Research On The Influence Of DC-side Limiting Reactor On DC Fault Characteristics And Stability Of VSC-MTDC System

As the proportion of renewable energy such as wind power,solar power in the field of electric power generation is increasing gradually,compared with traditional AC transmission system,high voltage direct current(HVDC)transmission technology has become more widely used,considering the decentralized and intermittent nature of clean energy.The modular multilevel converter(MMC)based VSC-HVDC,which has advantages such as highly modular structure and good waveform quality,has gained more and more attention.This paper is based on Zhangbei HVDC project which is a multi-terminal HVDC system based on MMC and a deeply study on fault protection and small-signal stability of the system has been given,considering the influence of the DC-side limiting reactor.Firstly,this article introduces the research status of the basic research about MMC and fault protection,stability analysis and other contents of VSC-HVDC system.Then,the overall topology of MMC,the basic topology of the sub-modules,and the basic operating principles of MMC are introduced in detail.The network topology and steady-state control strategy of the HVDC transmission system has also been investigated.In fault protection,the limiting reactor can suppress the rapidly rising fault current and provide operation time for the protection system.So it is necessary to establish the intrinsic link between the limiting reactor and the fault transient characteristics of the system in order to select the appropriate value of the limiting reactor.Take the pole-to-ground fault occurred on the DC side of a symmetrical bipolar MMC-HVDC system as an example.The simplified model of a single converter station is established according to the different operating status,before and after blocking of the converter station,and a simplified mathematical expression of the fault current on the L,value of limiting reactor,is obtained.On this basis,the influence of the limiting reactor on the fault characteristics of the single converter station and the entire system is discussed.From the point of satisfying the system fault protection requirements,the corresponding method for calculating the parameter range of the limiting reactor is obtained.In small-signal stability,the limiting reactor will reduce the system's dynamic response speed and stability,therefore a suitable limiting reactor is also critical.The Thevenin equivalent model of the power and DC voltage-controlled converter station are established respectively,based on the principle of power balance.Then these models are connected with the DC line network model,and the small-signal mathematical model of the whole system is obtained.After that,based on this model,the influence factors of the small disturbance stability of the system are explored,and the root trajectory curve is drawn to clarify the influence of the limiting reactor on the small-signal stability of the system,so that the parameter range of the limiting reactor can be obtained according to the stability requirements of the system.Based on the current design method of limiting reactor,combined with the requirements of fault protection and small-signal stability of the system,an improved parameter design method for limiting reactors is proposed.Finally,based on Zhangbei HVDC project,a simulation model is built to verify the correctness of the simplified fault current expression,the small disturbance mathematical model and the improved method for designing the limiting reactor parameters proposed in this paper.