Dynamic thermal ratings: monitors and calculation methods

Opening of the transmission system to independent generators and reduction in traditional regulation has led many utilities, both in the United States and around the world, to employ methods where transmission lines and equipment can be operated reliably at higher loadings. Since the mid-1980's, considerable attention has been paid to increasing the power flow of overhead lines, power transformers, underground cables, and substation terminal equipment by means of monitoring weather and the equipment thermal state and by developing more accurate thermal models. The resulting dynamic thermal rating techniques have typically yielded increases of 5 to 15% in capacity. In this paper, dynamic thermal rating models and monitoring methods for lines, underground cables, power transformers, and substation terminal equipment are discussed and explained. It is shown that overhead line ratings are very dependent on wind speed and direction, that line temperature and sag respond very quickly to changes in line current and wind, and that, in contrast, the soil temperature and thermal resistivity of the earth, which determine the thermal rating of underground cable, change very slowly with weather and current loading. While the thermal response of underground cable and overhead line circuits is quite different., they share the requirement for careful monitoring at multiple locations along their routes. Power transformers, substation terminal equipment and underground cables are shown to be sensitive both to circuit current and air temperature. Transformer ratings are determined not only by oil and winding temperatures but also by degradation of the winding insulation. Relatively simple thermal rating algorithms are suggested for substation terminal equipment and new, simpler, field temperature calibration methods are explored. Finally., the application of dynamic rating methods to complex interfaces is also explored. Path15 in California is discussed as an example of how dynamic rating methods can be combined with load reduction procedures to increase power transfer levels in complex interfaces