Many transmission line surge arresters (TLSA) have been suspended from phase conductors, adding mass that creates a new mechanical node between insulator suspension clamps and Aeolian vibration dampers. This placement interferes with damper performance and could shorten conductor service life. Mitigations include re-positioning dampers, inspecting more frequently or selecting other installation configurations. This article, contributed by Dr. William Chisholm, discusses this topic and a case study shows appropriate consideration of these factors for installation of arresters on a new 230 kV line as an alternative to continuous counterpoise.
The first successful lightning surge arresters for electric power systems used hollow ceramic insulators, filled with silicon carbide blocks to provide non-linear voltage limiting electrical characteristics. The silicon carbide arresters had internal components to quench and interrupt AC power system follow current that normally keeps flowing through a flashover arc. The hollow ceramic insulators were sealed with metal plates, providing over-pressure protection but also allowing long-term moisture ingress that shortened service life. These arresters were used mainly for protection of equipment. Two separate technologies converged to improve the reliability and service life of arresters:
• The non-linear electrical performance of metallic oxide varistor (MOV) materials was superior to silicon carbide. This technology was patented by Matsuoka, working at Matsushita Electric Company, in 1970 and had displaced silicon carbide in many applications by 1980.
• Polymer rubber materials, including ethylene propylene diene monomer (EPDM) and long chains of polydimethylsiloxane (silicone), protected the MOV elements against moisture while reducing weight and cost, compared to ceramic housings. The industry wide substitution of polymer for porcelain took place over the brief period from 1987 to 1995.
These improvements made it practical to use transmission line surge arresters (TLSAs) to improve lightning performance by applying them at some or all insulator positions, as well as at equipment or line terminals. Electrically, TLSAs using MOV materials have been successfully installed to limit transient overvoltages on line insulators for more than 30 years. For example, successful application of TLSAs on an unshielded 230 kV transmission line since 2001 used arresters with 60 mm block diameter. Experience with improving MOV technology allowed reduction in block diameter from the initial 61-76 mm to only about 40 mm for shielded lines, with overhead ground wires (OHGW) that limited their energy duty by diverting a large proportion of current to ground. Further reduction in cost and increase in performance has been achieved using a direct mold silicone construction that eliminates internal air spaces. Originally developed as components to be retrofitted to existing lines, TLSAs have now become an important design and mitigation alternative to investment in extensive grounding systems for new line designs. One case study set out the decision criteria leading to fitting of TLSA to all three phases of a 230 kV transmission line during construction, as an alternative to using continuous counterpoise in an area where rock resistivity was extremely high.
Mechanically, however, TLSAs have demonstrated in-service problems, the most common being rapid degradation of flexible leads and their terminals. Once a lead breaks away or pulls out from the tower or the bottom of an arrester, its uncontrolled motion in the wind can reduce electrical clearances. Rigid configurations, including the external gap EGLA type, do away with flexible leads and offer longer service life.
Unfortunately, many TLSA installations have been made in ways that affect or even completely defeat the mechanical protection given by Aeolian vibration dampers. Aeolian vibration is a low amplitude high frequency vertical vibration that accumulates millions of stress cycles. This is particularly a concern for lines that operate over a wide temperature range, e.g. -40 to +40°C. At low temperature, high conductor tension especially needs effective vibration control. Conductor damage has been associated with installation of components that have the same weight and size as TLSAs, notably aircraft marker balls, anti-galloping detuning pendulums and spacer dampers. Galloping is large amplitude low frequency oscillations involving complex vertical, longitudinal and torsional motions that accumulate thousands of stress cycles ad are common when there is an ice layer on conductors. Rocking motion along the line direction is a concern, especially if a resonant frequency of the span is synchronized to the pendulum frequency of the arrester itself. An arrester installation that causes galloping is theoretically possible in particular span lengths but has not been reported in practice – as of yet. There is considerable experience with spacer damper systems for bundle conductors. Lessons learned from these problems can be applied to applications of TLSAs, thereby achieving long service life – especially if arrester standards expand to include mechanical issues during the procurement and installation process. In this respect, IEC 60099-8 provides an example of leadership by setting out some vibration requirements for EGLAs.
A CIGRE Task Force consisting mainly of mechanical engineers evaluated one specific problem of non-gapped line arrester (NGLA) interactions with vibration dampers and formulated recommendations. This work, completed in 2016, set out the following recommendations:
1. On retrofit applications of TLSAs, Stockbridge type dampers should be moved along the conductor so that the distance from the TLSA clamp to the Stockbridge damper is at the optimum distance, as it presumably was from the suspension insulator clamp;
2. TLSA clamps on conductors should be mounted over reinforcing armor rods;
3. Loose connecting wires should be long enough to accommodate all motions possible of suspension insulator strings, without pulling them tight;
4. Alternatively, clamps on loose connecting wires should be applied over a reinforcing layer of armor rods at both ends;
5. There appear no Standards that relate to mechanical reliability of surge arresters. A survey of industry service experience of failures leading to realistic test requirements seems necessary;
6. Support hardware should provide freedom of motion of the surge arresters across and longitudinal to the conductor.
EPRI in the U.S. has initiated studies of the stress on flexible leads from lateral motion of conductors and insulators. An important side-benefit of this work has been ongoing documentation of installation errors at various utilities, e.g.:
1. Arresters that are fixed between tower and conductor, making an inverted braced-post configuration with insufficient tension and compression strength ratings to restrain conductor motion from suspension insulators;
2. Arresters installed with insufficient clearance to phase, considering the arrester fails as a short circuit to ground and brings line potential to the normally grounded end;
3. Explosive disconnects installed at the bottom of the arrester, thus allowing a long, energized lead to swing freely from the conductor;
4. Flexible leads that break part way between arrester and phase conductor, leaving a short stub that becomes a source of corona and electromagnetic interference;
5. Rigid leads that reduce electrical clearance and transfer cantilever forces to the arrester, which may not be rated for these loads;
6. Combinations of connection lead and strain relief chains that bypass the electrical isolation function of the explosive disconnect.
The EPRI report of installation problems and times to failure support Conclusion 5 of the CIGRE Task Force report but additional experience is needed.