Abstract
Focusing on the relative location of the centre of gravity of the pulse-width modulation waveform in each section, a novel direct digital sine pulse-width modulation (SPWM) method based on equal-area theorem for multilevel inverters is proposed in this paper. Three different direct digital SPWM schemes are illustrated and investigated thoroughly in terms of total harmonic distortion (THD) and amplitude of the fundamental-frequency voltage, and the best scheme with the lowest THD is identified. The THD and fundamental voltage produced with the proposed method with modulation index M varying from zero to its maximum value are calculated and compared with level-shifted carrier-based SPWM method. It is demonstrated that the proposed direct digital SPWM method can provide far lower THD, while its fundamental voltages are on par with the carrier-based SPWM method. Other advantages of the proposed modulation scheme include simplicity, digital oriented and generalised for multilevel inverter with any levels and any topologies, including cascaded H-bridge, neutral point clamped, flying-capacitor and any other types of inverters. Simulations and experiments are carried out. The experimental results agree well with simulations, which prove the validity of the proposed digital SPWM method.
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