What is the function of a transformer oil winding temperature indicator?

A winding temperature indicator estimates or measures the hottest winding temperature, which is the key driver of insulation aging and overload limits. Many designs use top oil temperature plus a heater element driven by load current to simulate winding hot spot rise. The indicator provides alarms and trip contacts to protect the transformer under heavy load, cooling failures, or abnormal conditions. Trending winding temperature helps operators manage loading safely and avoid accelerated paper degradation.