Temperature Profile along an Overhead Line Conductor in and near the Tension Clamp

摘要

Difficulties of building new overhead-lines and the increasing need for more electric power challenge the transmission grid of Switzerland and other countries. Therefore, it is required to improve the efficiency, safety and reliability of the existing lines. Changing from static to dynamic line rating can contribute to the solution, but this can lead to an operation of the overhead line closer to its thermal limits. As the maximum conductor temperature is a major limitation, the longitudinal conductor profile is measured in and near the tension clamp. This zone is studied because of the locally occurring elevated stresses and strains caused by the tensile and compression force induced by the clamp. The analysis should serve for the lifetime prediction of the existing lines, where the long-term material weakening and creeping are important and strongly depend on stress, strain and temperature. Since this study is aimed for the Swiss transmission grid, an all-aluminum alloy conductor with a core wire and four outer layers is investigated at laboratory conditions at a constant ambient temperature of 21.5°C. The steady-state temperature is measured at the surface of the conductor with thermocouples that are fixed in drilled holes of 1 mm diameter and depth. The 15 thermocouples are installed within the wedge clamp and in the thermal transition zone between the clamp and the point where the influence of the clamp is negligible. The tensile force is varied from 2 to 24 kN and an alternating current of 560, 780, and 880 A is used to heat the conductor. The high values of the tensile force and current are chosen according to a possibly extreme but realistic scenario. Qualitatively, the shape of the longitudinal temperature profile along the thermal transition zone is measured to be the same for all cases. Beyond a distance of 120 cm from the clamp, its influence on the conductor temperature is negligible. The variation of the tensile force has a rather small impact on the longitudinal conductor temperature profile with a maximum change of 1°C in the free span at 880 A. By increasing the electric current, the conductor temperature rises at all points. The rise of temperature is largest in the free span and lowest inside the clamp. The largest temperature gradient occurs next to the clamp and decreases in an exponential manner towards the temperature in the free span. The temperature difference between the clamp and the free span decreases with decreasing current. For the highest measured current of 880 A the temperature in the free span is 73°C and 50°C within the clamp. This significant reduction of 23°C at the clamp can be caused by decreased Joule heating and increased convective cooling due to the larger cross-section and surface of the wedge clamp, respectively. The direct current resistance was measured for obtaining a first approximation of the lowered Joule heating. The averaged DC resistance of the conductor with the clamp is 34 μΩ/m and therefore approx. 41% lower than the 58 μΩ/m resistance of the conductor in the free span.