Temporary Dynamic Circulating Current Injection-Based Overload Support Strategy of Modular Multilevel Converters Under AC Faults

Grid-connected modular multilevel converters (MMCs) typically reach gigawatt-level unit capacity. During AC faults, their temporary overload operation provides emergency grid support, prevents fault escalation, and accelerates recovery. This paper proposes a dynamic circulating current (CC) injection strategy to significantly enhance the temporary overload support capability of MMCs under AC faults. The precise safety margins of three-phase arm currents are first derived for both symmetrical and asymmetrical AC faults. Under these constraints, the optimal CC for each phase is calculated to consistently follow the positive-sequence fundamental current. Through independent decoupled control, the CC is dynamically injected according to actual fault conditions, thereby improving the temporary overload capability. The proposed method increases the available safety margin of the arm current while keeping the peak arm current (PAC) and submodule capacitor voltage ripple within safe limits, enabling safe high-overload operation during AC grid faults. Theoretical analysis and calculations show that the maximum reactive overload capability is enhanced by approximately 90% under symmetrical faults and by over 200% under asymmetrical faults. Consequently, the grid voltage level, especially under weak-grid fault conditions, is improved, providing strong support for post-fault grid security and rapid recovery. Comprehensive simulation results validate the effectiveness of the proposed strategy.