The 400 kV Gutinas- Brasov OHL is located in a mountainous area of Romania with rocky soil. Because of thick ice covering conductors during winter that caused shield wire breakage, shield wires had to be dismantled in sections 94-100 and 130-145. Without this shielding screen, these sections were left open to lightning strike, with some 4 to 10 outages recorded per year. To reduce the impact, a solution of installing line surge arresters was selected. The project was carried out in two stages: in 2006 for one section and again in 2013 for the second. Based on experience of the first stage, various improvements were applied during the second.
This edited article, based on a contribution by Marian Florea of Transelectrica, outlines lessons learned from the first phase and also operating experience over these years.
Lightning discharges are the cause of a significant number of interruptions to electricity supply. Traditionally, shield wires are installed and good lightning performance can be achieved with good earthing. However shield wires can also become liability when they ice up, sag or gallop and flashovers can occur between them and phase conductors below. An alternative to shield wires is using transmission line surge arresters (TLSAs) in parallel with insulator strings. The TLSA conducts the lightning surge around the insulator string, thereby avoiding a flashover in air – versus the traditional approach whereby the shield wires conduct a lightning strike to ground before flashover occurs. Shield wires require a low resistance earthing but performance of line arresters applied to insulator is not dependent on this (even though some manufacturers recommend maximum values for earthing resistance).
When selecting transmission line arresters for any specific application, there are several factors to consider. For example, arrester rating has to be coordinated with arresters at the substations; line arresters should be the same rating or greater so that they do not end up protecting substation arresters. Line arresters have to be sized based on line voltage and line insulation.
To prevent the problem of line lockout, arresters can be equipped with a frangible link, the disconnector, which functions to separate the arrester from the line if the arrester becomes internally faulted. The disconnector does not clear the fault current but does allow successful reclosure after breaker operation. If an arrester fails and power frequency current passes through the disconnector, an explosive charge is ignited, shattering the disconnector. Sometimes, mechanical failure of disconnectors, without any fault in the arrester, can lead to a situation whereby the arrester is unnecessarily taken out of service. Another situation may occur when surge currents set off the explosive charge and cause a disconnect operation when the arrester has successfully withstood a surge event.
TLSA construction is similar to that of non-ceramic insulators (NCI), being constructed with line and ground end fittings connected by a fiberglass reinforced core that provides the mechanical strength. Surge arresters for line voltages above 230 kV require grading rings to avoid corona and to maintain a satisfactory voltage distribution on the gapless MOV disks. During icing conditions, galloping can cause torsional conductor oscillations at frequencies that excite the natural pendulum frequency of the arrester. Service experience has shown that many TLSA failures can be ascribed to installation-related issues. Often, connections between the arrester and energized conductor or grounded structure are subjected to static and dynamic loads that lead to fatigue or overloading, resulting in broken connections or damage to the arrester. (EPRI, Application of Transmission Line Surge Arresters Product ID:1019954 Date Published:16-Dec-2010)
The 400 kV Gutinas-Brasov Line, designed and erected between 1970 and 1973, has a total length of 126 km and connects Brasov and Gutinas 400 kV substations. The line – equipped with two sub-conductors (ACSR 450/75 sq.mm) per phase – crosses the East Carpati Mountains where there is risk of high thickness of ice on conductors each winter. Initially, the line was provided with 2 shield wires over its full length. But in 1985, due to failures of the shield wire in two line sections (between towers #94 to 100 and #130 to 145, these were dismantled and both sections left vulnerable to direct lightning strike. The initial phase sub-conductors on towers #94 to 100 were replaced by a single conductor phase (ACSR 973/228 sq.mm). After more than 30 years service, a solution of mounting TLSAs on the towers was undertaken in order to diminish outages of the line due to lightning.
The first stage, i.e. mounting arresters on towers #130 to 145, was decided upon in 2005 and implemented in 2006. The second stage, covering towers #94 to 100 section was decided in 2011 and implemented in 2013. The solution adopted was one whereby polymer-housed MOV type arresters are connected with the ground pole to the cross-arms and equipped with disconnectors between the active pole and the phase. This eliminated risk of arrester damage due to dynamic loads from Aeolian vibrations or galloping, as in the case where the arresters are installed under the phases.
The arresters had to fulfill conditions specified by IEC 60099 to meet certain minimal characteristics, i.e. continuous operating voltage of 255 kV, residual voltage at switching impulse 900 at 20,000 A 8/20 micros wave, withstand voltage at lightning impulse of 2100 kVmax, withstand voltage at 50 Hz 1 minute, wet conditions of 2100 kVmax, energy dissipation of 9.0 kJ/kV. Another requirement was that the disconnecting device had to be connected between the arrester bottom pole and the flexible connection to the phase. Before mounting, accurate measurement of tower earthing had to be performed and, in the case of earthing resistance below 25 ohms, earthing improvements had to be carried out.