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WTI and OTI of Transformer

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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.

Details of Winding Temperature Indicator(WTI):
Winding temperature indicator(WTI) consists of a sensor bulb placed in the oil filled pocket in the transformer tank top cover. The bulb is connected to the instrument housing by means of two flexible capillary tubes. One capillary is connected to the measuring bellow of the instrument and the other to a compensation bellow.

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 instrument is provided with a maximum temperature indicator. The heating resistance is fed by a current transformer associated with the loaded winding of the transformer. ( The heating resistance is made out of the same materials as that of the winding ) The increase in the temperature of the resistance is proportionate to that of the 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. In the WTI, there are four nos. of the mercury switch. Two of them is used for Fan and motor pump control and another two nos. The switch is used for high-temperature warning alarm and trip circuit contact.

The switch is S1, S2, S3, S4. The setting of the switches is given below:-

Fan controlMotor pump control WTI alarm WTI Trip
Fan on : 640 C ,
Fan Off:580 C
Pump on:720 C ,
Pump Off :680 C
WTI alarm: 850 C WTI Trip: 950 C

Oil Temperature Indicator (OTI):

The (OTI)oil temperature indicator consists of a sensor bulb, capacity tube, and a dial thermometer, the sensor bulb is fitted at the location of hottest oil. The sensor bulb and capacity tube are fitted with an evaporation liquid. The vapour pressure varies with temperature and is transmitted to a bourdon tube inside the dial thermometer, which moves in accordance with the changes in pressure, which is proportional to the temperature. In OTI, there are 2 (two) nos. of mercury switch i.e ( S1 and S2 ). S1 is used for Alarm and the S2 switch is used for Trip.

The setting of the switches is given below:

OTI alarm OTI Trip
OTI alarm: 800 C OTI Trip: 900 C

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