Application of temperature field modeling in monitoring of optic-electric composite submarine cable with insulation degradation

To effectively monitor the insulation state of the optic-electric composite submarine cable, the finite element numerical model for the temperature field of a 110 kV YJQ41 x 300 mm(2) buried submarine cable was constructed in COMSOL, based on its thermoelectric coupling module. The results were validated by the improved IEC 60287 standard method and the optimal size of the physical dimensions of the model was obtained. On the basis of study of the effects of ambient temperature, buried depth, soil thermal conductivity, heat transfer coefficient of seawater and ampacity on the optical fiber's temperature in normal insulation state, the optical fiber's temperature rise in different levels of global insulation degradation was investigated. The results reveal that, when the cable insulation is globally degraded, the relative temperature rise of optical fiber is proportional to dielectric loss factor. Comparatively, the ambient temperature has little influence on it. A formula for estimation of dielectric loss factor based on relative temperature rise and ampacity of cable is presented. According to the formula, a method for evaluation of the submarine cable insulation degradation based on relative temperature rise of optical fiber is proposed. By tuning the structure and electrical parameters of a part of cylinder area in the insulation layer of submarine cable, the cable with different local insulation states was modeled by the combination of ATP and COMSOL. The effects of ambient temperature, volume resistivity of insulation degradation area, ampacity of cable and the size of the degraded area on the maximum relative temperature rise of optical fiber were investigated. When the cable insulation is locally degraded, the maximum relative temperature rise of optical fiber is inversely proportional to the volume resistivity of the insulation degradation area. It is proportional to the central angle of the insulation degradation area and increases with the square of length of the insulation degradation area. If the threshold of the relative temperature rise of optical fiber is set to 1 degrees C, the range of optical fiber's temperature rise caused by local insulation degradation decreases exponentially with volume resistivity of the insulation degradation area and always has a spatial scale of meters for the cases with severe insulation degradation, which is similar to the spatial resolution of optical fiber distributed temperature sensing system based on BOTDR (Brillouin Optical Time Domain Reflectometer). This work can be taken as a reference to evaluation and location of insulation degradation of submarine cable based on temperature rise of optical fiber. (C) 2018 Elsevier Ltd. All rights reserved.