Stability Analysis of Phase Locked Loops for AC Microgrids With Hybrid Power Sources

Synchronous instability of the multiple distributed generations has been an important issue for microgrids. Therein, the phase locked loop (PLL) instability is one of the significant reason for synchronous instability of multi-inverter microgrids. Although this instability issue has been widely studied, it is ignored for microgrids with numerous inverters controlled through different controllers. To full this gap, this paper proposes a multi-inverter parallel microgrid stability analysis method based on PLL. Firstly, this paper establishes a PLL small signal model based on the droop control strategy and the P&Q control strategy. Furthermore, the whole PLL small signal analysis model of multi-inverter parallel microgrid is proposed by signal flow graph and Mason Gain Theorem, which is more convenient for power engineers. Eventually, the closed-loop transfer function of PLL is obtained. The proposed integrated small signal model can predict the synchronous stability regarding the PLL of the multi-inverter under grid-connected mode and the island mode. Finally, the simulation and experimental results can well validate the feasibility and effectiveness of the proposed stability analysis strategy based on PLL in the multi-inverter parallel microgrid. Therein, the simulation and experimental results illustrate that the proposed stability identification approach can provide an effective stability control parameter selection method.