Transformer Oil: Testing, Types & Properties

Transformer oil (also known as insulating oil) is a special type of oil which has excellent electrical insulating properties and is stable at high temperatures. Transformer oil is used in oil-filled electrical power transformers to insulate, stop arcing and corona discharge, and to dissipate the heat of the transformer (i.e. act as a coolant).

Transformer oil is also used to preserve the transformer’s core and windings – as these are fully immersed inside the oil. Another important property of the insulating oil is its ability to prevent oxidation of the cellulose-made paper insulation. The transformer oil acts as a barrier between the atmospheric oxygen and the cellulose – avoiding direct contact and hence minimizing oxidation. The level of transformer oil is typically measured using a MOG (Magnetic Oil level Guage).

Transformer Oil

Transformer Oil Types

There are two main types of transformer oil used in transformers:

  1. Paraffin based transformer oil
  2. Naphtha based transformer oil

Naphtha oil is more easily oxidized than paraffin oil. But the product of oxidation – i.e. sludge – in the naphtha oil is more soluble than the sludge from the paraffin oil. Thus sludge of naphtha-based oil is not precipitated in the bottom of the transformer. Hence it does not obstruct convection circulation of the oil, means it does not disturb the transformer cooling system.

Although Paraffin oil has a lower oxidation rate than Naphtha oil, the oxidation product (sludge) is insoluble and precipitated at the bottom of the tank. This sludge acts as an obstruction to the transformer cooling system.

Another problem with paraffin-based oil that the dissolved waxes inside of it can lead to a high pour point. Although this is not an issue in warmer climate conditions (such as India).

Despite the disadvantages mentioned above, paraffin-based oil is still commonly used in many countries (such as India) due to its high availability.

Transformer Oil Properties

Some specific properties of insulating oil should be considered to determine the serviceability of the oil.

The properties (or parameters) of transformer oil are:

  1. Electrical properties: Dielectric strength, specific resistance, dielectric dissipation factor.
  2. Chemical properties: Water content, acidity, sludge content.
  3. Physical properties: Interfacial tension, viscosity, flash point, pour point.

Electrical Properties of Transformer Oil

Dielectric Strength of Transformer Oil

test kit for measuring breakdown voltage of transformer oil

The dielectric strength of transformer oil is also known as the breakdown voltage (BDV) of transformer oil. Breakdown voltage is measured by observing at what voltage, sparking strands between two electrodes immersed in the oil, separated by a specific gap. A low value of BDV indicates presence of moisture content and conducting substances in the oil.

electrodes gap in test kit for measuring BDV of transformer oil

For measuring BDV of transformer oil, portable BDV measuring kit is generally available at site. In this kit, oil is kept in a pot in which one pair of electrodes are fixed with a gap of 2.5 mm (in some kit it 4mm) between them. Now slowly rising voltage is applied between the electrodes. The rate of rising voltage is controlled at 2 KV/s and observe the voltage at which sparking starts between the electrodes. That means at which voltage dielectric strength of transformer oil between the electrodes has been broken down.

This measurement is taken 3 to 6 times in the same sample of oil, and we take the average value of these readings. BDV is an important and popular test of transformer oil, as it is the primary indicator of the health of oil and it can be easily carried out at the site.

Dry and clean oil gives BDV results, better than the oil with moisture content and other conducting impurities. Minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in transformer, is considered as 30 KV.

Specific Resistance of Transformer Oil

This is another important property of transformer oil. The specific resistance of oil is a measure of DC resistance between two opposite sides of one cm3 block of oil. Its unit is ohm-cm at a specific temperature. With increase in temperature the resistivity of oil decreases rapidly.

Just after charging a transformer after long shut down, the temperature of the oil will be at ambient temperature and during full load, the temperature will be very high and may go up to 90oC at an overload condition. So resistivity of the insulating oil must be high at room temperature and also it should have good value at high temperature as well.
That is why specific resistance or resistivity of transformer oil should get measured at 27oC as well as 90oC.

