Review of modular multilevel converter based multi-terminal HVDC systems for offshore wind power transmission

The limitation of existing point to point high-voltage DC connections for offshore wind farm transmission is that it does not support power exchange or trading between two or multiple DC systems. However, proposed by many researchers, interconnected voltage-source converter based multi-terminal high-voltage DC systems provide system redundancy, higher flexibility and capability of exchanging power between multiple areas. This paper presents a detailed review of several key technical and operational aspects of multi-terminal high-voltage DC systems for large scale offshore wind farm transmission. This review not only summarizes the development of high-voltage DC system technology in the past decades such as line-commutated converter based and traditional 2-level voltage-source converter based high-voltage DC technology, but also provides an overview of contemporary challenges and opportunities for multi-terminal high-voltage DC systems including modular multi-level converter technology, DC breakers and network topologies. A case study of a six-terminal MTDC system was carried out in order to better compare these different topologies in terms of transmission line capacities, total length of transmission line, capital cost, and running costs.