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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.
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 may have a gas detector relay (Buchholz relay). These safety
devices detect the build up of gas inside the transformer due to corona discharge, overheating, or an
internal electric arc.
On a slow accumulation of gas, or rapid pressurerise, these devices can trip a protective circuit breaker to
remove power from 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).Large transformers for indoor use must either be of the
dry type, that is, containing no liquid, or use a less-flammable liquid.
Testing and oil quality
Transformer oils are subject to electrical and mechanical stresses while a transformer is in operation. In
addition there is contamination caused by chemical interactions with windings and other solid insulation,
catalyzed by high operating temperature. The original chemical properties of transformer oil change
gradually, rendering itineffective for its intended purpose after many years. Oil in large transformers
and electrical apparatus is periodically tested for its electrical and chemical properties, to make
sure it is suitablefor further use. Sometimes oil condition can be improved by filtration and treatment.
Tests can be divided into:
1.Dissolved gas analysis
4.General electrical & physical tests:
- Color & Appearance
- Breakdown Voltage
- Water Content
- Acidity (Neutralization Value)
- Dielectric Dissipation Factor
- Sediments & Sludge
- Interfacial Tension
- Flash Point
- Pour Point
- Kinematic Viscosity
The details of conducting these tests are available in standards released by IEC, ASTM, IS, BS, and testing
can be done by any of the methods. The Furan and DGA tests are specifically not for determining the
quality of transformer oil,but for determining any abnormalities in the internal windings of the transformer or
the paper insulation of the transformer, which cannot be otherwise detected without a complete overhaul of
Suggested intervals for these test are:
- General and physical tests - bi-yearly
- Dissolved gas analysis - yearly Furan testing - once every 2 years, subject to the transformer being in
- operation for min 5 years.