Research On Coordinated Control Method Of VSC-MTDC System

Currently,VSC-MTDC projects that are under constructing or have been put intooperating in the world mostly are two-terminal flexible system.Although,thedevelopment of two-terminal VSC-MTDC is mature,this system can only transmitpower point to point.If one end of the two converters malfunctions,the whole systemwill paralyze.The problem of VSC-MTDC technology mainly focus on thecoordinate operation between the converters.Therefore,the main purpose of thispaper is to realize the coordinate operation between many converters in themulti-terminal flexible VSC-MTDC and long term reliably operating of it by doingresearch on the principle and control strategy of it.In the end,five-terminalVSC-HVDC system which was verified and analyzed respectively by electromagnetictransient simulation software PSCAD/EMTDC is set up to prove the validity andrationality of the control strategy,and it provides application theoretical basis formulti-terminal flexible VSC-MTDC technology.This article includes the following content: Compares the advantages and disadvantages of traditional DC transmission and its application range,Studies the main engineering and research status of flexible HVDC transmission at home and abroad,Summarizes the technical advantages of flexible HVDC transmission comparing to traditional HVDC transmission,Analyzes the mathematical model of flexible VSC-HVDC transmission in d-q rotating coordinate system,Summarizes and analyzes the three control methods applied to the multi-terminal flexible VSC-MTDC system,including single point DC voltage control,DC voltage deviation control and DC voltage slope control.A five-terminal VSC-HVDC system which was verified and analyzed respectively by electromagnetic transient simulation software PSCAD/EMTDC is set up.The simulation results show that the three control methods have different defects in rigidity of control performance and dynamic response speed and so on.Combining the advantages of the three control methods,a new DC voltage control strategy—DC voltage deviation slope control strategy is proposed,the control strategy combines the deviation characteristics of DC voltage deviation control and slope characteristics of DC voltage slope control to realize the tracking of DC power in converter station and speed up the dynamic response ability.Finally,A five-terminal VSC-HVDC system is set up by using the electromagnetic transient simulation software PSCAD/EMTDC,and the steady-state simulation analysis for the characteristics of the DC voltage deviation slope control strategy is done under the three running states,It proves that once the VSC-MTDC adopts this control strategy,It will have good stability and power supply reliability.