Under what conditions would you expect a fire on a transformer? Most of us are more concerned about an electrical fault, that might start a fire. Thus we would be concerned about detecting and removing an electrical fault.
Cranky 108, a transformer fire requires three conditions-High temperature, oxygen, and inflammable gas. If anyone misses, fire chances are low.
1) Any transformer internal fault creates an arc under oil. Arc energy released depends on arc voltage(based on service voltage), arc current, and duration. The temperature inside the core of the arc may reach above 3000 C. This intense heat vaporizes the oil and the gas volume generated is huge compared to liquid volume. This generates a dynamic pressure wave that may rupture the tank welds. The combustible gases coming out of the tank rupture will catch fire if it is not cooled down. If the fault is on the secondary side, the current will be low and the arc energy released is low and may not cause a damaging pressure wave. But the pressure rise may activate PRD and excess pressure will be safely released into the atmosphere. In the former case, PRD may not even operate as the tank will rupture even before the wave reaches the PRD.
2) When oil-filled bushings fail, upper porcelain shatters due to the gas volume generated, releasing hot carbon gases into the atmosphere which immediately catch fire.
"Best" can be defined so many different ways . . .
Cheapest?
Quick-acting to disconnect / remove faulted / burning elements from the power system so as to minimize power system disturbance?
Quick-acting to disconnect / remove faulted / burning elements from the power system so as to minimize equipment damage?
Quick-acting to disconnect / remove faulted / burning elements from the power system so as to minimize / eliminate hazards / injuries / fatalities to employees / the public?
Selectivity/ reliability, i.e. for outdoor equipment will not be fooled into erroneous operation for weather-related phenomena?
Tied directly into fire suppression systems that will promptly extinguish fires and capture spills so as to minimize environmental impact?
Note that a good number of the factors mentioned in the far-from-exhaustive list above will in many cases suggest conflicting solutions. It is therefore at this point that the input of numerous specialists and stakeholders as demanded by the situation is very valuable, viz., electrical, chemical and civil engineers, risk analysis experts, your insurer, the relevant AHJs, etc., etc.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]