Over-distention operation of cascaded multilevel inverters

Established research has shown that cascaded multilevel inverters can provide more voltage vectors per number of active semiconductors compared to typical multilevel converters. This feature is significant for increasing the drive performance as well as reducing the drive complexity and losses. When two inverters are cascaded, the maximum number of effective levels (or maximal distention operation) is the product of the number of levels of the individual inverters. It is possible to operate the cascaded inverter beyond maximum distention. The over-distention operation is desirable since it effectively increases the number of voltage levels in spite of some missing switching levels. This paper studies over-distention operation based on an inverter system where two three-level inverters are cascaded, which can generate eleven equivalent converting levels instead of nine levels under maximal distention condition. An advanced modulation technique is introduced to handle both the missing line-to-ground voltage levels and the balance of DC link capacitor voltages in over-distention operation. Computer simulation and experimental validation are presented to verify the proposed methods.