A Generalized Multilevel Inverter With Extended Linear Modulation Range and Instantaneously Balanced DC-Link Series Capacitors for an Induction Motor Drive

The voltage drift control of series-connected capacitors across a single dc-link is the major challenge for a multineutral point-clamped (NPC) converter. This drifting is caused due to uneven currents drawn from the neutral points (NPs) during the PWM operation. This work presents the working principle of instantaneous voltage control of “<inline-formula> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula>” dc-link series-connected capacitors for an induction machine load. The mechanism of instantaneous voltage balancing is realized by ensuring zero instantaneous currents at each dc-link NP. To get zero instant currents at a dc-link NP branch, the phase terminals tapping on the dc-link are manipulated with the help of cascaded hybrid bridges (CHBs). Charge balancing of CHB capacitors is also taken care of by proper utilization of pole voltage redundancies. The detailed results are presented for a small-scaled inverter (nine-level) with three NPs (four dc-link capacitors). The simulation and experimental results reflect the capability and limitations of such inverters. The results are presented at all modulation indexes operated under low-power-factor (LPF) and high-power-factor (HPF) load conditions. It is found that the linear modulation range (LMR) can be increased to full base speed even for an HPF load.