Many transmission line surge arresters (TLSA) are suspended from phase conductors, thereby adding mass that creates another mechanical node between insulator suspension clamps and Aeolian vibration dampers. Such a placement can interfere with damper performance and shorten conductor service life. Possible mitigations include re-positioning the dampers, inspecting more frequently or selecting other installation configurations. This contribution to INMR by Dr. William Chisholm explains the relevance of these issues and presents a case study illustrating proper consideration of such factors for TLSAs installed on a new 230 kV line in Canada.
The first lightning arresters for power systems were mainly for protection of equipment and used hollow ceramic insulators filled with silicon carbide blocks. Silicon carbide arresters had internal components to quench and interrupt AC power system follow current that normally keeps flowing through a flashover arc. The ceramic insulators were sealed with metal plates to provide overpressure protection. But these also allowed for long-term moisture ingress. Two separate arrester technologies subsequently converged to improve arrester reliability as well as service life:
• The non-linear electrical performance of metal oxide varistor (MOV) materials was superior to silicon carbide. This technology, patented by Matsuoka at Matsushita Electric Company in 1970, displaced silicon carbide in many applications within only 10 years;
• Polymeric materials (e.g. EPDM and the polydimethylsiloxane chains in silicone rubber) protected MOV elements against moisture while reducing weight and cost compared to ceramic housings. Industry-wide substitution of polymers for porcelain also took place over a relatively brief period from 1987 to 1995.
These improvements made it practical to use TLSAs for improved lightning performance by applying them at some or at all insulator positions as well as at equipment or line terminals. From the electrical perspective, TLSAs using MOV materials have been installed for some 30 years to limit transient overvoltages on line insulators. For example, their application on unshielded 230 kV line since 2001 involved arresters with 60 mm block diameter. Studies and experience with improved MOV technology allowed block diameters to be reduced 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 fraction of current to ground. Continued reduction in cost and increase in quality has been associated with the directly molded silicone construction that eliminates internal air volume. Originally developed mainly as components to be retrofitted to existing lines, TLSAs have since become an important design and mitigation alternative to investment in extensive grounding systems for new lines. The case study discussed below sets out decision criteria leading to fitting TLSAs to all three phases of a 230 kV transmission line during construction instead of using continuous counterpoise in an area where rock resistivity is extremely high.
Mechanically, TLSAs have demonstrated in-service problems. The most common up to now has been 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 wind can reduce electrical clearances. Rigid configurations, as offered by the external gap type arrester (EGLA), do away with flexible leads and offer the promise of longer service life.
Unfortunately, some TLSA installations have been made in ways that detract from or even defeat the mechanical protection given by Aeolian vibration dampers. Aeolian vibration is 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 temperatures, for example, high conductor tension needs particularly effective vibration control. Conductor damage has been associated with installation of components having the same weight and size as TLSAs, notably marker balls, anti-galloping de-tuning pendulums or spacer dampers. Galloping is large amplitude, low frequency oscillation involving complex vertical, longitudinal and torsional motions that accumulate thousands of stress cycles, most often when there is ice on conductors. Rocking motion along the line direction is a real 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 along particular span lengths but has apparently not yet been reported in practice.1.
There is considerable experience with spacer damper systems for bundle conductors. Lessons learned from these problems can therefore be applied to existing and future applications of TLSAs. Long service life can be achieved, especially if arrester standards expand to include mechanical issues during their procurement and installation. In this regard, IEC 60099-8 provides leadership by setting out 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. This work was completed in 2016 and offered the following recommendations:
1. On retrofit TLSA applications Stockbridge type dampers should be moved along the conductor so that the distance from the TLSA clamp to the damper is at an 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 the 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 to be no standards relating to mechanical reliability of arresters. A survey of experience with failures leading to realistic test requirements seems necessary;
6. Support hardware should provide freedom of motion of surge arresters across and longitudinal to the conductor.
EPRI initiated studies of stress on flexible leads from lateral motion of conductors and insulators. One side-benefit of this research was ongoing documentation of installation errors at some utilities, including:
1. Arresters 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, allowing a long, energized lead to swing freely from the conductor;
4. Flexible leads that break partway 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.
Update on Vibration Damper Placement
Many utilities have already installed TLSA bodies or leads onto phase conductors, adding the new components somewhere between insulator clamp and vibration damper. Comparatively few seem to have followed Recommendation 1 above, namely to move the Stockbridge dampers at the same time as arresters are installed and thus preserve the original damping design. In cases where TLSAs have been placed close to dampers, the advice to move the dampers into the spans and away from the TLSA should be implemented immediately. It would also be worthwhile to inspect the conductor beneath the TLSA to establish whether there has been damage to inner layers from service without proper damping.