Minimum standard specific resistance of transformer oil at 90oC is 35 × 1012 ohm–cm and at 27oC it is 1500 × 1012 ohm–cm.

Dielectric Dissipation Factor of Tan Delta of Transformer Oil

Dielectric dissipation factor is also known as loss factor or tan delta of transformer oil. When a insulating materials is placed between live part and grounded part of an electrical equipment, leakage current will flow. As an insulating material is dielectric in nature the current through the insulation ideally leads the voltage by 90o. Here voltage means the instantaneous voltage between live part and ground of the equipment. But in reality, no insulating materials are perfect dielectric in nature.

 tan delta of transformer oil

Hence current through the insulator will lead the voltage with an angle little bit shorter than 90o. Tangent of the angle by which it is short of 90o is called dielectric dissipation factor or simply tan delta of transformer oil. More plainly, the leakage current through insulation does have two component one capacitive or reactive, and another one is resistive or active. Again it is clear from the above diagram, the value of ′δ′ which is also known as loss angle.

If the loss angle is small, then the resistive component of the current IR is small which indicates a high resistive property of the insulating material. High resistive insulation is a good insulator. Hence it is desirable to have loss angle as small as possible. So we should try to keep the value of tanδ as small as possible. The high value of this tanδ is an indication of the presence of contaminants in transformer oil.

Hence there is a clear relationship between tanδ and resistivity of insulating oil. If the resistivity of the insulating oil gets decreased, the value of tan-delta increases and vice verse. So both resistivity test and tan delta test of transformer oil are generally not required for the same piece of the insulator or insulating oil.

In one sentence it can be said that tanδ is a measure of the imperfection of dielectric nature of insulation materials like oil.

Chemical Properties of Transformer Oil

Water Content in Transformer Oil

Moisture or water content in transformer oil is highly undesirable as it affects the dielectric properties of the oil adversely. The water content in oil also affects the paper insulation of the core and winding of a transformer. Paper is highly hygroscopic. Paper absorbs the maximum amount of water from oil which affects paper insulation property as well as reduced its life. But in a loaded transformer, oil becomes hotter, hence the solubility of water in oil increases.

As a result, the paper releases water and increase the water content in transformer oil. Thus the temperature of the oil at the time of taking a sample for the test is critical. During oxidation, acids get formed in the oil the acids give rise to the solubility of water in the oil. Acid coupled with water further decompose the oil forming more acid and water. This rate of degradation of oil increases. We measure the water content in oil as ppm (parts per million unit).

The water content in oil is allowed up to 50 ppm as recommended by IS–335(1993). The accurate measurement of water content at such low levels requires very sophisticated instrument like Coulometric Karl Fisher Titrator.

Acidity of Transformer Oil

Acidic transformer oil is a harmful property. If oil becomes acidic, the water content in the oil becomes more soluble in the oil. The acidity of oil deteriorates the insulation property of paper insulation of winding. Acidity accelerates the oxidation process in the oil. Acid also includes rusting of iron in the presence of moisture.

The acidity test of transformer oil can be used to measure the acidic constituents of contaminants. We express the acidity of oil in mg of KOH required to neutralize the acid present in a gram of oil. This is also known as neutralization number.

Physical Properties of Transformer Oil

Inter Facial Tension of Transformer Oil

Interfacial tension between the water and oil interface is the way to measure the attractive molecular force between water and oil. in Dyne/cm or milli-Newton/meter. Interfacial tension is exactly useful for determining the presence of polar contaminants and oil decay products. Good new oil generally exhibits high interfacial tension. Oil oxidation contaminants lower the IFT.

Flash Point of Transformer Oil

Flash point of transformer oil is the temperature at which oil gives enough vapors to produce a flammable mixture with air. This mixture gives momentary flash on the application of flame under standard condition. Flashpoint is important because it specifies the chances of fire hazard in the transformer. So it is desirable to have a very high flash point of transformer oil. In general it is more than 140o(>10o).

