Posted in Recycle Silicone

Silicone Oil Usage for Transformers

1. Introduction

There is a variety of transformer oils types from which to choose, including air-cooled dry-type, cast resin and liquid-filled transformers. Liquid-filled transformers can contain mineral oil, chlorinated hydrocarbons or silicone fluid. Depending on the application needs, each type offers distinct advantages. But many also have drawbacks, The key is to decide which engineering and operating compromises are acceptable and estimate the long-term effects they will have on the specific application.

Liquid-filled transformers were developed more than 90 years ago. Today, many users continue to prefer this design over dry-type transformers, especially for demanding applications such as networks and medium and large power transformers. There are several important reasons for this preference as follows:

• Unlike solids, liquids cool as well as insulate. As a result one can select a liquid-filled transformer that is more compact that a comparable dry-type or cast-resin type.
• Liquid-filled transformers provide high efficiency and high BIL at reasonable cost. Similar electrical performance can be obtained from dry-type or cast-resin transformers, but usually only at additional cost.

The high dielectric strength of liquid-filled transformers provides greater design flexibility. As a result one can optimize the design to meet specific load requirements and thereby reduce operating costs.

Liquid systems also have an advantage stemming from their superior ability to remove heat from the core and coil assembly. This results in greater overload capacity and corresponding savings in maintenance and operating costs, as well as longer insulation life. This is especially true for silicone-filled transformers because silicones have the highest  thermal stability of all available liquids. Long-term costs resulting from energy loss in a unit can exceed the capital costs of purchasing the transformer. As a result, it is very important to evaluate the rate of loss and select the best design for the load and service conditions.

Silicone Transformer Fluid (oil) was introduced in the mid 70’s, and has become widely accepted for transformers where the location or environment presents a risk that demands a fire safe and/or environmentally friendly alternative to traditional transformer oils.

These fluids are also used in transformers designed to operate at temperatures above the typical 55/65°C rise transformers. The physical product, known generically as ‘silicone transformer fluid’, has not changed in its 25-year of history, although the specification of silicone fluid transformers and information regarding the handling has.

There are both IEC 836 and ASTM D 4652 specifications for silicone fluid, useful for transformer makers requiring fluid or when fluid is needed for repair or top-off of existing units. These standards cover both the physical and electrical properties of fluid suitable for dielectric applications. Although the properties of the Pure Silicone Fluid 50cSt  are very similar to the STO-50, the PSF-50cSt does not meet the required electrical properties for use in electrical transformers.

The ASTM standard also sets a maximum level of volatile material that can be lost within the first 24 hours at 150°C.

This test, ASTM D 4559, measures both the volatile content and its resistance to decomposition, important in high temperature applications. It also indicates a fluid’s long-term performance.
These fluids won’t pass this test if they are inadequately stripped during the manufacturing process or if the remaining polymerization catalyst is not neutralized or stripped.

Silicone fluid filled transformers may be specified generically as transformers designed for meeting the above specifications. In the case where the National Electrical Code must be met then the unit has to be built to Factory Mutual Approval or the UL or Underwriters Laboratory Classification requirements. Both of these standards now require physical changes to the transformer construction including a heavier tank and protection beyond normal ANSI standards.

If silicone is not specified and the broader ‘less flammable’ is requested,  the specifier may receive a unit with a high molecular weight hydrocarbon, ester or vegetable oil-based fluid. Whilst these two have fire points above 300°C they have substantially different fire properties if ignited.

2. Benefits of Silicone Transformer Oil over Petroleum-based oils*)

Silicone transformer oil provides major benefits when considering safety issues. These advantages include:

• High flash point and fire point – can be placed close to a building or installed indoors within National Code guidelines
• Self extinguishing – provides safest operating environment where fire potential is a concern
• Low rate of heat release, smoke evolution and toxicity – minimal damage from fire, as itself extinguishes and low heat evolution during a fire.
• Not petroleum based, non-bio accumulating and non-water soluble – thus may not be subject to petroleum  requirements for clean up.
• Non-hazardous material with excellent environmental product life cycle – regulatory friendly, does not biodegrade ‘especially in the transformer , contains no corrosive or acid causing materials.
• Same performance as PCB’s without environmental hazards attached
• Non toxic (cosmetic grade & food additive) base oil
• May be used in food processing plants, and near waterways
• Highly compatible with most other transformer fluids and construction materials
• Polydimethylsiloxane base is non-solvent and chemically inert
• Will not sludge or break down
• Longer transformer life, with reduced maintenance

Silicone transformer fluids allow for transformer systems that are fire safe, environmentally friendly and offer lower operating costs when compared to their counterparts.

Silicone-based transformer fluid is a synthetic transformer oil composed primarily of dimethylsiloxane polymers, and follows a very different series of production steps than does mineral oil-based transformer oil.

3. Raw Materials

While silicone-based fluid is produced both in the United States and abroad, the only publicly-available data is European. European data is used to model the main component of the product, cyclical siloxane.

4. Manufacture

The production of dimethylsiloxane starts with the production of dimethylchlorosilane using chloromethane and silicon.  Dimethylchlorosilane undergoes hydrolysis reactions to produce dimethylsilanediol, which undergoes another series of hydrolysis reactions to condense into cyclical siloxane.

The average density of the fluid is assumed to be 0.9565 kg/L.

5.Transportation**)

Trucking is the mode of transport used to represent transportation from the transformer oil production plant to Silicon production:

6. Use

The amount of oil used in a transformer depends on the size of the transformer.  A relatively small-sized (1000 kV•A) transformer is assumed, which requires about 1.89 m3(500 gal) of fluid to cool. It is assumed that the use phase of the transformer oil lasts the lifetime of the transformer, approximately 30 years. Included in the modeling is the electricity required to recondition the oil when dissolved gas analysis tests indicate the need.

Reconditioning is assumed to occur every five years.

7. End of Life***)

With periodic reconditioning of silicone-based transformer oil during the 30-year life of the transformer, the oil is in good enough condition for half of it to be further reconditioned and reused in another transformer. The other half is sent back to the manufacturer for restructuring for production into other silicone-based products.

End-of-life options for transformer oil do not include material disposal, as it is generally a well-maintained product and can be used in other applications.  Therefore, none of the product is assumed to be landfilled.
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*) www.clearcomproducts/PDMS
**) Dimethylsilanediol and cyclic siloxane production,Carette, Pouchol (RP Silicones), Techniques de l’ingénieur, vol. A 3475, p.3.
***)Life Cycle Data,National Renewable Energy Laboratory (NREL): U.S