Design of discontinuous switching sequences in the case of grid-connected three-level voltage-source converter

The paper studies discontinuous space-vector modulation and switching sequence design in the case of grid-connected three-level neutral-point-clamped voltage-source converter operating with low switching frequency. Discontinuous pulse-width modulation (DPWM) methods of two-level modulation theory are used in three-level modulation and the effect of discontinuous switching sequence design on the switching waveforms is examined. A simple method is presented to design and arrange the discontinuous switching sequences such that switching waveform symmetry is retained during the transitions between the subsectors of the space-vector diagram. It is demonstrated with hardware-in-the-loop simulations that the loss of switching waveform symmetry results in the distortion of grid current. The results show that with the help of the method proposed the switching sequences can be arranged so that switching waveform symmetry is retained.