Silicone in Medium-to-High Voltage Electrical Applications

Silicones are used in a range of electrical applications due to their high electrical resistivity, resistance to environmental degradations and to electrical aging, and cost effectiveness due to the hydrophobicity [1-2]. They are used at the end of underground high-voltage cables insulated with polyethylene, in the cable-end terminations, and as insulators for power lines.

Silicone Cable End Terminations

Modern materials allow pre-assembly and thus avoid problems associated with the use of molten casting material or mistakes made during manual assembly on the construction site. Today cable accessories are completely built at the supplier. Typically they consist of rubber terminations made of different insulating silicone rubbers.

Silicones allow for two types of design:

  • Push-on technique where a PE ring acts as a space holder until placement, and using silicone rubbers with hardness from 35 to 50 Shore A
  • Cold shrink technique using softer silicone rubbers with hardness from 25 to 35 Shore A

Insulation is made without chemical bonding between the termination and the cable, and it relies on the elastomeric characteristics of the silicone termination to exclude any entrapped air, particularly in areas of high electrical field and around the edges at the cable end. The high gas permeability of silicones allows any included air to diffuse out to leave an air-free joint.

Such silicone rubber cable end terminations are produced by rubber injection molding using a silicone high consistency rubber (HCR) or by liquid injection molding using a two-part liquid silicone rubber (LSR).

Silicones provide overall electrical insulation because of their high dielectric strength. In addition to their good resistance to high temperature, UV and ozone, they are hydrophobic and so do not promote surface insulation failures. But more important, specially formulated silicones have been developed to smooth the electrical fields within the connection end and to ensure long-term performance. This is achieved in composite cable terminations either using some electrically-conductive silicone rubbers or, in more modern and smaller accessories, shaped deflectors made from silicone rubbers with medium electrical permittivity (see Figure 1).

Figure 1. Field line density in a cable end termination at the cut of the screen without control (upper figure) or with a nonconductive/high permittivity field control silicone rubber (in green; lower figure).

Silicones are appreciated in cable end terminations because of their resistance to erosion caused by radiation. As silicones do not absorb UV-visible sunlight, they are not prone to chalking or cracking. Such phenomena are typical with organic-based materials and, associated with dirt pickup and humidity, can lead to a significant reduction of insulation properties.

Silicone resistance to so-called “tracking” is also higher than with organic-based insulation materials. Tracking is the formation of electrically-conductive surface paths under intensive electrical surface leaks and discharges. In organic materials, this leads to the formation of carbon-based decomposition products that unfortunately show high conductivity. With silicones, even if poorly designed or not properly assembled, decomposition leads to nonconductive silica, and silicones will meet the highest class of electrical erosion resistance.

Silicone Insulators

Another key property is hydrophobicity, particularly for electrical insulators, or devices installed between power lines and supporting structures. Water on an insulator made of a silicone elastomer remains as droplets and does not form a continuous film because of the low surface energy of the silicone elastomer surface [3-4-5]. This reduces surface currents on the insulator. Surface hydrophobicity is maintained even after surface discharges or deposition of airborne pollution because of the presence of low molecular weight, unreacted polydimethylsiloxane species in the composition of the silicone elastomers. These species can migrate to the external surface and maintain low surface energy or hydrophobicity [6]. Insulators made of silicone elastomer therefore need little cleaning or maintenance and perform over a long period of time (see Figure 2).

Figure 2. Comparison of an insulator after 23 years of use and exposure to pollution (left) vs. a retained sample kept at RT (right). Both still show excellent hydrophobicity as indicated by the high contact angle of the water droplets. (Picture courtesy of Lapp Insulators GmbH & Co.KG).

References

1 Gorur, R.S.; Cherney, E.A. ; Burnham, J.T., Outdoor Insulators; Ed. Ravi S. Gorur May 1999 Phoenix, Ariz,; 185-193.

2 Oesterheld, J. Dielektrisches Ver-halten von Siliconelastomer-Iso-lierungen (Dielectric Behaviour of Silicone Insulations), Dissertation TU Dresden, 1996, Fortschrittsberichte Reihe 21 Nr. 178 VDIVerlag: Düsseldorf, 1995.

3 Bärsch, R.; Lambrecht, J.; Winter, H. J., On the Evaluation of Influences on the Hydrophobicity of Silicone Rubber Surfaces, 10th International Symposium on High Voltage Engineering, Montreal 1997; 13-16.

