PERFORMANCE EVALUATION OF FLYING CAPACITOR MULTILEVEL INVERTER BASED INDUCTION MOTOR DRIVE

Abstract

In recent years, industry has begun to demand higher power equipment, which now reaches the megawatt level. Controlled ac drives in the megawatt range are usually connected to the medium-voltage network. Today, it is hard to connect a single power semiconductor switch directly to medium voltage grids (2.3, 3.3, 4.16, or 6.9 kV). For these reasons, a new family of inverters namely multilevel inverters has emerged as the solution for working with higher voltage levels. The general structure of the multilevel inverter is such as to synthesize a sinusoidal voltage from several levels of DC voltage. Increasing the number of levels produces a fine staircase waveform approaching close to a sinusoidal wave with minimum harmonic distortion. In this thesis simulation of various inverters (from 2-level to 5-level) was carried out and the performance of all these inverters was evaluated. The line current THD variation of the inverters with power factor and frequency was observed for the R-L load and a fine comparison of the performance of all the inverters was also made. The performance of the various inverters fed induction motor drive was evaluated using scalar control (v/f control). Both the steady state and dynamic responses of the drive were observed under varying conditions of load torque and speed. Motor line current THD variation with frequency was observed and finally a comparison of all the inverters fed drive was made. It was shown that the drive performance will be improved with increase in the level of multilevel inverters.