Research On Direct Torque Control Of Three-Level Inverter

High voltage great power frequency converter based on multilevel topology has become one of hot research directions now in the field of power electronics and motor drives. The topology of neutral-point-clamped(NPC) three-level inverter is studied comprehensively in this paper. The method of sine pulse width modulation(SPWM) and the method of space vector pulse width modulation(SVPWM) of a three-level inverter are demonstrated in detail and simulated, and the implementation method of direct torque control(DTC) for three-level inverter is addressed. The drive circuit and control circuit of three-level inverter are designed, and the printed circuit board(PCB) have been designed, and a test platform of three-level inverter based on DSP2407 has been built. The test waveforms of three-level SPWM inverter are tested. Its main content is as follows:This paper introduces the current situation and development of power electronics and high voltage frequency converter, and the implementation methods of high voltage frequency converter which are widely used are summarized. The research status at home and abroad is analyzed in this paper, and the development and characteristics of modern AC variable speed are studied, also, the significance of the study are demonstrated.The working principle and implementation methods of three-level SPWM inverter are demonstrated in detail, and the action of the overlap of adjacent carrier on the total harmonic distortion(THD) of inverter is studied and simulated, thus a new control method of the overlap of adjacent carrier is proposed in this paper.The conventional method of DTC with looking up switching vector table of two-level is analyzed, thus the method of DTC with looking up swithing vector table of three-level is obtained. A real-time caculation method of reference voltage vector in the next sampling sycle is derived, which has the least torque ripple. If the calculation of the reference voltage vector is error, the method of DTC with looking up switching vector table should be used. In this paper, the reference voltage vector is comprised with the SVPWM method of seven-sement vector. The synthetic method of reference voltage vectors is simulated,and the simulation results shows that the synthetic method of reference voltage vectors is feasible. The implementation method and simulation results of the method of SVPWM for three-level inverter is compared with the method of SPWM, which proves the method of SPWM and SVPWM for three-level inverter are uniform in nature.In the aspect of hardware design,the control circuit and drive circuit of three-level inverter are completed.Compared the simulatation results and experimental results with theoretical conclusion, both are consistent.The sampling circuit of output current and bus voltage are built, and the printed circuit boards have been designed. The main circuit of three-level inverter has been completed,and a test platform of three-level inverter based on DSP2407 is built.The implementation method of asymmetric regular sampling method SPWM for three-level inverter is derived, and the software based on DSP2407 is designed,and the waveform of three-level inverter is tested.The experimental results proved the correctness of the hardware circuit design and the feasibility of the method of SPWM for three-level inverter in this paper.