Since 2006, some utilities have experienced an increasing number of failures of polymeric insulators on 115 and 138 kV transmission lines. Investigation has shown that these failures could often be attributed to high electric fields (E-field) occurring near or on the high-voltage end fittings. Such findings suggest that, contrary to common practice, it might be necessary to consider application of corona rings (also called grading rings) on polymeric line insulators even at system voltage below 161 kV. Reliability can be affected if utilities do not have measures in place to minimize the effect of corona discharges on the rubber shed material.
Over recent years, EPRI in the U.S. has been working to help affected utilities develop a strategy to address premature ageing of polymeric insulators on 115 and 138 kV lines due to high E-fields. This includes providing reference information, technical support and recommendations to assess the existing population and to specify polymeric insulators for new or replacement units. This edited INMR article from late 2013, contributed by Andrew J. Phillips and Chris Englebrecht of EPRI, offered an overview of such insulator failures and of the best strategy to manage the problem.
EPRI is credited as one of the first research organizations to identify high E-field and resulting discharge activity as an important contributor to premature ageing of polymeric insulators. Based on results from the multi-stress ageing chamber and other testing at the laboratory in Lenox, Massachusetts, this was identified as a primary ageing mechanism on 230 kV and 500 kV insulators. Appropriate E-field limits were subsequently established. More recently, insulator failures at 115 and 138 kV suggest that the same phenomenon may be present at lower system voltages as well.
Ageing of insulator sections subjected to local high electric field is usually the result of stresses associated with one or more of the following 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 can contain enough energy to directly cause erosion on rubber as well as loss of galvanization on metal end fittings.
On hydrophobic insulators individual drops or relatively limited water patches 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, this enhancement could result in corona activity from the edge of the water. Research suggests that it is unlikely that water drop corona alone will result in significant degradation of the polymeric housing since temperature increase due to this type of corona is minimal. There is, however, ample evidence that the chemical by-products of corona – together with moisture – can result in serious material degradation. In this respect, formation of nitric acid is considered important. For example, it has been found that 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 been found that some formulations of silicone rubber can be particularly vulnerable to deterioration when exposed to nitric acid.
Available evidence suggests that water drop corona can be just the initial phase of the following, more severe, degradation mechanism that affects long-term insulator performance. Present understanding of this process is 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. 1).
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. The 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.