Study On DC Fault Characteristics And Control Strategy Of MMC-HVDC System

The modular multi-level converter-high voltage direct current(MMC-HVDC)technology is an effective way to realize the integration of renewable energy and build a multiple energy structure.However,the direct current(DC)fault of the MMC-HVDC system may cause various over stresses,which should be blocked and disconnected instantaneously to avoid the damage of equipments.If the MMC-HVDC system cannot be tripped off from the fault,it could not operate as a normal condition,which seriously affects the safety,reliability and economics of the whole system.Consequently,enhancing the safety and stable operation of the MMC-HVDC system during DC faults is the key to solve the renewable energy integration problems at present.In this paper,the characteristics of the DC fault and control strategy of the full-bridge MMC-HVDC system are investigated in depth.1)Firstly,based on the analysis of the fundamental principles and the switching states of the full-bridge sub-modules,the operation principles and mathematical model of the MMC are developed.In addition,an enhanced pre-start control strategy of the full-bridge MMC is proposed,and the capacitor-balance method is further improved during the MINUS operating mode.2)Secondly,based on the mathematical model of the full-bridge MMC,the inner-loop,outer-loop and internal current circulation controller of the MMC-HVDC system are designed respectively.The simulation results in PSCAD/EMTDC verify the correctness of the control strategy for the steady-state operation,including the inner-loop control strategy,outer-loop control strategy,the current circulation suppression strategy and the capacitor balance method of the full-bridge MMC-HVDC system presented in this paper.3)Thirdly,the mechanism and progress of the over-voltage or over-current stress of the pole-to-pole DC fault,the DC unipolar earth fault and the DC disconnection fault are analyzed in depth.The simulation results in PSCAD/EMTDC verify the analysis of the characteristics of full-bridge MMC-HVDC system during different DC faults.4)Finally,the mathematical models of the full-bridge MMC-HVDC system under three kinds of faults are derived,including DC bipolar short circuit fault,DC unipolar earth fault and DC disconnection fault.In addition,a DC fault ride-through control strategy is further proposed.In order to realize the economic operation of the full-bridge MMC-HVDC system under the DC unipolar ground fault and DC disconnection fault,an optimized strategy is proposed by using the reasonable distribution of the active and reactive power for the upper and lower arm of MMC,which can satisfy the demand of the active and reactive power of full-bridge MMC-HVDC system.The simulation results in PSCAD/EMTDC are used to verify the correctness and feasibility of the proposed control strategy for the full-bridge MMC-HVDC system during DC faults.The works in this dissertation may effectively improve the operation safety,reliability and economics of the full-bridge MMC-HVDC system under DC fault,which lays a good foundation for the integration of renewable energy and building a multiple transmission and distribution system.