Study On Bottleneck Sections And Improvement Methods Of Ampacity Of Export Submarine Cable In Offshore Wind Farms

The export submarine cable line in an offshore wind farm usually goes through multiple laying sections with different laying environments.And in some of the sections the heat dissipation condition may be unfavorable,thus the submarine cable may be overheated if the load is high.In order to improve the safety of the export line,it is necessary to study the cable ampacity in each section and find the bottleneck sections,together with the methods to improve the ampacity of these weak sections.And this can also help improve the maximum allowable output of the offshore wind farm and increase the long-term revenue of generation.Depending on the installation environment,the export cable line is divided into different sections,namely the J-tube section,seabed section,shoal section and landing section.And the installation modes include the installation in the J-tube and the burial laying(direct burial or laying in the cable trench).The cable ampacity in the J-tube section is then studied.The thermal circuit methods including the 2D method and the quasi-3D method are first introduced.And based on the finite element software COMSOL Multiphysics,the thermal-flow coupled simulation model using the CFD technique is constructed,with heat conduction,turbulent natural convection and radiation coupled.The different ampacity methods are compared in an appropriate scene selected,and the heat-transfer characteristics inside the J-tube are analyzed.Afterwards,the ampacity of buried cable is studied,with the emphasis on the cable trench in the landing section.A general simulation model for different direct burial modes in different sections is established,and the ratings of these sections are compared.Then a thermal-flow coupled model for the cable trench is established,and the thermal-flow coupled field inside the trench is simulated and analyzed.Finally,the bottleneck sections are identified based on the above research,namely the landing section and the J-tube section.And based on the analysis of the limiting factors of the ampacity in the bottleneck sections,the methods to improve the ampacity of these sections are proposed,namely the ventilated J-tube and the water-filled cable trench.And the thermal-flow coupled models are built to test these methods,and it is found the improvement is significant.Also,the inside heat-transfer characteristics and the reason for the increase of the ampacity are analyzed.In addition,sensitivity analysis is conducted to study the factors which exert great influence on the ampacity of the landing section and J-tube section.