A Crowbarless Low Voltage Ride Through Method For Multi-kind Faults Of Brushless Doubly-fed Wind Generator

As smallrequired converter capacity,theconventional Doubly-Fed Induction Generator(DFIG)become one of the most widely used models in wind power system.But due to the presence of brush and slip ring,the DFIG has low operation reliability,high maintenance cost.The Brushless Doubly-Fed Induction Generator(BDFIG),a kind of new type of motor developed in recent years,has the same advantages with DFIG,but there is no brush and slip ring,thus BDFIG improves the system reliability and reduces maintenance costs,especially suitable for application in less human activity of mountains,wilderness,island or in remote areas such as wind power generation system,the research of design and control methods ofBDFIG motor made remarkable progressin recent years,greatly accelerate the process of from the lab to the engineering application stage,but there are still many basic problems to be solved.With the increasing of the capacity of wind turbines and wind power within the grid,wind power grid discipline has more and more requirements for the operation of wind turbines,one of the most challenging is requirements in high voltage transmission system side consecutive symmetric or asymmetric short circuit fault can stay connected to the power grid and power supply to the system,namely the Low-Voltage Ride-Through(LVRT).The research of LVRT control methods for the brushless doubly-fed wind turbine is the fledgling abroad,and related literature has not been reporteddomestic.In this paper,some research will be carried out about this problem,the research content is as follows:1.On the basis of studying the basic theory and the different reference frame mathematical models of BDFIG are studied in detail,especially,double winding stationary reference frame model,the power winding static reference frame model,the control winding static reference frame model and the conversion method between different frame model,and lay a solid theoretical foundation for the following research.2.This paper does thefurther study on the indirect power control principle of brushless doubly-fed induction generator.In MATLAB/simulink,the simulation model is built up to further demonstrate the validity of the indirect power control method in power generation mode,which lay the theoreticalfoundations for the following research.3.The research of the operation of the power grid failure cases is the foundation of the research of LVRT behavior.So far,BDFIG short-circuit response has been only summarily analysis in existing literature and has not quantitative calculation analysis.In this paper,using the mathematical model of BDFIG,under the condition of ignoring winding resistance,derived a approximate generalized equations,analyzed the maximum short-circuit current,and analyzed factors affecting the maximum short-circuit current in detail.The maximum short-circuit current values obtained in this way are compared with the values obtained from time domain simulations,and the differences are less than 5.39%,the results show that the generalized equations derived in this paper can serve as a approximate calculation formula of short circuit current.4.In order to reduce low voltage across a large torque ripple during pressure of wind turbine transmission chain,this paper proposes a a crowbarless LURT control strategy based on flux tracking,the method was deduced based on the mathematical model,and on the flux trace coefficient is derived and analyzed in detail.Constructed structure diagram based on indirect power control of low voltage through the system,the system can realize smooth transition betweenthe normal operation and LVRT operation.The simulation results show that the proposed control method can realize the symmetric and asymmetric through low voltage and low torque ripple during LVRT is small.BDFIG can provide a certain amount of reactive power to the gridduring LVRT,and control winding voltage and current are limited in the permitted range.5.In the laboratory existing brushless doubly-fed motor indirect power control hardware and software platform,write the related program,completed the low voltage through the preliminary work,including identifying power grid failure,the flux amplitude and phase calculation and observation,and the experimental results are given,which laid a foundation for further realizelow voltage through the control strategy.