Use of Insulation Resistance for Determining Drying Time
Drying time depends on the condition of the transformer, size, voltage, amount of moisture absorbed, and the method of drying used.
The measurement of insulation resistance is of value in determining the status of drying. Measurements should be taken before starting the drying process and at two-hour intervals during drying. The initial value, if taken at ordinary temperatures, may be high even though the insulation may not be dry. Because insulation resistance varies inversely with temperature, the transformer temperature should be kept constant during the drying period to obtain comparative readings. As the transformer is heated, the presence of moisture will be evident by the rapid drop in resistance measurement. Following this period the insulation resistance will generally increase gradually until near the end when it will increase more rapidly. Sometimes it will rise and fall a short range before steadying due to moisture from the interior of the insulation working out through the initially dried portions. A curve with time as abscissa and resistance as ordinate should be plotted and the run should be continued until the resistance levels off and remains relatively constant for three to four hours.
Insulation resistance measurements should be taken for each winding to ground with all windings grounded except the one being tested. Before taking insulation resistance measurements, the current should be interrupted and the winding short circuited and than grounded for at least one minute to drain off any static charge.