Transient Over-Voltage Caused by Converter Valve Failures in MMC-UPFC System

The Unified Power Flow Controller (UPFC) is regarded as a flexible AC transmission device by which various functions—parallel and series compensation, phase shifting, and terminal voltage regulation—can be implemented without necessitating structural changes to the power grid. Compared to traditional two-level or three-level UPFC systems, the topology and operational characteristics of the MMC-UPFC system are more complex. Transient overvoltage may be caused by a failure in the MMC converter valve, posing a threat to the insulation integrity of the main equipment. Based on the MMC-UPFC circuit topology, mathematical models for the parallel and series converters are derived, and a dual-loop control strategy is developed, in which PI control is applied to the parallel converter and cross-decoupling control is employed for the series converter. Simulations under steady-state operating conditions and power step scenarios confirm that the established model exhibits excellent dynamic performance. The transient overvoltage characteristics of the system, in the event of failures in the parallel and series converters, are simulated and analyzed. The results indicate that transient overvoltage primarily impacts the insulation of the DC pole line, the transformer valve side, and the converter components.