Abstract—Integration of large-scale offshore wind farms with power systems using HVDC transmission systems is technically advantageous and cost-effective in cases requiring long distance submarine cables. In view of system opportunities from wind turbine generators to grid interconnection points, the DC connection strategies at medium voltage can be employed in the wind farm collection grid. MVDC collection systems in wind farms collect power produced from wind turbine generators and feed the aggregated power into the HVDC transmission systems through DC/DC step-up converters. Such extended DC wind power systems could potentially reduce the total cost of the power conversion and improve the overall efficiency and operational performance of wind power systems. This paper reviews literature on DC connection strategies for integrating large-scale wind farms into transmission grids. Discussions are around connection system configurations, DC/DC converters, control of DC connection systems. Technical challenges and research needs for DC wind power systems are also discussed. Index Terms—Wind power, wind farm, DC connection, HVDC transmission system, MVDC collection system, DC/DC converters, control. I. INTRODUCTION ARGE-scale offshore wind farms are drawing more and more interests for the future wind power generation [1][2]. With several large-scale offshore wind farms now under construction or in the advanced stages of planning, particularly in Europe and North America, it becomes a common interest of the industry to research and develop technologies for reliable and efficient connection between offshore wind farms and transmission grids. For the large offshore wind farms with several hundred MW power levels and long distance to the onshore grid connection point (theoretically beyond 60 km), voltage source converter based high voltage direct current (VSC- HVDC) transmission systems is technically advantageous and cost-effective in cases requiring long distance submarine cables over conventional AC transmission [3][4]. In addition to compact converter station, flexible voltage and frequency control, it is practical to build multi-terminal VSC-HVDC systems to interconnect multiple large-scale Jiuping Pan is with ABB Corporate Research, 940 Main Campus Drive, Raleigh, NC, 27606, USA (email: Jiuping.pan@us.abb.com). Li Qi is with ABB Corporate Research, 940 Main Campus Drive, Raleigh, NC, 27606, USA (email: lisa.qi@us.abb.com). Jun Li is with North Carolina State University, 1791 Varsity Drive, Suite 100, Raleigh, NC, 27606, USA (email: jli2@ncsu.edu) Muhamd Reza is with ABB Corporate Research, 72178 Vasteras, Sweden (email: muhamed.reza@se.abb.com). Kailash Srivastava is with ABB Corporate Research, 72178 Vasteras, Sweden (email: kailash.srivastava@se.abb.com). wind farms with different AC power grids or different portions of the same AC grid. Such multi-terminal DC connection allows flexible and optimum operation of wind power farms in a broad regional scope [5]. In view of system opportunities from wind turbine generators (WTGs) to grid interconnection points, the DC connection strategies could be extended to use VSC-based medium voltage DC (MVDC) collection systems in wind farms. MVDC collection systems in wind farms collect power produced from wind turbine generators and feed the aggregated power into the transmission HVDC systems through DC/DC step-up converters. Such extended DC wind power systems could potentially reduce the total cost of the power converters from WTG up to the grid interconnection points and improve the overall efficiency and operational performance of wind power systems. In this paper, an overview of DC connection strategies for integrating large-scale wind farms with transmission grids is presented from three aspects: DC connection system configurations, DC/DC converters, and control of DC connection systems. The main technical challenges and research needs for DC wind power systems, such as DC/DC converters, DC breakers, protection systems and coordinated control are also discussed. The paper is organized as follows. Section II discuses various DC connection system configurations and highlights the one employing common DC bus, as well as modular DC/DC converters. In section III, various DC/DC converter configurations are discussed. In section IV, basic control requirements for different DC wind farms are summarized. Technical challenges and research needs are described in section V. In section VI, some conclusions are drawn. II. CONNECTION SYSTEM CONFIGURATIONS A. Two-stage DC Connection Systems For offshore wind farms with HVDC transmission, an internal electrical collection system is needed to collect the AC output of the wind turbines and feed the aggregated power to the HVDC cable terminals. The collection system of wind farms can be either medium voltage AC networks or medium voltage DC networks. Fig.1 (a) shows the system configuration with AC collection system. The converter output voltages are stepped-up via the 50/60 Hz medium voltage (MV) transformers and collected at a common coupling. Then, the voltage is stepped-up again with the 50/60 Hz high voltage (HV) transformer and rectified with a AC/DC converter located at the offshore platform to feed the HVDC link [6] [7]. Fig.1 (b) shows the system configuration with DC collection system. Similar to the system configuration with AC collection system, the first-stage DC/DC converters at DC Connection for Large-Scale Wind Farms Jiuping Pan, Li Qi, Jun Li, Muhamad Reza, Kailash Srivastava L