Transformer Oil: Testing, Types & Properties

What is Transformer Oil?

Transformer oil (also known as insulating oil) has high electrical insulating strength and stays stable at high temperature. Oil-filled electrical power transformers use it to insulate, stop arcing and corona discharge and to carry heat away from the transformer (as a coolant).

Transformer oil also preserves the transformer’s core and windings, which sit fully immersed in the oil. Another useful property of the insulating oil is that it limits oxidation of the cellulose paper insulation. The oil sits as a barrier between atmospheric oxygen and the cellulose, so they do not meet directly and oxidation is slower. Oil level is typically read on a MOG (Magnetic Oil level Gauge).

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 oxidizes more quickly than paraffin oil. Its oxidation product, sludge, is more soluble and does not settle at the transformer bottom, so oil can still circulate through the transformer cooling system.

Paraffin oil oxidizes slower than naphtha oil, but its sludge is insoluble and collects at the tank bottom, which slows the transformer cooling system.

Dissolved waxes in paraffin-based oil also raise the pour point. A warmer climate (such as India) is not affected.

Despite those drawbacks, paraffin-based oil is still commonly used in many countries (such as India) because it is widely available.

Transformer Oil Properties

Some specific properties of insulating oil should be checked to decide whether the oil is still fit for service.

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

The dielectric strength of transformer oil is also known as the breakdown voltage (BDV) of transformer oil. Breakdown voltage is the voltage at which sparking starts between two electrodes immersed in the oil, separated by a set gap. A low BDV points to moisture and conducting substances in the oil.

For measuring BDV of transformer oil, a portable BDV kit is generally available at site. Oil is held in a pot with one pair of electrodes set 2.5 mm apart (in some kits 4 mm). A slowly rising voltage is then applied between the electrodes. The voltage is raised at 2 kV/s until sparking starts. The recorded voltage is the breakdown of the transformer oil between the electrodes.

This test is repeated 3 to 6 times on the same oil sample and the results are averaged. The BDV test is a common on-site check of oil condition.

Dry, clean oil gives a higher BDV than oil with moisture and other conducting impurities. The minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in a transformer is taken as 30 kV.

Specific Resistance of Transformer Oil

This is another useful 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. As temperature rises, the resistivity of oil falls rapidly.

Just after charging a transformer after a long shutdown, oil temperature is near ambient. On full load it is much higher, and it may reach 90^oC on overload. Resistivity of the insulating oil must therefore be high at room temperature and still useful at high temperature.
Specific resistance or resistivity of transformer oil should be measured at 27^oC as well as 90^oC.

The minimum standard specific resistance of transformer oil at 90^oC is 35 × 10¹² ohm–cm and at 27^oC 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 an insulating material sits between a live part and a grounded part of electrical equipment, leakage current flows. In an ideal dielectric that current would lead the voltage by 90^o. Voltage here means the instantaneous voltage between the live part and ground. In reality no insulating material is a perfect dielectric.

So current through the insulator leads the voltage by an angle a little shorter than 90^o. The tangent of the angle by which it falls short of 90^o is the dielectric dissipation factor, or tan delta of transformer oil. More plainly, leakage current through insulation has two components: one capacitive or reactive and another resistive or active. The diagram also shows ′δ′, the loss angle.

If the loss angle is small, the resistive component I_R is small, which means the insulating material has high resistance. High-resistance insulation is a good insulator. The loss angle should therefore be kept small, and tanδ should be kept small. A high tanδ points to contaminants in the transformer oil.

Tanδ and resistivity of insulating oil therefore move in opposite directions. If resistivity falls, tan-delta rises. The reverse also holds. A resistivity test and a tan delta test of transformer oil are generally not both required on the same piece of insulator or insulating oil.

In short, tanδ measures how far an insulating material such as oil falls short of a perfect dielectric.

Chemical Properties of Transformer Oil

Water Content in Transformer Oil

Moisture or water content in transformer oil is highly undesirable because it harms the dielectric properties of the oil. Water in oil also harms the paper insulation of the core and winding. Paper is highly hygroscopic. It takes up most of the water from the oil, which weakens the paper and shortens its life. In a loaded transformer the oil is hotter, so more water can dissolve in the oil.

