Study On Static Voltage Stability Assessment And Optimization Of Multi-Terminal AC-DC Hybrid System

China has a vast territory and a large land area,but the concentrated areas of electric energy consumption and energy production and supply base are in reverse distribution.How to transfer large capacity,long distance and low loss power from the place far away from the load center to the client is a hot research topic.HVDC transmission technology has the advantages of low cost,good economy,long transmission distance,small loss and so on.Therefore,it is increasingly widely used in long-distance transmission.HVDC transmission technology mainly includes transmission technology based on line current converter(LCC)and voltage source converter(VSC).A new hybrid LCC-VSC multi-terminal DC transmission technology is produced by the combination of these two transmission technologies.Moreover,with the development of distributed energy,HVDC is gradually developed into a multi-terminal Direct Current(MTDC)system.When MTDC is connected to the traditional AC system through the converter station,the voltage stability of the traditional AC system will be profoundly affected.At the same time,the voltage stability problem also exists in the multi-terminal DC grid itself.At present,the research object of static voltage stability is mostly pure AC system,and there are few researches on the static voltage stability of multi-terminal AC/DC hybrid system.Therefore,in view of the shortcomings of existing studies,this paper improves the traditional modal analysis method and applies it to the multi-terminal AC/DC hybrid system for the first time to study its static voltage stability.The main contents of this article is as follows:(1)Modal analysis method is often used in static voltage stability analysis of pure AC system.Its participation factor index can only analyze the influence of reactive power on voltage stability.In order to solve this problem,this paper improves the traditional modal analysis method and proposes the decoupling analysis of AC-DC hybrid system,which is firstly applied to the static voltage stability analysis of AC/VSC-MTDC hybrid system.In addition,the index of traditional modal analysis method is improved in this chapter.The improved index can consider the variation of voltage stability with reactive power and active power of AC/DC hybrid system.Finally,an improved IEEE-57 node computation example is used to illustrate the correctness of the methods and indicators presented in this chapter.(2)HVDC transmission system has a variety of control modes,which have different effects on the voltage stability of AC/DC hybrid system.In view of this characteristic,this chapter introduces voltage droop control on the basis of the previous chapter,and analyzes the influence of droop control on the voltage stability of AC/VSC-MTDC system.In addition,based on the reality,this paper improves the model of the HVDC transmission system,constructs the AC/LCC-VSC-MTDC hybrid system,analyzes the variation of voltage stability of AC/DC hybrid system after LCC converter station is connected and the influence of the control mode of the converter station on the voltage stability of the AC/LCC-VSC-MTDC system.The voltage stability of AC/LCC-VSC-MTDC hybrid system is improved by coordinating the control mode of VSC converter station with the transmission power of LCC converter station.Finally,an improved IEEE-57 node computation example is given to illustrate the feasibility of the research content in this chapter.(3)At present,researches on the optimization of voltage stability are mainly focused on pure AC system or AC-DC system based on LCC converter station,and researches on the optimization of voltage stability of AC/VSC-MTDC and AC/LCC-VSC-MTDC hybrid system are rarely reported.The active and reactive power of VSC station can be controlled flexibly and independently.In view of this situation,taking the master-slave control as an example,this paper realizes the voltage stability optimization of the AC/DC hybrid system by optimizing the reference power of the VSC converter station controlled by fixed power.The sensitivity of the minimum eigenvalue index of the AC/DC system to the reference power of the VSC converter station was obtained,and the sensitivity index was taken as the weight coefficient to construct the objective function.Then,by optimizing the objective function through genetic algorithm,the obtained reference power value of converter station is the most favorable to the voltage stability of AC/DC hybrid system.Finally,an improved IEEE-57 node computation example is used to illustrate the correctness of the research content in this chapter.