| Multi-level diode clamped inverter has significant advantages over the conventional two-level inverter. The main advantages are as follows:(1) Voltage across the switches is only half the dc bus voltage; (2) Better output voltage quality and current waveforms;(3) The peak transient voltage at the motor terminals is reduced. The research on multi-level diode clamped inverter PWM schemes is becoming popular. This dissertation emphasizes PWM Method and relative techniques of multi-level diode clamped inverter. The main contents of this paper are as follows:1. Based on the analysis of the SHEPWM principle, a method of the selective harmonic elimination technology based on Homotopy algorithm applied to three-level NPC inverter is presented. The method of solving the SHE-PWM nonlinear and transcendental equations has many advantages, such as large convergence margin, quick convergence, and insensitive initial iteration. Simulation and experiment results show that the method is satisfied.2. The principal limitation with the application of SHEPWM technique is the analytical difficulty in solving the nonlinear and transcendental equations. To obtain convergence with numerical techniques, commonly, some methods must be taken to preset the initial values and predict the trend of these values over whole range of modulation index. Therefore, genetic algorithm and chaotic ant colony algorithm are adopted here replace numerical algorithms to overcome this difficulty, because of its intrinsic ability to begin searching randomly, easy convergence and less computing-time. This method can achieve online SHEPWM control of the three-level NPC inverter.3. In order to perform the quantitative analysis of the harmonic characteristic mathematically in carrier disposition PWM, the Fourier series of the three-level NPC inverter output voltages with two kinds of carrier PWM strategies respectively have been derived by means of dual Fourier transforms. Simulation and experimental results are proposed to verify the comparison.4. A fast space vector pulse width modulation algorithm using 60°coordinate frame applied to multi-level diode clamped inverter is introduced. The position of reference vector and the nearest base vectors don't need to compute trigonometric function, so the computation is extremely simple. Simulation and experimental results show that the algorithm is effective.5. A significant problem with three-level NPC inverters is the fluctuation in the neutral-point potential. The mathematical models of neutral-point circuit and neutral-point potential are developed. The neutral-point potential leads to the distortion of output voltage space-vector waveform of the inverter. The SVPWM method is proposed while neutral-point potential is unbalanced. A novel and simple judg rule based on the synthetic space vector is introduced. 6. The region of neutral-point potential which can be unbalanced is described. And it introduces a novel modulation approach for the complete control of the neutral-point potential. The new modulation approach, which based on the virtual space vector concept, guarantees the entire balance of the neutral-point potential. 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 algorithm is proposed in which the same positive or negative small vectors are adopted as first active vector. Some experimental results are given to validate the method.This dissertation researched on the technique of SHE, PWM and the neutral-point potential balance, which improved the performance of multi-level diode clamped inverter. The study had theoretical meaning and offered value for engineering application.
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