Design and Control of Unidirectional DC–DC Modular Multilevel Converter for Offshore DC Collection Point: Theoretical Analysis and Experimental Validation

Abstract

This paper presents the design and control of an advanced unidirectional DC/DC Modular Multilevel Converter (MMC) which enables the integration of off-shore windfarms with the High-Voltage Direct Current (HVDC) transmission system. The proposed converter consists of a single-phase MMC inverter, coupled with series-connected rectifier modules through a medium frequency transformer of multiple secondary windings. The modularity feature of the proposed converter enables scalability for different voltage levels. In addition to the galvanic isolation, the transformer also provides stepping gain to the output voltage. The proposed converter shows superior performance in terms of efficiency, losses and devices utilization, when compared with the most competitive unidirectional cascaded DC/DC converters such as input series output series (ISOS) and input parallel output series (IPOS). Furthermore, unlike the conventional d-q control method, which involves multiple transformations, this paper employs a simple proportional resonant (PR) control strategy that directly acts on the AC output of the MMC, under the stationary reference frame. The analytical design along with the simulation and experimentally validated results, confirmed the excellent performance of the proposed converter.

Isolated and Nonisolated DC-to-DC Converters for Medium-Voltage DC Networks: A Review

“…Connecting offshore wind farms to MVDC collection system leads to a low production cost of energy and expanding both the power rating of wind farms and voltage rating of the converter without limitations on generator types [13], [64]. This technology provides a promising future for offshore wind farms applications by eliminating the additional conversion stages and getting better system reliability [79], [95]. MVDC collection system is composed of MV AC-DC, DC-DC converters, and MVDC bus.…”

“…It is noted that the capability of bidirectional power flow is unnecessary for the wind farm DC collectors [95]. DC autotransformer cannot be an alternative option since it loses its features in the high transformer ratio [72], [74].…”

Soft‐switching and sensorless voltage balancing modulation for unidirectional modular multilevel DC–DC converter

“…Medium‐voltage direct current (MVDC) distribution network is the key link to interconnect high‐voltage direct current transmission lines and low‐voltage direct current (LVDC) microgrids [1, 2]. DC–DC converters are important in such scenarios [3–5].…”

“…When the MVDC distribution network is connected to LVDC loads such as industrial facilities, data centres and electrical vehicle (EV) charging stations, galvanic isolation is mandatory [4, 6–8]. In such scenarios, multilevel structures should be utilised on the MVDC side to handle the high voltage, and uncontrolled rectifiers could be used on the LVDC side for cost‐efficient purposes, and a transformer is applied as the AC link for galvanic isolation [8, 9].…”

Suppression of Reactive Power in Isolated Modular Multilevel DC–DC Converter Under Quasi Square-Wave Modulation

“…Replacement of twolevel bridges in a DAB converter with MMCs results in an isolated modular multilevel dc-dc converter (IMMDCC). Combining the advantages of DAB and MMC, IMMDCC is regarded as a promising solution for dc-dc conversion in dc grids [12]- [15].…”