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Research On Some Key Techniques Of Multilevel Voltage Source Inverter

Posted on:2008-01-20Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q ChenFull Text:PDF
GTID:1102360215951328Subject:Power electronics and electric drive
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Multilevel inverters have drawn tremendous interest recently due to their advantages regarding power export, harmonics, power factor, and common-mode voltage. However, the present research on its theories and applications have not matured enough and suffer from restrict of application in wider area. This dissertation will stress on those unsolved or imperfect solved problems. The main contents of this paper are as follows:1. This dissertation presents a review of multilevel inverter modulation methods, harmonic suppression, losses, and control strategy. The basic multilevel theory is introduced from the point of topology structure, and the three-level one is stated explicitly.2. By analysis of the principle of the optimal space vector sequence and the relevant modulation methods, the viewpoint of the novel modulation method produced for multilevel inverter should consider more system performances is proposed. Under this guidance of this idea, a PWM strategy for the three-level inverter with an ability to reduce losses according to different load is established.3. The nonlinear, harmonic elimination equations solved by traditional iterative algorithm exists some weaknesses. After analyzing the equations deeply, this dissertation presents an approach to eliminate harmonics in neutral-point-clamped inverter using the theory of multivariable polynomial. Taking computing 4 switching angles for instance, the results show that the elementary symmetric polynomials can be exploited to reduce the degree of the polynomials and the resultant theory can be utilized to eliminate variables. The research indicates the method has many merits, such as hurdling the difficulty of determining the initial values when solving the selective harmonic elimination nonlinear equations, obtaining all solutions accurately. Experimental and simulation results of two groups of solutions are presented to confirm the validity of the technique.4. The scheme of study on multilevel inverter conduction losses is provided. Conduction loss expressions are derived based on the knowledge of conduction devices, which are determined by the working condition, and the conduction duty cycle. The loss model is used to analysis and comparison on two different modulation strategies, sinusoidal PWM (SPWM) and third harmonic injection PWM (THIPWM), for neutral-point-clamped three-level inverter.5. The multilevel inverter switching losses is studied. The algebraic equations that represent the voltage and current waveforms during the switching event are developed based on the fact that the current and voltage waveforms during switching are principally a function of the pre- and post-switching voltage and current, the character parameters, and the physics of the switching process. The switching losses model of neutral-point-clamped three-level inverter with widely used IGBT and fast recovery diode is developed. The simulation and experimental results demonstrated the validities of turn-on and -off models and switching losses models of the semiconductor devices.6. Direct torque control (DTC) for induction motor-fed by three-level inverter is studied theoretically. Especially, the demagnetization phenomenon of stator flux and the torque pulsations caused by space vector voltage are analyzed. This paper also lays out an improved switching lookup table and compares with simulation results.7. The design of a three-phase three-level NPC inverter based on SVPWM is accomplished in this paper. A speed-loop-open, constant ratio of voltage and frequency controlled AC motor drive is fulfilled, which employs TMS320LF2407A DSP as control core. The experimental results testify the correctness and validity of the designed software and hardware.
Keywords/Search Tags:multilevel inverter, modulation methods, selective harmonic elimination, conduction losses, switching losses, neutral-point-clamped, direct torque control, AC drive
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