Control Strategy for Multilevel Inverter with Non-ideal DC Sources

Multilevel converters have attracted much interest due to increasing applications of medium and high voltage inverters in drives and in FACTs. A large number of topologies and control strategies have been proposed in the literature. Among the topologies, the modular multilevel inverter using several cascaded H-bridges is rather popular, with each H-bridge supplied by an isolated DC voltage source. Most control methods for these inverters are based on the assumption that the DC source voltages all have the same constant value. Some authors have discussed the potential of using unequal DC source voltages, e.g. in a relation of 1:3:9, sacrificing the benefit of having equal ratings for each cell. This paper discusses the potential of cascaded multilevel inverters when their DC source voltages are not exactly equal, but differ from each other in the range of plusmn20 %. It is shown that this has almost the same benefit in terms of output voltage quality as a relation of 1:3:9, but with similar ratings of each cell. In reality, the instantaneous values of the DC source voltages tend to differ from each other due to operational imbalances. The proposed control strategy is able to take advantage of these nonidealities.