AC MACHINES-1 (66761) Theory

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.

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

a.  Dielectric Strength of Transformer Oil

The dielectric strength of transformer oil is also known as the breakdown voltage (BDV) of transformer oil.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.

b. 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.

the temperature will be very high and may go up to 90^oC 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 27^oC as well as 90^oC.

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.

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.

he 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.

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.

lash 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 140^o(>10^o).

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.

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.

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.

Common Problems When Testing

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