Modeling, analysis and PWM control of a new multilevel voltage source inverter including high voltage high power application

Abstract

In general, increasing the switching frequency in voltage source inverters(VSI) leads to the better output voltage/current waveforms. Harmonic reduction in controlling a VSI with variable amplitude and frequency of the output voltage is of importance and thus the conventional VSI``s, which are now categorized into two-level inverters, have required increased switching frequency along with various pulse width modulation(PWM) switching strategies. In the case of high power/high voltage applications, however, the conventional two-level inverters have some limitations to operate at high frequency mainly due to switching losses and constraints of device rating itself. Futhermore, the semiconductor switching devices should be used in such a manner as problematic series/parallel combinations to obtain elther capability of handling high power and enduring high input voltage possible, which further lower the highest possible switching frequencies. In this thesis, a general circuit topology of multilevel inverters above three level is suggested. When compared with conventional two-level inverters, the multilevel structure allows to raise the power handling capability in the conversion process in a very powerful and systematical way while the level of inverter employed can be determined according to the inverter input voltage, output power, device rating, waveform quality required, etc. The proposed multilevel inverter can realize any multilevel pulsewidth modulation scheme which leads to harmonic content reduction and provides full utilization of semiconductor devices like GTOs especially in high power range where high voltage could be applied. Thus, with our multilevel inverter, 6600V direct conversion could be possible requiring no additional coupling component such as transformer. Moreover, high quality output waveform in higher level inverter can be applicable to, for example, high power uninterrupteble power supply system such as TRIPORT type with low switching fr...