Research And Optimization Of Direct Torque Control For High Power Traction Drive System

Based on the national scientific and technological support project "Development of traction inverter for high-speed freight locomotive", high power traction drive system of HXD1electric locomotive as the research object, direct torque control algorithm for asynchronous traction motor is deeply studied, including the optimization scheme of conventional direct torque control and speed-sensorless.Conventional direct torque control is the basis of this paper. Firstly, the basic theory of the conventional direct torque control system is discussed, and the mathematical model of two-level inverter and asynchronous traction motor and the basic structure of direct torque control system are briefly described, focused on the basic control algorithm for circular, eighteen-gon, and hexagonal flux.In this paper, the limits of conventional direct torque control are analyzed from flux and torque, for its large torque ripple and non-fixed switching frequency. In low-speed zone, instead of the circular flux trajectory control with switching table, indirect stator control sets switching frequency constant and reduces the ripple of starting torque. In order to retain the advantages, in medium-speed and high-speed zone, precision torque control is added to improve torque ripple, which conduces to the stable operation of the control system. The simulation results of the optimization programs are compared with conventional direct torque control, which show that the optimization program can effectively reduce the torque ripple.Speed-sensorless technology is also studied based on the optimization program of direct torque control. The basic principles of a closed loop stator flux observer and a full order state observer are focused on, and the speed estimation method of MRAS and Luenberger are discussed in detail. And then, the two flux observers and two speed observers are simulated in the case of parameter changes, which show that, compared to MRAS, the estimated speed of Luenberger is more accurate, and more suitable for direct torque control.Hardware in loop simulation platform with dSPACE+DSP2812is set up, on which, the main circuit model in dSPACE and the control program in DSP are finished, and the optimization program of direct torque control and its speed identification are completed for high power traction drive system. Simulation and experimental results verify the algorithms are feasible. The optimization programs of direct torque control are effective in reducing the torque ripple, and speed observer is accurate.