Accurate determination of ambient temperature at burial depth for high voltage cable ratings

Recently installed high voltage cable circuits often contain temperature monitoring systems that measure the outer surface temperature of one phase of the circuit at discrete intervals over its entire length. This greatly assists in the calculation of the available cable rating. However, well established buried high voltage cable circuits very often have no additional temperature monitoring and the rating is determined using IEC287. In order to improve the accuracy of this rating calculation a model based on thermodynamic laws has been developed that determines the soil or backfill ambient temperature at the cable burial depth. This model uses weather data and can be used to calculate a predicted soil temperature for a given cable depth. This work will present the results obtained comparing ambient temperature measurements made at National Grid Transco´s substation at Skelton Grange, Leeds, UK with predictions made using the ambient temperature model. The sensitivity of the model to input parameter sample rate and accuracy has been investigated. Results indicate that providing the meteorological data is sampled hourly and the weather station is no more than 25 miles distant, the ambient temperature can be predicted to within ±2 °C.