Voltage Ripple Minimization in Modular Multilevel Converters using Modified Rotative PWM Scheme

Carrier-based pulse width modulation (PWM) schemes are usually employed in MMC fed variable speed drive (VSD) applications due to ease of implementation. Though this is the case, the phase-disposition PWM (PD-PWM) technique is a better choice as it is able to generate high-quality waveforms with a greater possibility of optimal switching. This technique suffers as it may provide uneven energy distribution among the submodules (SMs) and also non-uniform switch utilization. The use of carrier rotative PWM techniques as applied to MMC's will alleviate the problem of uneven energy distribution among the SMs, but the issue of non-uniform switch utilization still remains. A modified rotative PD-PWM (MRPD-PWM) technique is proposed in this paper, which retains the uniform energy distribution among the SMs while evenly distributing voltage distribution across SM capacitors. Moreover, this technique minimizes SM capacitor voltage ripple which can reduce the capacitance requirement in MMC-fed VSD applications. The performance of the proposed PWM scheme is compared with aforementioned PWM techniques for half-bridge submodule (HBSM) based three-phase MMC in PLECS platform and verified experimentally under the same converter switching frequency.