Reducing Overvoltage Stress on FB-SMs in the Coordinated Operation of Hybrid MMCS and Hybrid CBs During DC-side Fault Conditions

This paper presents a method to limit overvoltage stress on full-bridge submodules (FB-SMs) in Hybrid Modular Multilevel Converters (H-MMCs) when a DC short-circuit fault happens. The H-MMCs will employ a reduced number of FBSMs per arm to minimize conduction losses during normal operation, while being coordinated with hybrid circuit breakers (H-CBs) to enable fast fault isolation in multi-terminal DC (MTDC) systems. After confirming a DC fault, the CB initiates the opening of the mechanical switch, and the H-MMC is blocked to prevent the discharge of MMC submodules, thereby limiting the current rise in the MMC and CB components. However, due to the blocking of the H-MMC, the FB-SMs in certain upper and lower arms may experience significant overvoltage stress. To mitigate this stress, two measures are implemented: (1) the voltage across the contacts of the mechanical switch in the H-CB is gradually increased during its opening process, and (2) the FB-SMs in the arm experiencing higher overvoltage stress are bypassed once a critical threshold is reached. These steps alleviate the stress on the FB-SMs without requiring an increase in their capacitance or the addition of auxiliary circuits in the H-MMC. The validity of this approach is verified through simulation results obtained in the PSCAD/EMTDC environment, demonstrating its effectiveness in HVDC systems.

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