A study into the variability of environmental parameters used in cable ratings

Abstract

The restriction on the allowable temperature rise of power cable insulation requires the use of a rating specification. Cable ratings are defined for steady state, cyclic and transient loads. The cable temperature is determined by a power balance in which the heat generated by the cable is equated to the heat flow through the surroundings to ambient. The rating is dependent upon a number of fixed parameters. These include cable geometry, materials and burial depth. The rating is also dependent upon a number of variable parameters which include soil thermal resistivity, soil temperature and climatic conditions. The rating of cables installed in surface troughs is also affected by air temperature, solar radiation and windspeed. In addition to the above parameters, the load history is required for transient ratings. This paper details an investigation into improving cable ratings and parameters as they are applied to surface trough cable systems. Results are presented which show that there is substantial unrealised cable capacity with existing cable rating procedures and that the use of time variant parameters may result in more reliable forecast of cable capacity.