Silicone polymers for high-voltage insulators.

Composite insulators with sheds molded from silicone rubber have been used in high-voltage service for nearly 30 years, and their popularity is on the rise. Silicone compounds retain hydrophobicity better than ceramics or glass, especially in coastal regions and contaminated areas. In general, composite insulators are lightweight, and they can significantly reduce losses from breakage and vandalism. The lack of an industry consensus on polymer performance testing complicates the development of a standard silicone product line, however, leading some suppliers to opt for a custom formulation approach.

Most electric power transmission and distribution is done via overhead lines, with insulators providing mechanical support and electrical protection. The size and specific design of high-voltage insulators will vary according to line voltage, environmental conditions, materials of construction and manufacturer. The two predominant designs are suspension- and post-type. Both are characterized by angled "sheds" that maximize the leakage distance between line and tower, directing water off the device. The longer the insulator, the greater the leakage distance.

Insulator service life can be affected by electrical, mechanical and environmental factors. Airborne contamination, UV exposure and vandalism are typically the greatest physical threats. Electrical stress results primarily from corona discharge (when the voltage gradient exceeds critical value), leakage current from the line and arcing or flashover discharge from the line to the support structure.

Ceramic and glass have long been the materials of choice for high-voltage insulators, offering long-term resistance to electrical stress and outdoor exposure without significant deterioration in most environments. The shortcomings of porcelain and glass designs are well recognized, however. For one, they are typically heavy and prone to breakage. Breakage estimates on ceramic insulators range as high as 30% during transportation and installation alone, not including damage that occurs in service.

Surface contamination

One of the biggest problems in high-voltage applications is airborne contamination that settles on the insulator surface. In some cases, airborne particles are of natural origin, such as in a coastal region exposed to salt spray. Industrial sources, agricultural spraying and automotive exhaust can also contribute to reduced water repellency and diminished electrical performance. When the contamination is exposed to moisture, an electrolytic film can develop, leading to corona discharges, surface deterioration, dry band arcing and eventually to flashover. Ceramic and glass designs are particularly susceptible to this buildup, and in coastal areas or contaminated environments, they must be maintained by periodic washing or re-greasing with a dielectric grease.

During a flashover, the intense localized energy causes abrupt temperature elevation and severe arcing. The stress of these conditions can lead to catastrophic fracture of a ceramic or glass insulator. Coastal environments have a particular problem, as the weather creates cycles of contaminant deposit followed by high humidity or fog. For example, these conditions caused more than 170 power outages for one U.S. utility over just a nine-day period in 1991 (ref. 1).

Polymeric materials

The potential benefits of composite insulators have interested utilities and equipment manufacturers worldwide since polymeric designs were first introduced in the 1960s. Lighter in weight and less susceptible to breakage than glass or porcelain materials, they showed immediate promise for lowering transportation costs, easing installation and reducing maintenance. The savings could be especially dramatic in areas with difficult accessibility. Typically smaller than ceramic insulators, composites can also reduce tower heights and right-of-way space requirements.

In particular, polymeric materials have been found to drastically reduce the effects of vandalism on high-voltage insulator. Polymers typically resist mechanical shock much better than ceramics or glass. The inherent flexibility of these materials helps an insulator absorb an impact when struck by a bullet or other projectile, offering a less appealing target for vandalism.

Initial materials testing for HV insulator applications focused on a number of candidates, including ethylene propylene rubbers (such as EPDM), epoxies, polyolefins, polyurethanes, polyethylene, silicones and PTFE (Teflon). Water repellency has been a fundamental design parameter, regardless of the material or specific insulator design.

Silicone rubber

Silicone materials of various types have been used in electrical service for over 50 years. In fact, one of the first applications for a silicone was electrical insulation on aircraft in World War II. In the 1970s, Dow Coming developed a room-temperature curing elastomer, designed for spray application to porcelain insulators to reduce insulator maintenance and resist flashovers, particularly in salt fog environments. The product is still used today, allowing utilities to improve the electrical performance of porcelain arrestors without replacing them.

