transformer oil

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Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Explanation

The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures for an extended period. To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of thousands of KVA) may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.
Large, high voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.
Oil filled transformers with a conservator (an oil tank above the transformer) tend to be equipped with Buchholz relays. These are safety devices that detect the build up of gases (such as acetylene) inside the transformer (a side effect of corona or an electric arc in the windings) and switch off the transformer. Transformers without conservators are usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.
The flash point (min) and pour point (max) are 140 °C and −6 °C respectively. The dielectric strength of new untreated oil is 12 MV/m (RMS) and after treatment it should be >24 MV/m (RMS).

Transformer Oil Testing

Transformer oils are subject to electrical and mechanical stresses while a transformer is in operation. In addition there are contaminations caused due to chemical interactions with windings and other solid insulations, catalyzed by high operating temperature. As a result the original chemical properties of transformer oil changes gradually, rendering it ineffective for its intended purpose after many years. Hence this oil has to be periodically tested to ascertain its basic electrical properties, and make sure it is suitable for further use or necessary actions like filtration/regeneration has to be done. These tests can be divided into:

1. Dissolved gas analysis
2. Furan analysis
3. PCB analysis
4. General electrical & physical tests:

• Color & Appearance
• Breakdown Voltage
• Water Content
• Acidity (Neutralization Value)
• Dielectric Dissipation Factor
• Resistivity
• Sediments & Sludge
• Interfacial Tension
• Flash Point
• Pour Point
• Density
• Kinematic Viscosity

 Transformer oils (insulating oils) are dielectric oils used in oil-filled transformers, some high voltage capacitors and circuit breakers, fluorescent lamp ballasts.

Oil-filled transformers are more efficient, more reliable more compact, more quiet and with longer lifespan than dry transformers.

Transformer oils have the following functions:

• Prevention of electrical discharges between the transformer coils: corona and arc (insulating).
• Removal of heat generated by the transformer (cooling). Transformer oil forced by natural convection circulates through a radiator carrying the heat out of the transformer.
  

Transformer oils (insulating oils) have replaced toxic PCB (polychlorinated biphenyl).

Mineral oils, vegetable lubricants (soybean oil, vegetable based esters) or synthetic lubricants (polydimethylsiloxane) are used as basics for transformer oils.

Properties of some transformer oils

(Materials Data)

• Mineral transformer oil
• Silicon transformer oil
• Synthetic transformer oil