Research On Control Technology Of Vsc-hvdc Converter Based On Virtual Synchronous Generator(VSG)

At present,the modeling,analysis and control of VSC-HVDC have been deeply studied and explored.At the same time,focusing on the advantages of VSC-HVDC,more and more projects have been built or running.However in some application areas,there are still some problems to be solved urgently.With the widespread access of VSC-HVDC,the rotational inertia of the power grid is reduced,which makes it difficult to support the system with voltage and frequency.At the same time,for wind power,photovoltaic,energy storage and other application scenarios,VSG technology has been proposed.Introducing VSG technology into VSC-HVDC converter control,which can improve the converter's ability to participate in power grid frequency modulation and realize friendly support for power grid.Therefore,this paper focuses on the control technology of VSC-HVDC converter based on VSG.Firstly,the development of VSC-HVDC is summarized from two aspects: theoretical research and engineering application.At the same time,the development status of VSG is briefly described.The application scope,development prospects and defects and shortcomings of VSC-HVDC system using VSG technology are comprehensively analyzed.The system structure and basic principle of VSC-HVDC are introduced.The steady state control strategy of HVDC system using MMC topology is studied.Secondly,the basic principle of VSG is described in detail,and the VSG control strategy applied to the converter of VSC-HVDC system is designed.Combined with the virtual speed control system and the virtual excitation system,a general strategy based on VSG control is proposed,which can be applied to both active network and passive networks The two different operating state transition processes are analyzed.The vector control is introduced.At the same time,a switching strategy of VSG operation state based on virtual current control is proposed.Then the analysis of the unbalance problem in the power grid is carried out,and the positive and negative sequence separation methods are used to realize the separation of the positive and negative sequence components of the system voltage and current.Considering three different faults at the same time,the corresponding improved VSG control strategy is proposed.Aiming at the unbalance voltage in active power network,the reference current generation link is used to compensate the reference current in the inner loop current control,which satisfies the constraints of voltage and current vectors under three control objectives,so as to suppress the fluctuation components of negative sequence current,active power and reactive power.Aiming at the problem of unbalance loads in passive network,the negative sequence loop is used to control the negative sequence component of voltage without canceling the internal voltage-current cascade control,which effectively reduces the voltage imbalance.And A VSG power flow balancing control is designed to prevent the rapid drop of DC voltage for the groud fault in AC grid.