Current sharing control of a DC-DC topology for offshore wind HVDC systems

One of the bottleneck issues associated with offshore wind HVDC power transmission systems lies in the high-voltage-gain high-power DC-DC converter. Currently, many DC-DC converter topologies are developed in the literature. One of the popular structures is the use of multi-level converters (MMCs) with a low-frequency high-turn-ratio transformer but with some drawbacks. All the energy passes through the transformer that will increase the complexity in the system design and manufacturing. Due to the fluctuating nature of wind energy, the converter efficiency may decrease remarkably at low input power. Alternative structures include dual-active bridge converters, thyristors-based DC/DC converters, half-bridge and full bridge-based multi-level converters, and multiphase resonant dc-dc topologies. Again, each has advantages and limitations. This paper proposes a matrix transformer based DC-DC converter with the features of modular structure, electrical isolation, high voltage step-up ability, low voltage stress on switching devices, robustness, fault tolerance and high efficiency over a wide input power range to meet the HVDC requirements. This paper focuses on the current sharing control strategy to fully exploit the advantages the proposed converter.