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This paper describes a five-level diode-clamped PWM inverter intended for a medium-voltage motor drive without regenerative braking. It is equipped with a specially-designed circuit that plays an important role in achieving voltage balancing of the four split dc capacitors connected in series at the dc input terminals of the inverter. The voltage-balancing circuit frees the inverter to concentrate on the motor control. Then, this paper has an intensive discussion on the modular multilevel cascade converter (MMCC) family that are characterized by the modular structure of either bridge cell or chopper cell. The four family members are compared from various points of view, along with applications of two promising members to a medium-voltage battery energy storage system and a medium-voltage motor drives for energy savings.
Hirofumi Akagi (2011) studied this question.