An insulator performs dual functions: mechanically, it holds the conductor or busbar at a certain distance from ground; and electrically it also provides the necessary insulation. Therefore, one way to define insulator failure is when either or both of these functions are no longer being fulfilled. A less technical way to define failure is to say that an insulator has failed whenever it is removed from service either because of degradation revealed by inspection or because of unsatisfacto unsatisfactory ry service experience. Alternative insulator types fail in different ways. The failure modes of toughened glass or porcelain insulators, for example, are entirely different from the failure modes of composite insulators and this could have consequences consequences for continuity of service. It is therefore difficult to properly compare failure rates of different types of insulators. Many factors can bring about failure. The insulator can contain a manufacturing defect defect that grows over time and lead to failure. Such a defect can be within the dielectric material, the metal end fittings or in the materials used to keep the dielectric and fitting together. Failure can also be caused by extreme service conditions such as when an insulator separates due to heavy ice accretion or high winds that cause applied mechanical load to exceed its rated value.
Damage from Power Arc
Similarly, extreme pollution events can cause degradation of the dielectric or corrode fittings, thereby preventing the insulator from fulfilling its electrical or mechanical functions. A flashover leading to a power arc of extended duration can also bring about the failure of the dielectric or of the fittings. Various acts of vandalism can cause severe insulator damage or even failure. Finally, an insulator can fail if it was not properly designed for its intended service environment, such as having too little creepage or even too much creepage – since both can prove detrimental to service life. Below is a compilation of 12 notable examples of insulator ‘failure’ due to either intrinsic or external factors:
1. Radial Cracking
1. Radial Cracking This type of failure can be attributed to the phenomenon of cement growth and applies mainly to porcelain insulators that have been assembled using overly expansive cement.
2. Pin Corrosion 2. Pin Corrosion Problems of pin corrosion could occur in places with severe contamination and are basically independent of bulk dielectric. Use of newer designs with sacrificial zinc sleeves for corrosion protection has been proven to limit onset of corrosion but of course has no impact on ageing of any existing discs from earlier generations.
3. Brittle Fracture 3. Brittle Fracture
Brittle fracture of a composite insulator leads to mechanical separation of the fiberglass core rod and is characterized by the presence of some smooth fracture surfaces. This failure mode was first reported in early generation insulators and attributed to attack on the rod by nitric acid produced by corona in a moist environment. The exact source of the acid later became a topic of debate. While there is evidence that electrical stress is not required for brittle fracture to occur, it can accelerate the process. It has also been proposed that certain chemicals used in manufacturing fiberglass can themselves produce acids, even without moisture from the outside. Still, it is commonly accepted that effective sealing of the rod from the environment and the use of corrosion resistant rods is the most effective way to minimize risk of such failure. In the end, brittle fracture could be due to a combination of factors and not always explained by any single mechanism.
4. Damage to Bulk Dielectric
4. Damage to Bulk Dielectric The type of failure shown in these breaker bushings affects mostly porcelain insulators and is most typically caused by vandalism. Unlike toughened glass insulators that can disintegrate under sharp external stress, only sections of porcelain sheds are broken off. Still, while the insulators continue to operate, they should ideally be replaced as soon as possible since any related internal cracking cannot easily be determined.
5. Spontaneous Shattering 5. Spontaneous Shattering This failure mode applies only to glass disc insulators and is due to sudden release of internal stresses induced in the shell during the toughening process. The incidence of the problem is linked to inferior quality control such as the presence of inclusions (hard particles that have not fully melted in the furnace) or impurities in the tensile zone of the glass body. Since the average size of these impurities is less than 100 mic rons, there is no practical method to detect them other than to screen out glass shells containing inclusions through thermal shocks prior to assembly of fittings. Even though the thermal shock test is carried out twice during production, there is a very low probability that some shells containing inclusions survive and shatter subsequently during service. The time when this finally happens can be triggered by a variety of environmental factors or even occur with no special external circumstance. Statistics reveal that typical annual rates of spontaneous shattering are on the order of up to 1 per 10,000 units and will depend on the manufacturer. 6. Flashunder
Unlike porcelain disc insulators, composite insulators are not normally at r isk of internal puncture since the electrodes of different potential are separated by a long fiberglass core rod. However, punctures in the housing have been detected or occur due to possible mishandling or other factors such as bird pecking and these can provide access for moisture into the core. Tracking along the rod under the housing material has then been known to occur, possibly resulting in permanent loss of electrical strength – just as in the case of puncture of a porcelain dielectric. If internal tracking is considerable, the insulator will not be able to support the electrical stress and the rod will ‘carbonize’ along its length.
7. Non-Brittle Type Fracture
7. Non-Brittle Type Fracture This failure mechanism has appeared on some composite insulator designs in recent years and has been linked to moisture penetration into the core along with poor
bonding along the rod-to-housing interface. While it may be termed brittle fracture at first glance, it is actually quite distinct since there is an absence of any planar fracture surfaces.
8. Explosive Shattering
8. Explosive Shattering As a pressurized housing, porcelain has a well-known failure mode that represents a potential safety threat to workers and the public as well as a risk factor to any nearby equipment. Here bushings have ruptured violently from flashover shock triggered by contact with wildlife. Explosive failures of this type are also due to internal failures that cause sudden heating and expansion of gas volume.
9. Shed Tearing
9. Shed Tearing This type of failure was first reported in northwestern China where constant sheer by winds caused larger diameter sheds to tear. The solution, still being researched, is ensuring composite insulator geometries are adapted for service in areas of frequent high wind.
10. Erosion
10. Erosion
Insulators that are not properly specified in terms of geometry and specific creepage for the pollution severity of their service environment may experience excessive leakage current activity that eventually triggers erosion of the polymeric housing. When such damage occurs on sheds, repair is possible but if the damage is to the shank, the insulator must be replaced as soon as possible.
