Asymmetric overlap and hysteresis current control of zero-current switched alternate arm converter

The alternate arm converter (AAC) is a multilevel converter of the same family as the modular multilevel converter (MMC). Contrary to the MMC, the AAC offers dc-fault tolerant capabilities but requires complex submodule (SM) capacitor voltage/energy regulation and circulating current control. Such control is also limited due to the small overlap period where both arms conduct and energy between the arms can be exchanged. This paper develops a double-band hysteresis-based circulating current control strategy by introducing an asymmetric overlap period control of the director switches (DSs). The proposed controller ensures zero-current switching operation of the DSs, and increases the flexibility of SM capacitor voltage regulation owing to the asymmetric overlap period. It improves the performance of current control and energy regulation for the AAC at both near sweet-spot and non sweet-spot operation without disturbing the output voltages and currents. The performance and effectiveness of the proposed controller are illustrated through simulations on MATLAB-Simulink and PLECS.