Temperature Rise Test of Transformer

A temperature rise test of a transformer checks whether winding and oil temperature rise stay within the specified limits. IEC 60076-1 treats it as a type test. The method below is the short-circuit heat run used on oil-immersed units.

Temperature Rise Test for Top Oil of Transformer

1. First, the LV winding of the transformer is short-circuited.
2. Then one thermometer is placed in a pocket in the transformer top cover. Other two thermometers are placed at the inlet and outlet of the cooler bank respectively.
3. The voltage of such value is applied to the HV winding that power input is equal to no load losses plus load losses corrected to a reference temperature of 75^oC.
4. The total losses are measured by three wattmeters method.
5. During the test, hourly readings of top oil temperature are taken from the thermometer already placed in the pocket of the top cover.
6. Hourly readings of the thermometers placed at inlet and outlet of the cooler bank are also noted to calculate the mean temperature of the oil.
7. Ambient temperature is measured by means of a thermometer placed around the transformer at three or four points situated at a distance of 1 to 2 meter from and half-way up the cooling surface of the transformer.
8. Continue the temperature rise test for top oil until the temperature increase is less than 3°C in one hour. This steady value is the final temperature rise of the transformer oil.

9. There is another method of determination of oil temperature. Here the test is allowed to be continued until the top oil temperature rise does not vary more than 1^oC per hour for four consecutive hours. The loweest reading during this time is taken as the final value for the temperature rise of the oil.

During the top-oil temperature rise test the LV winding is short-circuited and voltage is applied to the HV winding. That voltage is far below rated voltage because core loss depends on voltage. With little core loss, the current is raised so copper loss makes up the missing heat. IEC 60076-2 injects no-load loss plus load loss at the reference temperature, commonly 75°C for oil-immersed windings. The frozen 3°C per hour and 1°C per hour rules on this page are workshop end-of-run checks, not a reprint of IEC 60076-2.

The table below is a teaching set of oil-immersed temperature-rise limits above the cooling medium. Current IEC 60076-2 defaults are often higher, for example 65 K winding and 60 K top oil on many ONAN ratings. Use the guaranteed values on the rating plate for a factory test.

NB: The table values equal winding or oil temperature minus cooling air or water temperature.

Winding Temperature Rise Test on Transformer

1. After completion of temperature rise test for top oil of transformer the current is reduced to its rated value for transformer and is maintained for one hour.
2. After one hour the supply is switch off and short circuit and supply connection to the HV side and short circuit connection to the LV side are opened.
3. But, the fans and pumps are kept running (if any).

4. Then resistance of the windings are measured quickly.
5. But there is always a minimum 3 to 4 minutes time gap between first measurement of resistance and the instant of switching off the transformer, which can not be avoided.
6. Then the resistances are measured at the same 3 to 4 minutes time intervals over a period of 15 minutes.
7. Graph of hot resistance versus time is plotted, from which winding resistance (R₂) at the instant of shut down can be extrapolated.
8. From this value, θ₂, the winding temperature at the instant of shut down can be determined by the formula given below-
   

   

   
   Where, R₁ is the cold resistance of the winding at temperature t₁.

Winding temperature rise is found indirectly. Hot winding resistance is measured and converted to temperature with the resistance-temperature formula, because the winding is not open for a thermometer. IEC 60076-2 uses that shutdown-resistance method and extrapolates to the instant the supply is cut.