Common-mode voltage reduction methods for medium-voltage current source inverter-fed drives

Common-mode voltages (CMVs) can lead to premature failure of the motor insulation system in medium-voltage (MV) current source inverter (CSI)-fed drives. A series of nonzero-state modulation (NZSM) techniques are proposed for the CMV reduction. In these methods, the zero states are avoided because they generate the CMV peak values in common operating conditions of a CSI drive. The overall performance of the proposed modulation techniques with the corresponding switching patterns is investigated. The simulation results indicate that the CMV magnitude is decreased by half using NZSM. Although there are tradeoffs between the CMV reduction and increase in the device switching frequency, shrink in the modulation index range and deterioration in the waveform quality, an active-zero-state modulation (AZSM2) method is recommended for low-speed CSI-fed motor drives with the best CMV reduction effect. The near-state modulation (NSM-S1) technique can be applied to a CSI working at a modulation index higher than 0.67.