Modular multilevel DC/DC converter architectures for HVDC taps

Abstract

DC/DC connections between DC voltage levels in High Voltage Direct Current (HVDC) schemes are useful for interconnection between links and as taps to inject/extract relatively small amounts of power. Taps with high voltage transformation ratios (such as 500 kV to 50 kV) are particularly challenging to achieve with reasonable use of semiconductor ratings. Four architectures of DC/DC converter covering both direct connection and transformer-interfaced are analysed and compared in terms of the ratio of their apparent power rating (rated current times rated current summed across all stacks of the cells) to the throughput power. It is found that the direct conversion architectures have reasonable power capacity factor only for low transformation ratios (of 2:1 or less) but the inclusion of a transformer and intermediate square-wave operation allows much wider transformation ratios with a reasonable power capacity factor. For each design the nature of the cell-balancing currents has been determined since this is a major factor in determining the total current flow in each cell stack.

A Modular Multilevel DC–DC Converter With a Compact Sub-Module Stack Suited to Low Step Ratios

“…The stack volt-ampere rating expression for transformer-less front-to-front MMC RSP-FF [35], [36] and direct-chain-link MMC RSP-DL [25], [37] are given in (26) and (27) The detailed comparison result among these three dc-dc converters are presented in Fig. 4.…”

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

“…Along with isolated topologies, several non-isolated configurations have been proposed in pursuit of more compact dc-dc converter stations [23][24][25][26][27]. The so-called dc autotransformer was presented in [27] utilizing typical dc/ac MMC bridges in analogy to the ac autotransformer.…”

Sizing and Short-Circuit Capability of a Transformerless HVDC DC–DC Converter

“…Alternatively, the voltage transformation can be achieved by means of a series or parallel connection of several dual-active bridges (DABs) [6]. However, for low and medium voltage transformation ratios, these topologies result in a low utilization of the installed power semiconductor devices and relatively high losses [7], [8].…”