Optimised phase disposition (PD) modulation of a modular multilevel converter using a state machine decoder

This paper presents a theoretical harmonic analysis of phase disposition (PD) and phase shifted carrier (PSC) pulse width modulation (PWM) strategies for MMC converters. It is shown that when these strategies are implemented on a per MMC arm basis, their spectral performances converge because of cancellation of odd carrier sideband groups between each phase leg's arms. An improved PD modulation strategy is then presented that uses a single PD modulator for the entire phase leg, followed by a state machine decoder that evenly distributes switching pulses to all sub-modules across the phase leg upper and lower arms to balance the distribution of sub-module commutation events. The resulting strategy achieves optimum phase leg PD spectral performance, and also achieves natural voltage balancing of the MMC sub-modules. All theoretical findings are supported by simulation and experimental results obtained using a five level MMC prototype.