Study On The Control Strategies Of Multi-terminal VSC-HVDC System

Voltage source converter based high voltage dc(VSC-HVDC)transmission technology,based on pulse width modulation technology and the power electronics device,has a strong engineering application.It has a lot of advantages compared with traditional LCC-HVDC,such as active and reactive power controlled independently,avoiding commution failure,supplying power for the weak AC system and less harmonic,which making VSC-HVDC has a broad application in the area of intergrating of renewable energy,isolated power supply and city power supply.VSC-MTDC transmission technology is a combination of VSC-HVDC transmission technology and multi-terminal HVDC technology,and has more than three converters.It can transmit power from a plurality of geographic locations to multiple load centers,which is more flexible,reliable and economical.This paper focuses on VSC-HVDC,the control strategies of VSC-HVDC and VSC-MTDC are studied through simulation.The main contents are as bellows:Firstly,this paper establishes the mathematical model of VSC-HVDC system in the dq synchronous rotating coordinate system,and derives the double closed loop direct current control strategy.Then,different forms of outer loop control are analyzed,which can be used to achieve the different purpose of control.The advantages and disadvantages of parallel topology,series topology,hybrid topology of the system are discussed,then master slave control,voltage droop control and margin control are analyzed.At the same time,the Zhoushan VSC-HVDC model in China is built in PSCAD/EMTDC,which tests dc voltage control strategy.In PSCAD/EMTDC has set up a three terminal VSC-HVDC system supplying passive network,the main converter station uses constant dc voltage control,auxiliary station adopt constant active power control with U-P droop characteristic,the passive side converter station uses constant AC voltage control,and presents the parameters tuning method of outer-loop controller based on the symmetrical optimum method.Then a passive double closed loop controller based on the passivity of VSC converter is established.At the same time,any converter station exiting,passive side active load increasing and ac voltage drop have carried on the simulation.In PSCAD/EMTDC has set up a four terminal VSC-HVDC system consisting of two wind farms and two ac power grid.Based on dc current in the system,this paper designs the control strategy of the two ac power grid under three different power allocation methods,and presents a system fault crossing method by using discharge circuit,which is validated by simulation in PSCAD/EMTDC.