Transient Synchronization Stability Analysis and Enhancement of Paralleled Converters Considering Different Current Injection Strategies

Renewable energy sources (RESs) are highly penetrated in power systems through phase-locked loop (PLL) synchronized converters. Under severe grid voltage sags, PLL-synchronized RES system is susceptive to transient instability and tripping from the grid due to inappropriate current injection. The existing literatures mainly focus on synchronization instability issues of a single grid-connected RES system, and the effect of the interaction between them is rarely studied. To fill this gap, this paper aims to clarify the interaction mechanism and characterize stability boundary of paralleled PLL-synchronized RES system with different converter parameters. More specifically, the effect of current injection angle and magnitude on transient synchronization stability is investigated for guiding current injection reference design during low voltage ride-through (LVRT) and rated capacity option of RESs. First, the transient interaction model of the paralleled converter system is developed, and interaction mechanism under different current injection angles is revealed. Next, stability region analysis is performed to evaluate the impact of current injection magnitude, wherein current injection boundary is obtained. Then, a current injection design method is proposed for improving both transient stability and voltage support. The theoretical analysis and the proposed method are validated by simulation results.