In cases where the TLSA have been placed about halfway between dampers and insulator clamps, CIGRE’s analysis provides breathing space for developing an action plan. Fig. 1 shows that vibration damper placement is a compromise, absorbing some energy at wind speeds of 1 and 7 m/s and a larger fraction of energy from wind with 3-5 m/s velocity.
Mechanical engineers recommend vibration damper placement at 80% of the loop length at 7 m/s as a good overall choice, given usual distribution of wind speeds over the year. When a heavy TLSA is placed at 40% of the loop length, halfway between insulator clamp and an existing damper, Fig. 2 shows there is reduction in damper effectiveness. However, the original design may have included some extra margin in damper power dissipation at vibration frequencies associated with certain wind speeds.
Placement of TLSAs in the photo above was approximately 20% of the original distance from damper to suspension clamp. Thus, the x-axis value in Fig. 2 is 0.2 and damper effectiveness has been reduced to 25-40% of its original value, i.e. too much to ignore. The dampers must ideally be repositioned. Since field tests have confirmed that TLSAs act as vibration nodes when suspended from phase conductors, cumulative damage to the conductor under the arrester clamps should also be evaluated by spot inspections. Electrical engineers have already mastered concepts of resonance with RLC circuits and the recent IEEE introductory guide to Aeolian vibration can facilitate thinking in the mechanical domain.
Reliable Conductor Clamping Systems
CIGRE TB 277 describes the following design characteristics for clamping of spacer systems to phase conductors:
A spacer clamp should be capable of easy, reliable installation, preferably verifiable by ground based inspection and should provide a safe, reliable, non-damaging, long-term grip on the sub-conductor. High, localized clamping stresses should be avoided and the clamp conductor groove should be smooth and free from irregularities. The clamp should be smoothly profiled to minimize Corona and RIV discharges at specified voltages. Component parts should be secured and an energy storage mechanism is required to prevent loosening due to the effect of vibration, thermal cycling conductor and elastomer creep (in the case of elastomer lined clamps). All materials should be compatible with the conductor and avoid corrosion.
The clamps should:
• Maintain suitable grip for the whole life of the line and over the range of service temperatures;
• Avoid conductor damage during installation and service;
• Be free of corona and RIV at maximum line voltage;
• Be installed without disassembly of parts;
• Be designed to minimize risk of improper installation;
• Be capable of being safely removed and re-installed on conductors;
• Allow installation survey from the ground;
• Ensure that individual components will not become loose in service.
Substitution of ‘arrester’ for ‘spacer’ is an appropriate goal for NGLA users and suppliers. Experience with rigid, articulated, spacer-damper and flexible spacers can guide choice of suitable TLSA hardware. Spacer clamps are generally made of primary aluminum alloy. Rubber-lined clamps, as used in many spacer-dampers, may not be appropriate for NGLAs since a separate electrical connection would need to be made to allow flow of lightning surge currents through the arrester’s MOV elements. However, a range of interesting articulation systems is in use for spacers and spacer dampers. Some spacer clamping systems and components could also be used in reliable NGLA installations. Safety plates with tabs that bend after tightening or breakaway nuts that confirm correct installation torque are also mentioned as ways to avoid conductor abrasion or hammering damage.
Alternate Mounting Configurations
There are a range of mounting configurations for EGLAs and NGLAs. For example, flexible leads can fail quickly in service and benefit from re-design that separates electrical and mechanical components. This results in designs with higher part counts, weight and cost that must be validated with the same limit-cycle tests (i.e. 10 cm, 106 cycles) as spacer systems.
There are ways to address limited durability of flexible leads, including:
• Specifications that require limit cycle testing, like for spacer dampers;
• Specifications that would apply an appropriate number of cycles and amplitude, considering clamp mounting location and damper let-through frequency;
• Installation practices that place the disconnect-lead clamp near or on the insulator clamp;
• Use of stress relieving loops;
• Frequent and specific inspections.
Lead failures are easy to find from helicopter or ground inspection, although it is difficult to establish whether the explosive disconnect has operated. It is possible that drone-based inspection will become cost effective and can bring a closer focus to areas of interest, near ring tongues and collars of flexible leads. If leads break at the arrester or tower and fall, they may reduce electrical clearances. Electrically, the greatest benefit of partial TLSA application is often gained with installation on the bottom phase. A mounting configuration with flexible lead presents minimal danger to electrical clearance if it fails as designed and disconnects from the phase. However, if the lead breaks away from the top of the NGLA, then nuisance flashovers may occur for side winds that blow the flexible high voltage connection towards the tower while still connected to the phase.
Recently, SaskPower in Canada completed construction of a shielded 230 kV transmission line, running parallel to an existing, unshielded 138 kV line for 300 km. Most access was by helicopter and this formed at least 10% of total line cost of CA$ 330M. These lines run across a geographic feature known as the Canadian Shield, which is granite of high electrical resistivity. With no overhead groundwire, every lightning stroke to the I2P 138 kV line caused a flashover and short circuit across its polymeric insulators.
