Transformer oil

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, 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.

Transformer oil is most often based on mineral oil, but alternative formulations with better engineering and/or environmental properties are growing in popularity.

Function and properties

Oil transformer with air convection cooled heat exchangers in the front and at the side

Transformer oil's primary functions are to insulate and cool a transformer. It must therefore have high dielectric strength, thermal conductivity, and chemical stability, and must keep these properties when held at high temperatures for extended periods. Typical specifications are: flash point 140 °C or greater, pour point −6 °C or lower, dielectric strength 24 MV/m (RMS) or greater.

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.^[1]

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 (oil reservoir) may have a gas detector relay (Buchholz 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.

Mineral oil alternatives

Mineral oil is generally effective as a transformer oil, but it has some serious disadvantages, of which the worst is its high flammability. If a transformer leaks mineral oil, it can easily start a fire. Fire codes often require that transformers inside buildings use a less flammable liquid, or no liquid at all (dry-type). Mineral oil is also an environmental contaminant, and has poor moisture tolerance.

Pentaerythritol tetra fatty acid natural and synthetic esters have emerged as an increasingly common mineral oil alternative, especially in high-fire-risk applications such as indoors or offshore, due to their low volatility and high fire point, which can be over 300 °C. ^[2] They also have a lower pour point, greater moisture tolerance, and improved function at high temperatures, and they are non-toxic and readily biodegradable. Silicone or fluorocarbon-based oils, which are even less flammable, are also used, but they are more expensive than esters, and less biodegradable.^[3]

A 380kV transformer with vegetable oil ^[4]

Researchers are experimenting with vegetable-based formulations, using coconut oil for instance. As yet these are unsuitable for use in cold climates or for voltages over 230 kV.^[5] 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 (PCBs) were formerly used as transformer oil, since they have high dielectric strength and are not flammable. Unfortunately, they are also toxic, bioaccumulative, 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. In many countries significant programs are in place to reclaim and safely destroy PCB contaminated equipment.

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.^[6]

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:

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
   • 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 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

References