The 400 kV Gutinas- Brasov overhead line is located in a mountainous area of Romania that has mostly rocky soil. Thick ice covers conductors during winter and long ago had caused breakage of shield wire to the extent that it had to be dismantled in the most severely affected line sections. Without shielding, these sections were left vulnerable to lightning strike. To reduce the negative impact on reliability, a solution involving application of line surge arresters was selected for these sections.
The project was carried out in two stages and experience during the first phase resulted in a number of improvements during the second installation. Further improvements were later also applied to increase arrester safety. Compared to the situation before installation of arresters, which saw from 4 to 10 lightning induced outages per year, no outages were recorded between 2006 and 2015. Marian Florea, Program Director at the affected Romanian TSO, Transelectrica, reviews details of the project engineering as well as key lessons learned.
Overview of TLSA Technology
Shield wires have been installed on transmission lines, in combination with good earthing, to assure good lightning performance in areas where lightning impacts reliability of electricity supply. But shield wires can at times themselves become a liability, such as if they ice up and sag or gallop, leading to risk of flashover between them and the phase conductors below. An alternative in such cases is to use transmission line surge arresters (TLSAs) in parallel with the insulator strings. While the traditional approach using shield wires has been to conduct a lightning strike to ground before flashover occurs, the line arrester approach involves conducting the lightning strike around the insulator string, without flashover in air. In the case of shield wire, low resistance earthing is necessary. But if arresters are applied to all insulator strings, performance no longer depends on earthing resistance (although some arrester manufacturers still recommend maximum values for resistance to earth).
There are several factors to consider when selecting TLSAs for any application. For example, arrester rating has to be coordinated with arresters at substations. The line arresters should be the same rating or higher so that they do not end up protecting the substation arresters. Moreover, line arresters have to be sized based on voltage as well as a line insulation evaluation. To avoid a problem of line lockout, arresters can be equipped with a frangible disconnector. The disconnector does not clear the fault current but rather allows successful re-closure after breaker operation. If an arrester fails and power frequency current passes through the disconnector, an explosive charge ignites, shattering the unit. Sometimes, mechanical failures of disconnectors, unrelated to an arrester fault, produce a situation where an arrester is unnecessarily taken out of service. Another situation that can occur is when surge currents set off the explosive charge and cause a disconnect operation even though the arrester has successfully withstood the surge event. Surge arresters for line voltages above 230 kV require grading rings to avoid corona and to maintain satisfactory voltage distribution along the MOV disks. Under icing conditions, galloping can cause torsional conductor oscillations at frequencies that excite the natural pendulum frequency of line arresters. Experience has shown that many TLSA failures can be linked to installation-related issues such as when connections to the energized conductor or grounded structure are exposed to static and dynamic loads that lead to fatigue and overloading. This can result in broken connections or possible arrester damage.
Project & Background
The 400 kV Gutinas-Brasov line, designed and erected during the early 70s, runs 126 km and connects the Brasov and Gutinas substations. The line is equipped with two sub-conductors per phase, each ACSR 450/75 sq.mm. The line crosses the East Carpati Mountains, a region where there is near constant ice build up on conductors each winter. Initially, the line was provided with two shield wires over its entire length. But due to frequent failures in two sections, i.e. Towers #94 to 100 and also between Towers #130 and 145, shield wires in both sections were dismantled in 1985. Moreover, the initial phase conductors on the line section from #94 to 100 were replaced by a single ACSR 973/228 sq. mm conductor. Line arresters were later installed to diminish outages of this line due to lightning.
Planning for the first stage began in 2005 and saw arresters mounted on Towers #130 to 145 a year later. The second stage of the project covered Towers #94 to 100 and was implemented in 2013. The solution adopted saw the polymeric-housed MOV line arresters connected with the ground pole to cross-arms and equipped with disconnectors between the active pole and the phase. This was intended to eliminate risk of arrester damage due to Aeolian vibrations as well as galloping that would introduce dynamic loads when arresters are installed under the phases. Arresters had to fulfill the conditions specified by IEC 60099 and also to meet minimum performance characteristics such as: continuous operating voltage of 255 kV, residual voltage at switching impulse, withstand voltage at 50 Hz for1 minute, wet conditions of 2100 kVmax and energy dissipation of 9.0 kJ/kV. Another condition was that of the disconnecting device between the arrester’s bottom and the flexible connection to the phase. Before mounting, measurements of tower earthing had to be performed and, in cases where earthing resistance was below 25 ohms, improvements had to be performed.
