Research Of Control Strategy For Brushless Doubly-Fed Induction Machine

The brushless doubly-fed machine(BDFM)is a kind special structure machine;There are two different pole-pair windings embedded in the same stator core,the rotor is specially designed to couple to both air-gap fields to realize energy conversion,two common rotor structures are the reluctance rotor(brushless doubly fed reluctance machine,BDFRM)and the cage rotor(brushless doubly fed induction motor,BDFIM).Thanks to its special structure,the BDFM exhibits some intrinsic advantages,compared to the induction machine and the synchronous machine,the BDFM shows great promise in reducing the rating of the converter need for adjustable speed drives(ASD)and variable speed constant frequency(VSCF)wind power generations to a small fraction of 25%?35%machine rating,therefore resulting in a low hardware cost;Compared to the doubly-fed induction machine,the BDFM is better suited for operations requiring minimal maintenance by virture of absence of brush and slip ring,these unique advantages make it show potential broad application prospects in ASD and VSCF.In recent 10 years,although a great of research results push BDFM from laboratory stage to practical application stage largely,there are many basic problems to be solved,such as low efficiency,harmonic problem,control method,etc.In existing BDFM control strategies,vector control strategy needs rotating coordinate transformation and field oriented,which make the system structure complicated,larger current fluctuations in low frequency exist in direct torque control system,indirect stator-quantities control(ISC)system lacks of reactive power control function.Aim at this situations,With the object of BDFIM,the control strategy have been intensively carried out in this paper,a reactive power control method for the BDFIM with ISC scheme and an indirect power control method are proposed,the main research contents completed are summarized as follows:1.In order to choose appropriate model used for control strategy,multiple reference frame model,the rotor reference frame d-q model and unified reference frame d-q model are studied,two general equivalent circuits corresponding to power winding(PW)and control winding(CW)model of unified reference frame d-q model are derived,respectively,unified reference frame d-q model is simulation studied.2.A reactive power control method for the BDFIM with ISC scheme is proposed in this paper,the analytical expression between reactive power and CW flux linkage vector dynamic phase and amplitude increments is deduced,control system structure diagram is established based on theoretical analysis,the dynamic performance of the prototype experimental results show the feasibility of the proposed scheme,as a result,the proposed control system has been able to control not only speed and torque,but also the reactive power effictively.3.In order to improve accuracy of CW flux estimation under a super low frequency,on the basis of analying and comparing several u-i flux estimation models,taking BDFIM's special structure and working principle into consideration,flux estimation methods are studied in this paper,with an improved orthogonal nonlinear feedback compensation flux estimation method adopted as the CW flux linkage estimation methods.The experimental results show this flux estimation method effectively eliminates the u-i model integrating drift and improves steady and dynamic performance of control system.4.The power-angle characteristic of BDFIM is studied in this paper,the BDFIM power-angle characteristic expressions are deduced on the base of PW static frame model,which are very similar to that of the cylinder synchronous machine,the correctness of the expressions are preliminary verified by simulation,on the base of these expressions,brushless doubly-fed induction generator's(BDFIG)active power and reactive power regulation method,the static stability are analyzed preliminary in this paper.5.In order to meet control requirement for active power and reactive power in the wind power generation,an indirect power control method for BDFIG is proposed in this paper,which is different from indirect stator-quantities control,this control method directly controls output active power and reactive power,the control system structure diagram is established based on theoretical analysis.The experimental results of the prototype show that the system has good dynamic performance reponse to the active power and reactive power change.6.In order to accurately calculate CW flux static angle increment,grid frequency should be detected in BDFIG indirect power control system,this paper uses a dynamic grid frequency detection method based on software phase-locked loop,whose real-time performance is so strong as to be suitable for grid frequency on-line detection.The experimental results show the feasibility of this method.