Research On Short Circuit Fault Of MMC-HVDC Based On Sub-module Topology Optimization

Modular multilevel converter(MMC)has become one of the most promising topology in high voltage AC and DC conversion field.With the depth research process and further expansion of the engineering applications of MMC-HVDC,its voltage level,transmission capacity and transmission distance gradually develop to a higher level.Because of better economic and technological advantages of the overhead transmission lines,it has become the mainstream choice of power lines in the future.But at the same time,the probability of short-circuit fault of the overhead line is much higher than that of the DC cable,this brings many problems in insulation,protection and operation.Therefore,it is important to study the DC short-circuit fault characteristics and control protection strategies.Firstly,the development history and research status of MMC-HVDC are introduced.Then,take the traditional half-bridge sub-module MMC as example,the topology and working principle are analyzed in detail,the mathematical model is established.The structure and function of MMC-HVDC hierarchical control system are studied,the direct current control strategy of converter station level and nearest level modulation strategy of valve level are mainly introduced in this paper.Secondly,the type of MMC-HVDC short-circuit fault is introduced,and the fault mechanism and transient characteristics are analyzed for the most serious bipolar short-circuit fault.On the basis of summarizing the existing temporary fault protection strategies,a control protection strategy for permanent faults is proposed.Control the AC side phase voltage equal to the DC bus voltage,it can extinguish the fault point without the need of system outage,thus the complicated relay protection action and the restarting process of the system are omitted and the rapid recovery of system after fault clearing can be achieved.Thirdly,the principle of capacitance voltage fluctuation in sub-module is analyzed,and the existing capacitor voltage monitoring and control method are introduced.In view of the hardware cost of this method and the high complexity of the control system,a capacitive voltage balancing control strategy based on self-balanced combined capacitance sub-module is proposed from the perspective of sub-module topology optimization.The number of measurement and transmission of the voltage signal were reduced by half,and the simulation verified the effectiveness of the control strategy.At last,a diode clamp reverse blocking double sub-module(D-RBSM)is designed to solve the problem of operation safety and stability caused by triggering non simultaneity.The advantage of triggering simultaneity and the capacity of fault current locking are verified by simulation.In addition,based on D-RBSM,it is improved to make it suitable for permanent fault control and protection strategy,verify the effectiveness of the design scheme in this paper.