In an oil-filled transformer, the windings and core are completely submerged in mineral oil. This oil is not just a coolant; it performs three vital functions, and its degradation is one of the earliest causes of transformer failures.
The Three Functions of Oil
- Electrical Insulation: Clean, dry oil is an excellent insulator that fills the gaps between the windings, core, and tank, preventing electrical discharge.
- Cooling: It transfers heat from the windings and core to the tank walls and radiators through natural or forced convection.
- Diagnostics: Oil comes into contact with everything inside the transformer, so fault byproducts (gases, moisture, contaminants) dissolve in it, making oil analysis a window into what is happening inside.
The Enemies of Oil
| Enemy | Effect | Detection Method |
|---|---|---|
| Moisture | Lowers the breakdown voltage and increases the likelihood of internal discharge | BDV test and water content measurement |
| Heat and oxidation | Forms acids and sludge that block cooling paths | Acidity and color inspection |
| Internal electrical faults | Decompose the oil and generate dissolved gases | DGA gas analysis |
A periodic oil sample costs very little and may save an entire transformer. Many catastrophic failures were preceded by clear warning signs in oil analyses that were ignored.
Sample answer: Oil serves two main functions: electrical insulation between live parts and between them and the tank, and cooling by transferring heat from the windings and core to the tank and radiators. An important diagnostic function is added: analyzing an oil sample (BDV, moisture, dissolved gas analysis DGA) reveals the transformer's internal condition without opening it.
Considering oil to be only a coolant and neglecting its insulating role. A drop in the oil's breakdown voltage due to moisture can cause internal discharge and a major failure even if the temperature is completely normal.
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