Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers (wet 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 discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor (material), conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone e ...
and arcing, and to serve as a coolant.
Transformer oil is most often based on
mineral oil
Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils.
The name 'mineral oil' by itself is imprecise, ...
, but alternative formulations with different engineering or environmental properties are growing in popularity.
Function and properties
Transformer oil's primary functions are to
insulate and
cool
Cool commonly refers to:
* Cool, a moderately low temperature
* Cool (aesthetic), an aesthetic of attitude, behavior, and style
Cool or COOL may also refer to:
Economics
* Country of origin labelling
* mCOOL - US consumer legislation to enforc ...
a transformer. It must therefore have high
dielectric strength
In physics, the term dielectric strength has the following meanings:
*for a pure electrically insulating material, the maximum electric field that the material can withstand under ideal conditions without undergoing electrical breakdown and becom ...
,
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
, and
chemical stability
In chemistry, chemical stability is the thermodynamic stability of a chemical system.
Thermodynamic stability occurs when a system is in its lowest energy state, or in chemical equilibrium with its environment. This may be a dynamic equilibrium ...
, and must keep these properties when held at high temperatures for extended periods. Typical specifications are:
flash point
The flash point of a material is the "lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture". (EN 60079-10-1)
The fl ...
140 °C or greater,
pour point
The pour point of a liquid is the temperature below which the liquid loses its flow characteristics.
It is defined as the minimum temperature in which the oil has the ability to pour down from a beaker. In crude oil a high pour point is generally ...
−40 °C or lower, dielectric breakdown voltage 28 kV (RMS) or greater.
To improve cooling of large power transformers, the oil-filled tank may have external
radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics.
A radiator is always a ...
s through which the oil circulates by natural
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
. Power transformers with capacities of thousands of
kVA may also have
cooling fan
A fan is a powered machine used to create a flow of air. A fan consists of a rotating arrangement of vanes or blades, generally made of wood, plastic, or metal, which act on the air. The rotating assembly of blades and hub is known as an ''imp ...
s, oil pumps, and even oil-to-water
heat exchangers
A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...
.
[Kenneth R. Edwards, ''Transformers'', American Technical Publishers Ltd., 1996 pp.138-14]
Power transformers undergo prolonged drying processes, using electrical self-heating, the application of a
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
, or both to ensure that the transformer is completely free of
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
before the insulating oil is introduced. This helps prevent
corona
Corona (from the Latin for 'crown') most commonly refers to:
* Stellar corona, the outer atmosphere of the Sun or another star
* Corona (beer), a Mexican beer
* Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
formation and subsequent
electrical breakdown
Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating mate ...
under load.
Oil filled transformers with a conservator (oil reservoir) may have a gas detector relay (
Buchholz relay
In electric power distribution and transmission, a Buchholz relay is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a "conservator".
The Buchholz relay is use ...
). These safety devices detect the buildup of gas inside the transformer due to
corona discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor (material), conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone e ...
, overheating, or an internal
electric arc
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...
. On a slow accumulation of gas, or rapid pressure rise, these devices can trip a protective
circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
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.
Mineral oil alternatives
Mineral oils are still widely used in the industry.
Mineral oil is generally effective as a transformer oil, but it has some disadvantages, one of which is its relatively low flashpoint versus some alternatives. If a transformer leaks mineral oil, it can potentially start a fire.
Fire code
Fire safety is the set of practices intended to reduce the destruction caused by fire. Fire safety measures include those that are intended to prevent the ignition of an uncontrolled fire and those that are used to limit the development and eff ...
s often require that transformers inside buildings use a less flammable liquid, or the use of dry-type transformers with no liquid at all. Mineral oil is also an environmental contaminant, and its insulating properties are rapidly degraded by even small amounts of water. Transformers are well equipped to keep water outside the oil for this reason.
Pentaerythritol
Pentaerythritol is an organic compound with the formula C(CH2OH)4. Classified as a polyol, it is a white solid. Pentaerythritol is a building block for the synthesis and production of explosives, plastics, paints, appliances, cosmetics, and many o ...
tetra
fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...
natural and synthetic
esters
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides are ...
have emerged as an increasingly common mineral oil alternative, especially in high-fire-risk applications such as indoors due to their high
fire point
The fire point of a fuel is the lowest temperature at which the vapour of that fuel will continue to burn for at least five seconds after ignition by an open flame of standard dimension. At the flash point, a lower temperature, a substance will ig ...
, which can be over 300 °C.
They are readily
biodegradable
Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
.
