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Investigation and Design of a Modular Multilevel Converter in AFE Mode with Minimized Passive Elements

Fig. 1 : (a) MMC AFE with grid connection and (b) Half-bridge submodule as building-block of MMC AFE
This paper investigates the design procedure of a modular multilevel converter (MMC) in an active front-end (AFE) mode of operation. The design is performed with the aim of minimizing the number of passive elements, including arm inductances and submodule capacitances. This has been accomplished using the basic controllers needed for the operation of the MMC in AFE mode, without employing any additional controllers in the circuit. In other words, the design takes advantage of properly locating arm inductance and submodule capacitance magnitudes with respect to the intrinsic resonance phenomenon in the MMC converter. A model is devel-oped in the MATLAB/Simulink environment in order to show the feasibility of the proposed de-sign and also to provide a comparison between the proposed MMC AFEs and the formal de-signs. The comparison in the shown case study reveals a 90% reduction in arm inductance magnitude and a 40% reduction in submodule capacitance values, at the expense of adding fil-ter inductances with a much smaller size.

Proper sizing of the arm inductances is crucial for the MMC AFE. At higher powers and cur-rents, the main issue with the MMC AFE is the design of large arm inductances capable of re-stricting the input current in case of a fault on the MMC's DC bus. In high current MMC AFEs, where it becomes non-beneficial to design huge inductances, using an H-bridge-based MMC is proposed as a solution, as it has an intrinsic capability to suppress fault currents.

In this paper, an AFE MMC is designed with special focus on minimizing the passive elements' size, while keeping the same power quality and fault protection level. No additional con-roller is added in order to reduce the size of the passive elements. The proposed design can extend the operation range of the half-bridge-based MMC AFE by decreasing the amount of inductances needed. The performed reduction of passive elements (i.e., arm inductances and submodule capacitances) will result in reduced initial investment costs, and of active power loss in the passive elements.
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