1.School of Electrical Engineering, Southeast University, Nanjing 210096, China;2.State Grid Electric Power Research Institute (NARI Group Corporation), Nanjing 211106, China;3.State Key Laboratory of Technology and Equipment for Defense Against Power System Operational Risks, Nanjing 211106, China

Due to differences in parameters such as inertia, damping, and impedance of grid-forming converters operating in parallel, the transient interactions between converters are intensified, increasing the risk of transient synchronization instability of converters. Multi-machine transient stability cooperative control is key in improving the transient stability of grid-connected systems. To this end, based on the transient interaction model of a parallel grid-forming converter system, this paper reveals the multi-machine instability mechanism that may be induced by the power angle driving effect of transient interaction energy and proposes a cooperative frequency synchronization stabilization control method for multiple grid-forming converters. On one hand, the transient interaction energy is reduced through frequency synchronization control to maintain synchronous and stable operation among converters. On the other hand, by calculating the center frequency through weighted virtual kinetic energy, the acceleration energy of converters with larger virtual kinetic energy is dissipated, thereby improving the transient stability of the converters relative to the system. The improvement of system stability under the condition of the parallel system connecting to a large power grid is demonstrated using a Lyapunov energy function for the proposed control method. Furthermore, by constructing the stability region of the converter parallel system considering cooperative control, it is verified that the cooperative control method can effectively expand the system stability region, providing guidance for parameter optimization. Finally, the correctness of the proposed method is validated through simulations and experiments.