Posted in Recycle Silicone

Applications of Silicone Transformer Oil

1. Introduction

The use of silicones in these applications is much related to cable end terminations or silicone rubber connections made at the end of underground high voltage cables insulated with polyethylene, as well as to silicone insulators for power lines.

Key benefits from silicones are their high electrical resistivity, resistance to environmental degradations and to electrical aging as well as their hydrophobicity, which results in lower assembly and maintenance costs

2. 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 60 site. Today cable accessories are completely built at the supplier. Typically they consist of rubber terminations made of different insulating silicone rubbers.

Silicones allows 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.

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.

3. 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 . 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 . Insulators made of silicone elastomer therefore need little cleaning or maintenance and perform over a long period of time.

When selecting a dielectric fluid, there are a number of criteria that must be contingent upon the application. One of the strongest advantages that a silicone fluid has over a fluid such as mineral oil is its much higher thermal stability, flash points and fire points. This is critical for fluids used in transformers that are located inside or near to a building where flammability is a significant concern. A silicone transformer oil can be subjected to very high temperatures, well above normal transformer operating temperatures, without creating excessive vapor pressure, breaking down or creating corrosive by-products.

Silicones are chemically inert, have good oxidation resistance and are compatible with conventional insulating materials at transformer operating temperatures.

There are two modes of degradation for silicone fluid : thermal breakdown and oxidation. whese temperatures the longer polymer chains will slowly begin to degrade in order to form more volatile cyclic silicone material.
Oxidation of silicone fluid takes place very slowly ( in the presence of oxygen ) at temperatures above 175°C ( 342°F ). When it oxides, silicone fluid polymerizes, gradually increasing in viscosity until gelation occurs. This process occurs without the formation of objectionable acids or sludges. Additionally, the dielectric properties of the longer-chain silicone molecule are similar to the dielectric properties of fresh silicone fluid.

The temperatures at which thermal and oxidative degradation take place are well in excess of the hot-spot temperatures expected in 65°C-rise transformers. In the limited-oxygen atmosphere of sealed transformers, silicone transformer fluid can be used at temperature rises that are above standard rises of other transformer fluids.

The silicone transformer fluid is not expected to degrade in any significant manner over the useful service life of a 65°C-rise transformer.

Insulation systems using silicone fluid in combination with solod insulating materials having high-temperature capabilities have shown significantly improved thermal capabilities and longer insulation life ( i.e. insulation system consisting of silicone fluid combined aromatic polyester amide/imide, Nomex paper and glass materials).

The Pure Silicone Fluids are 100% linear Polydimethylsiloxane Fluids (CAS# 63148-62-9) with viscosities measured @ 25C. They contain no additives such as pour-point depressants or heat-stabilizers. In addition, they contain no chlorine or other halogens. Pure Silicone Fluids with viscosity >5cSt are chemically inert, non volatile, thermally stable, have excellent oxidation resistance, and are compatible with conventional insulating materials. Material compatibility and thermal stability are closely related. A large number of materials have been tested for compatibility with silicone transformer oil. Below is a list of materials tested and found suitable in STO-50 Silicone Transformer Oil.**)

Transformer Materials that are compatible with Silicone Transformer Oil:

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*) Silicone in Medium to High Voltage Electrical Applications , E. Gerlach, Dow Corning GmbH, Wiesbaden (Germany),2002
**) www.clearcoproducts.com