The I2P 138 kV transmission line was constructed with continuous counterpoise, although it had no shield wires, to provide a suitable return path for fault current that would allow rapid operation of protective relays. Early on in project planning for the new I1K line, grounding investigations were carried out to establish whether continuous counterpoise would improve backflashover performance since the new design was fitted with an optical fiber groundwire (OPGW) for improved communication between terminals. A ‘Zed Method’ applied a voltage step between the base of existing I2P towers and a test lead, laid on the rock. The surge impedance of the test lead provided the transient connection to ground, yielding 400 mA pulse current. Tower base voltage rise was monitored in response to this impressed current. In a few cases, measured surge impedance of the tower base with its counterpoise in place was almost the same as the impedance of the insulated wire draped on the ground. More typically, the guyed structure had an impedance of 140 W in isolation that fell to a constant 77 W when the counterpoise was reconnected. The tests concluded that continuous counterpoise draped across the Canadian Shield rock surface did not provide much of a grounding path. Each leg of counterpoise had a surge impedance of about 360 W, meaning that more than 10 radial wires would be needed to achieve a footing impedance of 35 W. At that stage, many options were considered and refined down to two choices for the new I1K line:
1. Moving the counterpoise up and suspending it from the tower in an underbuilt location below the phases, where it could be inspected by helicopter; or
2. Installing surge arresters across all three phases.
The arrester selected met the IEC 60099-4 Edition 2.2 Line Discharge Class 3, with a 58 mm MOV block diameter. In Edition 3 of this standard, the new classification is SM with Qrs of 2.0 Coulomb, corresponding to Energy Class E of IEEE C62.11/2012. Each arrester weighs about 60 kg with additional mass from the corona ring. Given the choice of arresters over grounding made early in the project planning stage, it was possible to advise the manufacturer of the vibration dampers that their design, a twin pairs of dumbbell weights, would also need to protect the conductor under the arrester clamp. Close inspection reveals that the armor rod has been placed between the conductor and the NGLA saddle clamp, further improving mechanical durability and hopefully leading to long service life. Another consideration in application of polymer-housed TLSAs is that the region is prone to forest fire. For example, one test along the I2K line near Island Falls showed that the ground temperature after a recent fire was hot enough to melt the connection from tower to counterpoise. In this case, the counterpoise was found to be otherwise intact, since it provided a continuous reduction in footing impedance when re-connected to the tower. Such damage is difficult to inspect by helicopter and offers an additional argument for selecting TLSAs (or underbuilt groundwires) as an alternative to grounding with continuous counterpoise.
Improvements in Arrester Mechanical Standards
There are standards that describe mechanical vibration test methods for transmission line components and some important definitions are found in Figs. 3, 4 & 5.
Bundle spacer systems are tested with mechanical oscillations that simulate in-service conditions. For example, a galloping test applies a 1 Hz oscillation frequency with 10 to 20 cm amplitude for 106 cycles. Components pass if there are no visible changes. This would also seem a reasonable requirement for complete arrester/flexible lead systems. However galloping tests are severe. An adaptation of the vibration test was set up to simulate galloping effects on a 44 kV NGLA located about 710 mm from the suspension clamp. Galloping amplitude of about 0.8 m was achieved on a 20 m span with the purpose of simulating damage found in the field. This was achieved.
The damage was partially mitigated if the clamp was tight and flush to the top of the arrester. The stud fatigue damage occurred after 20k to 66k cycles, consistent with findings about endurance of Stockbridge dampers on galloping test lines.
Transmission line surge arresters (TLSAs) are large and heavy enough to affect performance of vibration dampers. Field tests have confirmed that TLSAs act as vibration nodes when suspended from phase conductors. CIGRE guidance recommends that existing vibration dampers should be displaced along the conductor, treating any conductor-mounted arrester as a new clamp and node to be protected. Damper re-positioning should be an inspection and remediation priority for installations where arresters have been placed close to pre-existing dampers. Concerns with long-term durability of the conductor should include re-selection of appropriate dampers for the new span as well as improved arrester attachment using armor rods or other stress reduction clamps. Guidance, including standards for limit-cycle testing, should be obtained from clamping systems that have proven suitable for bundle spacers. TLSA accessories, including flexible lead clamps, corona rings for NGLAs and arc receivers for EGLAs, should meet appropriate vibration and flexion criteria for an anticipated 20 to 50 year service life. Placement of light accessories close to existing insulator clamps is recommended to reduce level of vibration. Since some accessories are about the same weight as vibration dampers, placement near existing dampers should be reviewed with manufacturers and supported by additional bending amplitude verification testing. Industry should continue to consolidate reports of TLSA mechanical failures through appropriate CIGRE, IEEE, EPRI and other technical groups or through contributions to INMR.