Fault-tolerant control of MMCs based on SCDSMs in HVDC systems during DC-cable short circuits

This paper proposes a fault-tolerant control of the high-voltage direct current (HVDC) systems based on modular-multilevel converters (MMC) under DC-cable short-circuit faults, where the MMCs are configured by hybrid scheme of series-connected double submodules (SCDSM) and half-bridge submodules (HBSM). Under faults, the SCDSM-based MMCs are able to control the real and reactive powers, which assists to adjust the fault currents in an allowable range and stabilize the grid voltage. For this operation mode, the SCDSMs are utilized to generate the bipolar output voltages, which are based on the arm current direction. In a converter leg, an arm is operated in the blocked state, while the other arm is controlled in the conducting mode to produce desirable output voltages for regulating the phase currents of the MMC. With this configuration, the SCDSM-based MMCs in the HVDC system still provide the capabilities of fault-current blocking as well as the reactive power compensation. Furthermore, the power loss and investment cost of the MMC based on the SCDSMs are lower than those of the HVDC system based on the MMCs with the hybrid SMs and clamp double SMs (CDSM). Simulation results for 400 MW-320 kV HVDC system are presented to verify the proposed control scheme and the fundamental operation of the MMC based on the SCDSMs is proved by experimental results for a rescaled prototype in laboratory.