By making a "Thermal Image" of the winding the Winding Temperature Indicator, simulates the winding temperature. The temperature of the winding depends on the transformer load (i.e. the current through the winding) and the temperature of the cooling medium (the oil). These two parameters are measured and made to interact in the instrument.
The oil temperature is measured as usual with a bulb in a pocket. The measuring system also has a specially designed heating element, to measure the transformer load. This heating element is a thermal model of the winding. The heating element is connected to the current transformer (CT) via a Matching Resistance or a Matching Unit, to allow setting the correct winding temperature gradient.
The winding temperature indicator shown in figure (47) is consists of four contacts which are normally open nd closes in series according to pre set closing value(temperature). These contacts can be assigned as follows:-
- The first contact is used for automatic operation of first fan group.
- The second contact is used for automatic operation of second fan group, this value is almost higher than the first contact setting.
- If the cooling fans are not sufficient to retain the btransformer temperature to its normal value, the third contact is applied to feed alarm circuit.
- As a last step, the fourth contact is applied for tripping to prevent the transformer from high temperatures.
These values can be chosen as (55,65,80,90) respectively.