Oil does three jobs at once inside a transformer: it insulates, it carries heat away from the windings and core, and it tells you — through laboratory analysis — what condition the transformer is in. Take the oil out and a transformer of the same rating would have to be considerably larger, or run considerably cooler.
Insulation: doing what air cannot
Clean, dry mineral oil has a breakdown voltage in the order of 70 kV across the standard test gap, several times that of air. Higher dielectric strength means the clearances between windings, between winding and core, and between winding and tank can be smaller for the same voltage class. That is why an oil-filled unit at 20 kV is compact compared with a dry-type unit of the same rating.
The oil works as a system together with cellulose insulation — pressboard barriers and paper wrapping on the conductors. The paper provides mechanical and dielectric strength; the oil fills every gap and pore in it. The system only performs as designed while the oil stays dry, because dissolved water sharply reduces breakdown voltage.
Cooling: moving heat to the tank wall
Losses in the core and windings appear as heat. Oil expands as it warms, becomes less dense and rises, then falls again once it has given up heat at the radiators. This thermosiphon circulation needs no pump, and the IEC cooling code calls it ONAN: oil natural, air natural.
Where natural circulation is not enough, fans are added (ONAF) or pumps (OFAF, ODAF). Directed oil flow, ODAF, guides the oil through the winding ducts rather than around them, which lowers the hot-spot temperature for the same average oil temperature. Hot spot is what governs insulation ageing: roughly every 6 K of extra hot-spot temperature halves the expected life of the paper.
Diagnosis: the oil as a condition report
Because the oil circulates through the entire active part, it collects the evidence of what is happening inside. Dissolved gas analysis identifies fault types from the gas mixture: hydrogen and methane point to partial discharge, ethylene to local overheating, acetylene to arcing. Furan content indicates how far the paper has degraded.
- Breakdown voltage to IEC 60156 — the practical measure of dielectric health.
- Water content — a few tens of ppm is normal in a service-aged unit; a rising trend needs attention.
- Acidity and interfacial tension — indicators of oxidation and general ageing.
- Dissolved gas analysis to IEC 60599 — fault detection and long-term trending.
Mineral oil is not the only option
Synthetic and natural esters have a far higher fire point, typically above 300 degrees Celsius against roughly 150 for mineral oil. That allows a K-class fluid classification and less demanding fire separation indoors. Esters are also readily biodegradable, which helps near water or in protected areas. They cost more, absorb more water and are more viscous, so the cooling design has to account for it.
The practical point is that oil is not a filler — it is an active component with a service life of its own. Kept dry and clean, it will comfortably outlast several decades of operation; ignored, it becomes the fastest route to failure. Sample it on a defined interval, trend the results rather than judging single readings, and treat a rising trend as a reason to investigate rather than an immediate reason to replace the oil.
