Overmodulation of Six-Phase Cascaded-CSI Using Optimal Harmonics Injection

Abstract

This paper proposes a new and straightforward method to extend the modulation range of the six-phase cascaded current source inverter (CSI). The proposed technique employs vector space decomposition (VSD) to mitigate the inverter current harmonics and extend the linear modulation region by about 8%. For motor drive applications, increasing the fundamental output component can reflect higher torque production capability for the same drive size, given that thermal limits are not exceeded. The extension can be realized by injecting optimized xy harmonics while keeping the amplitude of the resulting phase currents under the maximum value. The method utilizes a look-up table of optimized values of the injected harmonics to extend the modulation range. The output filter capacitor effects are also studied in this paper, and a selection approach is introduced. Finally, the experimental results of a C-CSI laboratory prototype are presented and discussed to verify the feasibility of the proposed modulation technique.