Research On Modelling And Decoupling Control Strategies For BDFM

The Brushless Doubly-Fed Machine(BDFM), which removes the brush gear and slip ring of the doubly-fed machine as well as retains the advantages of small inverter capacity and adjustable power factor, requires low maintenance costs and enhances system reliability. BDFM shows significant commercial benefits for use in variable speed drives, water and wind power generation and new energy vehicles as a power coupling device. However, the control of BDFM is still a difficulty on account of the strong coupling and nonlinear characteristics of the machine. Therefore, more in-depth study on BDFM is important. This article presents the modeling and decoupling control strategy for BDFM. The main contents of this dissertation are summarized as follows:Firstly, the history and prospects of BDFM and the research status in the structure and control strategy for this machine are reviewed briefly. And the structural features and operation principle of BDFM are illustrated.Secondly, according to the structural characteristics and working principle of BDFM, the vector model of the machine is established in the rotor reference frame and then generalized into an arbitrary rotating frame by rotating transformation. The presented model has a compact form and provides more convenience for analysis and control purposes. Then, considering the control motor is generally designed with a sufficiently rapid inner current loop, the machine can be seemed as supplied by a current source. By choosing appropriate state variables, a state space model, with the power motor voltage and control motor current as inputs, is established in an arbitrary rotating frame. This state space model is in an affine nonlinear form.Thirdly, on this basis of the state model, the feedback linearization control strategy is proposed to deal with the strong coupling and nonlinear characteristics of BDFM. The state model in the control motor synchronous frame is obtained by feild orientation, based on which the feedback linearization controller is designed to achieve the decoupling control of torque and control motor flux. Then the sufficient condition for the proposed strategy is presented, and the stabilities of the zero-dynamics and the system are proven. A series of simulations are made to verify the effectiveness and superiority of the proposed control strategy. Simulation results show the decoupling capability and excellent performance of the proposed strategy. And the robustness of the proposed control system is also verified.Finally, to deal with the fact that the inner nonlinear coupling effects and the external disturbances are difficult to obtain in practice, a simpler strategy--active disturbance rejection control(ADRC) strategy is presentd for BDFM. This strategy is independent of the model and parameters of the machine. The ADRCs for the control motor flux and the speed are designed, respectively. To reduce the workload of parameters selection and improve the performance of ESO, two optimized design methods are proposed. On this basis, the complex nonlinear system is transformed to a linear one including two separate parts, and an excellent performance can be obtained by a simple controller. Simulation results show that the proposed controller, which brings out an excellent performance and increases the robustness of the system, rejects the external disturbances successfully and achieves the decoupling control of the system.