Research On Control Strategy For DC-line Fault Of MTDC System Based On MMC

The Multi-Terminal Direct Current technique,especially MTDC technique of voltage source converter based on full-controlled device receives emphasis in various countries and develops rapidly in recent years,because it has incomparable advantages in power controllability and power transmission expansibility.The Multi-Terminal Direct Current gives full play the characteristics in wind power plant network,interconnection of big power grid and islands power supply.Modular Multilevel Converter developing from voltage source converter owns advantages of big capacity and small loss.Therefore,the control mode at different levels,DC-line fault characteristic and fault protection strategy are analyzed and studied in this paper by taking MTDC system based on MMC converter station as research object.By taking three-Terminal MMC DC power transmission system as an example.The mathematical model of single MMC converter and three-Terminal MMC DC power transmission system is analyzed thoroughly.Through research and analysis,the nearest level modulation way is selected.In the aspect of station level control,internal loop current control and outer loop control providing current reference value for internal loop is adopted in this paper.And,in the aspect of system-level control,voltage margin control is selected in this paper after comparing different control modes and considering the demand of DC voltage and power.In addition,the simulation model of three-Terminal MMC DC power transmission system is erected in PSCAD/EMTDC and simulated research is made on the condition of normal operation of the model and operation exiting owing to fault of main converter station.At last,three faults on the DC-side of MMC-MTDC system as single-pole grounding fault,line to line fault and line break fault is simulated and the characteristics of each fault is analyzed deeply.After that,corresponding protection strategy is proposed to ensure stable operation of system.The simulation result on simulation software PSCAD/EMTDC indicates that the voltage margin control strategy proposed and controller designed in this paper enable the system to transmit the control mode of converter station so as to balance the active power of the overall system when the power demand changes.Even if the main converter station quits,the remaining system will run stably and keep power supply;when DC-side fault happens,voltage margin control strategy coordinating with corresponding protection strategy enables the system to recover stable quickly,eliminate the impact caused by fault and enhances the capability of system in handling the DC-side circuit fault.