Thermal Analysis and Debottlenecking of HVAC Export Cables for Offshore Windfarms

The substantial costs of HV export cables connecting offshore windfarms to the grid combined with their influence on energy availability requires extensive thermal evaluation of these cables. The analysis in this paper reveals that the offshore windfarm cables can be divided into onshore (underground), landfall, offshore (subsea) and J-tube sections based on their unique thermal behaviour. The utilization of long-term Distributed Temperature Sensing (DTS) data from a specific windfarm export cable in UK reveals that the J-tube and landfall sections are the major thermal bottlenecks, along with the cable joints/transition-points at onshore and offshore. Landfall is found to comprise of the most thermally strenuous environment due to large burial depths. While J-tube is influenced significantly by the ambient conditions in its air-section, thereby making it most prone to be affected by the local weather. Whereas, the thermal instability of the lengthy offshore section comes from the diffusion of thermal properties of surrounding material due to seabed geology and variation of cable burial depth over time due to seabed movement and sedimentation. Onshore cable section is found to be thermally stable and free from hotspots not only because of the difference in cable design and utilization of ducts for installation but also because the fibre optic cable for DTS is not embedded inside the cable. The temperature offset therefore needs to be established for the land and submarine cables respectively in order to obtain a more accurate account of the conductor temperature. In conclusion, employment of modern techniques combined with thermal analysis of each section can improve the existing design and operation practices.