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A multiport ac–dc converter topology is proposed that offers bidirectional power transfer capability between the two dc ports and the ac port. The decoupling of power flows can be exploited as a pole voltage balancing mechanism to accommodate unbalanced dc side loads in bipolar dc distribution systems. The proposed topology gains bipolar operation by using two three-phase two-level voltage-sourced converters connected in a differential fashion with a center-tapped transformer. The analysis shows that no penalties are incurred in terms of increased power ratings for either the grid interface transformer or the semiconductors; only access to the center point of the converter side winding is required. A control scheme for the topology is proposed with the ability to independently control power flows between the different ports. Both the topology and control scheme are validated by extensive simulations and experiments. [ABSTRACT FROM AUTHOR]

A new class of modular multilevel dc/dc converters has recently emerged that eliminates the traditional full-rated intermediate ac stage inherent to two-stage dc/dc converter structures, that is, two cascaded dc/ac modular multilevel converters (MMCs). This paper introduces the first averaged dynamic state-space model for this new class of single-stage dc/dc converter, called the DC-MMC, which captures all input and output terminal dynamics as well as internal capacitor dynamics. Based on the developed analytical model, a dynamic phasor model is derived to accommodate the multifrequency components that exist at steady state. The phasor model enables: 1) solving for the full steady-state solution of the DC-MMC under arbitrary loading and 2) evaluation of DC-MMC open-loop stability via eigenvalue analysis. These analyses are not possible with existing DC-MMC models. Simulations of a comprehensive switched model verify the accuracy of the presented analytical models, which represent valuable study tools for the analysis of emerging modular multilevel dc/dc converters containing multiple frequency components. [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]