Oil Immersed Transformers: Principles, Components, and Benefits

Do you have an idea that the power system would not be complete without the use of transformers? Actually! They assist optimally in the optimal transmission and distribution of power. One of these kinds of transformers is an oil-immersed transformer, which has an advantage over other types in that it can withstand more power and higher temperature settings.

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In the following article, we shall discuss the principles, components, types, advantages, and basic maintenance routines of oil-immersed transformers.

1) What is an Oil Immersed Transformer?

“As the name suggests, the transformer is placed in a steel tank filled with insulating oil with moves around to keep it cool via convection but at the same time keep insulating it”

Generally, transformers alter the voltage of alternating current (AC), either raising or lowering it. For transmission purposes over long distances, high voltages are preferred as they facilitate efficient and quick transmission. Lower voltages on the other hand allow safety in the residential, commercial, and industrial sectors.

Power distribution companies use oil-immersed transformers in medium and large ranges due to their durability, efficiency, and good cooling properties.

2) Components of Oil Immersed Transformer

To better understand the phenomenon of Oil Immersed Transformers, it is crucial to know each part of the transformer along its role. The following are the major components:

• Core: The core is commonly constructed out of laminated sheets of steel and it is constructed to be the path for the passage of the magnetic flux. It is mostly architectured in such forms as to lower energy losses.

• Windings: Two windings are available in transformers:

• Primary Winding: Has the function of receiving the input voltage.

• Secondary Winding: Connects with the transformed voltage.

Windings are composed of copper or aluminum and are protected from each other by some insulating organ.

• Insulating Oil: The typical insulating oil used is generally mineral oil which serves two purposes cooling and insulation of the interior. This oil is incombustible and it can tolerate high temperatures which makes it ideal for high-power operations.

• Conservator: The conservator is a tank that is situated at the top of the transformer to allow the oil to expand and contract with heat and cooling. It has an oil level and oil pressure drive to control the steady-state ratio within the transformer.

• Breather: Breather contains silica gel and when the transformer oil is heated and expands due to different reasons this gel assists in keeping the moisture out of the transformer hence no contamination is caused and oil life is boosted considerably.

• Radiators: Collecting all trapped heat inside the oil, the radiators are units installed outside of the transformer tank allowing for efficient heat dissipation.

• Bushings: Bushings are insulating fittings that provide connections between high- and low-voltage windings and the external circuit safely.

• Buchholz Relay: This safety device automatically senses gas production as a result of insulation failures and alarms the operators of possible trouble.

3) Working Principle of Oil Immersed Transformer

The Oil Immersed Transformers are used in power distribution areas. These transformers simply work on the principle of electromagnetic induction.

Step 1) Input voltage supplied: An alternating current (AC) passes through the primary winding which surrounds a laminated steel core of the transformer. It causes a magnetic field to be generated.

Step 2) Magnetic flux induction: The generated magnetic field expands and collapses and AC cycles which in turn produces a changing flux in the core. The change in magnetic flux induces a voltage in the secondary winding, which is placed in such a way that it intersects with the magnetic field.

Step 3) Voltage Transformation: The primary and secondary windings have turns in a relative ratio and this determines the amount of voltage that will either step up or step down.

Step 4) Cooling and Insulation: The whole transformer is placed in insulating oil, usually mineral oil, which also acts as a cooling agent. During normal operation of the transformer, oil is used to disperse heat that is generated to prevent any overheating and insulating the windings and the core to avoid electrical breakdown.

Step 5) Output voltage supplied: The secondary winding transfers the voltage to the external circuit. So, the process of voltage adjustment is completed here.

In this way, oil-immersed transformers provide effective and safe means of voltage transformation for the various stages of electricity distribution.

4) Types of Oil-Immersed Transformers

Oil-immersed transformers are classified based on their cooling methods and come in a variety of options:

• Oil Natural Air Natural (ONAN): In this type, the oil and the air surrounding it cool on their own with no fans or pumps needed. It is employed among smaller transformers.

• Oil Natural Air Forced (ONAF): Here, the oil does cool on its own, but the fans cool the radiator by pushing the surrounding air. This arrangement is used in medium-sized transformers.

• Oil Forced Air Forced (OFAF): In this arrangement, the oil and the air surrounding it are both circulated by pumps and fans. This setup is suitable for large-capacity transformers.

• Oil Forced Water Forced (OFWF): This type employs both water and oil in extremely large transformers as cooling mediums so that overheating does not occur when the transformers are under high load.

5) Benefits of Oil Immersed Transformer

The oil-immersed transformers have high regard due to their ability to work under extreme conditions and factors influencing them. Here’s a quick look at the upshots of these transformers that would make them popular with any power management system.

