Abstract
The power flow limits of transmission lines are set in order to ensure a given level of security to the electric system; their improper definition can reduce system reliability, increase the curtailment of renewable energy sources or create barriers to the free trading of energy. Unlike the previous literature, the Dynamic Thermal Rating procedure here proposed takes into account not only that the temperature of conductors can vary by for different weather conditions, but also the mechanical interaction between spans, due to their different elongation and to the consequent rotation of insulator strings. The developed tool is able to forecast the time trend of conductor temperatures, tensions, sags and clearances at each span, or to indicate which current can be carried for a given time before a clearance or temperature constraint is violated. Several case studies compares the results of this novel method with the outcomes of the traditional ruling span technique, especially when using High-Temperature Low-Sag (HTLS) conductors, having non-linear behaviour with respect to temperature.
Highlights
All 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
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
The present paper proposes two novel models: 1) the first is oriented to overcome the singularity of the function “stress-deformation” of High-Temperature Low-Sag (HTLS) conductors and the non-linearity of its derivatives; 2) the second elaborates the traditional “equation of change of state” of the power line, in order to take into account the non-linear HTLS model above mentioned
Summary
All 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 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]. This traditional method strictly assumes one uniform conductor temperature over the entire power line, neglecting the fact that different spans basically share the same current, but not always the same weather conditions (solar radiation, wind speed).
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