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Winding Resistance Test of a Power Transformer

The winding resistance test is a fundamental diagnostic tool to assess the condition of the current-carrying conductors in the power transformer.

This blog post provides an in-depth understanding of the purpose, methodology, and practical aspects, ensuring effective application of the test in the field.

Purpose of the Winding Resistance Test

The primary purpose of the winding resistance test is to:

  1. Detect Manufacturing Defects: Identify issues such as loose connections, poor solder joints, or incorrect winding.
  2. Assess Operational Health: Monitor transformer condition over time by identifying issues like aging or degradation of the windings insulation.
  3. Verify Repairs: Ensure that maintenance or repairs, such as tap changer replacements, are executed correctly.
  4. Confirm Design Integrity: Verify that the windings meet design specifications during factory acceptance tests.

Type of Test

The winding resistance test is classified as an offline test and is part of the transformer’s routine, type, and special tests. It is performed in both factory and field environments under de-energized conditions to ensure safety and accuracy.

Problems Identified by the Test

The winding resistance test can reveal:

  • Open or Shorted Windings: Detects breaks or short circuits in the windings.
  • Tap Changer Issues: Identifies malfunctioning or high-resistance tap positions.
  • Loose Connections: Uncovers weak or loose connections and joints in the winding assembly.
  • Deformation or Damage: Indicates winding deformation caused by mechanical stress or fault events.
  • Thermal Degradation: Highlights increased resistance due to overheating or insulation breakdown.

Test Setup

Performing a winding resistance test involves the following steps:

Figure 1: 3 phase delta winding connection

Equipment Required:

  • Micro-ohmmeter or resistance measuring device.
  • Insulated test leads and clamps.
  • Temperature measurement tools (e.g., thermometers or temperature probes).
  1. Preparation:
    • De-energize the transformer and isolate it from the system.
    • Discharge any residual charge using grounding devices.
    • Verify the temperature of the winding to standardize resistance readings.
  2. Testing Procedure:
    • Connect the micro-ohmmeter to the transformer’s winding terminals.
    • Measure the resistance for each phase (e.g., R-Y, Y-B, B-R for three-phase transformers).
    • Record the readings at various tap positions if the transformer is equipped with a tap changer.

Passing Criteria and Tolerances

The winding resistance test results should align with the manufacturer’s specifications and the following tolerances:

  • Phase-to-Phase Variance: For three-phase transformers, the difference in resistance between phases should typically not exceed 2%.
  • Temperature Correction: Resistance values should be corrected to a base usually taken as 20°C or 75°C using a standard formula to ensure consistency.
  • Comparison with Historical Data: A deviation greater than 5-10% from baseline readings may indicate an issue.

International Standards

The test should adhere to internationally recognized standards, including:

  • IEC 60076: Power Transformers—Part 1, General.
  • IEEE C57.12.90: Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers.
  • IS 2026: Indian Standard for Power Transformers.

These standards outline testing procedures, equipment specifications, and acceptable tolerances.

Key Points and Practical Experiences

  1. Importance of Stabilization: Allow sufficient time for current stabilization during measurements to avoid errors.
  2. Avoiding Heating Effects: Prolonged testing can heat the windings, altering resistance values. Minimize the duration of high-current application.
  3. Regular Maintenance: Perform the test periodically as part of the maintenance schedule to monitor transformer health.
  4. Documenting Results: Maintain detailed records of resistance readings for future comparison and trend analysis.
  5. Skilled Personnel: Ensure the test is conducted by qualified professionals familiar with transformer diagnostics.
  6. Environmental Factors: Consider the impact of ambient temperature and humidity, as these can influence readings.

Conclusion

The winding resistance test is a crucial assessment in ensuring the reliability and efficiency of power transformers. By following proper procedures, adhering to international standards, and understanding the implications of test results, operators can effectively identify and address potential issues, extending the life of their assets and ensuring uninterrupted power delivery.

References

  1. IEC 60076: Power Transformers—Part 1, General.
  2. IEEE C57.12.90: Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers.
  3. IS 2026: Indian Standard for Power Transformers.

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