An Adaptive Phase-Locked Loop for the Transient Stability Enhancement of Grid-Connected Voltage Source Converters

The phase-locked loop (PLL) is widely used in voltage source converters (VSCs) for the purpose of grid synchronization. This paper presents a comprehensive analysis of the impact of the PLL on the transient stability of VSCs. By using the phase portrait, the large-signal nonlinear responses of the PLL with different controller parameters after transient disturbances are characterized. It points out that increasing the damping ratio and the settling time of the PLL is beneficial for the transient stability enhancement of VSCs. On the other hand, the parameters of the PLL are also constrained by the small-signal stability and the dynamic performance requirement of the VSC. In order to improve the transient stability of the VSC, an adaptive PLL is proposed in this paper. The method allows the PLL to operate with the pre-designed parameters in the normal operation, yet to increase its damping ratio adaptively during the transient period. Time-domain simulations and experimental tests are given to verify the effectiveness of the theoretical analysis.