Until recently, it was not uncommon that composite insulators on transmission lines below 161 kV would be installed without corona rings. But the past 15 years have been marked by growing numbers of failures of these insulators. Subsequent investigation showed that many of these failures were linked to high electric field near or on high-voltage end fittings. Such findings suggested that reliability can be affected if utilities do not have measures to minimize impact of corona discharges on the rubber insulator housing and sheds. Andrew J. Phillips and Chris Engelbrecht of the Electric Power Research Institute (EPRI) in the United States offer an overview of past such failures and propose strategies to address premature ageing of polymeric insulators due to high E-fields.
EPRI was among the first organizations to determine that high E-field and resulting discharge activity was a key contributor to premature ageing of polymeric insulators. This was identified as a primary ageing mechanism on 230 kV and 500 kV insulators based on results from the multi-stress ageing chamber and other testing in Lenox, Massachusetts and appropriate E-field limits were established. However, subsequent insulator failures at 115 and 138 kV later suggested that this phenomenon could also be a risk factor at lower system voltages. Ageing of insulator sections subject to localized high electric field is usually the result of stresses associated with one or more types of discharge activity:
• Continual corona activity from metallic end-fittings or grading rings under dry conditions;
• Discharges due to non-uniform wetting of the rubber material;
• Internal discharges, e.g. along the interface between the core and rubber housing or within the core itself.
Continuous corona activity from the metal end fittings contains sufficient energy to cause erosion on rubber as well as loss of galvanization on metal end fittings. Moreover, individual drops or relatively limited water patches on hydrophobic insulators can enhance localized E-field by a factor of up to 12 due to the high permittivity of water (εr = 80). In the high E-field regions of the insulator, such enhancement could result in corona activity from the edge of the water. Research has suggested that it is unlikely that water drop corona alone will result in significant degradation of the housing since temperature increase from this type of corona is minimal. However, there is ample evidence that the chemical by-products of corona – together together with moisture – can cause serious material degradation. Formation of nitric acid is considered important in this respect. For example, the pH on the surface of an insulator drops from an initial value of about 7 to 3.4 after only about 15 minutes of corona activity on a wet insulator surface. Moreover, it has also been found that some formulations of silicone rubber can be especially vulnerable to deterioration when exposed to nitric acid. Evidence suggests that water drop corona can be just the initial phase of a more severe degradation mechanism that can affect long-term insulator performance. This process is thought to be as follows:
1. Water drop corona in the high E-field regions results in localized loss of hydrophobicity. Regions affected have E-field magnitudes above the onset threshold for water drop corona (see Fig. 2);
2. Under wetting conditions, patches of surface water form in regions of lower hydrophobicity and are separated by dry regions or ‘bands’;
3. Localized arcs form, bridging gaps between water patches;
4. The energy and temperature of these localized arcs are significantly higher than that of water drop corona, further stressing the rubber;
5. With time, as affected regions lose hydrophobicity and completely wet out, E-field in the adjacent regions is enhanced above the water drop corona onset threshold under wetting conditions;
6. The ageing mechanism is then initiated in previously unaffected regions. In this manner, affected regions grow in size;
7. By-products formed by corona in combination with water, notably nitric acid, can be aggressive on the housing, resulting in cracks or corrosion of end fittings.
During wetting conditions, the surface of hydrophilic composite insulators, such as EPDM, is more likely to become covered with patches of water rather than distinct droplets. Dry regions separate these patches and, due to E-field enhancement, sparking can occur between patches. These discharges contain more energy than corona and could degrade rubber. Although such activity can also occur away from the high E-field region, observation during ageing tests suggests that it is more prevalent in regions with high E-field.
Sufficiently high E-field magnitudes can lead to discharge activity within any internal defects, e.g. voids, inclusions from poor bonding between sheath and core. This, in turn, could eventually lead to insulator failure either by destruction of the rod from discharge activity or by ‘flashunder’ (see Fig. 4).
Research has shown that not all insulators are equally affected by high electric fields. Important factors that influence the rate and level of degradation include:
• Type of rubber and design of weathershed system;
• Design of end fitting seal;
• Level, location and type of discharge activity, which is determined to a large extent by E-field along insulator, type and intensity of wetting, presence of contaminants and level of hydrophobicity of material.
Reports of insulator failures on 115 kV and 138 kV lines by 2012 prompted some utilities in the U.S. to initiate a study to better understand ageing mechanisms on insulators of this voltage class. EPRI’s failure database at the time showed that these were not isolated incidents but rather part of a trend of increasing failures on such insulators. In fact, on average, 7 failures/year had been reported to EPRI between 1998 and 2012. Breakdown of the different failure modes of the incidents recorded in the EPRI database for 115 kV to 138 kV insulators indicated that the dominant failure modes were stress corrosion cracking (brittle fracture) and flashunder. A high proportion of these were on the same insulator design and specifically on units manufactured between 1993 and 1999.
