The introduction of polymeric insulators has created more options and opportunities for the overhead line designer and system operator. With the high strength-to-weight ratios of the composite units, previously impractical tower configurations are now possible, leading to aesthetic and efficient compact lines and line upgrading solutions.

Unfortunately, reliability is sometimes compromised by damage during construction of the line. With simple precautions, such risks can be eliminated and the full benefits of composite insulator characteristics realized. Experiences in South Africa with long rod insulators should provide utility personnel with the insight necessary to ensure quality installations.

History

In February 1996, one month after commissioning the 132-kV Vulcan/Ekangala line in South Africa, the mechanical failure of a 132-kV composite long rod insulator resulted in the dropping of a phase conductor. Investigation of the breakage showed the insulator had been subjected to a large bending load during line construction, which resulted in the splitting and partial fracture of the rod.

In mid-1998, the Aurora/Saldanha 132-kV circuit experienced a line drop. To untangle the twin conductors, erection crews rotated the caps of the insulators using a pipe wrench. The severe torsional stresses delaminated the rod, leading to the ultimate failure. Some 31% of the inspected insulators showed the teeth marks of a wrench on their end fittings. These are just two of several similar breakages that local utilities experienced, making the improper handling during storage, transport and erection the most common cause of polymeric insulator failure today.

When first introducing composite insulators, technicians strongly promoted their resistance to damage. Thus, it isn't surprising that the insulators were treated roughly on site. However, the incorrect loading of the units or damage to the moisture seal can cause complete mechanical or electrical breakdown. Moreover, the damage may not be visible and latent failures can occur after a few years of service.

Mechanisms of Failure

The practices and precautions suggested in the handling of composite insulators are primarily designed to limit the probability that the units are subjected to bending and torsional forces for which they have not been designed and to prevent the exposure of the cores to moisture by either housing, sheath or end-seal damage. The failure mechanisms that can arise from these defects are core delamination and breakage, and brittle fracture.

Core Delamination and Breakage

The core of a composite long rod insulator comprises several million continuous, unidirectional glass fibers, running longitudinally from end to end, encapsulated in a resin. This creates an extremely high tensile strength but does not provide much resistance to cantilever, torsional or compressive stresses.

The bending of a rod places half of its cross-section in tension and the other half in compression (a neutral axis separates the two regions). When taken past its elastic limit, the rod starts to delaminate and split down the neutral axis. As the bending progresses, this split propagates over the full length of the rod. With further bending, the side of the rod in compression will start to fracture.

Brittle Fracture

The ingress of moisture to the insulator core can result in a mode of failure commonly referred to as brittle fracture. It is a stress corrosion mechanism precipitated by an acid attack of the glass fibers. The action of partial discharges in the presence of water vapor or the dissolving of acid crystals present in unreacted hardener of the insulator core generates the acid. With a constant applied tension, the crack advances, the stress at the crack front steadily increases and propagation accelerates. Ultimately, the remaining fibers can no longer support the load and a conventional tensile failure occurs. Some composite insulators available today are manufactured with acid-resistant glass fibers. For the others, it is critical to prevent moisture from reaching the core.

Transport

Wherever possible, transport the insulators to the site in their original closed shipping crates. If only part of a crate requires delivery and you remove the insulators from the manufacturer's packing, do not transport the crate without adequate protection. Do not place other materials on top of the insulators in transit. Furthermore, do not tie the insulators down or tie them together with chains and ropes.

On-site Handling

When left at their point of installation, the insulators become vulnerable to damage from improper stacking, impact from other equipment, vehicles and rodents. Consider using a temporary, reusable packing system. Such packing may take the form of a plastic tube or wrap-on shield, and should be applied as soon as you remove the unit from its crate. The latter devices are simply fitted and, as they leave the end fittings exposed, can be left in place until the line construction is complete.

Insulator Inspection

Prior to installation, thoroughly examine the insulators for:

• Damage to the sheath resulting in exposure of the core.

• Exposure of the core caused by movement of the sheds on insulators with sheds not bonded to the core.

• Damage to the end seals.

• Broken or torn sheds.

• A split in the sheath.

• A misalignment of the end fittings indicating torsional stress.

• Marks on the end caps indicating bending, torsional or impact forces.

• Severe deflection of line post insulators.

• Loose bolts, missing split pins and incorrectly applied corona rings.

• Insulator types in incorrect positions and errors in the string-hardware assembly.

