Beatrice Lang
Electrical transformers use oil, often mineral oil, for several reasons. The oil acts as a coolant, drawing heat away from the transformer's core and coils and conducting it outside, where it can be dissipated into the air. It also has excellent electrical insulating properties, suppressing corona discharge and arcing. Additionally, the oil protects the solid insulation (paper) inside the transformer. Oil-filled transformers are common in high-power applications, especially in mains distribution networks. However, they are not suitable for all environments due to the risk of fire and spills. As a result, alternative fluids with higher flashpoints and less flammability, such as natural and synthetic esters, are becoming more popular.
| Characteristics | Values |
|---|---|
| Purpose | Insulation and cooling |
| Composition | Petroleum base oil and additives like anti-oxidants and metal deactivators |
| Types | Mineral oil, vegetable-based oils, natural esters, silicone or fluorocarbon-based oils, transformer oil |
| Functions | Insulate, suppress corona discharge and arcing, serve as a coolant |
| Properties | High dielectric strength, thermal conductivity, chemical stability, flash point >140 °C, pour point <-40 °C, dielectric breakdown >28 kVRMS |
| Testing | Furan testing every 2 years, DGA testing, ASTM International, International Standard, British Standards |
| Maintenance | Replacing old oil, adding new oil, cleaning the transformer oil tank |
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What You'll Learn
Oil is a good insulator and prevents arcing
Oil is a key component of electrical transformers. It is used as an insulator and a coolant, with the ability to absorb and transmit heat, preventing transformers from overheating and suffering damage.
The insulation function of transformer oil is critical to preventing current from crossing a predetermined path. The oil fills the air gaps, improving the quality of the paper insulation. The oil's high dielectric strength ensures that it maintains its integrity when exposed to a strong electric field, preventing arcing and short circuits.
The type of oil used in transformers varies, with mineral oil being the most common. Mineral oil has a fire point of around 165 degrees Celsius, which does not qualify it as a suitable fluid for applications requiring less flammable options. Natural esters, with a higher flashpoint, are now often used in such cases. Vegetable-based oils, such as FR3® fluid, are also used due to their biodegradability, higher flashpoints, and ability to increase the lifespan of the transformer's insulation system.
The quality and condition of the oil are crucial to the smooth operation of the transformer. Regular testing and maintenance are required to ensure optimal performance and longevity. High acidity, for example, can decrease insulation performance and increase the risk of short circuits.
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It has high thermal conductivity and acts as a coolant
Transformer oil has a variety of functions, including insulation and acting as a coolant. The oil used in transformers is specially designed to have high thermal conductivity and high dielectric strength for cooling and insulation.
The core task of power transformers is to deal with high voltages. Thus, an insulation material is required to prevent current from crossing the predetermined path. Transformer oil, composed of petroleum base oil and additives, serves as an insulating material. It fills the air gaps and improves the quality of the paper insulation.
The resistance in a transformer converts electrical energy into thermal energy, generating a large amount of heat. Transformer oil acts as a coolant, absorbing and transmitting this heat to help maintain normal operating temperatures. It prevents the transformer from overheating and suffering damage due to current short circuits.
The high thermal conductivity of transformer oil is essential for its cooling function. The oil migrates towards less buoyant fluid spaces in the tank, dissipating heat from the core and coils. This natural convection process helps to cool the windings, which are the hottest spots in the transformer tank.
Maintaining the quality and condition of the transformer oil is crucial for the smooth operation of the transformer. Regular testing and maintenance, including monitoring moisture content, are necessary to ensure optimal performance and service life.
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Oil suppresses corona discharge
Electrical transformers use oil for several reasons, including insulation, cooling, and suppressing corona discharge and arcing. This text will focus on the latter, explaining how oil suppresses corona discharge.
Corona discharge is an electrical discharge caused by the ionization of a fluid, typically air, surrounding a conductor carrying a high voltage. It occurs when the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air, leading to a breakdown of the air and the formation of plasma. This can cause a reduction in power transfer capacity and, in critical situations, interrupt the electrical service.