First Stage Project
One of the factors in the decision to proceed with Phase 1 was the number of lightning related incidents reported between 2000 and 2005, which varied between 4 and 10 outages per year. Since this line was among the most important for the security of the Romanian power network, such high numbers of trip-outs provided sufficient incentive to search for a permanent solution. It was therefore decided to install TLSAs in parallel with each insulator string on line section #130 to 145, which had operated without ground wires, to protect against lightning. To begin, existing suspension towers were replaced by heavy tension towers and the two phase ACSR 450/75 phase conductors were replaced by a single ACSR 973/228 sq.mm conductor. Surge arresters were then suspended from tower cross-arms and connected to the phases. The TLSAs were installed for all phases on Towers #131 to 138 as well as on all phases of Towers #140 to 142 and also Tower #144. These were all suspension towers equipped with suspension insulator strings. The arresters were connected in parallel with the insulator strings and limited voltage surges across the line insulation by going into conduction at a voltage below the line insulation’s flashover voltage. Since Towers #139 and 143 are tension towers equipped with tension insulator strings, vertical lighting rods were installed on their peaks instead of installing line arresters. Measurements of the earthing resistance showed good results, i.e. under 10 ohms, so no further improvement had to be done.
Upon completion of Phase 1, operating statistics on the Gutinas-Brasov line began to show an immediate decrease in reported outages. This confirmed that the protective strategy applied was both technically and economically justified, since as numbers of outages decrease, so too does the cost of corrective maintenance. Nevertheless, there were still three incidents reported between 2006 and 2012 (i.e. Aug. 2007, Dec. 2009 and Feb 2011). Upon investigation, all were found to have been caused by weaknesses in arrester mechanical and/or electrical connections. It was also observed that sheds on the composite suspension strings had sustained damage due to hard knocking against arrester corona rings during asynchronous movements of insulator and arrester under wind. Damage was also observed on the corona protective tube covering the flexible connection between the bottom of the arrester and the phase. Analysis of these various incidents revealed that several errors had occurred during the engineering design phase. For example, unsuitable locations had been chosen to suspend the arresters. Basically, there was insufficient distance between arrester and suspension insulator string to allow their independent movement under wind conditions and asynchronous balancing. In addition, selection and design of arrester components had been done without regard for the difficult environmental conditions in the service area. This resulted in excessive rotation of the arrester around its axis, leading to the upper fixture unscrewing. The disconnector also had poor electrical and mechanical stability (as in Fig. 1) due to weak connection of the copper conductor in the clamps to the phase conductors (see Fig. 2).
To avoid damage linked to excessive rotation of the arrester around its axis, 2 semi-collars (see Fig. 3) were later mounted on the upper fitting of the arrester, together with a chain to prevent accidental unscrewing (see Fig. 4). Moreover, to limit possible future damage, the disconnectors originally installed in Phase 1 were replaced by double disconnectors (see Fig. 5).
Solution Proposed for Second Stage
Based on experience from the first stage of the Project where there we no lightning outages on the line between 2006 and 2011, it was decided to further reduce risk of such outages by adopting the same solution for the line section between Towers #94 and 100 that also had no shield wire. To avoid some of the problems experienced during the first stage, more precise design was needed. For example, this time there would have to be enough distance between arresters and suspension strings so that the insulators would not be damaged by contact with the corona rings of arresters. Moreover, special consideration had to taken for tension towers #94 and #100 where there were two possible solutions for mounting arresters: the first by a rigid insulating connection to the live part of arrester (i.e. normal conditions) and the second to the live part of the line (i.e. conductors, clamps) and no rigid connection. The second alternative was chosen.
Arrester locations had to be selected based on analysis of clearances between live and grounded parts, under high wind as well as with no wind. Asynchronous movement of the suspension insulator strings and arresters had to be considered too. Dynamic wind pressure in the area is 45 daN/sq.m and, in accordance with Romanian standard NTE 003/004/00, minimum required distance in air between live and grounded points had to be 2900 mm in still air and 1000 mm in case of maximum wind with no ice on conductors. Practically, these distances have to be observed between the arrester’s bottom terminal or its corona ring and all live points including conductors, clamps, etc. In the case of old existing lines such as this, installing line arresters becomes almost impossible if respecting these required distances. Therefore, to fulfill all clearance requirements, it would be necessary to change tower geometry involving long interruption in line operation.
