| Abstract |
To ensure the stable operation of grid-connected photovoltaic (PV) generation systems when grid voltage dips, the grid-connected inverters are required to have the low-voltage ride-through (LVRT) capability. Based on a two-stage grid-connected inverter which consists of a boost converter and a T-type three-level inverter, the effects of symmetric and asymmetric grid voltage dips on the PV grid-connected system with conventional control strategy is analyzed, and the conclusion that the main incentives of LVRT failure are dc-link overvoltage and grid-side overcurrent is drawn. In this paper, an improved control strategy to avoid LVRT failure for the two-stage grid-connected inverter is proposed. For grid synchronization under grid voltage dip, a dual second-order generalized integrator (DSOGI) pre-filtered based phase-locked loop is adopted. According to PV voltage control scheme during LVRT, a modified PV voltage reference generation method is interpreted, which can make the PV panels work at the point where the power balance is automatically achieved, and thus the dc-link overvoltage is limited. Then a full feedforward of point of common coupling (PCC) voltage is introduced, which can not only limit the overshoot of the grid-side current, but also eliminate its negative-sequence component under an asymmetric voltage dip. By using the proposed control strategies, the LVRT failure can be successfully avoided and the LVRT requirements can be met. The simulation results verify the effectiveness of these control strategies. |
