Thermo-mechanical dynamic rating of OHTL: applications to Italian lines

The power flow limits in the transmission lines are set so as to ensure a given level of reliability in the electric system; an improper assessment of these limits can increase the number of events where renewable generators are shed or curtailed, or restrict to the free trading of energy, with the consequent onset of zonal prices. In the new deregulated context and in presence of a high degree of renewable penetration, the loading patterns of transmission and interconnection lines differ from those for which they were originally planned, and the transmission system could be sometimes congested. Straightforward solutions to this problem include construction of new power transmission lines and/or upgrades to the existing ones. However, such solutions require societal and environmental concerns and often cause lengthy approval processes. Alternative solutions focus on a better utilization of existing transmission lines through improved methods of thermal rating assessment. Thermal rating of a transmission line is the highest current that the line can carry under assigned meteorological conditions. This current-carrying capacity is limited in practice by the maximum allowed operating temperature; if the temperature limit is exceeded, conductor ground clearances are reduced. As a consequence, compliance with safety codes and reliability of line operation may be violated. Dynamic thermal rating (DTR) of transmission lines represents a significant improvement over the more traditional static rating. This is because DTR uses actual operating conditions, rather than assumed conservative environmental conditions or historical averages. Dynamic methods can provide ampacity either directly, based on actual measurements of conductor conditions in some critical points (e.g. temperature or sag), or indirectly, using ambient weather conditions. Dynamic rating is particularly interesting for TSOs, because the thermal time constant of conductors is relatively high (more than 10 minutes); this fact allows exploiting the dynamic performances of conductors, i.e. currents significantly higher than the steady-state thermal limits, in the meantime that the system is re-dispatched. This reduces re-dispatching costs, enhances system reliability and minimizes the curtailment of renewable energy sources. The development of thermo-dynamic calculation methods for estimating the temperature of the conductor, under specific meteorological and power flow conditions, has been widely performed in the past, especially for low conductor's temperatures. However, from the analysis of the literature, it arises that the dynamic thermal stress due to line current and weather conditions has been faced in the past taking into account only one span, under the hypothesis that the insulator strings were rigid. Conversely, the DTR algorithm proposed in this paper takes into account that in a real multi-span line, with different span lengths armed by insulator strings, supporting the same conductor and subjected to different temperature variations, longitudinal displacements arise due to different conductor elongations in each span; as a consequence, sag elongations can significantly differ from those calculated under the hypothesis of rigid insulator strings. A tool, which combines the CIGRE thermal model of conductors and a complex multi-span mechanical model of the line, was developed by TERNA, the Italian TSO, in collaboration with the University of Pisa, then tested on existing transmission lines. After a description of the tool, the operational results already obtained in the perspective of a TSO, hence in terms of reduced curtailment of wind production and real-time assessment of margins of temporary overloading of a congested line, are here shown and discussed.