An Adaptive Phase-Locked Loop to Improve Stability of Voltage Source Converters in Weak Grids

Abstract

Three-phase voltage source converters (VSCs) usually utilize phase-locked loop (PLL) to realize grid synchronization. However, when the VSC is connected to weak grids featured by high grid impedances, its dynamics will be deteriorated if a conventional fixed-gain PLL is used, which can even lead to grid-synchronization instability of the VSC. To deal with this problem, an adaptive phase-locked loop (AdPLL) is proposed in this paper, which ensures that the VSC has an expected stability margin in weak grids by adaptively regulating the AdPLL’s control bandwidth according to the identified grid impedance. The grid impedance is identified in real time along the system trajectory, in which the recursive least square method is applied to reduce its computation burden and enable its implementation in the VSC controller. Simulation results verify the effectiveness of the AdPLL on improving the VSC’s stability in weak grids.