Pentaerythritol
Pentaerythritol is an organic compound with the formula C(CH2OH)4. Classified as a polyol, it is a white solid. Pentaerythritol is a building block for the synthesis and production of explosives, plastics, paints, appliances, cosmetics, and many o ...
tetra
fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...
natural and synthetic
esters
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides are ...
are more expensive than mineral oil. Transformers require special design change to operate with
Pentaerythritol
Pentaerythritol is an organic compound with the formula C(CH2OH)4. Classified as a polyol, it is a white solid. Pentaerythritol is a building block for the synthesis and production of explosives, plastics, paints, appliances, cosmetics, and many o ...
tetra
fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...
natural and synthetic
esters
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides are ...
. Natural esters have very poor oxidation stability (typically only 48 hours in the same test vs 500h for Mineral oils and they produce acids) as a consequence, natural esters are only really a viable solution in hermetically sealed transformers in a distribution context. As transformers get larger than around 1 MVA and above 33kV it becomes more challenging to achieve a hermetically sealed design (due to thermal expansion and contraction). Mid-size and large power transformers will typically have a conservator and even if a rubber bag is employed the use of natural ester should be carefully considered because if there is oxygen ingress the natural ester will experience much faster oxidation than utilities are accustomed to with mineral oils.
Silicone
A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking ...
or
fluorocarbon
Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commerci ...
-based oils, which are even less flammable, are also used, but they are more expensive than esters, and less biodegradable.
Researchers are experimenting with vegetable-based formulations, using
coconut oil
frameless , right , alt = A cracked coconut and a bottle of coconut oil
Coconut oil (or coconut butter) is an edible oil derived from the wick, meat, and milk of the coconut palm fruit. Coconut oil is a white solid fat; in warmer climates duri ...
for instance. As yet these are unsuitable for use in cold climates or for voltages over 230 kV.
Researchers are also investigating
nanofluids for transformer use; these would be used as additives to improve the stability and thermal and electrical properties of the oil.
Polychlorinated biphenyls (PCBs)
Polychlorinated biphenyls are synthetic dielectrics first made over a century ago and found to have desirable properties that led to their widespread use.
Polychlorinated biphenyl
Polychlorinated biphenyls (PCBs) are highly carcinogenic chemical compounds, formerly used in industrial and consumer products, whose production was banned in the United States by the Toxic Substances Control Act in 1979 and internationally by t ...
s (PCBs) were formerly used as transformer oil, since they have high dielectric strength and are not flammable. Unfortunately, they are also
toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
,
bioaccumulative
Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost or eliminated ...
, not at all biodegradable, and difficult to dispose of safely. When burned, they form even more toxic products, such as
chlorinated dioxins and
chlorinated dibenzofurans.
Beginning in the 1970s, production and new uses of PCBs were banned in many countries, due to concerns about the accumulation of PCBs and toxicity of their byproducts. For instance, in the USA, production of PCBs was banned in 1979 under the
Toxic Substances Control Act
The Toxic Substances Control Act (TSCA) is a United States law, passed by the 94th United States Congress in 1976 and administered by the United States Environmental Protection Agency (EPA), that regulates chemicals not regulated by other U.S. ...
. In many countries significant programs are in place to reclaim and safely destroy PCB contaminated equipment. One method that can be used to reclaim PCB contaminated transformer oil is the application of a PCB removal system, also called a PCB dechlorination system.
PCB removal systems use an alkali dispersion to strip the chlorine atoms from the other molecules in a chemical reaction. This forms PCB-free transformer oil and a PCB-free sludge. The two can then be separated via a centrifuge. The sludge can be disposed as regular non-PCB industrial waste. The treated transformer oil is fully restored, meeting the required standards, without any detectable PCB content. It can, thus, be used as the insulating fluid in transformers again.
PCBs and mineral oil are miscible in all proportions, and sometimes the same equipment (drums, pumps, hoses, and so on) was used for either type of liquid, so PCB contamination of transformer oil continues to be a concern. For instance, under present regulations, concentrations of PCBs exceeding 5 parts per million can cause an oil to be classified as hazardous waste in California.
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
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
. 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 Dissolved gas analysis (DGA) is an examination of electrical transformer oil contaminants. Insulating materials within electrical equipment liberate gases as they slowly break down over time. The composition and distribution of these dissolved gas ...
#
Furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen atom. Chemical compounds containing such rings are also referred to as furans.
Furan is a colorless, flammable, highly ...
analysis
# PCB analysis
# General electrical & physical tests:
#* Color & Appearance
#* Breakdown Voltage
#* Water Content
#* Acidity (Neutralization Value)
#* 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
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically conductive.
For diodes, the breakdown voltage is the minimum reverse voltage that mak ...
is measured on-site according to 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
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
References
* Less and nonflammable liquid-insulated transformers, approval standard class Number 3990, Factory Mutual Research Corporation, 1997.
* McShane C.P. (2001) 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
* IEEE Guide for loading mineral-oil-immersed transformers, IEEE Standard C57.91-1995, 1996.
External links
{{Electric transformers
Oils
Electric transformers
Liquid dielectrics