Transformer oil

[3] To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection.

Power transformers with capacities of thousands of kilovolt-ampere may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.

These safety devices detect the buildup of gas inside the transformer due to corona discharge, overheating, or an internal electric arc.

On a slow accumulation of gas, or rapid pressure rise, these devices can trip a protective circuit breaker to remove power from the transformer.

Mineral oil is also an environmental contaminant, and its insulating properties are rapidly degraded by even small amounts of water.

Pentaerythritol tetra fatty acid synthetic and natural esters have emerged as an increasingly common mineral oil alternative, especially in high-fire-risk applications such as indoors due to their high fire point, which are over 300 °C (572 °F).

Natural esters have lower oxidation stability in the 120C oxygen saturated test of approximately 48-hours compared to 500-hours for Mineral oils, and are therefore used in closed transformers.

[8] Polychlorinated biphenyls (PCB) are synthetic dielectrics first made over a century ago and found to have desirable properties that led to their widespread use.

For instance, under present regulations, concentrations of PCBs exceeding 5 parts per million can cause an oil to be classified as hazardous waste in California.

In addition there is contamination caused by chemical interactions with windings and other solid insulation, catalyzed by high operating temperature.

The original chemical properties of transformer oil change gradually, rendering it ineffective for its intended purpose after many years.