4 Lambrecht, J. Über Verfahren zur Bewertung der Hydrophobieeigen-schaften von Siliconelastomer- Formstoffen, Dissertation, TU Dresden, 2001.

5 Strassberger, W.; Winter, H. J. Silikonelastomere in der Mittel- und Hochspannungstechnik, (Silicone Elastomerics in Medium and High Voltage Technology), ETG Fachbe-richt Nr. 68 VDE-Verlag: Berlin, 1997.

6 Kindersberger, J.; Kuhl, M.; Bärsch, R. Evaluation of the Conditions of Non-Ceramic Insulators after Long-Term Operation under Service Conditions, 9th International Symposium on High Voltage Engineering, No.3193, Graz, 1995.

Contact Details

E. Gerlach, Dow Corning GmbH, Wiesbaden (Germany)

About Xiameter

In 2000, Dow Corning was facing an increasing number of competitors around the world that were getting into the standard silicones business. At the same time, for a growing customer segment called “price-seekers” who needed little or no service attached to the products, price had become the driving force.

Dow Corning needed to defend their position as a provider of innovative silicon-based materials and solutions (which, for the most part, came bundled and priced together). Dow Corning knew that they needed to find a better way to meet customers’ needs exactly.

It was time for a game-changing strategy – one that would simultaneously:

  • Meet customer needs for efficient, cost-effective silicone products
  • Empower Dow Corning to continue to innovate and grow the silicones market overall

Dow Corning decided the best way to take advantage of the potential in the mature market segment, without detracting from the value of the Dow Corning offering, would be to create a separate brand. This brand would offer a clearly defined value proposition and set of products via a web-enabled platform to provide competitive pricing.

The product set would be a wide range of standard silicone products generally thought of as “commodities” by customers. These standard products would be – and still are – manufactured by Dow Corning.

The tone would be “no frills” – just straightforward business terms and conditions, high-quality products, a reliable supply, and market-driven prices. Short. Sweet. To the point.

Defining success (2002-2009)

With the launch of the XIAMETER® brand in March 2002, Dow Corning revolutionized the way they do business. Specialty silicone products, service and solutions continued as part of the business under the Dow Corning® brand. The XIAMETER brand met the needs of “price-seekers” who purchased standard silicone products via www.xiameter.com.

The global drive to find greater efficiencies in business through a web-enabled business model worked in their favor. It helped Dow Corning maximize productivity and reduced human error, while keeping costs competitive for customers. Rather than cannibalizing the Dow Corning brand as some feared, the XIAMETER® brand made it stronger. The two brands worked in harmony, helping increase Dow Corning’s financial results dramatically during this period.

Between 2002 and 2009, the XIAMETER® brand was a web-enabled business with clear business rules, offering commonly used standard silicones at transparent, market-driven prices. However, those business rules only allowed for large-volume orders. There was no technical service. Lead times ran 7-20 days. And only 400 standard silicone products were available through www.xiameter.com.

Expanding the model (2009-present)

In 2009, Dow Corning expanded the XIAMETER® business model – for the same reason we launched it in the first place – to meet customer needs based on smart customer segmentation analysis. Xiameter saw a need for all customers – not just large-volume buyers – to be able to purchase standard silicone products at market-based prices. Based on the success of the original Xiameter model, company leaders were confident expanding the model would work – both for customers and Dow Corning. And it did!

The XIAMETER® business model, which was expanded in June 2009 and expanded again in 2011, now offers thousands of standard silicone products. Plus, the model is no longer for large-volume customers only. There are more volume-quantity options. With the Xiameter transparent price tiers, customers can choose the pricing most appropriate for them based on the volumes they need. They can purchase multiple items within a product family and receive greater discounts. They can lock in price and volume commitments through an online supply agreement. They also can choose credit terms that work best for them, which can be varied each time they order. Plus, customers who need smaller quantities, shorter lead times or more flexible ordering options can have them – by purchasing XIAMETER® brand products through local distributors.

With today’s XIAMETER® brand, customers can get all the standard silicones they need in the way that works best for them. Whatever they want. Wherever and however they want it. In a world of traffic jams, on-hold music and long supermarket lines, we think it’s a great option.

Source: Xiameter.

For more information on this source, please visit Xiameter.

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