Research on Mechanism and Suppression Strategy of Submodule Capacitors' Continuous Overvoltage in the MMC-HVDC Grid

Based on the operation principle of modular multi-level converter (MMC), the mechanism of submodule capacitors' continuous overvoltage under the single-phase grounding fault at the valve side of converter transformer or bridge arm reactor in the neutral-line grounded MMC-HVDC system is discussed. Continuous overvoltage and bypass overvoltage have contradictory demands for the setting value of overvoltage bypass protection of converter valves. In order to provide reference for the setting value, several influencing factors are analyzed to filtrate the severest circumstance of continuous overvoltage, including converter states, converter transformer ratio, voltage of PCC point, reactive power of the fault converter, power flow in DC grid and DCBs' state. Furthermore, the valve controlled unbalance protection with the minimum number of current measuring devices to identify faults quickly is proposed to suppress the continuous overvoltage, which is helpful to reduce the equipment area and avoid the effect of measuring devices' inconsistency on the accuracy of protection. Finally, a four-terminal MMC-HVDC grid is set up in PSCAD/ EMTDC. Simulation results show that the valve controlled unbalance protection can effectively suppress the continuous overvoltage and has good fault ride-through ability under AC/DC side faults and reliable protection selectivity under internal faults of converter stations.