Coupling characteristic analysis and synchronization stability control for Multi-Paralleled VSCs system under symmetric faults

Abstract

The grid-following voltage source converters (GFL-VSCs) are prone to lose synchronization with the grid under symmetrical faults, which seriously threatens the security of the power system. Different from the existing works, this article takes a multi-paralleled GFL-VSCs system as the research object and deeply explores the power flow coupling mechanism between VSCs and synchronization stability control method for the system during low voltage ride through (LVRT). Firstly, by a nonlinear dynamic model of PLL, the synchronization instability issue of the multi-paralleled system is revealed. Furthermore, the impact of power flow coupling effect on the VSC’s synchronization stability is analyzed from the two aspects: the existence of equilibrium points and the transient synchronization behavior. Subsequently, a stability criterion based on coherence properties is constructed, which can quantitatively evaluate the instability risk of the multi-paralleled system. Finally, a current optimization control strategy is proposed, which can ensure the maximum transient stability margin of the entire system and meet the reactive power requirements of the grid code. The simulation results validate the correctness of the theoretical analysis and the effectiveness of the proposed strategy.