Research On Dual-PWM Three-Level Variable Frequency Speed-regulating System For High Power Medium Voltage Applications

To meet the biggest two global challenges, energy shortage and environment pollution, China has set energy-saving and emission reduction its important goal of the eleventh Five Year Plan. With the development of modern power electronics, micro-electronics and computer technologies, research on medium-voltage high power inverter technology has been very popular in the power electronics and motor drive areas due to its significant advantage for energy-saving. Space vector modulation (SVM) for dual-PWM three-level converter, mechanism of neutral point voltage fluctuation and control strategy of three-level rectifier has been studied in this paper.A survey of the development and applications of medium-voltage high power variable frequency speed-regulating system as well as the significance of this dissertation has been given in the introduction.The mechanism and implementation of space vector PWM (SVPWM) for three-level neutral-point-clamped (NPC) inverter have been studied. The mechanism of neutral point voltage fluctuation is investigated, the average current model of neutral point is established and a modified modulation method is proposed. Smooth switching of output vector avoids not only vector jump but also low utilization of DC voltage while sector switching. The three-level PWM rectifier topology is studied, and the decoupled state-space equations in synchronous rotating reference frame (d-q frame) are derived. The hierarchical controller structure is adopted, namely, the inner current-loop responsible for reference current tracking and the outer voltage-loop responsible for load regulation. The design methodologies for proportional-integral (PI) controllers are presented, for the inner current-loop and outer voltage-loop, respectively. The proposed three-level PWM rectifier based on revised space vector pulse width modulation (SVPWM) strategy is studied by using MATLAB/SIMULINK. Simulation results demonstrate that the proposed three-level PWM rectifier provides excellent performance both in steady-state and dynamic load variations. Pure sinusoidal waveforms can be achieved in grid currents with a unity power factor. Besides, energy feedback is also realized.A three-level converter prototype is built based on microprocessor TMS320F2812 DSP, and relevant experiments are implemented on this experimental platform. Both simulation and experimental results validated the control strategies proposed in this dissertation.