Pour Point of Transformer Oil

It is the minimum temperature at which oil starts to flow under standard test condition. Pour point of transformer oil is a valuable property mainly at the places where the climate is icy. If the oil temperature falls below the pour point, transformer oil stops convection flowing and obstruct cooling in a transformer. Paraffin-based oil has a higher value of pour point, compared to Naphtha based oil, but in India like country, it does not affect the use of Paraffin oil due to its warm climate condition. Pour Point of transformer oil mainly depends upon wax content in the oil. As Paraffin-based oil has more wax content, it has higher pour point.

Viscosity of Transformer Oil

In few words, the viscosity of transformer oil can be said that viscosity is the resistance of flow, in normal condition. Resistance to flow of transformer oil means obstruction of convection circulation of oil inside the transformer. Good oil should have a low viscosity so that it offers less resistance to the conventional flow of oil thereby not affecting the cooling of a transformer. Low viscosity of transformer oil is essential, but it is equally important that the viscosity of oil should increase as less as possible with a decrease in temperature. Every liquid becomes more viscous if the temperature decreases.

Transformer Oil Testing

Transformer oil needs to be tested to ensure that it works for today’s standards. Testing standards and procedures are defined by various international standards, and most of them are set by the ASTM.

Oil testing consists of measuring the breakdown voltage, and other chemical and physical properties of the oil, either through portable test equipment or in a laboratory. Through proper testing, the transformer’s lifespan is increased, reducing the need to pay for replacement.

What Factors Are Tested

Here are the most common things to look for when performing a transformer oil test:

  • Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus (ASTM D3487)
  • Acid number (ASTM D664)
  • Dielectric breakdown voltage (ASTM D877)
  • Liquid power factor (ASTM D924-08)
  • Interfacial tension (ASTM D971)
  • Specific resistance (ASTM D1169)
  • Corrosive sulfur (ASTM D1275)
  • Visual examination (ASTM D1524)

Note: ASTM stands for the American Society for Testing and Materials.

These tests will help determine if the oils are clean and will create a baseline of properties that need to be tested periodically. Although there are a large number of tests that are available, they are expensive. So it’s best to use them as diagnostics if an issue occurs during primary testing.

The recommended frequency is dependant on the power and the voltage. If the results from the test are showing some red flags, the frequency will have to increase. Even if the cost of testing is high, the expense should be compared to the cost of replacing a transformer and the downtime associated with losing the transformer.

It’s important to understand the difference between excessive and normal gassing rates. The amount of dissolved gas in transformer oil can be found using a dissolved gas analysis (DGA) test. The gassing rate will vary based on the loading, insulation material, and transformer design.

Common Problems When Testing

The table below shows the most common issues that can occur when testing transformer oil:

FaultKey GasResults
Corona dischargeHydrogenLow energy discharges create methane and hydrogen and smaller quantities of ethylene and ethane.
ArcingAcetyleneLarge amounts of hydrogen or acetylene or minor quantities of ethylene and methane can be produced.
Overheated CelluloseCarbon MonoxideIf cellulose is overheated, then it will produce carbon monoxide
Overheated OilMethane and EthyleneOverheating oil will produce methane and ethylene (300 degrees F) or methane and hydrogen (1,112 degrees F). Traces of acetylene might be created if the unit has electrical contacts or if the problem is severe.

Why is Transformer Oil Testing Important

Transformer oil testing is important to:

  • Determine essential electrical properties of transformer oil
  • Identify if a certain oil is suitable for future use
  • Detect whether regeneration or filtration is needed
  • Reduce oil costs and enhance component life
  • Prevent untimely failures and maximize safety

Keep in mind, and transformer oils can last for up to 30 years. So taking the proper testing procedures now will save you thousands of dollars in the long run.

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