The Modular Multilevel DC Converter With Inherent Minimization of Arm Current Stresses

The modular multilevel dc converter (M2dc) is a partial power processing dc-dc converter that is gaining popularity for medium-voltage and high-voltage dc (HVdc) grid applications. However, internal ac current stresses go up as the step-down dc voltage ratio increases, leading to increased cost and losses, and ultimately renders the M2dc impractical for some applications. The HVdc autotransformer (AT) (HVdc-AT) is another class of the partial power processing dc-dc converter that circumvents this issue by using a transformer for interarm ac voltage matching, although the core must tolerate a very large dc voltage stress between windings that leads to increased magnetics size and weight. Interestingly, the M2dc does not suffer from interwinding dc voltage stresses. This article presents a new class of the partial power processing dc-dc converter that uses an integrated center-tapped transformer to merge the best traits of the M2dc and HVdc-AT. Comparative analysis reveals the proposed converter can minimize ac current stresses at all operating points while also achieving a significant reduction in transformer area product relative to the HVdc-AT. A dynamic controller is proposed that regulates dc power transfer while ensuring balanced capacitor voltages. The converter operation and dynamic controls are validated by simulation and experiment.