Insulators molded from silicone rubber are also gaining increased recognition as an effective alternative to traditional porcelain and glass designs (refs. 2-4). In fact, nearly every major manufacturer of high-voltage electrical insulators has now introduced a silicone product among its offerings. The primary advantages of silicone insulators include light weight (as little as 1/10 of the electrical equivalent in porcelain), impact resistance, and good performance in contaminated environments. Unlike EPDM and other organic materials for high voltage applications, silicone elastomers resist degradation from UV exposure, salt fog (figure 1) and extreme temperatures.

Silicone elastomers for high-voltage insulator applications are generally high-consistency rubber (HCR) compounds. Two types of filler are commonly used: silica is the reinforcement that lends physical strength to the polymer, while alumina trihydrate (ATH) improves arc resistance. Filler treatments, pigments and cure agents may also be part of the formulation in small amounts.

The polymer-filler combination is important in silicone insulators. Processing, physical properties and electrical performance are all affected by the molecular weight and structure of the polymer, as well as filler type, size, shape, surface treatment and residual catalyst or contaminants. In determining the optimum formulation for specific applications, device manufacturers and silicone suppliers must determine the best balance of properties, processing characteristics and economic considerations.

Silicone has demonstrated better hydrophobicity and lower surface energy than most organic polymers. The surface properties of silicone are unique, in that it recovers its hydrophobicity between contamination and/or corona episodes, while other materials progressively deteriorate. Corona exposure does temporarily increase the wettability of silicone rubber, a phenomenon associated with an increase in surface oxygen content, but the water-repellency returns after a period of rest. The material's ability to recover its hydrophobicity is thought to result at least in part from the diffusion of low molecular weight PDMS (polydimethyl-siloxane) fluid to the surface (refs. 6 and 7).

Another force which affects the hydrophobic recovery demonstrated by silicone insulators is surface reorientation. The extreme flexibility of the siloxane chain and the low molecular forces between methyl groups produce a low glass transition temperature and a high free volume of PDMS. These conditions readily permit surface reorientation of silicone rubber, which has the most mobile surface of all common polymers for HV applications.

Testing

Existing test standards for ceramic insulators have not shown good correlation with actual service experience when applied to composite materials. In particular, it has proven very difficult to develop test conditions that accurately duplicate material degradation which occurs during long-term service.

The refinement of test apparatus and procedures specifically for polymeric materials remains an area of focus for a number of utilities, universities and research organizations. The Electric Power Research Institute (EPRI) High Voltage Transmission Research Center in the U.S. has developed a fog chamber that allows testing with variable levels of contamination. A specific insulator design can be subjected to a number of stresses that closely simulate the environment in which it may be used. Results of the center's aging studies indicate that the performance of silicone rubber insulators, regardless of manufacturer, is less affected by the aging conditions than ethylene propylene rubber designs. Further, silicone rubber insulators have been found to better resist contamination.

A nine-year study conducted at Chalmers University of Technology in Sweden compared the effectiveness of molded composite insulators to porcelain and glass, finding that "... the silicone rubber insulator had very low current pulses in the range of 6 mA under severe weather conditions, while all EPDM insulators showed very high discharge activity" (ref. 4). Utilities have been conducting field service testing on non-ceramic insulators for a number of years, and overall satisfaction has been high (ref. 8). Some of these insulators have been in service as long as 20 years. For example, Florida Power and Light's experience with silicone has been so positive that the utility replaced many of its porcelain insulators with silicone units following the extensive damage from Hurricane Andrew. These replacements have not experienced any service interruptions from the salt contamination which plagued ceramic units, and have not displayed any of the UV deterioration found in other polymers (ref. 9).

Despite the advances in testing, much of the industry still relies on procedures developed decades ago for porcelain insulators. While many manufacturers have adopted specific techniques to evaluate polymer compounds, they vary from one company to another. The wide range of process conditions among fabricators also complicates the development of standard compounds for molding HV insulators. As a result, some silicone suppliers have opted for a custom formulation approach.

