Insulation testing on a transformer is commonly referred to as Meggar testing. The transformer must be disconnected. An insulation tester (Meggar) must be used. The voltage is selected (typically 500V) and the Test button is pressed until the insulation resistance is displayed.
The cooling water should be blown out of the cooling lines as the presence of water may show lower values.
Three tests must be performed:
Primary – Secondary
Primary – case
Secondary – case
A tested 650 kVA transformer produced the following values:
Primary – Secondary 300 MOhm
Primary – case 300 MOhm
Secondary – case 5 MOhm
It is not uncommon for the "Secondary – Case" to read lower than the other two.
Depending on who you ask, the lowest acceptable value is 5 or 10 MOhm.
This type of testing should be performed by personnel trained to test transformers. Transformer manufactures can perform this test.
Reference: RWMA Resistance Welding Manual 4th Edition