Fault Current Limiting And Protection For DC System

DC system has many advantages,such as higher efficiency for distributed generations,larger power transportation capacity,easier to network,being more economical,lower dissipation,higher power quality and so on.DC system is suitable for the access of electric vehicles and other DC loads,so it has broad prospects for development.However,the DC system contains a lot of support capacitor,when the interpoles fault occurs,support capacitor discharges rapidly,and the fault current with large peak rise quickly,causing huge threat to fragile element such as IGBT,diode in DC system.The development and promotion of DC system is greatly restricted because relay protection technology can’t solve above problems.So,it is of great value and significance to study the relay protection of DC system.First of all,this paper introduces the research status of the relay protection principle for low-voltage DC system,and the existing protection principles are divided into two kinds: protection based on single-ended information and protection based on communication.Then the advantages and disadvantages of various protection principles are discussed.The key problems and future development direction of relay protection for low-voltage DC system are summarized.Secondly,based on the establishment of the simulation model of low-voltage DC system,this paper analyzes the fault characteristics of VSC and DC-DC circuit.DC voltage and current characteristics of VSC in fault steady state are mainly analyzed,and the analytical equation of steady-state fault current is derived.On the basis of fault characteristics,the abilities to tolerate fault current of the fragile element in the system such as DC-Link capacitor,diodes are investigated.Then,the research status of limiting or cutting fault current is summaried,and based on the principle of resistive type SFCL(Superconducting Fault Current Limiter,SFCL),a current limit scheme which makes use of resistor is proposed.The reference value of current-limiting resistor is derived according to fault characteristics.Thirdly,the paper discusses a variety of grounding methods for low-voltage DC system,and the method of negative pole grounded via a small resistor is proposed.Then,a protection criterion which could act reliably in the situation of large transition resistance is designed.For the interpoles fault of radiated low-voltage DC system,a main protection utilizing transient current change and a backup protection utilizing R-L model are proposed.This protection scheme is based on fault current limiting,and it still has strong reliability in the case of communication failure.Finally,a novel protection principle for HVDC system is put forward.The protection adopts current change rate to start and select the fault pole,then,it adopts fault current component to determine the fault relative position.The action speed of the new protection is very fast,the selectivity is strong,and the protection principle is simple.The protection could be applied in the ring network,the radiation network and other topological transmission network,and it could act reliably in the condition of ground fault and interpole fault.