Ampacity derating factors for submarine transmission cables in J-tubes

With the development of deep-sea wind power, the temperature hotspots of submarine cables in J-tubes are getting increasing attention. Currently, the relevant standards do not provide a thermal rating method for such cable sections, and the only studies are difficult to apply in practice. This study derived an analytical solution for calculating the derating in ampacity of submarine cables within J-tubes. The proposed approach is efficient and readily integrated with IEC-related standards. Initially, an indoor experiment was conducted to validate the correctness of the analytical model. Subsequently, a numerical example utilizing an actual HVDC submarine cable is presented, demonstrating increased cable conductor temperature within the J-tube and a corresponding reduction in ampacity. A derating factor is employed to quantify the extent of ampacity reduction, which is a function of the J-tube length, outer diameter, ambient temperature, wind speed, and solar radiation intensity. It turns out that once the J-tube air section's length exceeds approximately 60 times the cable outer diameter, the transmission circuit ampacity can be based on the value installed in the infinite J-tube. Furthermore, factors resulting in substantial cable ampacity reduction, such as high solar radiation intensity, may yield a derating factor approaching 0.6.