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

Abstract

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. Index Terms-DC-DC power conversion, HVdc converters, HVdc transmission. NOMENCLATURE CCL Circulating current loop. F2F-MMC Front-to-front modular multilevel converter. HVdc High-voltage dc. HVdc-AT High-voltage dc autotransformer. MMC Modular multilevel converter. MVdc Medium-voltage dc. M2dc Modular multilevel dc converter. M2dc-CT Modular multilevel dc converter with integrated center-tapped transformer. P3T Partial power processing transformer. SM Submodule. I. INTRODUCTION D C POWER transmission and distribution technology has been garnering increased attention over the last several Manuscript

Comparative Assessment of Multi-Port MMCs for High-Power Applications

“…In the MP-CT and MP-MMC, the converter-side windings carry both dc and ac currents. However, due to the windings orientations, dc flux cancellation is imposed in the transformer cores [20], [26].…”

“…The MP-CT in Fig. 1(b) is a new topology proposed here by adding a grid interfacing winding to the dc-dc MMC proposed in [26], and uses a zig-zag arrangement for the converter-side windings to realize core dc flux cancellation. Alternatively, the MP-MMC in Fig.…”

“…But this comes with the benefit of eliminating inter-winding dc voltage stresses that plague the MP-AT, leading to overall reduced core areaproduct (and associated losses) for the MP-CT magnetics. Further details on this trade-off can be found in [26].…”

“…Îp and Îs can be minimized by maximizing ac arm voltages Vp and Vs [26], [27]. Note that if the generated arm ac voltage is greater than arm dc voltage, solely FBSMs are employed in that arm to ensure SM capacitor balance can be satisfied [28].…”

Multiport Converter With Enhanced Port Utilization Using Multitasking Dual Inverters

“…However, these converters either use a large number of components, have redundant conversion stages or have ports that are unidirectional. A number of high power converters use transformer taps to achieve multiport capability [3], [11], [26] - [28]. Among these, [27] is a dual inverter based topology with two DC ports and an AC port that taps the grid interface transformer to obtain the second DC port.…”