Stability Analysis Of Brushless Doubly-Fed Machine

The Brushless Doubly-Fed Machine (BDFM) has many advantages such as highreliability, small converter capacity, flexible speed adjustment and low system cost dueto its giving up the brushes and slip rings. So BDFM has good applicable potentials inthe frequency control of motors and the variable speed constant frequency operation ofgenerators. However, the BDFM operation mode is quite different with the conventionalmotors. One of the key issues to promote the BDFM’s application is how to ensure itsstable operation at the different situations, and the studies about this issue are quite fewyet. Based on the Project of State Key Laboratory of Power Transmission Equipment&System Security and New Technology of Chongqing University, this thesis focuses onthe studies of the steady-state operating characteristics and the stability of the BDFMwith cage rotor.Firstly, the basic structure，the working principle and the research methods of thestability about the BDFM with cage rotor were briefly introduced. Secondly, accordingto the coupling circuit and the basic equations of the BDFM with cage rotor, thesteady-state mathematical model was derived in detail, and thus its steady-stateoperating characteristics and the relationship of energy transformation were researched.Thirdly, the stable operation regions of the BDFM with cage rotor under the open andthe closed loop controls were analyzed by the small signal analysis method and theLyapunov stability criterion. The factors of impacting the stability were discussed andthe results of the stability analysis were verified by the dynamic simulation. Finally, thestabilities of the open and the closed loop controls were compared and some methodswere proposed to improve the operational stability of the BDFM with cage rotor.The results show that all the steady-state characteristics of the BDFM with cagerotor can be expressed as the functions of the control winding voltage, the controlwinding frequency and the power angle, the proposed mathematical model simplifiesthe analysis of the BDFM steady-state characteristics, the BDFM with cage rotor showsthe poor stability at the open loop control with the constant ratio of control windingvoltage and frequency, the operating stability will be increased by reducing the inertiamoment and selecting the appropriate voltage to frequency ratio. Compared with thestability of open loop control, the stability of the control winding current closed loopcontrol strategy is improved greatly, and the BDFM with cage rotor can operate within a wide speed range. The load capacity of the BDFM with cage rotor can be improved byincreasing the given maximum electromagnetic torque.The study of this thesis provides the theoretical foundation for the analysis of thesteady-state characteristics and the stability of the BDFM with cage rotor, and isvaluable for the research of the control strategy of the BDFM with cage rotor.