A Novel Thermo-Mechanical Model to Assess the Dynamic Thermal Rating of Multi-Span Overhead Transmission Lines

Abstract

Keywords Ampacity, Dynamic Thermal Rating, Overhead lines IntroductionAll over the world, increasing the thermal rating of existing overhead transmission lines is considered a valid alternative to the construction of new lines [1,2].The thermal rating of a transmission line is the highest current that the line can carry under assigned meteorological conditions, respecting all clearances. The ampacity is limited in practice by the maximum allowed operating temperature: if the temperature limit is exceeded, the distances between conductors and ground decrease to unacceptable values. Assessing the dynamic ampacity of a conductor is then strictly related to the precise calculation of its sag [3,4].In this context, Dynamic Thermal Rating (DTR) of transmission lines represents a significant improvement with respect to the traditional steady-state rating criteria, which usually result in standard seasonal ampacities. In fact the thermal time constant of conductors is relatively high (more than 10 minutes) [3]; this fact allows exploiting the dynamic performances of conductors, i.e. currents significantly higher than the steady-state thermal limits, during transient overloads, thus avoiding re-dispatching or the curtailment of Renewable Energy Sources [5].The margin of dynamic overloading of a power line can be evaluated focusing on the temperature of the conductor; such a temperature can be either evaluated using a thermal model, having as an input weather conditions and power flows [6,7], or operating a direct measure on the conductor [8].The analysis of the literature clearly shows that, no matter which method is used to assess the conductor temperature, the dynamic mechanical behaviour of multi-span power lines has been so far investigated under the hypothesis that the horizontal mechanical tension along the line is uniform; this means neglecting the contribution given to the tensions, span by span, by the insulators' strings rotation. This conventional technique is referred to as method of the "equivalent span", or "ruling span technique" [9][10][11]. Furthermore, this traditional method strictly assumes one uniform conductor temperature over the entire power line, thus neglecting the fact that different spans basically share the same current, but not always the same weather conditions (solar radiation, wind speed). Finally, this method