Introduction

Transformer oil, also known as insulating oil, is a critical component for the reliable operation of electrical equipment, particularly transformers. It serves as a coolant, insulator, and protector against oxidation, ensuring the efficient and safe functioning of these vital systems. For transformer oil suppliers and dealers, understanding the significance of this specialized fluid is paramount.

Transformer Oil: Definition and Functions

Transformer oil, also known as insulating oil or dielectric oil, is an oil that is stable at high temperatures and possesses excellent electrical insulating properties. It serves several critical functions in power transformers, high-voltage capacitors, fluorescent lamp ballasts, and certain types of high-voltage switches and circuit breakers.

What is Transformer Oil?

Transformer oil is an oil specifically designed for use in oil-filled wet transformers. It is characterized by its ability to withstand high temperatures and maintain its insulating properties even when subjected to extended periods of heat exposure.

Roles of Transformer Oil (Coolant and Insulator)

Transformer oil plays two primary roles within a transformer:

• Coolant: As a transformer operates, heat is generated due to the load and ambient temperature. The oil acts as a coolant, dissipating this heat from the core and windings, thereby maintaining optimal operating temperatures.
• Insulator: Transformer oil provides insulation between the transformer windings and other live parts, preventing electrical breakdown and ensuring safe operation. It has high dielectric strength, allowing it to withstand high voltages without breaking down, thus maintaining the integrity of the insulation system.

Additionally, transformer oil suppresses and extinguishes electrical arcs that may occur due to internal or external faults, protecting the transformer and associated equipment from damage. It is also compatible with the insulating materials used in transformers, such as paper insulation, helping to maintain the dielectric properties of the overall insulation system.

Types of Transformer Oil

There are two main types of transformer oil used in transformers: naphthenic oil and paraffinic oil.

Naphthenic Oil

Naphthenic oils are derived from naphthenic crude oil and have a chemical formula of CnH(2n+2). They are characterized by a high asphalt index and are practically free of paraffin (around 45%). Naphthenic oils have a low pour point, which means they maintain a constant flow at very low temperatures. However, producing “refrigeration grade” naphthenic oils requires complex physical and chemical treatments, such as dewaxing, bleaching, sulfur removal, and treatments with acids and solvents, making them more expensive.

Paraffinic Oil

Paraffinic oils are characterized by the CnH(2n+2) chain and contain a high proportion of dissolved paraffin (around 75%), making them relatively stable at high temperatures. Unlike naphthenic oils, paraffinic oils tend to solidify at high temperatures due to their high paraffin content. They typically contain saturated hydrocarbons and have a high aniline index and low solvent power.

Comparison of Naphthenic and Paraffinic Oils

• Oxidation Stability: Paraffinic oils generally have better oxidation stability compared to naphthenic oils, resulting in a longer service life.
• Sludge Formation: Naphthenic oils produce sludge that is more soluble and does not settle at the bottom of the transformer, ensuring unobstructed oil circulation and effective cooling. Paraffinic oils produce insoluble sludge that can accumulate at the bottom, hindering the cooling system.
• Pour Point: Naphthenic oils have a lower pour point due to their lower wax content, while paraffinic oils have a higher pour point due to their higher wax content.

Properties of Transformer Oil

Transformer oil possesses several crucial properties that make it suitable for its intended applications. These properties can be categorized into three main groups:

Electrical Properties (Dielectric Strength, Specific Resistance, Dissipation Factor)

• Dielectric Strength: Also known as breakdown voltage (BDV), it measures the voltage at which sparking occurs between two electrodes immersed in the oil, separated by a specific gap. A low BDV indicates the presence of moisture and conducting substances. The minimum acceptable BDV is typically 30 KV.
• Specific Resistance: It is a measure of the DC resistance between two opposite sides of a 1 cm³ block of oil, expressed in ohm-cm. The resistivity decreases with increasing temperature. At 90°C, the minimum standard is 35 × 10^12 ohm-cm, and at 27°C, it is 1500 × 10^12 ohm-cm.
• Dissipation Factor (Tan Delta): Also known as the loss factor, it represents the tangent of the angle by which the current through the insulator leads the voltage by less than 90°, as no insulator is a perfect dielectric.

