Reduce beat and harmonics in grid-connected three-level voltage source converters with low switching frequencies

This paper investigated the low frequency beat and harmonics associated with the grid-connected three-level PWM voltage source inverters with low and fixed switching frequencies. Tests and analysis show that the causes for subharmonic beat are PWM switching sequence when the switching frequency is low and is an imperfect multiple of the grid frequency. The low frequency beat can lead to increased voltage and current harmonics, voltage errors, and even voltage regulation issues at high modulation index region. Mitigation techniques to reduce the beat and associated harmonics are investigated and verified through experiments. It is concluded there is really no advantage to fix the switching frequency to be the multiple of the grid frequency with an asynchronous PWM scheme.