Transformers
Transformers
Posted By MMASBA

Insulation resistance test of transformer


Insulation resistance test of transformer

Insulation resistance test of transformer is one of the most important transformer tests … we can perform this test at factory or site … this test can be classified under type test, routine test, Pre-commissioning and commissioning tests, Periodic maintenance transformer tests … you can know more about transformer testing and its classifications here >>> transformer testing

Purpose of this test:

Insulation resistance test of transformer is performed on the transformer to:

  1. Verify insulation integrity due to the moisture and impurity contents of insulation.
  2. Ensure that no leakage paths between phases or transformer body.

Precautions before do this test:

  1. Disconnect the electrical current from the transformer and grounding it.
  2. We should discharge any electrical static charge that may have been accumulated on the transformer by means of the discharge stick or using a measuring device with automatic discharge of the charges.
  3. Clean the terminals of the transformer.

Steps of this test:

Insulation resistance test of transformer is divided into three parts:

Firstly… Measure insulation resistance between low voltage windings and the main body of the transformer

  • insulation resistance test of transformerWe use MEGGER test at low voltage side as shown in fig.
  • Adjust MEGGER tester at 1000 V DC.
  • Record the value of insulation resistance measured by MEGGER tester after applying 1 min.
  • Repeat this test between another phase and transformer body as example (a & TR body , b & TR body , c & TR body)
  • Insulation resistance of new transformer should be greater than 1 Giga ohm and old transformer should greater than 300 Mega ohm.

Secondly … Measure insulation resistance between low voltage windings and high voltage windings.

  • insulation resistance test of transformer2We also use MEGGER tester as shown in fig.
  • Adjust MEGGER tester at 2500 V DC.
  • Record the value of insulation resistance measured by MEGGER tester after applying 1 min.
  • Repeat this test between other phases as example (A & a , A & b , A & c , A & n , B & a , B & b , B & c , B & n , C & a , C & b , C & c , C & n ).
  • Insulation resistance at all mentioned phases should greater than 1 GIGA ohm.

 

Thirdly … Measure insulation resistance between high voltage windings and the main body of the transformer

  • insulation resistance test of transformer3We use MEGGER test at low voltage side as shown in fig.
  • Adjust MEGGER tester at 5000 V DC.
  • Record the value of insulation resistance measured by MEGGER tester after applying 1 min.
  • Repeat this test between another phase and transformer body as example (A & TR body , B & TR body , C & TR body)
  • Insulation resistance of new transformer should be greater than 1 Giga ohm and old transformer should greater than 300 Mega ohm.

The summary is that if the Insulation resistance test of transformer between high voltage windings and the transformer body is less than 300 Mega Ohm indicates on the weakness of insulation that means contact between transformer body and high voltage windings.

Also when measuring resistance insulation between the low voltage windings and the transformer body have a lower resistor is 200 Mega Ohm and less than that indicates the presence of contact between transformer body and low voltage windings .

And when measuring insulation resistance between high voltage winding and low voltage winding are less resistance is 300 mega ohm.

 


Electrical Engineer