Research On Topology Structure And Adaptive Reclosing Strategy Of Capacitor Clamped DC Circuit Breaker

Flexible DC transmission is a new type of DC transmission technology based on modular multilevel converter(MMC).With the unique advantages of zero commutation failure risk,power decoupling control and low harmonic content,it has broad development prospects and high engineering value in the field of new energy grid connection.However,due to the rapid development of DC system faults and the nature of no fault current zero crossing point in AC system,the requirements of DC system for the speed and reliability of fault protection device are further improved.As one of the key equipment to isolate faults,direct current circuit breaker(DCCB)is an indispensable part to ensure the stable operation of flexible HVDC transmission system.However,high voltage DC circuit breaker only has the ability of fault isolation.In order to ensure the reliable power supply of DC grid and reduce the impact of multiple instantaneous faults on the system,this paper studies and innovates the short circuit fault of DC grid.The main research results are as follows.Firstly,this paper introduces the structure of flexible DC power grid,analyzes the development mechanism of DC side fault,establishes the transient mathematical model of fault current,and proves the necessity of quickly clearing short-circuit fault.After that,the research process of DCCB is divided into fault clearing stage and reclosing stage.In the fault isolation stage,DCCB is taken as the research focus of line protection device,and the fault isolation principle and the condition of breaking fault current are analyzed.In the reclosing stage,the fault discrimination principle and the adaptive reclosing control timing are introduced,and the overvoltage and over-current hazards caused by blind reclosing under permanent faults are analyzed.At the same time,the electrical characteristics of the above two-stage DCCB are simulated by using the test model,and the simulation results also verify the correctness of the above theoretical analysis.In order to solve the problem that the fault clearing speed of the traditional hybrid DC circuit breaker is limited by the mechanical time constant,this paper proposes a clamp-type highvoltage DC circuit breaker based on the pre-charged capacitor,which realizes the decoupling of the fault isolation process and the fault energy release process.After that,the working principle,advantages and disadvantages of the same type of hybrid DC circuit breaker are analyzed and compared.Based on the theoretical basis of fault isolation,the topology of hybrid DC circuit breaker is optimized from the aspects of breaking speed,equipment cost and component performance.Finally,the transient process after fault is analyzed in detail by using the 200 k V ring DC network built by simulation software.The simulation results also verify the feasibility of theoretical analysis.Considering that the current research on the topology of DC short circuit often ignores the problem of subsequent power supply recovery,this paper develops an adaptive reclosing scheme for the proposed clamped DC circuit breaker.By using the advantages of the topology of DC circuit breaker,the pre-charged capacitor and the fault point form an external discharge circuit independent of the converter station.The voltage and current characteristics in the circuit are used to distinguish the fault nature.The two sets of fault criteria cooperate with each other to further improve the accuracy of fault nature identification,which greatly avoids the converter station from suffering secondary fault impact in a short time.The cooperation of DC circuit breaker and reclosing strategy forms a complete set of protection and control methods to ensure the stable operation of power system.