Influence of Two-Stage Coupled Arm Inductors on Modular Multilevel Converter

The arm inductors of a modular multilevel converter (MMC) system are the necessity for the operation of the system, and the two kinds of arm inductors exist: separated inductor and coupled inductor (CI). The impact of series connection CIs on the MMC has not been fully explored. This paper provides an accurate estimation of the circulating currents of the CIs based MMC with a variety of two-stage configurations by using the combinations of positive- and negative-coupled inductors. In addition to this, a decoupling equivalent method is developed and equivalent circuits of each CI configuration are proposed. The influences of two-stage CIs on the harmonic components in circulating currents are discussed with emphasis on the even-order harmonic through the proposed estimation methodology. Simulation results are presented to demonstrate the effectiveness and accuracy of the proposed estimation as well as decoupling equivalent method. Finally, the correctness of theoretical analysis is validated with experiments.