Oil-Filled Transformer Inspections

A transformer maintenance program must be based on thorough routine inspections. These inspections must take place in addition to normal daily/weekly data gathering trips to check oil levels and temperatures. Some monitoring may be done remotely using supervisory control and data acquisition (SCADA) systems, but this can never substitute for thorough inspections by competent maintenance or operations people.
After 1 month of service, and once each year, make an in-depth inspection of oil-filled transformers. Before beginning, carefully inspect the temperature and oil level data sheets. If temperature, pressure, or oil level gauges never change, even with seasonal temperature and loading changes, something is wrong. The gauge may be stuck, or data sheets may have been filled in incorrectly. Examine the DGAs for evidence of leaks or other problems.

Transformer Tank

Check for excessive corrosion and oil leaks. Pay special attention to flanges and gaskets (bushings, valves, and radiators) and the lower section of the main tank. Report oil leaks to maintenance, and pay special attention to the oil level indicator if leaks are found. Severely corroded spots should be wire brushed and painted with a rust inhibitor.

Top Oil Thermometers

These thermometers are typically sealed, spiral-bourdon-tube dial indicators with liquid-filled bulb sensors. The bulb is normally inside a thermometer well, which penetrates the tank wall into the oil near the top of the tank. As oil temperature increases in the bulb, liquid expands, which expands the spiral tube. The tube is attached to a pointer that indicates temperature. These pointers may also have electrical contacts to trigger alarms and start cooling fans as temperature increases. An extra pointer, normally red, indicates maximum temperature since the last time the indicator was reset. This red pointer rises with the main pointer but will not decrease unless manually reset, thus, it always indicates the highest temperature reached since it was reset. See the instruction manual on your specific transformer for details.

Winding Temperature Thermometers

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, 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 embedded 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.
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.

Temperature Indicators Online

Check all temperature indicators while the transformer is online. The winding temperature indicator should read approximately 15 degrees above the top oil temperature. If this is not the case, one or both temperature indicators are malfunctioning. Check the top oil temperature next to the top oil indicator’s thermowell with an infrared camera. Compare the readings with the top oil indicator. Reset all maximum indicator hands on the temperature indicating devices after recording the old maximum temperature readings. High temperature may mean overloading, cooling problems, or problems with windings, core, or connections.

Temperature Indicators Offline

When the transformer is offline and has cooled to ambient temperature, check the top oil and winding temperature indicators, both should read the same. If not, one or both temperature indicators are malfunctioning. Check the calibration according to the proper procedure. Also, compare these readings with the indicated temperature on the conservator oil level indicator, all three should agree.