Environmental parameters for cable ratings

This thesis details the development of accurate models for calculating the temperature rise and rating of high voltage cables. Existing methods used for calculating the temperature rise and rating of high voltage cables are reviewed and are compared with the temperature rise of a full size simulated cable. The existing methods are found to be wanting where cables are installed in surface troughs or at shallow depths. The inadequacy of the existing ratings is attributed to three major causes and each of these causes has been investigated.The first area of investigation is the influence of short term climatic changes upon the ambient temperature of the soil. It is shown that these variations are greater at shallow depths and assumption that the temperature is constant is invalid. Furthermore, a novel method for determining the ambient temperature of the ground at any level between the surface and 7 metres has been developed. The method uses values of air temperature, wind speed and solar radiation as input parameters. Using data from a weather station installed at the experimental site, the ground temperature model has been shown to be accurate to within 1-2oC of the actual ground temperature at cable depth.The second area of investigation pertains to the simplifying assumption that the ground surface is an isotherm. This assumption has been made from the earliest development of large scale power distribution. It is known that this is untrue but that the resulting error is acceptably small provided that the cable is buried at a sufficient depth. The effect of modelling the true boundary condition upon the predicted temperatures of cables buried at various depths has been studied using finite element analysis. It is shown that the assumption of an isothermal ground surface can result in a 10% to 20% under-prediction of temperature rise where cables are installed in surface troughs and a means of accounting for this error by adjusting the value of the thermal resistivity parameter has been proposed.The third area of investigation was a survey of ambient soil conditions in England and Wales. The effect of rainfall and soil type upon the thermal resistivity of soils has been investigated and three specific soil types have been defined each having a different thermal resistivity. Ambient temperature data have been obtained from across England and Wales to produce more accurate ambient temperature values for use in cable ratings. These data and have also been used to provide additional data inputs for existing probabilistic ratings methods.