The internal pressures in the insulation of two 5-m long pieces of a state-of-the art massimpregnated non-draining subsea cable have been measured under load current cycling at ambient temperatures ranging from 3 to 37 °C. Thermal expansion of the mass gives in some cases rather extreme internal pressures (>30 bar) and radial pressure gradients (>1.5 bar/mm) in the insulation. This leads to an outwardly directed flow of mass during loading. After a load turn-off, the pressure drops rapidly. Values below 100 mbar were measured in the inner parts of the insulation. The backflow of mass proceeds much slower because the pressure gradient now is smaller. Such a radial redistribution of the mass and the low pressures are assumed to influence the dielectric properties of the insulation as the risk of creating harmful shrinkage cavities is believed to increase. For all the investigated ambient temperature levels the internal pressure in the insulation became around 1 bar or less when isothermal conditions were reached after the load turn-offs. This demonstrates that thermal expansion and contraction of the mass are not the sole mechanisms determining the internal pressures in such cables. Other, still unidentified phenomena are also contributing.

Inter-array power cables are used to connect wind turbines to the collector and export cable. In the transition from turbine tower to sea, the cable is installed in a J-tube, which has an unfavourable thermal environment and can thus be the thermal bottleneck of the cable installation. To optimize cable installation and reduce CAPEX, improved transient ampacity calculations can be used to determine the dynamic rating. In this work FEM have been applied to calculate the ampacity of a three-core HV cable situated in a J-tube. It was found that by including the trajectory of solar influx, the maximum temperature increased above the admittable cable core temperature compared to the steady-state case. High cable loads will always coincide with wind and thus increased convective heat transfer. By increasing the heat transfer coefficient to a value corresponding to wind speed of 20 m/s at high power production and thus large current, it was found the highest core cable temperature decreased by 18 °C compared to the steady-state case. These more accurate ampacity calculations can be exploited by either increasing the admissible current in the cable by 17% or decreasing the cable cross section.

No abstract