There are two types of temperature indicator OTI and WTI.
1)The Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI) work on either the liquid expansion or bimetallic principle and provide indications of top oil and average winding temperature via dial type indicator. In all eases the indicators should be mounted vertically for the most accurate reading
2)The thermometer hall probe in connected by capillary tube to the indicator
3) The buff probe should be fitted into the thermometer pocket provided in the transformer main tank cover near the hottest oil region. Before fitting the bulb probe, thermometer pockets should be partially filled with transformer WTI is the same as OT1 except for the current feed circuit from a CT to a heating element either on the thermometer pocket, or in the instrument itself
4) Normally the current feed to WTI is provided from a CT at one phase of the LV winding. Such CT has a suitable ratio to feed the required current to heat the heating element.
5) This additional temperature component from the heating element, together with the prevailing top oil temperature, gives the average winding temperature seen at WTL.
