Modular multilevel structure of a high power dual active bridge DC transformer with stepped two-level output

Abstract

This paper addresses the issues of dc voltage matching and dc fault protection in potential super-grids. An approach for high power dc-dc conversion is proposed and analyzed. A front-to-front connection of modular-multilevel converters (MMC) forms a dual active bridge (DAB)-like structure. Near two-level operation is achieved by sequential switching of half-bridge chopper cells. This alleviates dv/dt stress exercised by two-level DAB configurations on the ac stage. The operating mode, switching patterns necessary are distinct from conventional MMC. Furthermore, common-mode currents, cell capacitance size, arm inductances are significantly reduced. The considered structure is shown to be useful for high power applications due to the low switching frequency, modularity, reliability and dc fault isolation capability. Various operation aspects are illustrated using simulations. A reduced-scale test rig provides an experimental proof of the concept

Hybrid and Modular Multilevel Converter Designs for Isolated HVDC–DC Converters

“…But known non-isolated dc-dc converter designs cannot offer as simple dc fault blocking capabilityparticularly for dc pole-toground faultsas F2F dc-dc converter topologies can. In the latter, dc fault blocking for pole-to-pole and pole-to-ground dc faults is simply realized by inhibiting solid-state switches of the healthy converter side [1,2]. Yet, F2F dc-dc converters are likely to remain limited to onshore installations where galvanic isolation is mandatory.…”

Semiconductor loss calculation of DC–DC modular multilevel converter for HVDC interconnections

“…The loss of the semiconductors is composed of two parts, the conduction loss and switching loss. Based on the submodule voltage (V dc_p /N p or V dc_s /N s ) and current calculated by the models (6), (7) and (10), both types of losses can be calculated according to the different switch states. In this section, the conduction loss and switching loss are calculated, respectively, by taking the submodules of upper arm on primary side as an example.…”

“…Compared with traditional multilevel converters such as flying capacitor converter, neutral-point clamped converter or cascaded converter [1][2][3][4], modular multilevel converter (MMC) is a promising candidate due to its low switching frequency, low harmonic content, high level of modularity and simple implementation of redundancy. Many DC-DC topologies have been proposed based on MMC, which can be classified into nonisolated [5][6][7][8] and isolated types [9][10][11][12][13][14][15][16]. Non-isolated DC-DC MMC has the advantage of reduced investment cost and is preferable in high voltage, but common ground applications.…”

“…Different modulation technologies are feasible for such DC-DC MMCs. The quasi-2-level (Q2L) modulation and 2-level (2L) modulation were proposed in [9][10][11][12]14], respectively, where submodules are almost switched synchronously and the AC-link voltage is modulated as trapezoidal or rectangular waveform. With Q2L and 2L modulations, the harmonics of current and voltage will be relatively high and thus leads to more conduction loss.…”

The transition arm multilevel converter — A concept for medium and high voltage DC-DC transformers

“…A solution to this problem was proposed and analyzed in [14]- [16] where trapezoidal modulation was proposed for standard MMC resulting in the so called quasi two-level converter (Q2LC) of Fig. 1b with smaller footprint and higher efficiency.…”