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A dual-modular-multilevel-converter (MMC) structure is presented that allows multifrequency power transfer between its ports. By exploiting a three-winding transformer, where the converter-side windings can multitask by accommodating currents with multiple frequency components, the proposed MMC structure can be deployed for two different applications: 1) a dc–ac MMC with independent control of currents in the two dc poles to accommodate unbalanced conditions in bipolar dc systems, and 2) a three-port MMC that can achieve simultaneous dc–dc and dc–ac conversions between an ac grid and two dc systems. DC flux cancellation is imposed in the transformer core by using a center-tapped connection of the converter-side windings. A unified control scheme is developed that independently controls dc and ac terminal power transfers while keeping submodule capacitor voltages balanced. Converter operation and dynamic controls are validated by simulation and experiment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Explosive levels of research into modular multilevel dc–dc converters (dc–dc MMCs) for dc and hybrid ac–dc systems are currently being witnessed. A new class of dc–dc MMCs has recently emerged that obviates the traditional intermediate ac link with galvanic isolation by internally circulating ac power among the converter switching cells. This novel power transfer mechanism opens the door to an array of new non-isolated converter structures with broad application in MVDC and HVDC systems. This paper gives a comprehensive overview of dc–dc MMCs that exploit the circulating ac power concept. Emergent topologies suitable for use in dc and hybrid ac–dc grids are identified and collated into the following categories: 1) two-terminal and multi-terminal dc–dc MMCs; 2) dc–dc MMCs with ac grid interface; 3) dc line power tapping devices; and 4) dc line power flow controllers. The survey provides researchers with an overall snapshot of technology development for this promising new class of converters in key areas relevant to smart grids. [ABSTRACT FROM AUTHOR]

Multiport HV DC-DC converters are required to facilitate future HVDC infrastructure with the ability to interconnect and manage power flow between multiple HVDC networks. Existing topologies offer limited modularity and scalability, making them difficult to implement in the fast-growing HVDC industry. In this paper, a multiport modular multilevel converter (MP-MMC) is proposed as the first truly modular multiport HV DC-DC converter. The MP-MMC is made up of multiple subconverters that can be controlled individually with de-centralized controllers, allowing easy reconfiguration and high reliability of the converter power circuit. The MP-MMC is compared with other prominent multiport HV DC-DC converters based on modularity, reliability, semiconductor effort, magnetics requirement, and fault-blocking performance. Operation and performance of the MP-MMC are verified by simulation. [ABSTRACT FROM AUTHOR]

This paper begins by presenting a generalized methodology for conceptualizing modular multilevel converter (MMC)-based dc–dc topologies, which is predicated on the concept of harmonic power balance. A compelling implication is that MMC-based variants of conventional switched-mode converter structures can be realized. As an example case study, this paper introduces a new dc–dc MMC for HVdc applications, which is derived from the classical buck–boost dc–dc converter. This new topology, which is revealed to be an alternative option to the well-known dual active bridge (DAB) converter with an intermediate transformer, offers buck–boost functionality and bidirectional dc fault blocking, using only two quadrant switching cells. Comparative analysis shows the proposed topology has lower operating losses and a lower total magnetics rating in comparison to an MMC-based DAB solution for dc stepping ratios around unity. A dynamic controller is developed that regulates the converter dc power throughput while maintaining balanced capacitor voltages. The converter operating principle, dynamic controller performance, and dc fault blocking are verified by simulation. [ABSTRACT FROM PUBLISHER]

This paper introduces a modular multilevel dc/dc converter, termed the DC-MMC, that can be deployed to interconnect HVDC networks of different or similar voltage levels. Its key features include: 1) bidirectional power flow; 2) step-up and step-down operation; and 3) bidirectional fault blocking similar to a dc circuit breaker. The kernel of the DC-MMC is a new class of bidirectional single-stage dc/dc converters utilizing interleaved strings of cascaded submodules. The DC-MMC operation is analyzed and an open loop voltage control strategy that ensures power balance of each submodule capacitor via circulating ac currents is proposed. Simulations performed in PLECS validate the DC-MMC's principle of operation and the proposed control strategy. Experimental results for a 4-kW laboratory prototype illustrate the single-stage dc/dc conversion process for both step-down and step-up operating modes. [ABSTRACT FROM AUTHOR]