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Common domestically produced transformer oils include DB-10, DB-25, and DB-45, categorized by their freezing points:
DB-10: Freezing point -10°C, suitable for southern regions.
DB-45: Freezing point -45°C, suitable for northern regions.
1.Insulation:
Transformer oil provides insulation superior to air, enabling effective insulation between windings, as well as between windings and grounded parts like the tank and core, which helps reduce the transformer’s size.
2.Heat Dissipation:
Through internal temperature differentials, transformer oil transfers heat from the core and windings to cooling devices.
3.Arc Suppression:
In on-load tap changers, transformer oil extinguishes arcs generated during tap changing.
The freezing point is the highest temperature at which oil ceases to flow when cooled under specified conditions and tilted at a 45° angle for 1 minute.
The pour point is the lowest temperature at which oil can flow after cooling under specified conditions. It is generally 2°C higher than the freezing point.
Lower viscosity indicates better fluidity, improving the oil's cooling performance.
The flash point is the minimum temperature at which the vapor of the insulating oil forms a flammable mixture with air under specified conditions. The flash point of transformer oil should not be less than 135°C. A severely degraded oil will exhibit a significantly reduced flash point.
The acid value indicates the aging level of the oil, measured by the milligrams of potassium hydroxide (KOH) required to neutralize free acids in 1 gram of oil.
In-use transformer oil: < 0.1 mg KOH/g
New transformer oil: < 0.03 mg KOH/g
Residual mineral acids, alkalis, or oxidation products in transformer oil can dissolve in water, accelerating oil aging and corroding metal and insulation materials.
Measured by breakdown voltage and dielectric strength:
1.Breakdown Voltage: The minimum voltage causing insulation breakdown under specified conditions. Contaminants, water, and impurities lower this value. New transformer oil should have a breakdown voltage of ≥35 kV (test gap: 2.5 mm).
2.Dielectric Strength: Maximum electrical field strength that the insulating medium can withstand without breakdown, expressed as kV/cm.
Interfacial tension measures the resistance of the oil surface to expansion, serving as an indicator of soluble impurities.
Impurities may originate from external contamination, carbon particles from arcs, or detached insulation fibers. These can block oil flow, impair cooling, or form conductive bridges that degrade insulation.
Water in transformer oil can corrode metal components, reduce insulation strength, and accelerate aging.
Contact with air causes chemical reactions that degrade insulating oil. Oxidation increases acid value, density, and viscosity, lowers insulation strength, and darkens the oil.
1.Dielectric Loss: Loss caused by polarization and conductivity under an alternating electric field.
2.Dielectric Loss Angle (δ): Indicates deviation from 90° phase angle due to dielectric loss.
3.Dielectric Loss Factor (tan δ): Represents dielectric loss magnitude. The greater the value, the higher the degree of aging. At 90°C, new oil should have tan δ ≤ 0.005.
Perform a mixing test before combining oils of different grades or new oil with in-use oil. Test samples should match the actual mixing ratio, and results must meet relevant standards.
1.Prefer using the same oil grade to maintain consistent operating characteristics.
2.Avoid mixing oils with different additives to prevent chemical reactions that may compromise performance.
3.Do not improve the quality of degraded oil by mixing it with new oil. Severely aged oil may precipitate residues and form sludge when mixed, further deteriorating the oil’s properties.
This guide ensures safe and efficient use of transformer oil, maintaining optimal transformer performance and longevity.