Oil, specifically dielectric oil, can be used to suppress corona discharge. When corona discharge occurs, the dielectric oil is released, creating an isolated layer that protects the transmission line from the high electrical fields. This mechanism helps to reduce the impact of corona discharge, preventing power transfer capacity limitations and potential interruptions in electrical service.
The use of oil to suppress corona discharge is particularly important in high-power applications, such as mains distribution networks, where the voltage levels are high enough to cause corona discharge. In these cases, the oil-filled transformers are often used to provide effective corona discharge suppression.
It is worth noting that the quality of mineral insulating oils can deteriorate over time due to electrical, thermal, and chemical stress. This deterioration can impact the oil's ability to suppress corona discharge effectively. Therefore, regular testing and maintenance of the oil in transformers are necessary to ensure its continued effectiveness.
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It is stable at high temperatures
Transformer oil, also known as insulating oil, is stable at high temperatures and has excellent electrical insulating properties. It is commonly used in oil-filled wet transformers, high-voltage capacitors, and circuit breakers. Its stability at high temperatures is crucial for its primary functions of insulation and cooling.
The stability of transformer oil at high temperatures is essential to prevent the oil from breaking down and losing its insulating properties. A high dielectric strength allows the oil to maintain its integrity when exposed to strong electric fields. This is critical for the safe and effective operation of transformers, preventing current from deviating from its intended path and causing short circuits.
The oil's stability at high temperatures also contributes to its cooling function. Transformer oil acts as a coolant by absorbing and transmitting heat, helping to maintain the transformer's normal operating temperature. High thermal conductivity ensures that the oil effectively dissipates heat from the core and coils of the transformer, preventing overheating and potential damage.
The stability of transformer oil at high temperatures is further enhanced by the presence of additives, such as anti-oxidants and metal deactivators. These additives improve the oil's oxidation stability and reduce the buildup of sludge inside the transformer tank. Regular maintenance, including testing for moisture content, acidity, and flashpoint, is crucial to ensure the oil's stability and performance over time.
In summary, the stability of transformer oil at high temperatures is a critical aspect of its functionality. It enables the oil to maintain its insulating properties, prevent short circuits, and effectively cool the transformer by absorbing and dissipating heat. This stability is maintained through the use of additives and regular maintenance, ensuring the reliable operation of electrical transformers.
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Oil improves the quality of paper insulation
Electrical transformers use oil for cooling and insulation. The oil used in transformers is typically mineral oil, though other types of oil are growing in popularity. Transformer oil must be stable at high temperatures and have excellent electrical insulating properties.
The type of oil used in transformers is important. For example, vegetable-based oils are claimed to have benefits such as reduced fire risk, better moisture absorption, and being more environmentally friendly. However, they are usually only available as a "special option" due to their higher cost. Coconut oil-based formulations are unsuitable for voltages over 230 kV. Natural esters are used for applications requiring less flammable fluids, while silicone-based oils are even less flammable but more expensive.
The quality of the oil directly affects the performance and service life of the transformer. Regular testing and maintenance of transformer oil are important to ensure normal operation. The oil's colour, clarity, refractive index, acidity, moisture content, and flash point are all important parameters that are evaluated during testing. High acidity, for example, may lead to a decrease in insulation performance and an increased risk of short circuits.
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Frequently asked questions
Electrical transformers have oil because it is a good coolant, and it has excellent electrical insulating properties.
Transformer oil is often mineral oil, but alternative formulations are becoming more popular. Natural ester fluids, such as Cargill's FR3, are biodegradable, less flammable, and have a higher flashpoint than mineral oil.
Oil-filled transformers are often used in high-power applications. The oil helps to disperse heat from the core of a transformer, prolonging its life. It also protects the solid insulation, which is one of its most important functions.
