The WTI means winding temperature Indicator and OTI means Oil Temperature Indicator which indicates the winding temperature & oil temperature of the transformer and operates the alarm, trip, and cooler control contacts.This instrument operates on the principle of thermal imaging and it is not an actual measurement.
The measuring system is filled with a liquid, which changes its volume with rising temperature. Inside the instrument is fitted with a heating resistance which is fed by a current proportionate to the current flowing through the transformer winding.
The sensor bulb of the instrument is located in the hottest oil of the transformer, therefore, the winding temperature indicates(WTI) a temperature of hottest oil plus the winding temperature above hot oil i.e the hot spot temperature.
