Use of high thermal limit conductors in the replacement of 220 kV overhead lines in the Tirolean Alps

The distribution system operator TIWAG-Netz AG operates a 220 kV grid in the alpine region of Tirol. Due to the age of the conductors, the fittings and the pylons (approximately 50 years), the question has arisen as to whether, for parts of these double overhead lines about 85 km long between Zell am Ziller and Silz, there may be other options available besides complete replacement which ought to be examined. Since on these 220 kV double-circuit lines, secure operation (n−1 criterion) cannot even now be maintained at several times of the year, that is, even transgressions of the rated current have occurred, different solutions to expand the grid have been examined. The results of the analysis were that as an interim solution for approximately the next 15–20 years, the use of high thermal limit conductors (ZTACIR conductors) would be technically and economically the most feasible solution. To begin with in the report, operational aspects in using ZTACIR conductors, e.g. load flow, grid losses, reactive power demand are explained and depicted, to illustrate why the use of ZTACIR conductors was decided upon, despite the higher costs of the conductors compared to conventional ACSR conductors and higher costs of fittings and installation. Using a ZTACIR conductor several crucial aspects had to be considered: ground clearance without selective tower elevations, mechanical structure loads without increase of tensile stresses and weight- and diameter-identical conductor design, which has proven its reliability over decades in alpine regions. Extensive conductor tests were used to determine the transition point, the rate of thermal expansion and the elasticity modulus. The design of the conductors and components such as fittings and isolators needed to ensure that at the maximum operating current under design conditions neither conductor nor components assumed temperatures that would lead to a reduction in their mechanical strength or diminish the holding power of screw terminals or the long-term quality of electrical contacts. Special installation requirements such as roll diameter, suitable attachment points for conductor pre-stressing, tower statics, the use of appropriate work clamps and slings and the choice of installation points of the rope winch equipment in Alpine terrain presented some particular challenges.