Quarter-wave symmetric space vector pwm with low values of frequency modulation index in control of three-phase multilevel voltage source inverter

Abstract

The paper deals with the three-phase multilevel voltage source inverter (MLVSI) load current quality issue, which is important, in particular, for the MLVSI application to adjustable-speed AC drive. Here the problem of MLVSI control is solved by the use of the low-frequency space-vector PWM with the quarter-wave symmetric output voltages (QWS-SVPWM), based on the approach operating with the integer and fractional parts of the reference delta voltages relative values. The specified technique has low dynamic losses in power semiconductor switches. QWS-SVPWM is applied to control MLVSIs of any arbitrary numbers of the equal levels and any arbitrary topologies. Comparisons of dependences on a modulation depth for the THD and the first three orders’ integrated voltage harmonics factors (IHF) of the simulated MLVSI output voltage waveforms are carried out between the three lowest appropriate values of frequency modulation index. To obtain the most appropriate voltage waveform for the best quality MLVSI load current, the amplitude modulation index ranges for preferable use of each of the three considered frequency modulation index values, under the terms of minimization of the appropriate orders’ IHF values (depending on the particular kind of the load) are revealed.