Research On Wavelet Modulation For Three-level Inverter

Because of small switching voltage stress, simple structure and flexible control,and many other advantages, the diode-clamped three-level inverter is widely used inhigh-voltage high-power application fields. For the inverter control method, modulationscheme is a core part, so the study of modulation techinique has very importantsignificance. Good modulation strategy not only can effectively improve output voltagecharacteristics, but also has the advantages of simple control and easy implementation.Wavelet transform has good time-frequency local features and multi-resolutionanalysis characteristics, so it has been widely applied in the field of communicationsignal modulation. With the improvement of wavelet theory, foreign scholars havesuccessfully applied wavelet modulation in power electronic technology, and study thewavelet modulation of two-level inverter largely. Researches have shown that, waveletmodulation (WM) as a novel pulse width modulation technique can significantlyincrease magnitude of fundamental components, reduce the output voltage totalharmonic distortion rate, decrease switching loss, and easily digital realization.This thesis takes the diode-clamped three-level inverter as researching object, andanalyzes the performance of inverter output voltage waveform quality, switching loss,neutral-point potential balance, and other aspects under wavelet modulation strategy.Firstly, based on the analysis of the basic principle for wavelet modulation, the WMtechnique for single-phase two-level H bridge inverter is reseached. Secondly, accordingto the working principle of the diode-clamped single-phase three-level inverter, the WMtechnique is studied, and the simulated results are compared with SPWM modulationstrategy. In addition, the relationship between the numbers of sample groups for waveletmodulation and voltage harmonic distortion rate is studied, and the Fourier analysisexpression is deduced of the inverter output waveform to get the theoretical value of thefundamental voltage and harmonic distortion to verify the correctness of waveletanalysis. After that the paper also studies the principle of three phase waveletmodulation, and the WM technique for diode-clamped three-phase three-level inverteris implemented. The advantages of WM technique are analyzed by comparing itssimulated results with carrier disposition PWM control method and space vectormodulation strategy. At the same time, the neutral voltage waveform of WM strategy isstudied, and the hardware circuit is applied to control the neutral-point potential balance.At last, according to the theory analysis and simulation, the hardware and softwareplatform based on TMS320F2812DSP for realizing the WM technique of three-levelinverter is seted up, and inverter experimental waveforms are obtained to verify thecorrectness of the WM algorithm. The studies show that the WM strategy can effectively achieve the three-levelinverter output control. And the high quality of the output voltage, low switchingfrequency, simple algorithm and easy implementation can be achieved by using WM inthe three-level inverter. Furthermore, the amplitude of neutral-point voltage fluctuationis small, and the average of neutral-point voltage is close to zero so that the neutral-point potential has no significant derivation from zero potential for the WM technique.Therefore, the research of WM technique for multilevel inverter is valuable.