Utilities continue to face an abundance of technical hurdles, not the least of which is the transmission and distribution infrastructure. As the demand for electricity continues to grow, so do the increasing challenges of upgrading the existing infrastructure and building new transmission circuits. As a result, utilities are looking for new and innovative ways to increase circuit capacities and maintain system reliability.
The Israel Electric Corp. (IEC), which is currently experiencing an increase in demand of 3.5% per annum, has a transmission system comprised primarily of 400-kV and 161-kV ac overhead lines. The older lines were built with aluminum conductor, steel-reinforced (ACSR) conductors, but more recent lines have been constructed with low-strength, high-conductivity all-aluminum alloy conductors (AAAC). The difficulty of securing rights-of-way for new lines and budget restrictions some 15 years ago forced IEC to consider uprating existing lines rather than building new lines or reconstructing older lines.
During this period, most of the transmission lines with conductor operating temperatures in the range of 60°C to 80°C were uprated to 100°C, providing an ampacity increase of 70% or 40%, respectively. However, the demand for power has now reached the limit of the prior thermal uprating. Therefore, IEC has had to launch a second phase of conductor uprating whereby existing conductors will be replaced by high-temperature, low-sag (HTLS) conductors. The new conductors will effectively increase the circuit ampacity by an additional 40% to 50%, as the HTLS conductors will allow a continuous conductor temperature of at least 200°C.
Existing Line Limitations
A precondition of this reconductoring project was to avoid design changes to the existing supporting structures, other than repairs needed to restore them to their original performance. This stipulation meant the HTLS conductors had to meet the same structure loading and high-temperature sag limitations as the existing line structures.
The spare ground clearance on the original lines was used when the first phase of circuit uprating to 100°C operation was undertaken. In addition, methods such as structure and suspension hardware modifications were used. The uprating objective for the second phase was to ensure the sag associated with the new HTLS conductors, operating at 200°C, should not exceed that of the existing conductors, operating at 100°C.
The wind standard in Israel was revised recently to increase the design wind loading on existing line supports. As the majority of IEC’s transmission lines were built before the higher loading requirements, the utility is not required to strengthen these lines. However, the lines are now considered overloaded so no additional structure loading is permitted. For the recently constructed transmission lines, the structures have sufficient surplus strength to absorb the additional loading.