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.