Oil and Winding Temperature Indicator of the Transformer
Oil and Winding temperature Indicator are types of accessories that are used to protect the transformer operation process.
Table of content
There are some important functions of this equipment as follows:
- Indicate instantaneous temperature of oil and windings of the transformer and record maximum temperature rises of oil and windings.
- Operate high-temperature alarm at a predetermined value of allowable temperature limit.
- Trip the circuit breakers associated with the power transformer when the temperature of oil or winding reaches a predetermined limit.
- Control the cooling system of the transformer. Turn on the cooling equipment when the winding attains a preset high temperature and turn it off when the temperature decreases by a set differential.
There are two main types of temperature indicators used in transformers:
- OTI (Oil Temperature Indicator): equipment is used for indicating oil temperature
- WTI (Winding Temperature Indicator): equipment is used for indicating oil temperature
These indicators are supplied with a sensing bulb that is located in a pocket on the roof of the transformer tank.
The bulb is linked to the instrument housing by means of flexible connecting tubing. There are two capillary tubes: one is linked to the operating bellow of the instrument and one is linked to a compensating bellow.
Compensating below compensates for variation of ambient temperature. The pointer is fixed with a steel carriage on which normally four mercury switches are mounted. The make and break temperature of these mercury switches can be adjusted separately. One mercury switch is used to operate cooling fans, one mercury switch is used to switch on oil pumps, one mercury switch is for high-temperature alarms, and the last switch is used to inter-trip the transformer in extremely high-temperature conditions.
In addition, the dial of the oil temperature indicator is also supplied with a dummy pointer which is pushed by the main pointer when the temperature rises. When the main pointer moves back because of decreasing temperature, the dummy pointer stays at the maximum temperature position of the main pointer. This indicates the maximum temperature rise of the transformer during a preset period of time.
This device is used to measure the top oil temperature and protect the transformer.
Operating Principle of Oil Temperature Indicator
Thanks to the sensing bulb immersed in the pocket, this device measures top oil temperature by using a liquid expansion in the bulb to an operating mechanism through a capillary line. This movement is amplified by a link and lever mechanism to the mercury switches and the disc-carrying pointer. When the volume of the liquid in the operating mechanism changes, the bellow attached to the end of the capillary tube expands and contracts. This movement below is transmitted to the pointer through a lever linkage mechanism in the temperature indicator of the transformer.
This device measures the LV and HV winding temperature and is also used to protect the transformer.
Operating Principle of Winding Temperature Indicator
The basic operating principle of WTI is the same as OTI. But the only difference is that the sensing bulb pocket on the transformer top cover is heated by a heater coil surrounding it. This heater coil is fed by secondary current transformers associated with transformer winding. Hence the current through the heater coil is directly proportional through transformer winding to the current flowing. This is because there is no scope for direct measuring the temperature inside a winding. When the load of the transformer increases, the winding temperature is also increased and this increased temperature is sensed by the sensing bulb as the heater coil surrounds it. The rest of the working principle of the winding temperature indicator is the same as the principle of the oil temperature indicator.