Improvement of PV grid-tied inverters operation under asymmetrical fault conditions

This paper proposes a novel DC-side discrete-time voltage controller for grid-connected voltage-sourced converters during asymmetrical faults. The main objective is to eliminate the second-order voltage oscillation on the VSC's DC-side, as well as the positive sequence third-order harmonic current injection into the grid during asymmetrical faults. The compensator is designed based on the comprehensive discrete-time model of the DC-side dynamics. It is shown that DC-side average voltage can be controlled by the positive sequence direct axis current, while the oscillating component can be fully controlled by negative sequence dq-axes currents. The negative sequence current references are obtained using a discrete resonant controller plus a lead filter in order to achieve zero steady state error and asymptotically stability, without injecting harmonics currents into the grid. Simulation results showed the effectiveness of the proposed strategy under severe fault conditions.