The winding is the component with the highest temperature within the transformer and it is the component which is subjected to the fastest temperature increases as the load increases. Therefore for the control of the temperature parameter within the transformer, the temperature of the winding, as well as top oil, is measured. The temperature of winding of Transformer is measured using Winding Temperature Indicator (WTI) and the temperature of Transformer Oil is measured using Oil Temperature Indicator.
As can be seen from the above figure, Black needle shows the current winding temperature while the Red needle shows the highest winding temperature reached for a particular day. This Red needle needs to be reset manually.
Also, for setting winding temperature for Alarm and Trip, two knobs are provided. The Green knob shows the setting of winding temperature for Alarm and Red knob shows the Trip temperature setting as shown in figure.
Construction Detail of Winding Temperature Indicator, WTI:
Winding temperature indicator consists of a sensor bulb placed in an oil filled pocket in the transformer tank 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 compensating below.
Working Principle Winding Temperature Indicator:
An indirect system is used to measure winding temperature, since it is dangerous to place a sensor close to the winding due to the high voltage. The indirect measurement is done by means of a Thermal Image.
The measuring system is filled with a liquid which changes its volume with rising of temperature. Inside the instrument is fitted a heating resistance which is fed by a current proportionate to the current flowing through the transformer winding. To do this we connect the terminal of the heating resistance with the Bushing Current Transformer so that reflection of change in load is reflected in the Winding Temperature Indicator, WTI.
The Winding Temperature Indicator, WTI is provided with a maximum temperature indicator. The heating resistance is fed by a current transformer associated to the loaded winding of the transformer. 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 a temperature of hottest oil plus the winding temperature rise above hot oil Level the hot spot temperature.
Winding Temperature of Transformer may rise due to increased loading of Transformer or due to some internal fault. Normally the Winding Temperature Indicator, WTI gives alarm at 85 °C and Trip signal at 95 °C in India.