As a result the paper releases water and raises the water content in transformer oil. The oil temperature at the moment a sample is taken is therefore critical. Oxidation also forms acids in the oil, and those acids raise the solubility of water in the oil. Acid plus water then decompose the oil further, forming more acid and water, so degradation speeds up. Water in oil is measured in ppm (parts per million).

IS–335(1993) treated water content up to 50 ppm as acceptable. Measuring such a low level accurately needs an instrument such as a coulometric Karl Fischer titrator. Later unused-oil specifications (IEC 60296) set a tighter moisture limit of about 30 to 40 mg/kg, so 50 ppm should be read as the 1993 IS figure, not as the current unused-oil IEC limit.

Acidity of Transformer Oil

Acidic transformer oil is harmful. If the oil is acidic, water dissolves in it more readily. Acidity also attacks the paper insulation of the winding and speeds oxidation of the oil. Acid plus moisture also rusts iron.

The acidity test of transformer oil measures the oil’s acidic contaminants, expressed in milligrams of KOH needed to neutralize the acid in one gram of oil, known as the neutralization number.

Physical Properties of Transformer Oil

Inter Facial Tension of Transformer Oil

Interfacial tension at the water and oil interface measures the attractive molecular force between water and oil, in dyne/cm or millinewton/metre. IFT is useful for spotting polar contaminants and oil decay products. Good new oil generally has high interfacial tension. Oxidation products lower the IFT.

Flash Point of Transformer Oil

Flash point of transformer oil is the temperature at which the oil gives enough vapour to form a flammable mixture with air. That mixture gives a momentary flash when a flame is applied under a standard test. Flash point matters because it indicates fire hazard in the transformer. A high flash point of transformer oil is therefore wanted. In general it is more than 140^o(>10^o).

Pour Point of Transformer Oil

It is the minimum temperature at which oil starts to flow under a standard test. Pour point of transformer oil matters mainly where the climate is icy. If the oil temperature falls below the pour point, transformer oil stops circulating by convection and cooling is blocked. Paraffin-based oil has a higher pour point than naphtha-based oil, but in a warm country such as India that rarely limits the use of paraffin oil. Pour point depends mainly on wax content. Paraffin-based oil has more wax, so it has a higher pour point.

Viscosity of Transformer Oil

In short, the viscosity of transformer oil is its resistance to flow under normal conditions. Resistance to flow obstructs convection inside the transformer. Good oil should have a low viscosity so it does not block that flow and thereby the cooling. Low viscosity of transformer oil is wanted, and viscosity should also rise as little as possible as temperature falls. Every liquid becomes more viscous if the temperature decreases.

Transformer Oil Testing

Transformer oil needs to be tested against current standards. Testing methods are set by several international standards, and many of them come from ASTM.

Oil testing measures breakdown voltage and other chemical and physical properties, either with portable test equipment or in a laboratory. Proper testing can extend transformer life and reduce the chance of an early replacement.

What Factors Are Tested

Here are the most common items to check in 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 show whether the oil is clean and they set a baseline to recheck later. Many further tests exist, but they are expensive. Use them as diagnostics if a primary test raises a concern.

How often to test depends on power and voltage. If the results raise concern, test more often. Even when testing is costly, compare that cost with the cost of replacing a transformer and the downtime if the unit is lost.

It is important to tell excessive gassing from normal gassing. Dissolved gas in transformer oil is found with a dissolved gas analysis (DGA) test. The gassing rate depends on 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:

Fault	Key Gas	Results
Corona discharge	Hydrogen	Low energy discharges create methane and hydrogen and smaller quantities of ethylene and ethane.
Arcing	Acetylene	Large amounts of hydrogen or acetylene or minor quantities of ethylene and methane can be produced.
Overheated Cellulose	Carbon Monoxide	If cellulose is overheated, then it will produce carbon monoxide
Overheated Oil	Methane and Ethylene	Overheating 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 used 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

Transformer oils can last up to 30 years with good care. The tests above help catch problems early so the oil and the unit last longer.