The purity of transformer oil is crucial for its performance in electrical insulation and cooling. It is typically assessed based on several key parameters:
1. Chemical Purity
Free from contaminants like sulfur, acids, and oxidation by-products.
Low moisture content (typically < 10–20 ppm for new oil).
Low dissolved gases (e.g., hydrogen, methane, acetylene, indicating degradation).
2. Dielectric Strength (Breakdown Voltage)
Measures electrical insulation capability.
New oil: ≥ 30–60 kV (as per ASTM D877 or IEC 60156).
Used oil: Degrades over time; values below 25 kV may indicate contamination.
3. Interfacial Tension (IFT)
Indicates presence of polar contaminants (oxidation products, sludge).
New oil: > 40–50 mN/m (ASTM D971).
Degraded oil: < 25 mN/m suggests impurities.
4. Acid Number (Neutralization Value)
Measures acidic contaminants.
New oil: < 0.01–0.03 mg KOH/g.
Used oil: > 0.1 mg KOH/g indicates oxidation.
5. Dissipation Factor (Tan Delta)
Measures dielectric losses due to impurities.
New oil: < 0.005 (at 90°C, per IEC 60247).
Degraded oil: Higher values indicate contamination.
6. Particle Count
Solid impurities (dust, fibers) should be minimized (NAS class 6 or better for new oil).
Standards for Purity
IEC 60296: Specifies requirements for unused mineral insulating oils.
ASTM D3487: Standard for mineral insulating oil in electrical equipment.
Maintenance of Purity
Filtration & Drying: Removes moisture and particles.
Reclamation/Reconditioning: Restores used oil by removing acids and sludge.
Regular Testing: Ensures ongoing compliance with standards.
Would you like details on a specific test method or standard?