Wave-Velocity Based Real-Time Thermal Monitoring of Medium-Voltage Underground Power Cables

Underground power cables are a bottleneck in congestion management for the medium-voltage grid because their ampacity is constrained by the maximum allowed insulation temperature. Network operators usually lack real-time information on the thermal state of the cables, which is a requirement for safe dynamical loading. This paper investigates wave-velocity based thermal monitoring of medium-voltage power cables. Variation in wave velocity arises from the temperature dependent dielectric permittivity of the insulation material. A method is proposed and tested to calibrate a thermal model, which can be used to estimate the temperature of a power cable in practical network conditions. The obtained resolution for PILC is below 1 degree celsius. For XLPE, the resolution depends on the absolute temperature, but the fast-changing relative permittivity near its maximum operating temperature suggests that there the resolution is similar. Moreover, a method to estimate the load profile from the relation between the propagation velocity and insulation temperature is provided. The model has been tested on an operational cable circuit and the estimated current aligned well with the measured current by the network operator. The resulting absolute temperature error, caused by systematic inaccuracies from calibrating, is estimated to be about 5 degree celsius.