Low/Zero-Voltage Ride-Through Control Strategy of Single-Stage Grid-Connected PV Systems Based on Voltage–Current Joint Compensation

For single-stage grid-connected photovoltaic (PV) systems, severe low- and zero-voltage faults may cause synchronization instability and even the disconnection of large-scale PV inverters. To investigate this issue, a nonlinear large-signal dynamic model of a phase-locked loop (PLL)-based grid-connected PV system is established. Based on this model, the mechanism of synchronization instability under severe voltage sags is analyzed from the perspective of equilibrium point (EP) existence. It is found that uncoordinated active- and reactive-current injections will cause an excessive voltage drop across the line impedance, which acts as a significant disturbance. Moreover, a significant reduction in the grid voltage amplitude will result in an insufficient synchronization-restoring force; their combined effect will lead to the disappearance of EPs and synchronization instability. Based on these findings, a low- and zero-voltage ride-through (LVRT/ZVRT) control strategy based on voltage–current joint compensation is proposed. The active current is coordinated to compensate for the voltage disturbance caused by the reactive current injection required for grid support, while an additional virtual grid voltage compensation term is introduced into the PLL to enhance the synchronization-restoring force. MATLAB/Simulink simulations demonstrate that the proposed strategy achieves improved synchronization performance compared with the conventional PLL strategy, two strategies reported in the literature, and schemes employing only active-current compensation or virtual voltage compensation under LVRT/ZVRT conditions. This limits the maximum phase-angle deviation to below 33°, keeps the fault current within the permissible range, and restores synchronization stability within approximately 0.1 s after fault clearance.