Comparison of Hybrid Asymmetric and Conventional Multilevel Inverters for Medium Voltage Drive Applications Master of Science Thesis in the Master’s programme Electric Power Engineering

Power electronic converters are becoming more and more popular for various industrial applications. To overcome the limitation of semiconductors current and voltage ratings in high power applications, series and parallel connection of switches is often considered an effective solution. In addition, stepped waveform in the output of inverter has better harmonic spectrum than 2-level waveform in low switching frequencies. So, in recent years multilevel inverters have gained great interest in industry. Among the different solutions available for multilevel converters, the asymmetric topologies allow to generate more voltage levels with less number of semiconductors and thus increase of output performance and system reliability. For these reasons, this kind of topology has attracted a lot of attention both from the customers and from the manufacturers. Application of appropriate semiconductor switches in the different cells of the inverter leads to increase of inverter efficiency. This inverter is typically known as hybrid inverter. In this work, different topologies of multilevel inverters consisting cascaded symmetric, diode-clamped, flying-capacitor, and hybrid asymmetric are investigated. It will be shown that hybrid asymmetric inverter has more reliable topology than others, due to less number of power semiconductor switches and higher voltage levels. Also different multilevel modulation techniques will be studied form voltage waveforms and harmonic spectra aspects. This study proves that Phase Disposition Pulse Width Modulation shows less harmonic distortion than other techniques. Comparison of hybrid asymmetric inverter with conventional multilevel inverters will be lead in two states of constant frequency and constant efficiency. The results indicate that, hybrid asymmetric topology has better performance in power losses, total harmonic distortion and first distortion factor than other topologies that leads to energy saving, better power quality and reduce in size, weight and volume of its LC filter.