Current test data indicate that highly-filled elastomers offer the best electrical protection, and silicone suppliers generally try to retain as high a level of filler loading as possible and still achieve good processing characteristics. However, if a utility or insulator manufacturer insists on passing conventional tracking wheel tests or other standards that do not allow recovery time between stress incidents, it forces the compounder to reduce the level of alumina trihydrate filler.

Material development

The growing market for alternative materials in high-voltage components is spurred largely by a need to reduce overall costs. With a standard product line, material suppliers would be forced to offer a range of materials that demonstrate the best dielectric properties, physical properties and processing characteristics when molded on commonly-used equipment. Unfortunately, no two fabrication processes are exactly alike, making it difficult to achieve the highest level of performance in either area. By custom compounding silicone materials for specific criteria, it's possible to develop a product that will not only pass the required physical and electrical tests, but also help molders optimize processing techniques for their specific equipment.

The newest generation of silicone insulators how under development builds on the progress of the last 25 years. Much of the current study focuses on surface chemistry to better understand the effects of long-term aging and electrical discharge. The analytical tools have become more sophisticated, as researchers use electron microscopy, electron spectroscopy for chemical analysis (ESCA), scanning auger microprobe (SAM) and other techniques to make precise measurements at varying depths and angles. Insulator manufacturers and material suppliers are combining what they learn from test results and field experience to develop a more complete understanding of silicone compounds that will help produce insulators which improve on the longevity of existing materials and more effectively resist the stresses of high-voltage service.

Progress is facilitated by the custom compounding approach, which by design incorporates the ability to adjust material formulations quickly. The supplier has no resistance to change which might be experienced with a standard product line, and insulator manufacturers are able to reach a level of performance and processing efficiency that would be difficult to duplicate with off-the-shelf products. Further, a custom compounded material requires fewer process adjustments, as it has been formulated with that specific process in mind.

With the latest advances in polymer technology and insulator design, manufacturers are now finding other applications traditionally served by ceramic materials which can take advantage of silicone's unique properties. Engineers are testing various compounds for lightning arrestors, terminations, splices, bushings and other high-voltage components.

References

[1.] Burnham, J. T, D. W. Busch and J.D. Renowden, "FPL's Christmas 1991 transmission outages," paper presented at the IEEE/PES Winter Meeting, 1993.

[2.] Hall, J. F., "History and bibliography of polymeric insulators for outdoor applications," paper presented at the IEEE/PES Winter Meeting, 1992.

[3.] Burnham, J.T, "Silicone rubber insulators improve transmission line performance," Transmission and Distribution, August, 1992, pp. 20-25.

[4.] Vlastos, A.E. and E. Sherif, "Experience from insulators with RTV silicone rubber sheds and shed coatings," paper presented at the IEEE/PES Winter Meeting, 1989.

[5.] Kim, S.H., E.A. Cherney, and R. Hackam, "The al characteristics of RTV silicone rubber coatings as a function of filler level," IEEE Trans. Electr. Insul., 27 (6):1065 (1992).

[6.] Smith, P.J., M.J. Owen, P.H. Holm and G.A. Toskey, "Surface studies of corona-treated silicone rubber high-voltage insulation," IEEE Conference on Electrical Insulation and Dielectric Phenomena, 1992 annual report, paper no. 10-21, p. 843.

[7.] Kindersberger, J. and M. Kuhl: "Effect of hydrophobicity on insulator performance," paper for Sixth International Symposium on High-Voltage Engineering, August 28, 1989.

[8.] Schneider, H.M., J.F. Hall, G. Karady and J. Renowden, "Nonceramic insulators for transmission lines," paper delivered at the IEEE/PES Winter Meeting, 1989.

[9.] "Silicone rubber insulators gain popularity for use in power transmission lines," Materials News, May/June, 1993, page 6.