OTI and WTI in Transformer

OTI (Oil Temperature Indicator) and WTI (Winding Temperature Indicator) are two important components of an electrical transformer. OTI and WTI are the essential components of an electrical transformer, as they ensure the safe and reliable operation of the transformer.

The primary function of both these indicators is the same i.e., monitoring and displaying the temperature. OTI (Oil Temperature Indicator) monitors and shows the temperature of transformer oil filled in the tank, while WTI (Winding Temperature Indicator) shows the temperature of windings of the transformer.

Read this article to learn more about these two types of temperature indicators.

What is an Oil Temperature Indicator (OTI)?

In an electrical transformer, an OTI (Oil Temperature Indicator) is a temperature indicating instrument used for monitoring and displaying the temperature of dielectric oil filled in the transformer tank.

The dielectric oil filled in the tank of a transformer plays a crucial role in insulation and cooling of the machine. This oil maintains the temperature of the transformer within a safe limit and improves the energy efficiency. Therefore, it is important to continuously monitor the temperature of the oil to ensure that it is within the specified limit.

OTIs are generally used in larger power transformer for the following main purposes −

• Continuously monitor the temperature of the dielectric oil,

• Tracking the oil temperature in real-time and prevent overheating of the oil and hence the transformer,

• Operate the alarm and trip circuits if the oil temperature becomes greater than the specified limit,

• Maintain the data records for analysis, etc.

Components of Oil Temperature Indicator

A typical oil temperature indicator (OTI) consists of the following main components −

• Sensor Bulb − It is the component provided for measuring the temperature directly by placing it into the oil.

• Capillary Tube − It is provided to connect the sensor bulb with the temperature gauge.

• Temperature Gauge/Dial − Its function is to show the reading of oil temperature. It is generally calibrated in °C or °F.

• Mercury Switches S1 and S2 − An OTI also consists of two mercury switches namely, S1 and S2. The switch S1 is used for triggering the alarm circuit and the switch S2 is used for triggering the trip circuit if the oil temperature exceeds the specified limit for the respective operations.

Working Principle of Oil Temperature Indicator

The operation of an oil temperature indicator (OTI) is based on the principle of temperature-induced fluid expansion. When the temperature of the oil in the transformer increases, the liquid filled inside the sensor bulb and capillary tube expands and deflect the temperature gauge in accordance with the change in the temperature.

In practice, the following oil temperature settings are used −

• OTI Alarm − Oil temperature equal to 80°C.

• OTI Trip − Oil temperature equal to 90°C.

Let us now discuss about another type of temperature indicator used in a transformer which is called WTI (Winding Temperature Indicator).

What is a Winding Temperature Indicator (WTI)?

A WTI (Winding Temperature Indicator) is an instrument used for monitoring and showing the temperature of windings of an electrical transformer. It is mainly used in large power transformers used in transmission systems.

The winding temperature is an important parameter that significantly affects the efficiency and performance of the electrical transformer. For desired efficiency, the winding temperature must be within a specified limit.

If the temperature of the transformer windings goes beyond a specified limit, it can cause insulation breakdown and burning of the windings. Therefore, it is desirable to continuously monitor the winding temperature and keep it within the specified limit for the safe and reliable operation of the transformer. For this purpose, the WTI or Winding Temperature Indicator is equipped with the transformer.

A WTI (Winding Temperature Indicator) performs the following major functions −

• Continuously monitor the temperature of primary and secondary windings,

• Track the winding temperature and prevent it exceeding the specified limit,

• Complete the alarm or trip circuits if the winding temperature crosses the threshold limit, etc.

Components of Winding Temperature Indicator

A WTI (Winding Temperature Indicator) consists of the following main components −

• Sensor Bulb − It is provided to sense the temperature rise of the oil due to the heating of the windings. The sensor bulb of a WTI is placed in an oil filled pocket in the top cover of the transformer tank.

• Capillary Tube − This is used for connecting the sensor bulb and the temperature gauge. The change in winding temperature is transmitted through this capillary tube to the temperature gauge.

• Temperature Gauge/Dial − It is used for showing the winding temperature in °C or F.

• Heating Element − It is a heating coil which is wrapped around the sensor bulb. Its function is to simulate the change of winding temperature more accurately.

• Mercury Switches S1, S2, S3, & S4 − A WTI is equipped with four mercury switches. Where, S1 is for cooling fan control, S2 is for motor pump control, S3 is used for winding temperature alarm, and S4 is used for winding temperature trip.

Working Principle of Winding Temperature Indicator

The operation of a winding temperature indicator (WTI) is based on the principle of simulation of winding temperature which is provided by a combination of oil temperature and heating element temperature.

When the transformer operates, the temperature of its windings increases due to copper loss. This heat produced in windings is transferred to the dielectric oil filled in the transformer tank and heat up the oil.

The sensor bulb of the WTI senses the variation in the oil temperature and the heating element placed around the sensor bulb heats up to simulate the winding temperature accurately. The fluid filled in the sensor bulb expands and transmits to the temperature gauge through the capillary tube joining them. Finally, the gauge shows the winding temperature reading.

In practical transformers, the following winding temperature settings are generally used −

• Fan Control − Fan OFF for winding temperature equal to 58°C and fan ON for winding temperature equal to 64°C.

• Motor Pump Control − Pump OFF for winding temperature equal to 68°C and pump ON for winding temperature equal to 72°C.

• WTI Alarm − Alarm circuit will turn ON for winding temperature equal to 85°C.

• WTI Trip − Trip circuit will trigger for winding temperature equal to 95°C.

Hence, this is all about functions and working of a WTI (Winding Temperature Indicator).

Conclusion

OTI (Oil Temperature Indicator) and WTI (Winding Temperature Indicator) are used for continuously monitoring the temperature of dielectric oil and windings of a transformer.

The temperature readings of these two meters must be within a specified limit to ensure safe and reliable operation of the transformer. If the temperature exceeds the specified limits, these meters send signals for alarming or tripping of the transformer.