Study On Direct Torque Control Technology For Variable Speed Constant Frequency Brushless Doubly-fed Wind Power Generator

Taking the Natural Science Foundation Project of Liaoning Province and the Science and Technology Planning Project of Liaoning Province as backgroud, direct torque control (DTC) method based on sliding mode flux observation method and speed sensorless control technology is adopted for variable speed constant frequency (VSCF) brushless doubly-fed generator (BDFG) applied to wind power generation in this dissertation, based on the relation between maximal absorbing power and speed of the generator. The maximal power tracking and VSCF operation are synchronously implemented by controlling speed and electromagnetic torque of BDFG. And the maximal power factor is achieved by setting the flux given value of control winding. The study contents of this dissertation are mainly as follows:DTC scheme suitable for VSCF brushless doubly-fed wind power generator is proposed, based on the corresponding relation between maximal absorbing power and speed of the wind power generator and on the electromagnetic relation between the two stator windings of BDFG. The optimal speed values of the generator corresponding to the maximal absorbing power points are acted as the speed given values of DTC to get the torque given values, according to the wind speed changing. The maximal power tracking control is implemented by controlling speed and electromagnetic torque of BDFG. In the process of estimating torque, the task of the output voltage of power winding side satisfying the grid connected condition before grid connected running is completed to implement VSCF operation. The maximal power tracking control and VSCF operation are synchronously achieved based on this control scheme. Furthermore, the flux given value is set according to the maximal power fator principle, in order to meet the requirement of the grid to the power fator and to improve the generation efficiency.For the problem of bad robustness of flux observation as brushless doubly-fed wind power generator running at lower excitation current frequency of control winding side, the flux observer of control winding is designed based on the sliding mode control theory. The current and flux of control winding are regarded as the state variables, and the flux observation value of control winding is corrected by current observation error. The stability of the designed sliding mode flux observer is analyzed by Lyapunov theorem. The switching function in common sliding mode observer is replaced by a continuous function to weaken the chattering. Through simulation comparison with the operation performance of DTC system based on u-i flux model back EMF integral algorithm, it can be seen that the designed flux observer can enhance the accuracy of flux estimation and can effectively improve the operation performance of DTC system, as the frequency of BDFG control winding side is lower.For the complexity of BDFG internal electromagnetic relation, a suitable speed estimation method is proposed to enhance the reliability of the wind power generation DTC system and to enable the system to be more applicable to harsh wind field environment. Taking the two-phase stationary frame as the reference frame, the generator speed is estimated by evaluating the synchronous rotating speed of the rotor flux and the slip speed to implement the speed sensorless control. Due to the accuracy of evaluating the slip speed suffering from the influence of the load changing, the fuzzy control is introduced to enable the speed sensorless control system to accurately estimate the BDFG speed in the case of the load changing, in order to improve the system robustness. The simulation study is carried out by Matlab_Simulink software to verify the validity of the proposed speed estimation method, and the BDFG speed estimating accuracy of the speed sensorless control system without or with fuzzy control is compared and analyzed.The experiment platform of VSCF brushless doubly-fed wind power generator DTC system is built, taking DSP ICETEK-F2812-A as the control core and acting the dc motor as the prime mover to drive the experimental BDFG. The experiment studies of noload and load on VSCF control and optimal given speed tracking control of BDFG DTC system, as well as the performance of sliding mode flux observer are carried out to verify the validity and feasibility of the proposed control scheme.