Analysis Of Voltage Stability Of Grid-connection Containing Front-end Speed Controlled Wind Turbines In Wind Plant Based On The Bifurcation Theory

With the rapid economic development in China,the degree of dependence on energy is increasing.At the same time,however,the over exploitation of fossil energy also leads to serious environmental issues.Fossil energy belongs to a non-renewable resource,so people have fixed their attention on renewable energy.Wind energy,a renewable energy with the characteristics of huge reserves,renewable and non-pollution,etc.,which makes wind power generation become one of the development tendencies of renewable energy globally.The improvement of position of wind power generation in China’s economy has promoted the rapid development of installed capacity of wind turbines in China and raised demand for wind power generation.Compared with countries in Europe,studies on wind power technology in China is quite deficient.Although in the past few decades,China has acquired various core technologies,there are still some problems because most of the high technologies of wind turbines are mastered in European countries.In addition,due to the high random volatility and intermittency of wind energy,wind power integration with large scale will bring many serious challenges to grid balance,grid security and power quality.The conventional wind plants more focused on the voltage stability of grid-connected required by safe and stable operation of the grid which is satisfied with the wind turbines and wind plants,which seriously affects the full utilization of resources and the safe operation of the grid.Currently,wind turbines for power generation in China mainly are asynchronous wind turbines with the problems of insufficient output capacity of reactive power and high workload of maintenance and so on.In 2006,the German company VOITH TURBO successfully developed a prototype of Front-end Speed Controlled Wind Turbine(FSCWT).The design and control technology of FSCWT can make wind turbines hold characteristics which are similar to that in conventional power plants,which maximally meets the requirements from generating performance and the safe and stable operation of the grid.Therefore,FSCWT was introduced and mastered in 2012,and it obtained some achievements in technology of overall control,complete machine components matching and debugging,systematical layout and optimization technique,even in security,reliability,and maintainable assessment of system.Because the FSCWT applies one mode that the hydraulic torque-converting drive replaces rigid drive,its speed adjusting method of is different from conventional doubly-fed wind turbines.Because this kind of turbines removes the transducer of wind turbines,they hold the characteristics of high reliability and low workload of maintenance.And by this way,the problem of in sufficient capacity of low voltage ride-through existing in conventional wind turbines is solved.The main contents of this basis are as follows:(1)After mastering the mechanism of FSCWT in wind plant,this study describes the control system and excitation system of wind turbines and establishes grid-connected model of FSCWT in wind plant,adopting system model at node 3 to analyze the stability of grid-connection.(2)This study introduces the bifurcation phenomenon from the bifurcation theory.At the same time,the static and dynamic bifurcations in the power system are defined and their analysis methods are introduced briefly.And on this basis,the classification of static and dynamic bifurcations is conducted in this study.(3)Based on the grid-connected model of FSCWT in wind plant,this study establishes a model at node 3 with equivalent value,and it employs the model at node 3 with equivalent value from FSCWT in wind plant under the simulation environment Mat Cont.On this basis,the influence of integration of FSCWT on the voltage stability of grid is analyzed under disturbance of wind speed parameter,even the node voltage impacted by excitation system is analyzed with the change of wind speed.What’s more,this study analyzes the voltage stability affected by fluctuation of load reactive power and excitation system,as well as the voltage stability when appearing the fluctuation of load active power.(4)The influence of the characteristics of load on stability of grid is analyzed under the simple grid system of FSCWT.Integration of FSCWT to the grid can be regarded as a complex and dynamic nonlinear system.Therefore,it is necessary to analyze the voltage stability of grid-connection containing FSCWT in this study based on the bifurcation theory.