A novel modulation technique for asymmetric multi-cell inverters of 27-level without regeneration

The asymmetric multi-cell inverters are an alternative to provide an AC voltage with low THD. Considering a voltage DC ratio 1: 3: 9 for three inverters connected in series an AC voltage featuring 27 levels is obtained. The most common modulation used in asymmetric inverters is the Nearest Level Modulation (NLM) which has the drawback of bringing some cells into regeneration for some values of the modulation index, regardless of the load conditions. This paper presents a new modulation strategy for this asymmetric inverter that forces all the cell to have identical power flow direction. This strategy is based on the fact that the fundamental voltage amplitude of each inverter is required to be always greater than or equal to zero. To confirm the proposed technique, an active power flow analysis is performed as well as simulated and selected experimental results are included.