The rating of a transformer depends on winding temperature rise limits. Therefore, continuous winding temperature monitoring is necessary. Thus, this ensures the safe operation of the transformer.
In a power transformer, winding conductors have thick insulation. Therefore, direct sensor installation becomes practically impossible. In fact, the insulation system prevents direct sensor placement. So, another temperature measurement method is required. Hence, the Winding Temperature Indicator, or WTI, performs this task.
Now, to understand WTI operation, recall some basic concepts. First, a current transformer is installed in the bushing turret. Obviously, its secondary current follows the transformer load current. Therefore, this current is directly proportional to winding current.
Next, connect a heater coil across the CT secondary. Then, the heater coil produces heat due to current flow.
Naturally, this heating effect depends on current squared value. Moreover, the generated heat is given by:
Now, the CT secondary current follows winding conductor current. Therefore, heater coil heating follows winding current variation. As a result, this heating effect can be calibrated into temperature values.
However, this gives only theoretical winding temperature rise. Actually, winding temperature also depends on oil temperature. Therefore, hot oil temperature measurement is also necessary.
For this purpose, a mercury bulb is inserted inside oil. Generally, the oil pocket remains near the transformer winding.
Moreover, this oil pocket is mounted on transformer top cover. Then, the mercury bulb connects to a spring-shaped bellows. Additionally, a capillary tube connects the bulb and bellows.
Now, when oil temperature rises, mercury inside the bulb expands. Consequently, the expanded mercury creates pressure inside the capillary tube. As a result, this pressure deforms the spring-shaped bellows. Therefore, the attached pointer rotates.
Now, this arrangement measures surrounding hot oil temperature. Next, place the heater coil near the bellows arrangement. Then, the heater further heats the bellows during load increase. Therefore, the bellows deforms further during higher loading. Consequently, the pointer deflects further.
Hence, the final pointer position indicates winding temperature value. This temperature includes winding current heating effect. Additionally, it includes surrounding hot oil temperature effect. Thus, this is the basic principle of WTI operation.
Previously, another WTI arrangement was also used. In that arrangement, the mercury bulb remained immersed directly.
Specifically, it remained inside the oil pocket itself. Moreover, the heater coil surrounded the bulb directly there. Therefore, the heater was not placed inside the WTI device.
As a result, mercury expansion depended on two factors. First, one factor was winding load current. Secondly, the other factor was surrounding hot oil temperature. Then, the capillary tube connected directly with the bellows. Meanwhile, the bellows remained inside the winding temperature indicator. Therefore, pointer deflection indicated final winding temperature. Additionally, this included the surrounding oil temperature effect also.
Now, the main WTI component is a spring-shaped bellows. This bellows connects with the mercury bulb. Meanwhile, the mercury bulb remains inside the oil pocket. Similarly, the oil pocket remains on transformer top cover. Then, a capillary tube connects bulb and bellows together.
Next, the heater coil connects across the WTI CT secondary. At the same time, the WTI CT remains inside the bushing turret. Additionally, the heater coil is arranged around the bellows. Now, let us discuss the pointer arrangement. The pointer is fixed with a semicircular rotating plate. Therefore, the plate rotates together with bellows movement.
Meanwhile, the rotation occurs around a hinge point.
Furthermore, four mercury switches remain mounted on the rotating plate. These switches remain at different angular positions. Moreover, their operating angles are carefully adjusted during calibration. Then, during pointer rotation, switches operate one after another. Consequently, each switch operates at a specific temperature range. Also, the temperature ranges appear on the WTI dial.
First, the first mercury switch operates at preset temperature. Then, it tilts from its balanced position. As a result, this closes the cooling fan starting circuit.
If temperature rises further, another switch operates. Then, the second switch starts the oil pump circuit.
If temperature increases further, pointer rotates additionally. Consequently, the third mercury switch operates. Then, this switch closes the alarm circuit. Therefore, this indicates excessive transformer temperature condition.
Finally, if temperature still increases, pointer rotates further. Then, the fourth mercury switch operates finally. As a result, this switch closes the transformer trip circuit. Consequently, the transformer trips from both HV and LV sides. Thus, this protects the transformer against overheating.