Chemical Properties (Acidity, Water Content)

• Water Content: Moisture in transformer oil adversely affects its dielectric properties and the paper insulation of the transformer’s core and windings. The recommended water content limit is typically 50 ppm.
• Acidity: Acidic oil is harmful as it increases the solubility of water, deteriorates paper insulation, accelerates oxidation, and promotes rusting in the presence of moisture. Acidity is measured by the amount of KOH required to neutralize the acid present per gram of oil.

Physical Properties (Viscosity, Flash Point, Pour Point, Interfacial Tension)

• Viscosity: Low viscosity is desirable as it offers less resistance to oil flow, ensuring effective cooling of the transformer.
• Flash Point: The temperature at which the oil produces enough vapour to form a flammable mixture with air. A higher flash point, typically above 140°C, reduces fire hazards.
• Pour Point: The minimum temperature at which the oil starts to flow under standard conditions. A low pour point is essential in cold climates to maintain convection cooling.
• Interfacial Tension: Measures the attractive molecular force between water and oil interfaces, indicating the presence of polar contaminants and oil decay products.

Transformer Oil Testing

Transformer oil testing is a crucial aspect of preventive maintenance for power transformers and other electrical equipment. Regular testing helps identify potential issues early, extend the equipment’s lifespan, and prevent costly failures and unplanned outages.

Importance of Oil Testing

Transformer oil serves as both an insulator and a coolant, playing a vital role in the safe and efficient operation of transformers. Over time, the oil degrades due to electrical and thermal stresses, leading to the formation of contaminants and byproducts. Regular oil testing allows for early detection of these issues, enabling timely maintenance and preventing further deterioration.

Testing Procedures and Standards

Several international standards, such as ASTM and IEC, provide guidelines for transformer oil testing procedures and acceptable limits for various parameters. These standards ensure consistency and reliability in testing methods and results interpretation.

Common Tests (BDV, DGA, Moisture, Acidity, etc.)

• Breakdown Voltage (BDV) Test: This test measures the oil’s ability to withstand electrical stress without failure, indicating the presence of contaminants like moisture and conducting particles.
• Dissolved Gas Analysis (DGA): DGA detects and analyzes the gases dissolved in the oil, providing insights into potential faults, such as partial discharge, overheating, and arcing.
• Moisture Content Test: Excessive moisture in the oil can reduce its insulating properties and accelerate the degradation of paper insulation, making moisture content testing crucial.
• Acidity Test: Increased acidity in the oil can promote corrosion and sludge formation, indicating the need for oil regeneration or replacement.
• Interfacial Tension (IFT) Test: This test measures the tension between the oil and water interfaces, detecting the presence of polar contaminants and oil decay products.
• Dielectric Dissipation Factor (Tan Delta) Test: This test measures the leakage current through the oil, indicating the presence of contaminants or deterioration products.

Regular transformer oil testing, combined with trend analysis, enables proactive maintenance, maximizes equipment lifespan, and enhances operational safety and reliability.

Conclusion

Transformer oil plays a pivotal role in ensuring the reliable and efficient operation of critical electrical equipment. By serving as an insulator, coolant, and protector against oxidation, it safeguards these systems from potential failures and breakdowns. Understanding the different types, properties, and testing methodologies of transformer oil is essential for transformer oil suppliers, dealers, and maintenance professionals.

Maintaining the quality of transformer oil through regular testing and monitoring is crucial for maximizing equipment lifespan, ensuring operational safety, and preventing costly downtimes. By prioritizing transformer oil maintenance and implementing industry-standard testing procedures, organizations can proactively address potential issues and optimize the performance of their electrical systems.