• Cost-Effective: They allow for cheap installation and maintenance and the need for change is minimal since the transformer is robust.

• Reliable and Durable: The transformers can sustain a variety of environmental conditions with low rates of failure which makes them dependable for long periods.

• High Efficiency: Less energy losses, enable oil immersed transformers to perform excellently under heavy loads.

• Efficient Cooling: Oil absorbs dissolves and radiates heat efficiently to avoid overheating of the transformer as well as consistent transformer operation at high loads.

• Enhanced Insulation and Safety: The oil acts as an insulating cover for the windings reducing the risk of electrical discharges and increasing the life span of the transformer.

• Versatile Applications: They can be used for power networks, renewable energy systems, and industrial applications since they come in different sizes and capacities.

• Safety Features: They contain protective devices such as the Buchholz relay which prevents damage by detecting acids. In addition, there is moisture protection which has a silica gel breather.

6) Preventive measures and maintenance of Oil Immersed Transformer

Oil–immersed transformers can operate at optimal levels and last longer due to the regular required maintenance and preventive actions. Some key measures include periodic oil analysis to identify foreign substances and oil conditions, monitoring of oil to avoid its deep frying, and ensuring that the cooling system (radiators and fans) operates optimally.

The breather must be serviced by replacing the silica gel so that moisture does not accumulate, and the transformer must be visually monitored for any oil seepage. Over-temperature conditions that may result in overheating need monitoring, while the Buchholz relay should be regularly checked so that internal fault traces can be found.

Regular examinations for abnormal sound or vibration, coupled with the monitoring of the surroundings, serve to foresee any likely problem in good time. To conclude, irrefutably, electrical examinations, for example, insulation resistance tests are essential for malfunction prevention. The implementation of these actions considerably enhances the dependability and lifetime of the transformer.

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7) Conclusion

For any business that needs uninterrupted electricity, oil-immersed transformers are essential. Their unique cooling, insulation, and design features enable them to efficiently cope with a number of functions, making them well-suited for modern power systems. Proper and regular maintenance work, especially preventive maintenance, increases their life span and keeps your operations free of interruptions.

Delixi manufactures state-of-the-art oil-immersed transformers that can withstand harsh conditions and perform at their peak. With Delixi, you get dependability and high technology suited for modern industrial power systems. Let Delixi bolster your power infrastructure.

Highly refined mineral 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 wet transformers,[1] some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. It functions to insulate, suppress corona discharge and arcing, and serves as a coolant.

Most often, transformer oil is based on mineral oil, but alternative formulations - with different engineering or environmental properties - are growing in popularity.

Function and properties

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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.[2] Typically, they have a flash point greater than 140 °C (284 °F), pour point less than −40 °C (−40 °F), and a dielectric breakdown at greater than 28 kVRMS.[3] To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Power transformers with capacities of thousands of kilovolt-ampere may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.[4]

Power 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 insulating 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 like a 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

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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 codes 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 tetra fatty acid synthetic and natural esters have emerged as an increasingly common mineral oil alternative, especially in high-fire-risk applications such as indoors due to their high fire point, which are over 300 °C (572 °F).[5] They are biodegradable, but are more expensive than mineral oil. Natural esters have lower oxidation stability in the 120C oxygen saturated test of approximately 48-hours compared to 500-hours for Mineral oils, and are therefore used in closed transformers.

Hermetic seals are important for larger transformers due to thermal expansion and contraction. Mid-size and large power transformers will typically have a conservator and employ a rubber bag with the use of natural ester to reduce oxygen ingress and prevent the natural ester from experiencing a faster oxidation than utilities are accustomed to with mineral oils. Silicone or fluorocarbon-based oils, which are even less flammable, are also used, but they are more expensive than esters.[citation needed]

There are over 3 million transformers in service with vegetable-based formulations, using soy or rapeseed based formulations in up to 500 kV transformers so far. However, coconut oil-based formulations are unsuitable for use in cold climates or for voltages over 230 kV.[7] 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.[8]

Polychlorinated biphenyls (PCBs)

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Polychlorinated biphenyls (PCB) are synthetic dielectrics first made over a century ago and found to have desirable properties that led to their widespread use.[9] Polychlorinated biphenyls 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 s, 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 under the Toxic Substances Control Act.[10] In many countries significant programs are in place to reclaim and safely destroy PCB contaminated equipment.[citation needed] 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.[11]

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.[12]

Testing and oil quality

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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.[13] 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 International Electrotechnical Commission, ASTM International, International standard, British Standards, 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

[edit] Main article: Transformer oil 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

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• Heat-transfer oil

References

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