Friday, July 20, 2007

Biodegradable Insulating Liquid

Another high voltage insulating liquid is Envirotemp® FR3® manufactured by Cooper Power Systems. It is a transformer fluid made from soybeans, and is biodegradable. It does not contain any petroleum, halogens or silicones. It is fire resistant, and is currently used in both indoor and outdoor transformers. Its dielectric strength is rated at 56kV for a 2.0mm gap.

Environmentally friendly insulators such as this could be good for products that are WEEE compliant. (Waste Electrical and Electronic Equipment directive 2002/96/EC.)

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Tuesday, July 17, 2007

High Voltage Insulators: Solids

As you no doubt expected, here is the final installment in this brief introduction to high voltage insulators, solids, of course. However, this is limited to common encapsulating solids.

For best results, insulating materials should be homogeneous, and should not contain air spaces or moisture. This is especially critical with solids, as corona and arcing can occur in air voids.

Epoxy is best when you need excellent adhesion. It has good thermal conductivity for a solid.

Silicones come in two basic types. The curing process of additive cure silicones can be accelerated with the addition of heat, so that it can complete in an hour. Condensation cure silicones can take 24 hours or more to cure, however, the adhesion properties are better. For both types, adhesion can be an issue.

Other potting materials include tar (low cost) and polyurethane.

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Monday, July 16, 2007

HIgh Voltage Insulators: Liquids

Oil is commonly used in high voltage transformers and high power systems due to its excellent thermal conductivity, which can be improved further by circulation. Handling oil can be messy, and there can be environmental regulations to deal with if large quantities are used. One common type is manufactured by Shell.

Silicone oil is another good liquid insulator. In addition, silicone heat transfer fluids such as Dow Corning Syltherm™ can also be used as high voltage insulators.

Perfluorinated liquids such as Fluorinert™ are clean and easy to use. They are great insulators and can be used for testing high voltage circuits before they are potted. However, they are expensive, some types evaporate quickly.

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Sunday, July 15, 2007

High Voltage Insulators: Gases

Air is the most common insulator for high voltage. Cost is certainly a big advantage. It can contain a variable amount of moisture. I would consider it to be “dynamic” in that there is movement such as convection currents, and can sometimes contain unwanted particles or ions. Thus, its dielectric strength varies with time.

Sulphur Hexafluoride (SF6) is a great gaseous insulator, however, it is extremely hazardous, so it must be handled carefully, and used in a sealed chamber. One hazard is that poisonous gases are generated if there is an arc. Second, it is denser than air. Thus, if it is breathed in, it will settle in the bottom of the lungs, and displace the air, causing suffocation.

A good compromise may be nitrogen. It is not as hazardous as SF6, does not contain moisture, and is homogeneous.

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