Transmission line surge arresters (TLSAs) have become an important design and mitigation alternative to improve grounding and reduce investment in overhead ground wire. However, they have also been known to experience a number of mechanical problems in service. Perhaps the most common of these is degradation of the components associated with flexible leads. Should a lead break away from a tower or the bottom of the arrester, its uncontrolled motion under wind can reduce electrical clearances. Moreover, some TLSAs have been installed in ways that negatively impact or even completely defeat the protection provided by Aeolian vibration dampers. This is of particular concern since current line design practice sees generally higher conductor tensions and therefore greater need for vibration control.
William A. Chisholm (Ph.D., P.Eng., FIEEE) and David G. Havard, Ph.D., P.Eng, LSMIEEE of Havard Engineering in Canada review cases of interaction of TLSAs with vibration dampers as well as other line hardware such as aircraft marker balls, anti-galloping de-tuning pendulums, clamps on spacer dampers and post insulator struts. Vibration modes considered are: Aeolian type low amplitude high frequency vertical vibration that accumulates millions of stress cycles; and galloping, which is large amplitude low frequency oscillation. The later involves complex vertical, longitudinal and torsional motions that accumulate thousands of stress cycles, almost invariably with an ice layer on conductors. A third vibration mode: wake induced oscillation affects only bundled conductors and is not discussed.
Perhaps the first recorded application of a metal-oxide type TLSA occurred in the early 1980s when a 138 kV line with single overhead groundwire was converted to a ‘compact’ construction by restraining the lateral motion of phase conductors. A hollow strut assembly provided an interior volume for the stack of cylindrical MOV blocks with diameters of 61 and 76 mm. The larger-diameter, heavier MOV elements were used on the top phases of the double circuit line to Fig. 2: 138 kV strut arrester assemblies mounted on 138 kV line in U.S. (left) Bottom phase only; (right) all three phases. deal with the anticipated energy associated with shielding failures. Lightning strokes that bypass the OHGW transfer all their surge energy through a single arrester into the phase conductor and this arrester placement is called on to dissipate significantly higher levels of charge and energy.
The TLSAs in this case were attached directly to the conductor clamps (see Fig. 2). Since this design was rated for 89 kN (20,000 lb) tension and similar load in compression, it satisfied mechanical requirements for lateral restraint. At the same time, it did not interfere with function of the double sets of Aeolian vibration dampers. The 457 mm (18”) series ring gap provided electrical isolation from normal AC line voltage and switching surges and connected the MOV elements across the insulator only when lightning overvoltage exceeded 500 kV. As such, this was an early example of an externally gapped line arrester (EGLA) as now defined in IEC 60099 Part 8. This same type of arrester strut configuration could also be used in a braced post design provided that mechanical and electrical failure modes are coordinated. If the arrester base becomes pinned, its mass does not have much impact on tension equalization among spans. Moreover, it would also not affect line clearances and thermal ratings. Function of vibration dampers and other motion control devices are also not affected by this arrester configuration.
Arrester Suspended from Conductor
Provision of an external gap in the EGLA design calls for use of an insulating element that can withstand full line voltage for weeks or even years should the series varistor unit (SVU) component fail, e.g. by typical short circuit. A low-cost alternative for two insulating housings in series has been use of an external disconnecting device. On overload, e.g. at a charge level of 2 to 10 Coulomb, electrical energy heats and fires an explosive charge that separates either the lead connecting the arrester to the power line or the lead connecting the arrester to ground (see Fig. 3).
The modest weight of the arrester body is supported from below by an insulated hanger. If the ground lead disconnector operates, it shatters (as in Fig. 4) and the ground lead falls away. The base of the distribution arrester remains energized but the broken lead is a signal that it needs replacement.
By 2001, polymerhoused TLSA technology using explosive disconnects in the ground lead had scaled from distribution systems up to ratings of 180 kV and 195 kV maximum continuous operating voltage (MCOV), i.e. suitable for application on 230 kV lines. It is argued that this is the highest voltage where a line arrester provides good value since the increasing dry arc distance of EHV lines makes OHGW and low-impedance grounding more effective in most regions. Fig. 5 shows that a 230 kV TLSA exceeds 2 m in length based on a 13-unit insulator string length of 1.9 m. Unfortunately, in this particular application the arrester was suspended from the conductor at a location halfway between conductor clamp and vibration damper. This placement is not ideal since it modifies the vibration mode shape of the span and essentially relocates the damper sub-optimally. The explosive disconnect is threaded into the base of the arrester and attached by a slack ground lead to the tower at a distance of about 5 m. As such, if the ground lead disconnector operates, it is unlikely that wind would blow the grounded lead upwards into the phase conductor causing a fault. Loop length must be managed carefully during installation to ensure this is always the case.