Research On DC Fault Traversal Of MMC-VSC-HVDC System

Modular multi-level converter-high voltage direct current(MMC-HVDC)technology is a novel technology for long-distance and large-capacity transmission,speeding up the utilization of clean energy and solving the problem of multi-DC feeding.However,when DC fault occurs in MMC-HVDC system,overcurrent and voltage will decrease rapidly.In order to avoid equipment damage,IGBT must be blocked in time,which will cause MMC-HVDC system to be unable to operate.Therefore,how to ensure the safe,stable and effective operation of MMC-HVDC system in the case of DC fault has become the key to solve the current large-capacity long-distance transmission.In this paper,the DC fault characteristics and control strategies of MMC-HVDC systems with different topologies are further studied.Firstly,the topology and working principle of the full-bridge MMC are analyzed theoretically,the correct mathematical model is established,and the modulation strategy and capacitance voltage sharing method of the full-bridge MMC are further improved,and the current start-up control strategy of the MMC is improved to speed up the start-up process of the system.Secondly,the causes and fault mechanism of overcurrent caused by bipolar DC fault,unipolar grounding fault and disconnection fault of full bridge MMC-HVDC system are analyzed in depth.This paper studies the traditional control strategy of full bridge MMC when bipolar DC fault occurs,and proposes a new control strategy for bipolar DC fault.This strategy does not require high-speed accident detection circuit,and can restore power transmission quickly after fault clearance.The simulation results show that the proposed control runs as expected,and no overvoltage or overcurrent occurs in the converter under DC fault.It has higher control capability than the traditional control system when DC fault occurs in the power grid.Finally,the mechanism of DC bipolar short circuit fault in half-bridge MMC-HVDC system is analyzed in detail,and a new topology for self-clearing DC fault current is proposed based on half-bridge sub-module.Compared with full-bridge sub-module,the new sub-module reduces the number of devices by nearly half,reduces the expenditure of practical projects,and its control is simpler than full-bridge sub-module.It also reduces the voltage stress at both ends of electrical devices and protects the safe operation of the system.