| The exploitation and development of renewable energy is becoming more and more important when coming to the dual pressure of environmental pollution and primary energy shortage.As the renewable and clean energy,wind energy has the advantages of retarding climate change,promoting low-carbon economic growth and alleviating the shortage of primary energy such as coal and so on.As a result,wind energy has been highly concerned by the Chinese government and many enterprises.In the early stage of the development of wind power in China,both of the installed capacity and scale of wind power are small.The main problems of wind grid-connected system are the voltage deviation and harmonic pollution which are caused by wind power fluctuation.China’s wind power generation is gradually becoming to large-scale,centralized and industrialized mode with the innovation of wind technology and the rapid development of wind industry.However,with the increasing penetration of wind power,the problems of voltage,frequency,power quality and sub-synchronous oscillation of the power system cannot be ignored when the wind power is connecting to the main grid.The interaction mechanism between wind power system and traditional power system becomes more and more complex.Thus,the old mechanism of voltage and frequency stability of power system needs to supplement and correct.The large-scale wind power integration has brought problems of security and stability of power system.Therefore,it is of theoretical significance and practical value to study the voltage stability of the wind power system and traditional power system.The static and transient voltage stability mechanism of large scale and high penetration wind farm were analyzed and the voltage stability index system is built,which is based on an actual power grid of Hami,China.The safety and stability of large scale high permeability wind power system are evaluated and the methods to improve the voltage stability are put forward.The main research results of this paper are the followings:(1)Firstly,wind and thermal power generator model and wind power farms were established by DIg SILENT software based on the DSASP 2016 winter data packet of Hami.Different aspects of wind power and thermal power were studied,such as the dynamic behavior,output characteristics and the complementary characteristics of wind power and thermal power.Besides that,this paper takes the actual power projects as the research base to study the voltage stability problems.(2)Propose a new method which is VSRO transmission limit surface to study the static stability limit of the wind power grid system based on the static voltage stability mechanism of the wind power system,and the correctness of the proposed method is verified by the load flow calculation results.At the same time,different kinds of methods to analyze the static voltage stability are used in this paper to testify the influence of high penetration of wind-connected power system.Then the time domain simulation method is used to analyze the transient stability of wind energy.The static and transient stability index system of wind power system is established through the analysis of the static and transient stability of large scale wind power,which lays the foundation for further evaluation of the voltage stability of high penetration wind power grid.(3)Different kinds of index evaluation of wind power stability will lead to different evaluation results.Considering that there exists strong correlation between different kinds of indexes,integrate multiple factors to determine the weak bas-bar of power system,and evaluate the voltage stability of the system is proposed in this paper.The multi-objective decision method(MCDM)is used to evaluate the voltage stability of large-scale wind-connected system.Moreover,a multi-angle and multi-level index system is built.And the comprehensive weight calculation method is proposed to calculate the weight of index,and to evaluate the voltage state of the wind power grid from the macro level.The method is applied to analyze the voltage stability in Hami grid,and the results of single index and multi index evaluation are compared to verify the superiority of the proposed method.(4)The comprehensive weight calculation method is introduced in order to avoid the phenomenon of "big data determine the results".The calculation results are not overall when using the single weight calculation method.The comprehensive weight method which combines subjective and objective method can reduce the uncertainty and incomprehensiveness of the evaluation index.At the same time,assume that the sum of the comprehensive value is at its maximum while the sum of the deviation values is at its minimum,then can determine the final results by an innovative index-weight optimization method based on the Lagrange conditioned extreme value.Moreover,the sensitive analysis was applied to verify the robust of the proposed method.(5)The original inertia of the power system will be decreased when large scale wind power integrate into the main power system of Hami grid,so it is necessary to combine the actual wind power grid and the reactive power compensation devices to make a comprehensive analysis of the large-scale wind-connected power system and find new ways to improve the stability of wind power grid.Based on this,the paper proposes to use the virtual inertia to improve the stability of the wind power system.At the same time,three kinds of structure of wind grid are analyzed,and the stability of the wind power can be improved by optimizing the external structure of wind farms.Both of these two methods are emphasizing to improve the stability of wind power by the inside system.Those two methods are applied to Hami power grid.The simulation results show that,when using the virtual inertia control method,the wind farm participates in the speed regulation,which can effectively prevent the active power reduction and the speed instability when the load is changed.What is more,it can effectively improve the transient stability of the wind power grid.When applied the second method,which is optimize of the structure of the wind grid frame.The active and reactive power margins of the bus bar are increased,as well as it can improve the static voltage stability of the nearby power grid. |
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