Analysis And Protection Strategy For The MMC-HVDC Under DC Line Faults

Modular multilevel converter based high voltage direct current because of its characteristics of structure,so that it has a high degree of modularity,low harmonic content and flexible control of the advantages,so it has a widely used in the field of power grid.At present,most of the existing research focus on the AC side of modular multilevel converter based HVDC system at home and abroad,but it has a little reaearch on fault characteristics on the DC transmission line.Therefore,the main contents of this paper is researched and analysised the fault characteristics and protection strategy on the DC transmission line of modular multilevel converter based HVDC.The Specific content structure as follows:First of all,this paper describes the research background and the current status technology of HVDC.It can be seen that flexible DC transmission technology has high feasibility and economy for solving the problems of distributed energy grid-connected,wind farm grid-connected,offshore operating platform,and remote distribut ion areas such as remote areas,it has important value and significance for clean grid.Secondly,this paper analyzes the topolo gy and working principle of MMC-HVDC system,the modulation mode of the system,capacitor voltage balanc ing control strategy of the sub-module and the control architecture of MMC-HVDC system.To provide analysis of the control strategy for MMC-HVDC system and a theoretical foundation and basis for the construction of system.Then,to analysis the control converter stage of MMC-HVDC system,from the direct current control,start control and interphase cir culation control.The three control strategies are analyzed and studied by using the method of establishing mathematical model.The control model is built on the simulation software of PSCAD / EMTDC.The results show the accuracy and reliability of the system control strategy and the model.Finally,to analysis and study the pole to ground fault,pole to pole short circuit fault and the DC transmission line breakage fault.Which from the fault mechanism,by setting the different AC and DC side protection value and increasing the insulation protection of the fault line for the pole to ground fault.Use overcurrent suppression strategy,blocked the converter and disconnect the AC circuit breaker for pole to pole short circuit fault.Change the control strategy of converter station for the DC transmission line breakage fault.Finally,According to the simulation results show that the protection strategy is feasible.