Sinusoidal PWM of multilevel inverters in the overmodulation region

The behaviour of the key multilevel carrier based PWM methods for diode clamped, cascaded and flying capacitor inverters are considered in the overmodulation region. The double Fourier series analysis method is applied to theoretically determine the spectral components of each PWM method, since this analysis technique provides spectral decomposition of a switched waveform into baseband, carrier and carrier sideband harmonic components. Mathematical expressions are determined for the phase shifted carrier PWM (PSCPWM) and the phase disposition (PD) modulation methods. The results show that the only additional spectral components developed in overmodulation are odd baseband harmonics, which are to be expected since the switched waveform moves towards a square wave. No additional high frequency terms are developed when operating in overmodulation. This means that the sideband cancellation that the PSCPWM method relies on is unaffected by pulse-dropping. Furthermore the carrier harmonic, triplen sideband and baseband cancellation that occurs with three phase inverters is also not affected by inverter saturation. Theoretical and experimental results are presented to confirm this conclusion.