Insulator Installation

One person holding the insulator core at a central point can safely lift a long rod insulator less than 2.5 m (6.5 ft). Two people should lift and carry the longer units, holding the insulator about 0.5 m (1.6 ft) from each end. As a general rule of thumb, the angle of deflection on either side of the holding point should remain at less than 30 degrees to the horizontal.

Lifting lines or ropes must be attached to the metal caps of the insulators and not the sheds or rods. Large line post insulators must be carefully lifted in a horizontal position using two slings.

Before attaching to the pole, check that the type number on the insulator agrees with that on the structure drawing. Often, posts of much lower strength rating are specified for jumper support positions, and you must not use these as normal intermediate insulators. Similarly, do not confuse long rods intended for suspension and tension structures.

Take care not to apply bending loads to long rod insulators during attachment of the hardware or lifting of the assemblies to the pole top. Attach the lifting line to the earth-end insulator cap or fitting only. The earth-end hardware must be designed to allow the insulator to swing freely in all directions. Even so, as shackles and clevis fittings can turn sideways and lockup, the attachment to the crossarm must be free before you apply any load to the insulator string.

Do not step or climb on the insulators or their corona rings. Owing to the nature and geometry of single pole structures, horizontal line posts and composite strain units are often stood on and/or used to crawl out to the conductor attachment point. Thus, sheath and shed damage from boots, safety belt buckles and the like is common. To prevent this, mount suitable working platforms on the pole or use a bucket truck for work on the live end of the insulators. Even when working from a bucket or cage, make sure the equipment does not strike the insulators. Prevent ladders, tools, blocks and other equipment from coming into contact with the insulator housings.

Strictly prohibit the practice of throwing a line over the insulator to pull other components to the pole top. This can totally abrade and remove the sheath exposing the core, precipitating future failure.

If you are fitting the corona rings, make sure they are properly located and the mounting bolts fully tightened. A loose ring lying on the insulator can wear through the core and cause the line to drop.

Conductor Stringing

During stringing operations, it is critical that the long rod insulators are not subjected to bending or torsional loads. Use a proper stringing swivel when tensioning the conductor. Furthermore, roll the conductor off the drums and handle it carefully to avoid the formation of loops and twists, which could, on tensioning, apply a torsional stress to the insulator.

If, when pulled into position, the conductor bundle rotates from its desired position or if the conductors become twisted, under no circumstances should you correct this by attempting to turn the insulator or any of the hardware attached to it.

When tensioning the conductor, attach the tensioning equipment to the pole, leaving the strain string out of the way. Alternatively, you can tension the conductor while still on the running-out block, the conductor marked when at the correct sag and then lowered to the ground for cutting and fitting of the compression deadend. Lift the entire insulator assembly and conductor — preferably by the live-end metal fittings or come-along — and attach to the structure. You can install the vibration damper at ground level to eliminate the need to climb out on the insulator after it is in position.

If turnbuckles are provided for sag adjustment, it is important to prevent the insulator end cap from rotating while the turnbuckle is tightened or loosened.

For vertical suspension strings, be sure the insulators are able to swing freely and follow the movement of the running-out blocks without being subjected to any bending stress.

Where line posts are used at the intermediate positions, jamming of the conductor in the running-out blocks will result in the application of a high and possibly damaging cantilever load to the insulator. Thus, it is important to check all blocks before the commencement of stringing.

Line Design Considerations

Although the design of the line cannot eliminate the risk of insulator damage during construction, certain considerations can reduce the probability of failure. For example, select the method of attachment of a composite long rod to the tower that will enable the insulator to move in all directions. If not, the chance of exposing the insulator to cantilever stresses is considerably increased.

For monopole designs, the provision of adequate, stable ladders from which to work (or attachments for their temporary installation) will reduce the necessity to climb or stand on the insulators.

Experiences and developments over the last 30 years have led to composite insulators of low cost and high reliability. Their characteristics have significantly improved line security in environments of high vandalism and extreme pollution severity. Care in the transport and installation of these insulators will enable utilities to gain full value of their investment.

Roy E. Macey, the managing member of Mace Technologies, has been involved in the study of the performance, maintenance and design of high-voltage insulators for the past 31 years. He is a distinguished member of CIGRÉ and is the South African representative on CIGRÉ Study Committee B2 Overhead Lines and Working Group B2-03 Insulators.

Wallace L. Vosloo holds a Ph.D. in electrical engineering from the University of Stellenbosch. Presently, he is the chief consultant (insulators) for Eskom Enterprises, a subsidiary of Eskom in South Africa.