Research On Direct Torque Control Of Three-level Inverter-fed Induction Motor

The high-voltage high-power AC motors variable speed techniques based on multilevel converter have become one of research hot points now in the field of power electronics and power drives. Space vector pulse width modulation (SVPWM) method of a three-level neutral-point-clamped (NPC) inverter is studied comprehensively in this dissertation. Then, direct torque control (DTC) and speed sensor-less methods of induction motor, which is driven by a three-level NPC inverter, are studied and explored further, also, a three-level inverter prototype system is built and a lot of experimental results are obtained based on this prototype.The high-voltage high-power multilevel converter and AC motors drive techniques are summarized compendiously for domestic and overseas studies in the prolegomenon of this dissertation. The evolution and research status of multilevel converter topologies are analyzed, the development of AC variable speed techniques and the characteristic of DTC are discussed, also, the aim and significance for the research are demonstrated.In this dissertation, SVPWM pattern and implement method have been addressed comprehensively for three-level NPC inverter. In order to avoid the abrupt change of output voltage vectors during the process of desired vector changing from one section to another, a novel space vector modulation (SVM) algorithm is proposed where positive or negative small vectors are adopted as first active vector, which is virtually a kind of scalar unipolar modulation pattern. Besides, based on the particular disposition of all the redundant voltage vectors, a SVPWM approach called semi-bipolar PWM pattern is present. The harmonic performance of this modulation strategy not only has better output voltage and current waveforms (lower harmonic and lower EMI) than the popular eight-stage centered PWM strategy but also evidently favors the neutral-point voltage balance control.The author investigates the variation mechanism of neutral point (NP), establishes the neutral point current model, therefore, the theory base is built for the balancing control of neutral point potential. Moreover, based on the inherent correlation between carrier-based PWM and SVPWM for three-level inverter and on the semi-bipolar PWM pattern, two control techniques of NP potential variation are proposed, only requiring the information of dc-link capacitor voltages and three-phase load currents, which is convenient to apply and is compatible of digital computer realization.The characteristics of the balancing control of NP potential for a three-level NPC inverter at high modulation depth are analyzed, and based on the analysis, a novel space vector modulation pattern where a middle vectors are synthesized by redundant small vectors is proposed. Theoretical analysis and simulation results indicate that the proposed SVPWM method vector synthesizing concept based can make the average NP current zero, and do not influence the NP potential within every sample period. Thus, the NP potential variation can be restrained effectively, especially at high modulation depth.The main problem of direct toque control of a three-level inverter and control technique virtual synthesizing vectors based are demonstrated in detail, also, the method of vector selection is discussed materially. Then DTC strategy of induction motor based on PI regulating concept is utilized in control method of the three-level inverter, which contains such characteristics as convenient NP variation control, fast torque response and lower torque ripple.The estimation methods of stator flux linkage based on mathematic model of induction motor are studied, and the performance of these estimation methods is analyzed in details. The design method of a full-order state observer, in which the state variables are stator current and stator flux linkage, is present systematically. Based on a adaptive full-order observer, speed estimation method is deduced according to Liapunov stability theorem. With the proposed full-order observer, the flux linkage of motor can be accurately observed and high performance can be achieved without shaft encoder.Based on microprocessor TMS320LF2407A, a three-level inverter prototype is built and correlative experimental researches are implemented by this prototype. The three-level inverter prototype system can work reliably in the whole experimental process. The simulation and experimental results validate the control strategies proposed in this dissertation...