Research On Control & Protection Strategy Of Multi-terminal Flexible DC Transmission System With DC Circuit Breaker And Superconducting Fault Current Limiter

Flexible DC transmission technology(MMC-HVDC)has become the development trend of DC power grid industry in recent years because it has no reactive loss,low line cost,suitable for long-term power supply and easy to build flexible interconnection between large-scale power grid and large-scale new energy.However,the bad effects that fault current do to the converter is becoming more and more severe along with the voltage level and capacity increase,while the natural properties such as arc-breaking also make it more difficult to clear the short current fault in DC line quickly.The cooperation method between DC current breaker and superconducting fault current limiter is raised to solve these problems in the way of cutting fault current right after restrict it,looking forward to reduce the technical demand and cost of DC circuit breaker.Control and protection strategies of the flexible DC transmission system will change after installing superconducting fault current limiter.As an operation and maintenance worker of related flexible DC transmission system,this paper is intended to focus on :(1)Analyzing the short-circuit current development process and system protection action strategy of a multi-terminal flexible DC transmission project in S City when a serious DC short-circuit failure occurs between a power-receiving station and a power-sending station,in view of the idea of installing a superconducting fault current limiter and a DC circuit breaker for DC fault clearance,the selection scheme and structure selection of the primary and secondary parameters of the superconducting fault current limiter are proposed.(2)Analyzing the control and protection requirements of the superconducting fault current limiter and put forward a transformation ideas and frameworks for the adjustment of the control strategy and protection strategy of the entire multi-terminal flexible DC system after the superconducting current limiter is installed;(3)The multi-terminal flexible DC engineering control system is analyzed,and a set of control strategies including superconducting current limiter main wiring design,five-prevention logic,operation sequence,tripping and fault handling are proposed.(4)The protection strategies of AC side and DC side of multi-terminal flexible DC system are studied,and the matching and coordinated rectification strategies of superconducting current limiter and DC circuit breaker are analyzed,and the transformation scheme of system-level control system,DC protection,converter and valve protection are analyzed.(5)Establish a PSCAD simulation model including flexible DC transmission system,DC circuit breaker and superconducting current limiter,perform steady-state simulation and transient fault scanning of the multi-terminal flexible DC transmission system after the the superconducting current limiter installed,and analyze the setting value modification scheme,verify that the control and protection strategy can achieve at least 20% of the fault current suppression rate when the DC line failure occurs,cooperating with the DC circuit breaker to limit the fault current before cutting off,and ensure the operation of the non-fault station.