A Novel Mode Switching Control Strategy of an MMC-DAB based Variable Frequency Drive

The cascaded topology composed of modular multilevel converter (MMC) and dual active bridge (DAB) has several advantages such as high voltage level, electrical isolation and bidirectional power flow. All those merits suit the application of variable frequency drive (VFD). However, MMC based VFDs have some common drawbacks such as the voltage fluctuation of sub-module (SM) capacitors during low frequency operation. To solve this problem, a novel mode switching control (MSC) strategy is proposed in this paper. It enables the controller to switch the operation mode according to the current system state and adjust the control strategy at the same time. Thus power can be flexibly transferred on demand, which contributes the MMC-DAB based VFD to regulate the voltage of the sub-module. The control theory is well illustrated, besides the conventional control strategy and the proposal are compared by simulation. The feasibility together with the performance improvement is verified by simulation results.