Why is transformer top oil temperature monitored during operation?

Top oil temperature is a key indicator of thermal stress and cooling system performance. If top oil rises, winding hot spots are likely higher, accelerating insulation aging and increasing gas generation in the oil. Monitoring supports control of fans and pumps, overload decisions, and early detection of blocked radiators, pump failures, or low oil conditions. Stable top oil temperature helps maintain dielectric margin, reduces risk of thermal runaway, and extends transformer life by keeping the paper oil insulation system within design limits.