Thermo-mechanical model of multi-span overhead transmission lines equipped with high-temperature low-sag conductors

Increasing the thermal rating of existing overhead transmission lines is a valid alternative to the construction of new links. Dynamic thermal rating (DTR) of power lines is particularly interesting for Transmission System Operators, because the thermal time constant of conductors is relatively high (more than 10 minutes); in case of temporary grid congestions, DTR allows exploiting the dynamic performances of conductors, i.e. currents much higher than the steady-state limits, instead of redispatching generating plants. DTR is strictly connected with the possibility of calculating the actual sag of the most critical spans of a transmission line, the power flow and the weather conditions being known. However, the conventional calculation tools do not take into account the non-linear nature of novel high-temperature low-sag (HTLS) conductors and usually adopt a simplified model (the so-called "ruling-span technique") to analyze multi-span transmission lines. In the present paper a non-linear mechanical model for bimetallic conductors is proposed, to be used for studying HTLS conductors. Such model is than combined with the "equation of change of state", written in a form that can be applied to multi-span lines. The novel resulting model allows the traction/compression status of the external section of the conductor to be correctly calculated span by span, thus estimating the sag of each span. The traditional ruling span technique is also applied for comparison purposes and the relevant results are discussed.