Preventing Low Insulation Resistance (IR) Value:
The insulation resistance might be lowered because of moisture content in the oil and in the winding insulation, and might cause transformer failure. To avoid this, the entire transformer core with windings should be placed in hot air chamber until the moisture content is erased from the core and winding insulation resistance could be measured along with the help of a Megger. The minimum safe insulation resistance for various voltage ratings of windings is given in Table .
Table : Minimum Safe Insulation Resistance in M? (Mega-ohm)
|
Rated Voltage of the
Winding
|
Minimum Safe Insulation Resistance in M? at
Different Temperatures
30°C 40°C 50°C 60°C
|
|
66 kV and above
|
600 300 150 75
|
|
22 kV and 33 kV
|
500 250 125 65
|
|
6.6 kV and 11 kV
|
400 200 100 50
|
|
Below 6.6 kV
|
200 100 50 25
|
Ensuring suitable insulation resistance enhances the life of transformer.