Mechanical DC circuit breakers and FBSM-based MMCs in a high-voltage MTDC network: coordinated operation for network riding through DC fault

Abstract

DC circuit breaker is of vital importance for security in a high voltage (HV) multi-terminal DC (MTDC) network, particularly in the HV MTDC based on voltage source converters (VSCs) that are typically based on two-level, neutral point clamed (NPC) three-level and modular multilevel. Mechanical DC circuit breaker characterized with low costs and losses has been widely used in the conventional LCC-based HV MTDC network, but cannot be directly applied in the VSC-based HV MTDC due to its slow response and limited breaking capacity. On the other hand, the technology of hybrid DC circuit breaker is immature at present, which has been demonstrated with prototype and cannot be employed in HV (320kV) applications in over ten years. As a result, DC circuit breaker has become an obstacle for HV MTDC development. This paper presents a kind of coordinated operation between mechanical DC breakers and full-bridge sub-module (FBSM) based MMCs to guarantee DC fault ride-through in a HV MTDC network. The proposed coordination strategy is dependent on the DC fault current controllability provided by FBSM-based MMC. Specifically, the capability of FBSM's negative level is adopted to change the common-mode component of MMC arm's reference voltage, which can restrain the fault current to the maximum operation threshold of mechanical DC circuit breaker. This scheme can reduce the demand of breaking capacity and response time for mechanical DC circuit breaker, which thus shows great potential in the applications of HV MMC-based MTDC network. The effectiveness and validity of the coordinated operation is verified by simulation results.