Different cross-arms had be mounted on existing towers to allow installation of arresters at the required distance from suspension insulator strings, both for suspension towers and tension towers. As such, tower elements had to be re-calculated taking into account new forces and moments from the new components. The same arresters as used in the first stage were chosen, satisfying requirements of standards such as IEEE C62.22, IEC 60099, IEC 60694, IEC 60060 and IEEE 1243, corresponding to 400 kV, nominal system voltage. The arresters were polymeric-housed and provided with disconnectors.
Deflection angle of the arrester under maximum wind can be calculated taking into account arrester weight as well as weight of the corona ring. The position of maximum deflection of the corona ring was 2.338 m from the arrester’s vertical axis in still air. The suspension insulator string cannot be allowed to deflect other than in transverse plane to the line axis, in normal conditions and with a maximum angle of 60°. In the case of the suspension insulator string on jumpers, this angle was 30° for monophase tension towers #94 and #100, In the case of asynchronous movement of arresters and suspension insulator strings, taking into account the arrester’s deflection angle of 25°, a horizontal distance of 2300 mm would be needed between both axes. Finally, for suspension towers a longitudinal distance of 2300 mm between arrester axis and axis of the suspension insulator string was necessary. Also, a transverse spacing of 2000 mm had to be considered. In the case of possible arrester damage with the bottom terminal coming to ground voltage, a distance of 1000 mm would still be maintained from there to any live points of the phase, which would represent an acceptable separation under no wind (see Figs. 6, 7, 8). Similar distances had to be considered for monophase towers (as shown in Fig. 9). If an arrester would fail, distance between its bottom, with ground voltage, and the phase would be 1474 mm, also allowing continued line operation under no wind or low wind velocity.
Improving Tower Earthing Resistance
Earthing resistance in the affected line sections was measured. For example, results for Towers #95, 96, 97, 98 was 25 ohms. Towers #99 and 100 had 25.43 and 25.39 ohms respectively. Soil resistivity in the area was measured as well. Between towers #94 and 95 this was 27.9 ohms, between #95 and #96 it was 501 ohms, between #96 and 97 it was 554 ohms, between #97 and 98 it was 1450 ohms, between #98 and 99 it was 127 ohms and between #99 and 100 it was 519 ohms. In cases where soil resistivity is over 500 ohms, allowable tower earthing resistance is a maximum of 30 ohms but, with arresters installed, the maximum accepted value is 25 ohms. Improvement to existing earthing by adding vertical rods and/or tape type horizontal electrodes was not feasible due to the impossibility of driving rods into the hard, rocky soil. Moreover, amplifying existing earthing by additional tape would lead to reduction in efficiency. Therefore, a counterpoise earthing system was proposed instead. The system consists of horizontal electrodes connected to the tower. These could be placed radially in the soil around each tower, with individual lengths less than the maximum efficient length, or by connecting all towers in that section with a horizontal electrode placed along the line route. The counterpoise system was used for Towers #95 to 99 and involved galvanized 40 x 4 mm steel tape. Effective lengths were 112 m for soil of 1500 ohms and 74 m for soil of 500 ohms. All towers in the section from #95 to 99 were electrically connected by this system.
During 2013 and 2014, no line outages were recorded. But in January 2015 some flexible connections between arresters and phases were damaged, either at the phase end or at the arrester end. The connectors at both ends of the flexible copper conductor were then analyzed and it was observed that one of the weaknesses was that assembly had been done using only one nut. The probability of a nut unscrewing due to vibrations during line operation is high. Therefore, to make it safer, a solution was either to employ a double nut or to place a splint through the bolt. The first solution was not possible in this case because bolt length did not allow for it, but the second proved suitable.
Application of surge arresters on high voltage overhead lines is a highly effective solution, especially in mountainous areas where shield wire cannot be installed because of high ice build-up on conductors. Romanian experience in this regard showed excellent results from the viewpoint of reducing lightning caused outages. No outages were recorded on the 400 kV Gutinas-Brasov line over the period 2006 to 2015 and thereafter. Problems were experienced after the first project stage, e.g. damage due to errors in design and installation, incorrect locations having been chosen for suspending arresters and use of arrester components that were not ideal for local environmental conditions. Based on this, all necessary improvements were considered during the second stage of the project. Minor details may still have to be implemented in manufacturing certain components to achieve even safer operation.