Because of its rigidity, the epoxy family of polymeric materials occupies a unique position in the field of outdoor insulation. For example, this property means that it cannot be classified as elastomeric, as are both silicone and EPDM rubbers. Also, epoxies can serve as the strength-bearing member and hence there is no need for a fibreglass core (whether rod or tube) that is integral to both composite line and apparatus insulators. The question is therefore whether tests and standards developed for composite housed insulators and apparatus can ever apply to epoxy insulated devices? And, if not, how should they be evaluated? Certainly, there is a need for this type of information since epoxy insulated devices are used extensively in such low and medium voltage applications as bus support insulators, enclosures for protective devices (e.g. vacuum reclosers, sectionalizers), fuse cutouts, instrument transformers (for current and voltage measuring devices in stations), bushings and pin and post type insulators.
Epoxy-insulated devices offer a number of advantages over porcelain. They are lighter and non-brittle, meaning they are easier to handle and will not chip or break. They can also be cast in a variety of shapes. Since the same material serves both electrical and mechanical functions, they eliminate an important interface, which can be the source of concern during manufacture and also during service. They also eliminate any need for oil and the obvious related maintenance issues. They can even be installed in any orientation and this is an advantage where clearances are tight. However, in order to realize all these potential benefits, it is imperative that the material be able to withstand all environmental stresses, just as do composite insulators, as well as any internal electrical stresses due to the presence of internal active components.
The epoxy being employed today for most outdoor applications is the cycloaliphatic type (CEP) as opposed to the bisphenol variety that is used for manufacturing fibreglass solid rods and hollow cores. CEP is characterized by a saturated molecular structure that results in better tracking and erosion resistance. This particular property is inferior in the case of bisphenol epoxy, even if it has very good mechanical and electrical properties. There are innovations happening as well. Almost two decades ago, hydrophobic cycloaliphatic epoxy (HCEP) was introduced and another advancement, namely a flexible variety of HCEP for the sheds of composite insulators, has also since been made. Yet another novel application – epoxy insulated cross-arms where the conductor can be attached directly to the structure – has also been proposed.
In the absence of standardized tests for epoxy, a judicious combination of laboratory as well as field testing, computer modelling of flashover and degradation and statistical analysis can be used to answer questions regarding expected service performance. This was the thrust of this past INMR article, contributed by Ravi Gorur, formerly of Arizona State University.
There are two prominent modes of failure for outdoor insulation that employs polymeric materials. One of these is surface degradation due to tracking and erosion and the other is flashover due to surface contamination that renders the device incapable of maintaining the system voltage. The focus of this project was medium voltage instrument transformers, although insulators were also evaluated.
Tracking and erosion resistance was determined in the laboratory using the ASTM D2303/IEC 587 Inclined Plane Test. Rectangular slabs of the same material as used in the final product were provided in this regard. To evaluate flashover properties in the laboratory, the clean fog test was utilized. Theoretical models developed in previous projects and published in IEEE papers were employed to extend the range of prediction. In addition, in order to better correlate results from laboratory testing with real world conditions, two outdoor tests sites were built several years ago – one on the roof of the Engineering Research Center at ASU, the other at the premises of an electrical apparatus OEM in North Carolina. These locations were chosen primarily for convenience but also covered a range of environmental conditions important for polymeric-insulated devices. For example, the ASU site experienced long, hot summers and was considered one of the most severe locations in terms of total UV radiation per year. The North Carolina site experienced high humidity throughout the year and was in a relatively mild place, typical of many service locations. A total of 8 samples were installed in each site, all in their new condition, and were energized at 15 kV (corresponding to a 25 kV system voltage) and representing a two-fold increase compared to the nominal service voltage. This overvoltage provided an acceleration factor and was essential to promote some ageing within a reasonable time in otherwise clean locations.
At the time, the OEM invovled in this work typically tested all their new products for at least a year at the Koeberg Insulator Pollution Test Station (KIPTS) in South Africa – very close to the sea and among the most severely polluted test sites (both marine and industrial) in the world. Indeed, the pollution index at KIPTS is extremely high and would be classified as ‘very heavy’ according to IEC 60815. Several samples that had been exposed there for a year were supplied for this project and both test sites equipped with instrumentation to monitor leakage current. Photos of the test sites are shown in Fig. 1 while Table 1 lists important weather data. Samples that had been exposed in service in Finland and Norway for up to 20 years were also made available. They provided vital information on the degree of physical and chemical changes that can be expected over such a service life.