Research Of Protection Principle For A DC Distribution System With A Resistive Superconductive Fault Current Limiter

DC distribution power system has a lot of advantages,such as high power quality,large transmission capacity,high reliability,simple structure,economical,low energy loss,independent controlled reactive power and so on.At the same time,it is suitable for the access of distributed generators and there are a few transitions of energy.As a result,DC distribution system can promotes the fast development of electric vehicles and appliances that consume DC power.In addition,it can improve the environment.However,there are a lot of capacitors in the DC distribution system,so when the DC short circuit fault happens,the capacitors discharges quickly and the fault current rises fast.The peak value of current is large,and poses a considerable threat with respect to the safety issue.So it is really important for the fault current limiter(FCL)to be applied in the DC distribution system.Nowadays,the protection of DC distribution system is not mature,and there are few practical engineering application and protection scheme.The research considering the access of FCL is few.So it is significant to study the protection of DC distribution system.Firstly,this paper introduced the research status of DC distribution system around the world in detail.It is presented that the FCL is necessary in the DC distribution system.The influence of superconductive fault current limiter(SFCL)on DC system is introduced in detail.Secondly,the types of SFCL and the mathematical model of resistive SFCL are introduced in detail.Thirdly,the structure of DC distribution system and mathematical mode of converter are introduced in detail,including the control mode and equivalent model.Then based on the simulation,the impact of resistive SFCL on fault characteristic and traditional protection is studied.Considering the access of SFCL,a new transient current protection principle based on Ip(the peak value of the current)and tp(the transient time that the current takes to reach its peak value)is introduced and investigated in detail.This principle of protection can operate within a short time for the feeder faults,and the coordination of the upstream and downstream protective relays is ensured.Finally,the fault characteristics are analyzed when the fault resistance changes,based on the stage characteristics of voltage source converter.When serious DC short circuit fault happens,the capacitance discharges fast,and the rate of change of DC current and the peak are very large,which pose a considerable threat with respect to the safety issue.Also,in the steady state,the diodes are damaged.Above all,the protection needs action in milliseconds.Besides,the change of fault resistance makes it difficult for current or voltage protection based on single-ended data to cooperate.So the protection principle that can act fast and cannot be influenced by fault resistance needs to be researched.In this paper,pilot protection principle based on the difference of transient currents is proposed,considering the shunt capacitor and fault resistance.The characteristic of capacity discharge in DC system is taken full advantage of and this method is not influenced by the fault resistance and system operation mode.Also,this method with adaptivity does not require the real-time synchronization of currents in both ends and it has high reliability.