The Preliminary Modification Scheme Studies Of Qinghai-Tibet HVDC Transmission System

The Line Commutaed Converter based High Voltage Direct Current(LCC-HVDC)tecnology has beeb applied widely in the word.However its thyristors in the converter can't be shut off by themselves,it will lead to commutation failure problems,such as the commutation problems in Qinghai-Tibet HVDCsystem.As the appearance of the power electronic devies which can bu shut off,the Voltage Source Converter based High Voltage Direct Curren(VSC-HVDC)has solved the problem fundamentally.And because of its unique technical advantages,the Modular Multilevel Converter have been applied widely in the HVDC system.But the problem of the small capacity limits its development.Therefore the Hybrid-HVDC combined with the advantages of LCC and VSC technology has become a development trend.At present,the study of LCC-HVDC commutation failure problems mainly concentrated in the reactive power compensation and control strategies.There are little scholars using the VSC-HVDC technology for technical renovation,For the above problems,this paper studies the use of Hybrid-HVDCto reform the Qinghai-Tibet HVDC transmission system,this paper proposes a Hybrid-HVDC structure :the positive LCC negative MMC retrofit scheme,study itscontrol strategy,and made simulation in the PSCAD/EMTDC software.The results proved that the design of the positive LCC negative MMC hybrid HVDC retrofit scheme can effectively restrain commutation failure problems.Firstly,this paper summarizes the commutation failure problems of LCC-HVDC technology,analyzes the MMC basic topology,operation principle,mathematical model,method and the research status of the hybrid HVDC modulation strategy.Secondly,this paper put forward two kinds of hybrid HVDC schemes,demonstrated the respective advantages and disadvantages,combined with the characteristics of Tibet grid system,choosethe positive LCC negative MMC hybrid-HVDC for retrofit scheme.Then,this paper analyses the working principle of LCC,MMC,demonstrates thethe operation principle of positive LCC negative MMC HVDC system,determine the connection mode of the positive LCC negative MMC,The circuit parameters of Hybrid-HVDC and the PI control parameters optimizationwere calculated,aolsthe control strategy of the positive LCC negative MMC hybrid HVDC system waputforward.Finally,the rationality of the retrofit scheme for the positive LCC negative MMC HVDC system was verifiedby building in PSCAD/EMTDC simulation model,and the scheme enhance the capacity of the Qinghai-tibet HVDC system.