A Method for Improving the Stability of Grid-Connected Inverters by Eliminating the Negative Effects of PLL in Weak Grids

Phase-locked loop (PLL) is an important unit for a grid-connected inverter (GCI) to synchronize with the grid. And its negative impact on the stability of the GCI cannot be ignored. Especially in the weak grid, the impact is more prominent. In this paper, by constructing a small-signal control block diagram of a GCI that considers the effect of the PLL, it’s pointed out that the PLL introduces a positive feedback loop at the current reference value and the existence of the positive feedback loop will seriously reduce the stability of the GCI in the weak grid. Therefore, in order to eliminate the adverse effect of the positive feedback loop introduced by the PLL on the stability of the GCI, a negative feedback-loop compensation control method is proposed in this paper. By introducing a negative feedback compensation loop at the current reference value, the negative influence of the PLL is suppressed, thereby improving the stability of the GCI in a weak grid. At the same time, the control parameters of the negative feedback compensation loop are also analyzed and designed in detail. Finally, the correctness and effectiveness of the control strategy proposed in this paper are verified by simulation and experiment.