How to do Insulation Resistance for Transformer?

 Insulation Resistance Test  

Test between H.V. & L.V., H.V. & earth, L.V. & earth.

Insulation resistance (IR) is a critical parameter in distribution transformer maintenance and testing. It ensures the proper functioning of transformers by measuring the effectiveness of the insulation materials used within the device. if not enough insulation, power transformers and liquid immersed transformer can suffer from breakdowns, leading to  repairs cost, operational downtime, or even safety hazards.  

During the handover test, the insulation resistance of the motor, distribution equipment and distribution lines at normal temperature should not be lower than 0.5MΩ(for operating equipment and lines, the insulation resistance should not be lower than 1MΩ/kV). The insulation resistance of low-voltage electrical appliances, their connecting cables and secondary circuits should generally not be lower than 1MΩ. It should not be lower than 0.5MΩ in a relatively humid environment. The insulation resistance of the small busbar in the secondary circuit should not be lower than 10MΩ. The insulation resistance of Class I handheld power tools should not be less than 2MΩ.

What is Insulation Resistance (IR) Value for Transformer?

1. Transformer Coil Insulation Resistance

   Voltage of the Coil	Size of Megger	Min.IR Value of Liquid Filled Transformer Coil	Min.IR Value of Dry Type Transformer Coil
   0 to 600 V	1KV	100 MΩ	500 MΩ
   600 to 5 KV	2.5KV	1000 MΩ	5000 MΩ
   5 KV to 15 KV	5KV	5000 MΩ	25000 MΩ
   15 KV to 69 KV	5KV	10000 MΩ	50000 MΩ

What is the Polarization Index Test?

The Polarization Index (PI) test is another essential test related to insulation. It provides a more comprehensive assessment of the insulation condition than a single IR test. The PI test involves measuring the insulation resistance over a period (testing 1 minute and 10 minutes)  

1. PI Value Interpretation:

   • PI < 1.0: Poor insulation, immediate action is required.
   • PI between 1.0 and 1.5: Questionable insulation, maintenance might be needed.
   • PI between 1.5 and 2.0: Adequate insulation, but closer monitoring is advised.
   • PI > 2.0: Good insulation, no immediate action required.

Factors Affecting Insulation Resistance in Transformers

1. Temperature 
2. Humidity 
3. Age of the Transformer 
4. Type of Insulating Material 

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