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pubblicazioni - Memoria

Creep behaviour of High Temperature Low Sag conductors

pubblicazioni - Memoria

Creep behaviour of High Temperature Low Sag conductors

L’articolo descrive le procedure di prova finalizzate alla misura dei diversi contributi all’allungamento permanente dei conduttori HTLS. Prove di laboratorio e relative configurazioni sono state realizzate su differenti tipologie di conduttori allo scopo di verificare i modelli teorici presenti in letteratura e di definire una procedura per la corretta valutazione dell’allungamento complessivo nei progetti di ri-tesatura con conduttori HTLS mono o bimetallici.

The scenario resulting from the introduction of the free market of electric energy requires to increase the power transferred by transmission lines and consequently to optimize the exploitation of the existing grid, due to the difficulty to obtain permissions to build new lines. The fastest and most effective solution to this problem is offered by reconductoring of existing lines with High Temperature Low Sag (HTLS) conductors. Materials and constructive characteristics used for these conductors allow to increase the capacity of the existing OHLs overcoming several problems that a high operating temperature may produce on traditional conductors (AAC, AAAC, ACSR, etc.) such as progressive decrease of mechanical strength and excessive thermal elongation with consequent reduction of the clearances to ground.

The above aspect is particularly critical due to the fact that technical feasibility of reconductoring allows minimum structural reinforcements on existing towers and requires an accurate design of HTLS conductors because of very small deviations on loads are generally available for these operations. Particular materials used for the construction of these conductors allow to preserve their mechanical characteristics, such as breaking load or corrosion resistance, up to high temperatures and to reduce the coefficient of thermal expansion; moreover a correct design allows to keep under control both elastic and thermal elongations. Nevertheless also the permanent elongation taking place on conductors after installation influence conductor clearance to ground.

It depends on two phenomena: plastic elongation occurring between EDS and maximum tension (related to heavy ice and/or wind conditions); creep elongation due to long-time permanence in average operating conditions. The creep rate, related to stress and temperature, comes from internal structural modifications of the materials and, among the metals used in conductor construction, takes place mainly in aluminum and aluminum alloys. As the short-time effects are easy to be quantified through a stress-strain test, it is more complicated tomeasure the creep elongation and understand how the two phenomena influence each other.

Test procedures to measure the different contributions to permanent elongation of HTLS conductors have been set-up and laboratory tests have been carried out on different types of conductors with the aim to verify the theoretical model in literature and to define a procedure for a right computation of the global permanent elongation in reconductoring projects with monometallic or bi-material HTLS conductors.

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