Low-Voltage Ride-Through Techniques for Two-Stage Photovoltaic System under Unbalanced Grid Voltage Sag Conditions

This paper proposes a low voltage ride through (LVRT) control strategy for two-stage photovoltaic power generation system (PVPGS) during unbalanced grid faults, where the two-stage PVPGS is composed of PV arrays, the frond-end Boost converter, and the rear-end three-phase inverter. Discharging circuit is added at the dc-link of the two-stage PVPGS, and the positive sequence and negative sequence vector current control in the double synchronous reference frame (DSRF) is employed for rear-end three-phase grid-connected inverter. The reference current calculation method considering current limitation is introduced, and the dual-loop control in DSRF is utilized for rear-end inverter, where the outer voltage loop is used for maintaining dc-link voltage, the positive sequence inner current loop is applied to improve dynamic response for current tracking, and the negative sequence inner current loop is designed for eliminating current overshoot. Moreover, a separate DC voltage loop is adopted in discharging circuit for the LVRT operation. A 500kVA two-stage PVPGS model is built in the PSCAD/EMTDC platform, the performance of proposed control strategy is evaluated by simulations under asymmetric conditions including single-phase and two-phase ground fault, and two-phase short-circuit fault.