Transformer oil, also referred to as insulating oil, stands as a specialised fluid renowned for its outstanding electrical insulation capabilities and remarkable stability under high temperatures. This unique oil finds its essential application within oil-filled electrical power transformers, serving several crucial functions. These encompass insulation provision, suppression of arcing and corona discharge, as well as effective heat dissipation to maintain optimal transformer performance by acting as a coolant.
Moreover, it serves as a protective agent for the transformer’s core and windings. These vital components are fully submerged within the oil, benefiting from its encompassing safeguard. Notably, the insulating oil boasts an additional noteworthy trait: its capacity to hinder the oxidation of paper insulation crafted from cellulose. This strategic barrier formed by the transformer oil prevents direct interaction between atmospheric oxygen and cellulose, thereby minimising the oxidation process.
In practice, the level of transformer oil is conventionally determined using a Magnetic Oil Level Gauge (MOG), aiding in the assessment of the oil’s quantity within the transformer. This concise overview highlights the critical role and multifaceted advantages of transformer oil in the realm of electrical transformers.
Know more about Different Parts of Transformers
Types of Transformer Oil
Two main types of transformer oil are commonly used: paraffin-based and naphtha-based. Paraffin oil has a lower oxidation rate than naphtha oil but tends to form insoluble sludge at the bottom of the tank, obstructing the transformer cooling system.
On the other hand, naphtha oil is more easily oxidised but produces soluble sludge, which does not hinder convection circulation. Despite the advantages of naphtha oil, paraffin oil is still widely used in many countries due to its availability.
Properties of Transformer Oil
To determine the serviceability of transformer oil, several properties should be considered: electrical, chemical, and physical.
Electrical Properties of Transformer Oil
2. Specific Resistance: Specific resistance measures the DC resistance of a unit volume of oil. It should be high at both room temperature and high operating temperatures. The values at 27°C and 90°C are critical, with minimum standards set at 1500 × 10^12 ohm-cm and 35 × 10^12 ohm-cm, respectively.
3. Dielectric Dissipation Factor (Tan Delta): Tan delta indicates imperfections in dielectric behaviour. High tan delta suggests the presence of contaminants, reducing resistive properties. Low tan delta is desirable for effective insulation
Chemical Properties of Transformer Oil
2. Acidity: Acidity accelerates oil oxidation, harms insulation, and encourages rusting. Acidity test measures acidic contaminants and is expressed as mg of KOH needed to neutralise acid per gram of oil.
Physical Properties of Transformer Oil
1. Interfacial Tension (IFT): IFT measures attractive forces between water and oil molecules. High IFT indicates good quality oil, while oxidation contaminants reduce IFT.
2. Flash Point: Flash point is the temperature at which oil vapours form a flammable mixture with air. A high flash point (>140°C) reduces fire hazards.
3. Pour Point: Pour point is the minimum temperature at which oil flows. It’s vital in cold climates to ensure proper oil circulation and cooling in transformers.
4. Viscosity: Viscosity is the resistance to flow. Low viscosity aids oil circulation and cooling. Maintaining viscosity stability across temperature changes is crucial.
In conclusion, the properties of transformer oil directly impact the efficiency, safety, and longevity of electrical transformers. Understanding and monitoring these properties ensure optimal transformer performance and reliability.
Transformer Oil Testing
Regular testing of transformer oil is essential to ensure its compliance with standards and identify any potential issues. Various tests, as specified by international standards such as American Society for Testing and Materials (ASTM), measure different properties of the oil. Some common tests include acid number, dielectric breakdown voltage, liquid power factor, interfacial tension, specific resistance, corrosive sulphur content, and visual examination. These tests help determine the suitability of the oil for future use, detect the need for regeneration or filtration, reduce costs, and prevent untimely failures.
Common Problems When Testing
During testing, certain problems can arise, indicating potential issues with the transformer. For example, corona discharge produces hydrogen, while arcing generates acetylene. Overheated cellulose produces carbon monoxide, and overheated oil generates methane and ethylene. The presence of these gases can indicate faults in the transformer and should be addressed promptly to avoid further damage.
Importance of Transformer Oil Testing
Transformers often use breathers to prevent moisture from entering the transformer and contaminating the oil. A breather is a cylindrical container filled with desiccant, usually silica gel, and is connected to the transformer tank. Its primary function is to maintain dry air within the transformer, preventing moisture from reacting with the insulating oil.
When air enters the breather, the desiccant absorbs the moisture, ensuring that the air inside the transformer remains dry. This helps preserve the insulation properties of the oil and prevents internal faults that can result from moisture contamination.
In Conclusion
Transformer oil is a critical component in electrical power transformers, providing insulation, arc suppression, corona discharge prevention, and heat dissipation. Its properties, including dielectric strength, specific resistance, water content, acidity, interfacial tension, flash point, pour point, and viscosity, affect the performance and longevity of transformers. Regular testing of transformer oil is vital to ensure its compliance with standards, identify potential issues, and optimise the lifespan of transformers. By investing in proper testing procedures, the costs associated with transformer replacement and downtime can be significantly reduced.
