As we know, Transformer oil often referred to as insulating oil, is a unique kind of oil recognized for its superb electrical insulation qualities and its stability under high temperatures. It plays a pivotal role in oil-filled electrical power transformers where it serves to insulate, suppress arcing and corona discharge, and helps manage the heat produced by the transformer, effectively functioning as a coolant.

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Certain specific attributes of insulating oil need to be taken into account to assess the oil’s serviceability. The properties (or parameters) of transformer oil include:

  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, also referred to as the breakdown voltage (BDV) of transformer oil, is determined by observing the voltage at which sparking occurs between two electrodes submerged in the oil and separated by a specific gap. A lower BDV value indicates the presence of moisture and conducting substances in the oil.

To measure the BDV of transformer oil, a portable measuring kit is commonly used on-site. The kit consists of a container where the oil is placed, and a pair of electrodes fixed with a gap of 2.5 mm (or 4 mm in some kits) between them. A gradually increasing voltage is applied between the electrodes at a controlled rate of 2 KV/s. The voltage at which sparking initiates between the electrodes indicates the breakdown of the dielectric strength of the oil in that region. This measurement is performed multiple times (3 to 6) on the same oil sample, and the average value of these readings is taken. BDV is an important and widely used test for transformer oil as it provides a primary indication of the oil’s condition and can be easily conducted on-site. Oil with low moisture content and minimal impurities demonstrates better BDV results compared to oil with higher moisture content or conducting impurities. The minimum acceptable breakdown voltage for transformer oil, or its dielectric strength, is generally considered to be 30 KV.

  • Specific Resistance of Transformer Oil

The specific resistance of transformer oil is another crucial property. It measures the direct current (DC) resistance between two opposite sides of a 1 cm3 block of oil and is expressed in ohm-cm at a specific temperature. The resistivity of oil decreases significantly with increasing temperature.

When a transformer is initially charged after a long shutdown, the oil temperature is at the ambient level. However, during full load operation, the temperature can rise considerably, potentially reaching 90°C under overload conditions. Therefore, the insulating oil should possess high resistivity at room temperature and maintain good performance at elevated temperatures. Consequently, the specific resistance or resistivity of transformer oil should be measured at both 27°C and 90°C. The minimum standard specific resistance for transformer oil is 35 × 1012 ohm-cm at 90°C and 1500 × 1012 ohm-cm at 27°C.

  • Dielectric Dissipation Factor of Tan Delta of Transformer Oil

The dielectric dissipation factor, also known as the loss factor or tan delta of transformer oil, describes the behavior of an insulating material placed between live and grounded parts of an electrical equipment. In such a setup, a leakage current flows through the insulation. Ideally, the current through the insulation should lead the voltage by 90°, considering that the insulation is dielectric in nature. However, real insulating materials are not perfect dielectrics.

Consequently, the current through the insulator leads the voltage by an angle slightly less than 90°. The tangent of the angle by which it falls short of 90° is referred to as the dielectric dissipation factor or tan delta of transformer oil. In simpler terms, the leakage current through the insulation has two components: capacitive or reactive, and resistive or active. The value of “δ,” known as the loss angle, represents this angle.

A smaller loss angle indicates a lower resistive component of the current (IR), which signifies a high resistive property of the insulating material. A high resistive insulation is desirable as it indicates good insulation. Therefore, it is preferable to have the loss angle (tan delta) as small as possible. A high value of tan delta suggests the presence of contaminants in the transformer oil. Thus, there is a clear relationship between tan delta and the resistivity of the insulating oil. When the resistivity of the insulating oil decreases, the tan delta value increases, and vice versa. Consequently, both resistivity and tan delta tests for transformer oil are generally not required for the same piece of insulator or insulating oil. In summary, tan delta serves as a measure of imperfections in the dielectric nature of insulation materials like oil.

Chemical Properties of Transformer Oil

  • Water Content in Transformer Oil

The presence of moisture or water in transformer oil has a negative impact on its dielectric properties. It also adversely affects the paper insulation used in the transformer’s core and winding. Paper is highly hygroscopic and absorbs water from the oil, which not only compromises its insulation properties but also reduces its lifespan. However, when a transformer is under load, the oil temperature increases, leading to higher solubility of water in the oil. Consequently, the paper releases water, increasing the water content in the transformer oil. Therefore, the temperature of the oil when taking a sample for testing is crucial. During the oxidation process, acids are formed in the oil, which further enhances the solubility of water. The combination of acids and water promotes the degradation of the oil at an accelerated rate. Water content in oil is usually measured in parts per million (ppm). The recommended limit for water content in oil, as per IS-335(1993), is up to 50 ppm. Accurately measuring water content at such low levels requires sophisticated instruments like a Coulometric Karl Fisher Titrator.

  • Acidity of Transformer Oil

Acidic transformer oil is a detrimental characteristic. When the oil becomes acidic, water content in the oil becomes more soluble. The acidity of the oil degrades the paper insulation used in the winding, accelerates the oxidation process, and contributes to the rusting of iron in the presence of moisture. The acidity test of transformer oil helps determine the level of acidic contaminants present. The acidity is expressed in milligrams of potassium hydroxide (KOH) required to neutralize the acid present in one gram of oil, also known as the neutralization number.

Physical Properties of Transformer Oil

  • Interfacial Tension of Transformer Oil

Interfacial tension refers to the measurement of the attractive molecular force between water and oil at their interface. It is expressed in units such as dyne/cm or milli-Newton/meter. Interfacial tension is a useful indicator for identifying the presence of polar contaminants and degradation products in the oil. High-quality new oil typically exhibits high interfacial tension, whereas oil oxidation contaminants lower the interfacial tension.

  • Flash Point of Transformer Oil

The flash point of transformer oil is the temperature at which the oil produces enough vapors to form a flammable mixture with air. When a flame is applied under standard conditions, this mixture momentarily ignites, indicating the flash point. The flash point is significant as it indicates the potential fire hazard associated with the transformer. Therefore, it is desirable for transformer oil to have a very high flash point, generally exceeding 140°C (>10°C).

  • Pour Point of Transformer Oil

The pour point of transformer oil is the minimum temperature at which the oil begins to flow under standard test conditions. The pour point is particularly important in regions with icy climates. If the oil temperature falls below the pour point, convection flow of the oil in the transformer is impeded, hindering effective cooling. Paraffin-based oil has a higher pour point compared to naphtha-based oil. However, in countries like India with warm climate conditions, the use of paraffin oil is not significantly affected. The pour point of transformer oil primarily depends on its wax content, with paraffin-based oil having a higher pour point due to higher wax content.

  • Viscosity of Transformer Oil

Viscosity refers to the resistance to flow under normal conditions. In the case of transformer oil, viscosity represents the hindrance to convection circulation within the transformer. Good-quality oil should have low viscosity to minimize resistance to oil flow and ensure efficient cooling. While low viscosity is essential, it is equally important that the viscosity of the oil increases as little as possible with decreasing temperature. Generally, liquids become more viscous as temperature decreases.

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