Insulation Resistance Test Procedure for UHV Transformers

Insulation Resistance Test Procedure for UHV Transformers

An insulation resistance (IR) test for a UHV (Ultra High Voltage) transformer is a critical diagnostic test to assess the health of the transformer's insulation system. It essentially measures how well the insulating materials (like paper, oil, bushings, and windings) resist electrical current leakage.
Procedure to perform the test:
1. Preparation
  • De-energize the transformer.
  • Disconnect it from the grid and isolate windings.
  • Ensure the transformer is dry, clean, and free of moisture on bushings.
    • 2. Testing

  • Apply DC test voltage from the Megger Insulation Tester according to transformer rated voltage . For UHV transformers, this can be in hundreds of kilovolts for field tests (or scaled down for insulation testers).
  • Measure resistance between:
    • Primary winding → ground
    • Secondary winding → ground
    • Between windings (primary → secondary)
  • Record values over time (usually 1–10 minutes) as insulation resistance often increases with time during the test.
  • High resistance (usually in GΩ range for UHV transformers) → Good insulation.
  • Low resistance → Moisture, contamination, or insulation breakdown.