Transformer oil quality

 Transformer oil quality is defined by technical specifications including electrical properties like high dielectric strength (measured by breakdown voltage), low dielectric dissipation factor (tan delta), and moisture content, as well as physical properties such as low viscosity, viscosity at various temperatures, and high interfacial tension. Chemical properties require low acidity, absence of corrosive sulfur, and good oxidation stability to prevent sludge formation. The oil should be pure hydrocarbon mineral oil, free from contaminants and suitable for use as a coolant and insulator.

Electrical Properties

Breakdown Voltage (BDV):

A measure of the oil's ability to withstand electrical stress. Unused oil should have a minimum BDV of 30 kV (or higher), and this value should be greater than 70 kV after treatment. 

Dielectric Dissipation Factor (Tan Delta):

Indicates the dielectric losses within the oil. A low tan delta at 90°C (e.g., <0.005) is required. 

Water Content:

Must be very low (e.g., <20 or <30 ppm) to maintain dielectric strength and prevent acid and sludge formation. 

Physical Properties

Viscosity: Low viscosity is crucial for efficient circulation and heat dissipation within the transformer. Specific values are given for different temperatures (e.g., <12.0 mm²/s at 40°C). 

Pour Point: The lowest temperature at which the oil will flow. 

Interfacial Tension: Measures the purity of the oil and its ability to resist the formation of contaminants. A minimum of 0.04 N/m is often specified. 

Chemical Properties

Acidity: The oil should be neutral and free from acidic compounds, with a maximum acidity limit (e.g., 0.01 mg KOH/g). 

Corrosive Sulphur: Must be non-corrosive to prevent damage to transformer components. 

Oxidation Stability: The oil's resistance to chemical degradation over time, which is tested to ensure low levels of acidity and sludge formation after an oxidation period. 

Purity and Origin

Composition:

Transformer oil is a pure hydrocarbon mineral oil derived from selected crude oil fractions. 

Contaminants:

The oil must be clean, free from moisture, and other suspended foreign materials. 

Standards:

The specifications are typically in accordance with national (e.g., IS 335) and international standards (e.g., IEC, ASTM, ISO).