Multi carrier PWM of the modular multilevel VSC for medium voltage applications

This paper presents integrated level-shifted and phase-shifted multi carrier modulation schemes that ensures PWM and local capacitor voltage balancing of the Modular Multilevel Converter (M2LC) for medium voltage applications. The integration of either of the modulation schemes with the cell voltage balancing algorithm ensures the floating capacitor voltages are balanced throughout the operation of the M2LC voltage source converter. A comparison of the two schemes is presented based on the harmonic content of the synthesized output waveforms and peak-to-peak ripple of the local capacitor voltages. The converter semiconductor losses are also evaluated and compared for these modulation schemes in a typical medium voltage grid application. It is shown that both schemes are competitive in terms of the synthesized output waveform quality. However the phase shifted scheme offers less capacitor voltage ripple while the level shifted scheme offers low converter loss. The concepts are confirmed with both PLECS simulation package and a 10kVA 9-level experimental prototype.