Influence of soil-moisture migration on power rating of cables in h.v. transmission systems

The paper is primarily concerned with the limitations on power ratings of transmission cables imposed by migration of moisture¿on the presence of which effective thermal dissipation usually depends¿owing to heat from the cables when operated at continuous full load during dry periods. The influence on cable operation of change in function of the British Grid and accompanying changes in load patterns of individual circuits are discussed, attention being drawn to increased liability to continuous full-load operation or extended periods of heavy loading under cyclic conditions. From analysis of the results of experimental installations in various soils, supported by information on service breakdowns due to thermal instability of two 132kV cables, a method of calculating current rating is determined, taking into account moisture migration from the soil around a cable. Using this method, modified rating factors for prevention of moisture migration are derived. These are based on cable external-surface temperature instead of maximum conductor temperature. Descriptions are given of means for avoiding reduced current ratings by special soil bedding materials or by forced cooling. Reference is made to development of a system for continuously monitoring and telemetering, for control purposes, the external-surface temperatures of cables throughout a route.