The Analysis Of DC Fault Mechanism And Bridge-arm Overcurrent Suppressing Method Of MMC-HVDC

The investment of Xiamen flexible DC transmission project has officially kicked off the research on the system design,operation principle,control strategy and fault characteristics of the bipolar MMC system by scholars all over the world.The analysis of DC side fault mechanism is of great significance for fault diagnosis,protection control strategy,fault recovery,device selection and overcurrent suppression.The research of DC fault of the MMC based High Voltage Direct Current(MMC-HVDC)is mainly focused on pseudo bipolar MMC system,while the research of DC fault in bipolar MMC system is still in its infancy.For the above,in this paper,Based on the introduce of the topology structure and working principle of half bridge sub-module of MMC-HVDC,the topology structures and working principles of MMC in bipolar and pseudo bipolar MMC systems are analyzed.The selection of main electrical parameters,which effects the system performance are detailed,and the connection mode of system transformer winding are analyzed in detail.The equivalent circuits of converter under fault conditions in both MMC systems are built according to electrical parameters of the AC/DC,the discharging mechanism of sub-module capacitance and inductor on bridge-arms.The expression of fault current is deduced.The four transient processes of the bridge-arm current after the fault occurs,the converter is locked and the AC circuit breaker operates are analyzed qualitatively.The flow paths of fault current on bridge-arm at different stages of two topologies are compared and analyzed.Based on the operating characteristics of the two systems under different fault types,the differences of the influences of fault short-circuit current on bridge-arm valve group of converter station are studied in detail.The relationship between the three electrical parameters and the change of fault short-circuit current is pointed out.Based on the analysis of DC fault,aiming at the overcurrent problem of bridge-arm under DC unipolar grounding fault in bipolar MMC system,a method of suppressing overcurrent over the bridge-arm based on virtual impedance is introduced in detail.The voltage increment and the power increment on the additional virtual impedance are fed back to the outer loop controller during the fault.The overcurrent of the bridge-arm is suppressed through adjusting the reference value of the outer loop controller.The correctness of the mechanism analysis and validity of the method are verified by establishing simulation models on the simulation software of RT-LAB.