I. Electrical Insulation Properties

· High Dielectric Strength

· Transformer oil must have a high dielectric breakdown voltage (typically ≥30 kV/2.5 mm in standard tests), preventing electrical breakdown between windings or to ground. Impurities (e.g., water, dust, gas bubbles) significantly reduce this strength.

· Low Electrical Conductivity

· The oil’s volume resistivity should be extremely high (e.g., ≥10¹² Ω·m at 20°C) to minimize leakage current and power loss. Moisture or ionic contaminants lower resistivity, increasing the risk of partial discharges.

II. Thermal Performance

· Effective Heat Dissipation

· Transformer oil has a high specific heat capacity (≈1.8 kJ/kg·K) and thermal conductivity (≈0.12 W/m·K), enabling efficient heat transfer from windings to cooling surfaces. Its low viscosity (e.g., ≤40 cSt at 40°C) ensures smooth circulation in oil-cooled systems.

· Thermal Stability

· It must resist thermal degradation at operating temperatures (typically ≤105°C for mineral oils). Prolonged exposure to heat causes oxidation, forming sludge and acidic compounds that degrade insulation and clog oil pathways.

III. Chemical Stability

· Oxidation Resistance

· Oxidation inhibitors (e.g., 2,6-di-tert-butyl-p-cresol, DBPC) are added to retard oil degradation. Oxidation byproducts (organic acids, peroxides) corrode metal parts and damage insulation materials.

· Compatibility with Materials

· The oil should not react with transformer components (copper, steel, insulation paper, seals). It must maintain physical properties without swelling or embrittling elastomeric seals.

IV. Physical Properties

· Viscosity

· Low viscosity at low temperatures (e.g., ≤1000 cSt at -40°C for cold climates) ensures pumpability, while high viscosity at high temperatures prevents excessive evaporation.

· Flash Point

· A high flash point (≥135°C for mineral oils) reduces fire hazards. Degradation lowers the flash point, indicating the need for oil replacement or treatment.

· Pour Point

· The pour point (e.g., ≤-20°C) ensures the oil remains fluid in cold environments, avoiding circulation failure in outdoor transformers.

V. Purity and Contamination Control

· Low Moisture Content

· Water content must be strictly controlled (≤30 ppm for new oil) to prevent insulation breakdown. Moisture also accelerates oil oxidation and freezing in cold climates.

· Freedom from Particulates

· Mechanical impurities (dust, metal shavings) are filtered out to avoid abrasion of pump components or short circuits. Filtration systems maintain particle counts below specified limits (e.g., ≤100 particles/100 mL for ISO cleanliness code 16/14/11).

· Gas Content

· Dissolved gases (O₂, N₂, H₂, CO) in new oil are minimized. Abnormal gas generation (e.g., CH₄, C₂H₂ from arcing) signals internal faults, detected via dissolved gas analysis (DGA).

VI. Environmental and Safety Aspects

· Biodegradability (for Synthetic Oils)

· Natural esters (biodegradable oils) have higher flash points (≥300°C) and lower environmental impact compared to mineral oils, suitable for transformers in ecologically sensitive areas.

· Non-Toxicity

· Mineral oils are non-toxic but persistent in the environment; synthetic alternatives like silicone oils offer better stability but higher cost.