical fault. Simulation results are presented to confirm the feasibility of the proposed control strategy.