Research On Protection Method Of Multi-terminal VSC-HVDC Lines

VSC-HVDC technology has lots of advantages,including high controllability,the ability of feeding to passive system and the expansibility in the construction of multi-terminal system,which will make it future development direction of HVDC technology.The paper proposed protection schemes of transmission lines in multi-terminal VSC-HVDC system connected to offshore windfarms with submarine cables.The major coverage can summarize as:(1).The structure and the control strategy of VSC-HVDC is considered in this paper,and the similarities and differences between fault features of different fault types is also analyzed on the basis of a VSC-HVDC simulation model using PSCAD.The weakness of traditional HVDC protection scheme in multi-terminal VSC-HVDC system has been discussed theoretically,and proved by the simulation result.(2).This paper proposes the necessarity and feasibility of using HVDC breaker and using current-limiting inductor to isolate short-circuit fault in multi-terminal VSC-HVDC system.It is illustrated that the current-limiting inductor will apparently reduce the rising rate and peak value of fault current,while produce different effect on fault voltage at DC line side and DC bus side.(3).Given the influence of current-limiting conductor on fault voltage and the requirement of protection system of HVDC lines,a protection scheme is proposed and well analyzed based on ratio of transient voltage to identify internal fault of high resistance and voltage differential rate to identify internal metallic fault.The Simulation result proved the reliability and that such scheme is insensitive to fault resistance.(4).A protection scheme based on voltage differential rate and trailing edge is proposed based on the analysis on influence factors of fault voltage wave.The analysis illustrated that the influence of inductor on smoothing voltage wave is more transparent although both of them can achieve that point.Therefore,the voltage measurement has a longer trailing edge which could be used to identify fault location.The simulation proved the analysis.At the end of this paper,conclusion and future research is discussed.