How to do Megger test/insulation resistance for transformer?

Transformers are one of the costliest equipment used in distribution and transmission process of power. The transformer has to go through different testing procedures periodically to prove that the transformer works properly.

Deterioration of the transformer insulation resistance is one of the most common causes of failure of the transformer: a failed transformer is a costly replacement in an electrical system with the potential for lengthy downtime.

The transformer resistance testing will reveal a lot of details about the transformer by measuring the winding resistance of a transformer from one HV transformer bushing to another. The megger test or insulation test is carried out to ensure the healthiness of the overall insulation system of an electrical power transformer.

Procedure of Megger test/Insulation Resistance test of transformer:

We calculate electrical leakage in wire and the results are very consistent as we are checking to transfer electrical current through the system. We check any system such as motor, cable, generator winding, or general electrical installation’s electrical insulation standard.

  1. First disconnect all the line and neutral terminals of the transformer.
  2. Megger connects to LV and HV bushing studs to calculate the importance of IR insulation resistance between LV and HV windings.
  3. Megger leads to connecting to HV bushing studs and earth point transformer tanks to calculate the importance of IR insulation resistance between HV windings and earth.
  4. Megger leads to the relation between the LV bushing studs and the transformer tank earth point to calculate the strength of IR insulation resistance between the LV windings and ground.

IR Value of Transformers:

Voltage  Test Voltage (DC)  LV side Test  Voltage (DC) HV side Min IR Value
415V 500V 2.5KV 100MΩ
Up to 6.6KV 500V 2.5KV 200MΩ
6.6KV to 11KV 500V 2.5KV 400MΩ
11KV to 33KV 1000V 5KV 500MΩ
33KV to 66KV 1000V 5KV 600MΩ
66KV to 132KV 1000V 5KV 600MΩ
132KV to 220KV 1000V 5KV 650MΩ
News Reporter
Instrumentation Engineer