These devices are supposed to indicate the hottest spot in the winding, based on the manufacturer’s heat run tests. At best, this device is only accurate at top nameplate rated load and only if it is not out of calibration . They are not what their name implies and can be misleading. They are only winding hottest-spot simulators, which are not very accurate. Normally, there is no temperature sensor embedded in the winding hot spot. At best, they provide only a rough approximation of hot spot winding temperature and should not be relied on for accuracy. They can be used to turn on additional cooling or activate alarms as the top oil thermometers do.
Winding temperature thermometers work the same way as the top oil thermometer (discussed in section 4.2), except that the bulb is in a separate thermometer well near the top of the tank. A wire-type heater coil is either inserted into, or wrapped around, the thermometer well, which surrounds the temperature sensitive bulb. In some transformers, a current transformer (CT) is around one of the three winding leads and provides current directly to the heater coil in proportion to winding current. In other transformers, the CT supplies current to an auto-transformer that supplies current to the heater coil. The heater warms the bulb, and the dial indicates a temperature, but it is not the true hottest-spot temperature.
These devices are calibrated at the factory by changing taps on either the CT or the autotransformer, or by adjusting the calibration resistors in the control cabinet. These devices normally cannot be field calibrated or tested, other than testing the thermometer, as mentioned. The calibration resistors can be adjusted in the field if the manufacturer provides calibration curves for the transformer.
In practice, most winding temperature indicators are out of calibration, and their readings are meaningless. These temperature indications should not be relied upon for loading operations or maintenance decisions.
Fiber optic temperature sensors can be imbedded directly into the winding as the transformer is being built; these sensors are much more accurate. This system is available as an option on new transformers at an increased cost, which may be worthwhile, since the true winding “hottest-spot” temperature is critical when higher loading is required.
Thermometers can be removed without lowering the transformer oil if they are in a thermometer well. Check your transformer instruction manual. Look carefully at the capillary tubing between the thermometer well and the dial indicator. If the tubing has been pinched or accidentally struck, it may be restricted. This is not an obvious defect, but it can cause the dial pointer to lock in one position.
If this defect is found, the whole gauge must be returned to the factory for repair or replacement; it cannot be repaired in the field. Look for a leak in the tubing system; the gauge reading will be very low and must be replaced if a leak is discovered.
Every 3 to 5 years, and if trouble is suspected, test the thermometer. Suspend the thermometer’s indicator bulb and an accurate mercury thermometer in an oil bath. Do not allow either thermometer to touch the side or bottom of the container. Heat the oil on a hotplate, while stirring, and compare the two thermometers while the temperature increases. If a magnetic stirring/heating plate is available, it is more effective than hand stirring. Pay particular attention to the upper temperature range at which your transformers normally operate (50 ºC to 80 ºC). An ohmmeter should also be used to check switch operations. If either dial indicator is more than 5 ºC different than the mercury thermometer, it should be replaced with a spare. A number of spares should be kept, based on the quantity of transformers at the plant. Oil bath test kits are available from the Qualitrol Company. After calling for Qualitrol authorization (716-586-1515), you can ship defective dial thermometers for repair and calibration to: Qualitrol Company, 1387 Fairport Road, Fairport, New York 14450.
The alarms and other functions should also be tested to see if the correct annunciator points activate, pumps/fans operate, etc.
If the temperature gauge cannot be replaced or sent to the factory for repair, place a temperature correction factor on your data form to add to the dial reading, so that the correct temperature will be recorded. Also, lower the alarm and pump-turn-on settings by this same correction factor. Since these are pressure-filled systems, the indicator will typically read low if it is out of calibration. Field testing has shown some of these gauges reading 15 ºC to 20 ºC lower than actual temperature. This is hazardous for transformers because it will allow them to continuously run hotter than intended, due to delayed alarms and cooling activation. If thermometers are not tested and errors corrected, transformer service life may be shortened or premature failure may occur.