Research On Self-disconnecting Function And Control Strategy Of HVDC Circuit Breaker

VSC-HVDC grid has low damping,which is sensitive to overcurrent and overvoltage,and has a fast rising fault current.Therefore,the development of VSC-HVDC technology relies heavily on the development of high voltage DC circuit breaker(HVDC CB).On the other hand,the high voltage fault has a large current surge on the DC circuit breaker,which seriously affects the performance and life of the circuit breaker.In this context,this paper proposes the need for the self-disconnecting function of DC circuit breakers.Due to the communication delay,the magnitude of the fault current will rise sharply in the VSC-HVDC grid,so that the internal power electronic switch will be greatly impacted when the DC breaker is breaking,which will damage the DC equipment.Therefore,taking the Zhangbei VSC-HVDC four-port power grid project which is under construction as an example,this paper uses PSCAD/EMTDC simulation software to build the relevant model,and proposes a fault diagnosis method with simple principle and rapid detection.And combined with the topology characteristics of hybrid DC circuit breaker,the current limiting measures are proposed to reduce the value of the fault current.Specific work includes:(1)In the aspect of fault detection,the conventional differential undervoltage protection principle is improved.The DC grid protection control strategy based on the rate of change of voltage(ROCOV)detection is proposed for the Zhangbei project,include line protection and busbar protection.The possible misoperations in the power grid are analyzed,and the solution is proposed,and the simulation is verified.(2)In the aspect of fault handling,based on the topology structure of the hybrid high voltage DC circuit breaker,the breaking method of operating arresters by sequence is proposed,and a series of control strategies are developed to make the high voltage DC circuit breaker optimal.(3)Combined with the first two works,a set of self-disconnecting function control system for high voltage DC circuit breakers is formed,and cooperates with the signals sent by the system to form a master-slave collaborative control strategy.According to the above research work,the following conclusions are obtained:(1)The fault detection method based on the voltage change rate can accurately determine it is internal fault or not,and has certain anti-interference and transition resistance capability;(2)The method of operating arresters by sequence of the hybrid DC circuit breaker can equivalently accelerate the operating time of the DC circuit breaker,reduce the peak value of the fault current,and have the effect of limiting the overcurrent;(3)The master-slave coordinated control strategy of the DC grid can further enhance the reliability of the grid operation,and protect the breaker from the impact of overcurrent in time,thus improving the safety of grid equipment operation.