Analysis of the Existence of Stable Equilibrium Points in the WPP-MMC System under Symmetrical AC Fault

Under the scenario of offshore wind power trans- mission, the system the wind power plant connecting to a modular multilevel converter (WPP-MMC) under faults is investigated, the fault interactions are modeled, and its impact on the existence of stable equilibrium point (SEP) is revealed, as a prerequisite for maintaining synchronization during fault ride-through. Considering that both WPP and MMC have their own control dynamics, e.g., reactive current limiting mode or low voltage ride-through mode for WPP and voltage source mode or current source mode for MMC, the WPP-MMC system may exhibit multiple operating conditions, based on which the fault interactions inside the system become more complex and diverse. Focusing on this phenomenon, this paper analyzes the existence of SEP under each operating condition, so that the optimal parameter settings for maintaining synchronization can be obtained. Moreover, this paper finds an interesting phenomenon that multiple SEPs may exist under the same fault, which brings fresh reflections on the application of classic theoretical tools in WPP-MMC systems. Finally, the correctness of the theoretical analysis and parameter settings is verified by simulation results.