Modified Phase-disposition PWM Technique for Modular Multilevel Converters

Modular multilevel converters (MMCs) are considered to be a suitable candidate to improve the energy conversion efficiency and fault tolerant ability of electric propulsion systems. These converters offer advantages such as modularity, low harmonic contents, high availability, lower electro-magnetic interface and no bulk dc-link capacitors. Phase-shifted pulse width modulation (PS-PWM) is a natural PWM method for MMCs as it ensures even power distribution and equal switching frequency on submodules (SM). However, this technique suffers from lack of waveform quality and optimal switching. In contrast, phase-disposition PWM (PD-PWM) technique ensures better voltage profile and lack of even power distribution. A modified rotative PD-PWM (MPD-PWM) technique is proposed in this paper, to overcome the problem of uneven power distribution. Moreover, a common mode voltage is injected to PWM modulator, which decreases the switching transitions and increases the fundamental voltage. The key performance of each PWM scheme for half bridge submodule (HB-SM) based three-phase MMC (4 SMs per arm) are holistically evaluated by simulations in PLECS software.