Transient Stability Analysis of Multiparallel Grid-Forming Converters Considering Active and Reactive Power Control Coupling and Current Limiting

During fault recovery stage, multi-parallel grid-forming (MP-GFM) converters may re-trigger the current limiter, leading to transient synchronization instability. While most existing studies focus on the transient synchronization stability (TSS) of single converters, and the well-established TSS studies designed for synchronous generators are not directly applicable to MP-GFM converters due to two key differences: active and reactive power control loop (ARPCL) coupling and current limiting. This paper aims to address the TSS issue of MP-GFM converters during fault recovery stage, considering these two factors. Firstly, a mathematical model of a system with MP-GFM converters is developed, considering ARPCL coupling and current limiting. Then, for the first time, the current limiting constrained largest estimated domain of attraction (CL-LEDA) is defined and constructed. Specifically, unlike traditional energy function that only considers active power control dynamics, an analytical energy function that incorporates ARPCL coupling is proposed in this paper. Furthermore, considering current limiting constraint, a new idea to calculate the critical energy value is proposed based on LaSalle's invariance principle. Then, the CL-LEDA can be constructed by the obtained energy function and critical energy value. Provided that the operating state is located within this CL-LEDA at fault clearance moment, all MP-GFM converters will achieve transient stability, without triggering current limiting during faults recovery stage. Finally, experimental results based on RT-LAB hardware-in-the-loop and complete hardware-based experimental verify the analysis.