Design and evaluation of AC inductors indispensable to a modular multilevel cascade converter (MMCC-TSBC) for medium-voltage motor drives

This paper provides a theoretical and experimental discussion on ac inductors used in a modular multilevel cascade converter (MMCC) based on triple-star bridge cells (TSBC) for motor drives. The TSBC converter is one of the most promising candidates suitable for low-speed, high-torque motor drives. It requires ac inductors for the cluster current control, and the three kinds of inductors exist: non-coupled inductors, zigzag inductors, and three-legged three-winding inductors. This paper proposes a new set of ac inductors consisting of three three-legged cores with special windings. The proposed set of inductors is characterized by canceling out the flux produced by not only the motor currents but also the supply currents, thus resulting in bringing a significant reduction in size and weight to it. A specially-designed downscaled motor drive system rated at 400 V and 15 kW is used to confirm the validity of the proposed set of inductors.