During the investigation, it was shown that all such failures could be attributed to continuous discharge activity from the end fitting under dry conditions. This continuing exposure to corona resulted in cracks in the rubber sheath and degradation of the end fitting seal. Once a seal is compromised, moisture can come into contact with the fiberglass rod, leading to brittle fracture. Brittle fracture is a mechanical failure of the rod due to acid attack and where the fracture exhibits one or more smooth planar surfaces – mainly perpendicular to the axis of the rod and giving the appearance of the rod being cut. As a consequence of these failures, utilities were forced to re-examine use of corona rings (or lack thereof) on 115/138 kV polymeric insulators. In cooperation with EPRI, utilities have since initiated a number of specific activities to assess risk to 115/138 kV polymeric insulators from premature ageing due to high electric fields. These included:
• Daylight discharge inspections;
• Detailed examination of insulators taken from service, failure investigations;
• E-field calculations.
These activities focused mainly on one particular design that suffered most failures.
Daylight Discharge Inspection
EPRI and 5 utility members together performed daytime discharge inspections on twelve 115 and 138 kV transmission lines to determine whether continuous discharge activity was occurring from end fittings under dry conditions. While these inspections were primarily directed towards one design, there were also opportunities to inspect other designs.
Conclusions from these inspections were:
• Corona discharge activity under dry conditions was observed on the end fittings of composite insulators installed on all twelve 115 kV and 138 kV transmission lines inspected, though not all insulators had corona activity;
• Corona discharges were more likely on dead-end strings and least on brace post configurations. This is not surprising since it is known from previous calculations that E-field is generally higher for dead-end insulators than for suspension units or brace post configurations;
• Corona activity was observed on 3 of the 4 insulator designs from different manufacturers (see Fig. 7);
• In one case, daylight corona observations were made before and after installation of a corona ring. This confirmed that addition of the ring eliminated corona from the end fitting.
EPRI worked with several utilities to evaluate degradation on over 200 units of 115 and 138 kV insulators removed from service. All had been installed between 1994 and 2006, without corona rings, and were of the same design. 74 insulators removed from service were subjected to detailed examination comprising (1) visual inspection, (2) hydrophobicity measurement, (2) dye penetration test, (3) dissection and, in some cases, (4) mechanical testing. The remaining units were evaluated only by visual inspection. In all cases, the most severe degradation was observed in the same regions, i.e. where dry corona activity was seen during daylight discharge inspections. On some units it was found that degradation of the sheath and end fitting seal had progressed to the point that the rod was exposed to the environment. These were considered as high-risk units where failure was inevitable.
3-D E-field calculations were performed at both 115 and 138 kV to obtain a better understanding of expected E-field distribution on these insulators as well as the parameters that influence it. Another goal was to evaluate possible remedial measures. Significantly, E-field calculations were made to account for the presence of all three phases (and in some cases even adjacent circuits) on structures. Calculations focused on structures where failures had already occurred or where corona has been observed. These calculations considered both E-field on end-fittings (to indicate likelihood of dry corona) and along the sheath (to indicate the likelihood for water induced corona). The following conclusions were drawn:
• Dead-end insulators have higher E-field magnitudes than suspension insulators;
• Single dead-end insulators have higher E-field magnitudes than double dead-end insulators;
• Addition of a hot line link results in slightly higher E-field magnitude on the insulator;
• There is a significant difference in E-field levels between different insulator designs (see Fig. 8). Small and slender end fittings tend to have higher E-fields in the region of the end fitting seal. Shape of end fitting dictates where the highest field occurs and accordingly whether or not any dry corona present will be in contact with the housing material;
• E-field magnitudes exceeded EPRI recommended limits on all designs of 115 and 138 kV polymeric insulators installed without corona rings;
• Addition of 8” corona rings at the live end of the insulator was in most cases sufficient to reduce E-field magnitude to an acceptable level;
• Results of E-field modeling combined with corona camera inspection confirmed that failures that occurred on 115 and 138 kV insulators as well as degradation observed could be associated with high E-fields on the insulators;
• E-field limits need to be adjusted downward for insulators installed at high altitudes i.e. above 3300 ft (1000 m).
Service experience suggests the need for corona rings on 115 kV and 138 kV polymeric insulators to protect against risk of premature ageing due to corona activity. Although this conclusion may seem simple, the implications could be quite profound, especially if large numbers of such insulators are already in-service. Utilities would then be faced with the challenge of identifying high-risk units and deciding the most appropriate and cost effective remedial actions. Fortunately, deterioration due to corona discharge activity develops slowly and this gives utilities time to conduct proper condition assessment. EPRI helped develop a population assessment strategy to address premature ageing of polymeric insulators on 115 and 138 kV lines due to high E-field. Key in this strategy is a set of tools that includes field guides, failure databases, E-field modeling techniques, corona inspection technologies and relevant accelerated ageing test results.
Since 2006, there have been increasing reports of failures of polymeric insulators on 115 and 138 kV lines. These were particularly serious since they mostly involved critical dead-end insulators that pose a threat to system integrity as well as risk a downed conductor. Investigation suggested that these failures were due to high electric fields occurring close to or on the high voltage end fittings of affected insulators. Consequently, corona or grading rings may also be necessary for polymeric insulators installed at these voltage levels. Levels of dry corona activity from end fittings that occurred in-service were higher than expected based on laboratory testing. E-field modeling offered two explanations: 1. At 115 and 138 kV, proximity of nearby phases significantly increases surface E-field magnitude; 2. Most laboratory testing is done on suspension configurations but E-field magnitudes on dead-end and hard angle insulators are higher.