Galasso Guidone

Transformers typically use highly refined mineral insulating oil, natural ester oil, or synthetic ester oil. The oil is selected for its high dielectric strength, good thermal conductivity, chemical stability, and compatibility with solid insulation. Mineral oil remains common due to cost and proven performance, while ester oils are chosen for fire safety and environmental benefits.

Acidity is measured by titrating an oil sample with a standardized alkaline solution and determining the neutralization number, expressed in mg KOH per gram of oil. The test indicates the amount of acidic oxidation byproducts present. Rising acidity signals oil aging and increased risk of sludge formation, corrosion, and dielectric degradation.

Key characteristics include high dielectric strength, low dielectric losses, high resistivity, low moisture content, good oxidation stability, suitable viscosity, and safe flash and fire points. The oil must remain chemically stable, avoid sludge formation, and stay compatible with cellulose insulation, metals, and sealing materials. It should also support heat transfer by maintaining predictable viscosity and thermal properties across operating temperatures. Utilities prioritize consistent quality, strong documentation, and proven performance under long term thermal and electrical stress.

Mineral transformer oil is typically a highly refined mixture of hydrocarbons, mainly paraffinic and naphthenic components, with very low sulfur and aromatics compared to raw petroleum fractions. Inhibited oils include an antioxidant additive package to slow oxidation and sludge formation. Ester fluids differ by being based on fatty acid esters rather than petroleum hydrocarbons. Exact composition varies by supplier, specification, and whether the oil is inhibited, passivated, or designed for low temperature performance.

The most common choice is mineral insulating oil, either inhibited or uninhibited depending on the utility’s specification and climate, because it offers a strong balance of dielectric strength, cost, availability, and cooling performance. For fire sensitive or environmentally sensitive sites, natural ester or synthetic ester fluids are increasingly used because they can provide higher fire point and improved biodegradability, though at higher cost and with different moisture behavior. The selected fluid must match transformer design, sealing system, and standards in the procurement specification, including compatibility with materials and gaskets.

Key properties include dielectric strength, low moisture content, low acidity, high resistivity, stable dielectric dissipation factor, suitable viscosity for cooling, adequate flash and fire points, and good oxidation stability. Interfacial tension is important because it indicates contamination and aging byproducts that can lead to sludge. Density and specific heat affect cooling behavior. Corrosive sulfur behavior matters for copper and paper compatibility. These properties collectively determine whether the oil can reliably insulate and cool the active parts while supporting long term insulation life and diagnostic monitoring.

Transformer oil provides electrical insulation by increasing dielectric strength between live parts and grounding surfaces. It suppresses partial discharge, fills insulation voids, and prevents arcing. Oil also protects solid insulation from oxygen and moisture, maintaining long term dielectric reliability.

A typical program includes dissolved gas analysis, moisture content, dielectric breakdown voltage, acidity or neutralization number, interfacial tension, resistivity, and dissipation factor. Particle count and inhibitor content may be added for detailed quality control. For asset health evaluation, furan analysis helps estimate cellulose insulation aging. Trending results over time is critical because rapid changes can indicate developing faults such as overheating, arcing, or oil oxidation. Field screening tests may be combined with periodic full laboratory testing.