Research On Low Voltage Ride-through Of BDFM Under Grid Symmetrical Fault

The Brushless Doubly-Fed Machine(BDFM)as a new type of adjustable-speed AC motor is developed in recent years.The machine removes slip ring and brush gear,which not only reduces maintence costs,but also enhances the system reliability,meanwhile the machine has the advantages of small inverter capacity.Therefore,BDFM `has a wide application prospect in wind power generation system.With the rapid development of wind power generation technology,wind turbine capacity is getting larger in the power grid.State Grid Corporation of China(SGCC)formulates standard of wind farms connected to the grid.When the grid voltage drops symmetrically,the capacity of low voltage ride-through about wind turbine generator has got more and more attention.The article presents the improved method of control theory about BDFM,when the grid voltage drops symmetrically.The main contents of this paper are as follows:Firstly,the development history of BDFM and research status in the structure of motor body and control theory are summarized.Then,the research status of low voltage ride-through about BDFM are introduced.Secondly,on the basis of structural features and working principle of the machine,the vector mathematical model is provided in the rotor coordinate system.In order to analyze control strategy,the mathematical model is converted into the arbitrary reference frame by rotation transformation.Then,the state-space equation is presented in arbitrary frame,with the control machine stator current as input.Thirdly,the direct power control strategy of BDFM is proposed in arbitrary frame,and then the complete design scheme of control system is presented.During the symmetrical grid voltage dip,we analyze the transient response of control machine stator current and control machine stator electromotive force.According to the expression,a type of low voltage ride-though mode is presented to improve the stability of control system and guarantee the ability of low voltage ride-though of BDFM.Simulation results validate the accuracy of theoretical analysis and the effectiveness of the proposed control algorithms.Finally,the feedback linearization control strategy has been implemented on BDFM.On the basis of this,the control system is designed completely.There is a linear proportional relationship between control machine rotor flux linkage and power machine stator voltage on the stability operation of BDFM.Therefore,an improved feedback linearization control theory is proposed to enhance the capacity of low voltage ride-though of the machine.According to the simulation results carried out in MATLAB,the improved control theory is verified.