M
AVTMMTO210 Rev 7 Sept 2013
30
Temperature Correction
It may be necessary to convert the resistance measurements to values
corresponding to the reference temperature in the transformer test report.
To estimate the winding temperature at the measurement is important. If the
transformer has winding temperature, use theses readings. If not, the winding
temperature is assumed to be the same as the oil temperature under the following
conditions:
The transformer has been out of service for at least 3 hours
The temperature of the insulating liquid has stabilized, and the difference
between the top and bottom temperature does not exceed 5ºC.
If the transformer is measured without oil, the winding temperature is normally
assumed to be the same temperature as the surrounding air.
Conversion of Resistance Measurements
Winding resistance measurements are normally converted to a standard reference
temperature.
The conversions are accomplished by the following formula:
Rs = Rm ( Ts + Tk )/( Tm + Tk )
where
Rs = resistance at desired temperature Ts
Rm = measured resistance
Ts = desired reference temperature
Tm = temperature at which resistance was measured
Tk = 234.5 (copper)
Tk = 225 (aluminum)
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