Performance Enhancement of Neutral Voltage Modulation in Cascaded H-Bridge Inverters Under DC-Source Voltage Imbalance Using Modified Level-Shifted PWM

The operation of a cascaded H-bridge inverter in unbalanced desource voltage conditions necessitates application of a suitable offset voltage injection to achieve extension of the linear modulation index range for balanced three-phase operation. This can be achieved to a certain extent with the min-max offset voltage injection and to an exceedingly greater degree by the so-called neutral voltage modulation (NVM). However, application of these methods using the phase-shifted (PS) pulse width modulation (PWM) leads to suboptimal results in terms of harmonic distortion of the phase voltages and currents, and equal power dissipation of the unequally charged desources - which is not ideal. This paper suggests an alternative application of these offset injections using the in-phase disposition (IPD) level-shifted (LS) PWM method which has been modified accordingly for application in dc-source imbalance conditions. The obtained results show significant performance improvement in the THD of the phase voltages and currents. At the same time, exceedingly greater balancing potential and control over the power distribution of the dc-sources is achieved.