Transformer oil or insulating oil is 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 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. These safety
devices detect the buildup of gas inside the transformer due to corona discharge, overheating,
or an internal electric arc. On a slow accumulation of gas, or rapid pressure rise, 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 and pour point are 140 °C
and −6 °C respectively. The dielectric strength of new untreated oil is 12 MV/m
and after treatment it should be >24 MV/m. Large transformers for indoor use must either
be of the dry type, that is, containing no liquid, or use a less-flammable liquid.
Recently, research is underway in making transformer oil nano fluids by mixing insulating or semiconducting
particles with transformer oil to enhance its thermal conductivity and electrical breakdown
strength. Mineral oil alternatives
Silicone-based or fluorinated hydrocarbons, where the added expense of a fire-resistant
offsets any additional costs of building a transformer vault, have also been presented
as a viable mineral oil alternative. However, silicone has been proven to be much less biodegradable
than esters in the event of a leak or spillage. Pentaerythritol tetra fatty acid natural and
synthetic esters have emerged as an increasingly common mineral oil alternative. They offset
all the main risks associated with mineral oil, such as high flammability, environmental
impact and poor moisture tolerance. Esters are also non-toxic to aquatic life, readily
biodegradable and provide a lower volatility and higher flash point.
Additionally, they have a high fire point of over 300°C and K-class fluids such as
these are often used in high-risk transformer applications, such as indoors or offshore.
They also have a lower pour point, greater moisture tolerance and improved function at
high temperatures.
Vegetable-based oils have also been suggested, but these are unsuitable for use in cold climates
or for voltages over 230kV. Some papers have also cited coconut oil as a potential substitute
for use in transformers. Polychlorinated biphenyls
Well into the 1970s, polychlorinated biphenyls were often used as a dielectric fluid since
they are not flammable. PCBs do not break down when released into the environment but
accumulate in the tissues of plants and animals, where they can have hormone-like effects.
When burned, PCBs can form highly toxic products, such as chlorinated dioxins and chlorinated
dibenzofurans. Starting in the early 1970s, production and new uses of PCBs have been
banned due to concerns about the accumulation of PCBs and toxicity of their byproducts.
In many countries significant programs are in place to reclaim and safely destroy PCB
contaminated equipment. Polychlorinated biphenyls were banned in 1979
in the US. Since PCB and transformer oil are miscible in all proportions, and since sometimes
the same equipment was used for either type of liquid, contamination of oil-filled transformers
is possible. Under present regulations, concentrations of PCBs exceeding 5 parts per million can
cause an oil to be classified as hazardous waste in California. Throughout the US, PCBs
are regulated under the Toxic Substances Control Act. As a consequence, field and laboratory
testing for PCB contamination is a common practice. Common brand names for PCB liquids
include "Askarel", "Inerteen", "Aroclor" and many others.
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 it ineffective 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 suitable for further use. Sometimes oil condition can be improved by
filtration and treatment. Tests can be divided into:
Dissolved gas analysis Furan analysis
PCB analysis General electrical & physical tests:
Color & Appearance Breakdown Voltage
Water Content Acidity
Dielectric Dissipation Factor Resistivity
Sediments & Sludge Flash Point
Pour Point Density
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 the transformer. 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.
On-site testing
Some transformer oil tests can be carried out in the field, using portable test apparatus.
Other tests, such as dissolved gas, normally require a sample to be sent to a laboratory.
Electronic on-line dissolved gas detectors can be connected to important or distressed
transformers to continually monitor gas generation trends.
To determine the insulating property of the dielectric oil, an oil sample is taken from
the device under test, and its breakdown voltage is measured on-site according the following
test sequence: In the vessel, two standard-compliant test
electrodes with a typical clearance of 2.5 mm are surrounded by the insulating oil.
During the test, a test voltage is applied to the electrodes. The test voltage is continuously
increased up to the breakdown voltage with a constant slew rate of e.g. 2 kV/s.
Breakdown occurs in an electric arc, leading to a collapse of the test voltage.
Immediately after ignition of the arc, the test voltage is switched off automatically.
Ultra fast switch off is crucial, as the energy that is brought into the oil and is burning
it during the breakdown, must be limited to keep the additional pollution by carbonisation
as low as possible. The root mean square value of the test voltage
is measured at the very instant of the breakdown and is reported as the breakdown voltage.
After the test is completed, the insulating oil is stirred automatically and the test
sequence is performed repeatedly. The resulting breakdown voltage is calculated
as mean value of the individual measurements. See also
Heat-transfer oil References
^ Kenneth R. Edwards, Transformers, American Technical Publishers Ltd., 1996 ISBN 0-8269-1603-1
pp.138-14 ^ "What's Your Transformer Got In The Tank?".
M&I Materials. ^ "Fluids Comparison". Midel.
^ "Coconut Oil As An Alternative To Transformer Oil". ERU Symposium. November 2001.
Less and nonflammable liquid-insulated transformers, approval standard class Number 3990, Factory
Mutual Research Corporation, 1997. McShane C.P. Relative properties of the new
combustion-resistant vegetable oil-based dielectric coolants for distribution and power transformers.
IEEE Trans. on Industry Applications, Vol.37, No.4, July/August 2001, pp. 1132–1139,
No. 0093-9994/01, 2001 IEEE. “The Environmental technology verification
program”, U.S. Environmental Protection Agency, Washington, DC, VS-R-02-02, June 2002.
[1] IEEE Guide for loading mineral-oil-immersed
transformers, IEEE Standard C57.91-1995, 1996. External links