Power Transformer Testing Overview

Posted on by Engr. Hamid Ali

In this post we present you Power Transformer Testing Overview as per IS:2026 (Part I). Power transformers are critical components in the electrical power system, and their reliability is ensured through rigorous testing as specified in IS:2026 (Part I) – Power Transformers: General Requirements. These tests are categorized into Type Tests, Routine Tests, and Special Tests. Check here different types of Transformers in substations in KSA

1. Type Tests

Type Tests are conducted to verify the design, quality, and performance of a transformer model. They are usually performed on a prototype unit.

1.1 Measurement of Winding Resistance

  • Purpose: Verify conductor integrity, cross-sectional area, and proper connections.
  • Method: DC resistance measured for all windings at various tap positions.
  • Acceptance: Measured resistance should match the manufacturer’s design values within acceptable tolerance.

1.2 Measurement of Voltage Ratio and Voltage Vector Relationship

  • Purpose: Confirm voltage transformation ratio and phase displacement.
  • Method: Voltage applied to the primary winding; secondary voltage and vector group are verified.
  • Acceptance: The measured voltage ratio and vector relationship must conform to the nameplate specifications.

Click here to view Vector Groups in Transformers Key Concepts

1.3 Measurement of Impedance Voltage/Short Circuit Impedance and Load Loss

  • Purpose: Assess internal impedance and copper losses under load.
  • Method: Circulate rated current with short-circuited secondary, measure voltage and losses.
  • Acceptance: Impedance and load losses must be within the specified tolerance limits in the design.

1.4 Measurement of No-Load Loss and No-Load Current

  • Purpose: Evaluate core losses and magnetizing current.
  • Method: Apply rated voltage to primary with open-circuited secondary.
  • Acceptance: No-load loss and current must conform to manufacturer-specified limits.

1.5 Measurement of Insulation Resistance

  • Purpose: Check the insulation quality between windings and to earth.
  • Method: Measured using insulation resistance tester (megger).
  • Acceptance: Insulation resistance should meet or exceed the minimum values specified in IS:2026 or manufacturer requirements.

1.6 Dielectric Tests

  • Purpose: Validate insulation strength under high voltage conditions.
  • Method: Power frequency withstand, induced overvoltage, and lightning impulse tests.
  • Acceptance: The transformer should withstand the specified test voltages without breakdown, flashover, or partial discharge exceeding limits.

1.7 Temperature Rise Test

  • Purpose: Ensure thermal performance under continuous load.
  • Method: Load transformer to rated capacity, measure oil and winding temperatures.
  • Acceptance: Temperature rise of oil and winding must be within the limits specified in IS:2026 (usually 55°C for oil and 65°C for windings above ambient)

1.8 Tests on On-Load Tap Changers (OLTC)

  • Purpose: Verify OLTC operation and contact resistance.
  • Method: Measure contact resistance at all tap positions and test mechanical operation.
  • Acceptance: Resistance values must be uniform, and tap changer operation must be smooth without arcing or misalignment.

2. Routine Tests

Routine Tests are performed on each manufactured transformer to confirm quality and operational readiness.

2.1 Measurement of Winding Resistance

  • Same procedure and purpose as in Type Tests.

2.2 Measurement of Voltage Ratio and Voltage Vector Relationship

  • Same procedure and purpose as in Type Tests.

2.3 Measurement of Impedance Voltage/Short Circuit Impedance and Load Loss

  • Same procedure and purpose as in Type Tests.

2.4 Measurement of No-Load Loss and No-Load Current

  • Same procedure and purpose as in Type Tests.

2.5 Measurement of Insulation Resistance

  • Same procedure and purpose as in Type Tests.

2.6 Dielectric Tests

  • Same procedure and purpose as in Type Tests.

2.7 Tests on On-Load Tap Changers (OLTC)

  • Same procedure and purpose as in Type Tests.

3. Special Tests

Special Tests are additional tests performed upon customer request or for special applications.

3.1 Dielectric Tests (Enhanced)

  • Purpose: Additional insulation validation under special conditions.
  • Method: May include chopped impulse waves, switching surges, or higher test voltages.

3.2 Measurement of Zero-Sequence Impedance

  • Purpose: Determine impedance path for unbalanced (zero-sequence) currents.
  • Method: Measure impedance with all phases tied and grounded.

3.3 Short-Circuit Test

  • Purpose: Validate mechanical integrity under fault conditions.
  • Method: Laboratory testing or validated by design experience.

3.4 Measurement of Acoustic Noise Level

  • Purpose: Assess sound emissions, especially in noise-sensitive areas.
  • Method: Measure sound pressure levels around the transformer.

3.5 Measurement of Harmonics of No-Load Current

  • Purpose: Analyze harmonic content generated by the transformer core.
  • Method: Measure no-load current using harmonic analyzers.

3.6 Measurement of Power Taken by Fans and Oil Pumps

  • Purpose: Determine auxiliary power consumption.
  • Method: Measure input power to fans and pumps under working conditions.

Conclusion

The comprehensive testing of power transformers as outlined in IS:2026 (Part I) ensures their reliability, safety, and operational efficiency. These tests validate the design, manufacturing quality, and readiness of transformers for service in various electrical networks.