Experts on transformer test equipment point out that testing three-phase transformers during commissioning or maintenance ensures performance, efficiency and reliability. Tests can be categorised as follows:

1. Pre-commissioning Tests (Done before energizing)

2. Routine Maintenance Tests (Periodic checks)

3. Type Tests (For design validation)

1. Pre-Commissioning Tests (Initial Checks)

These tests verify that the transformer is safe and ready for operation.

A. Insulation Resistance Test (Megger Test)

• Measures insulation health between windings and ground.

• Test Equipment: Megohmmeter (500V, 1000V, or 2500V DC).

• Acceptance Criteria:

• Primary to Ground & Secondary to Ground: ≥1 MΩ (higher is better).

• Primary to Secondary: ≥1 MΩ.

B. Winding Resistance Test (Ohmic Test)

• Checks for loose connections, broken strands, or high-resistance joints.

• Test Equipment: Micro-ohmmeter or DC resistance bridge.

• Acceptance Criteria: Compare with factory test results (deviation ≤ ±2%).

C. Turns Ratio Test (TTR Test)

• Verifies the transformation ratio of each phase.

• Test Equipment: Turns Ratio Tester.

• Acceptance Criteria: Should match nameplate ratio (±0.5% tolerance).

D. Polarity & Phase Relation Test

• Ensures correct phase sequence (R-Y-B) and polarity.

• Test Equipment: Phase sequence meter or oscilloscope.

E. Magnetizing Current Test

• Checks core integrity (detects shorted turns or core damage).

• Test Method: Apply 10-25% rated voltage and measure current.

• Acceptance Criteria: Should be balanced across phases (deviation < 10%).

F. Power Factor/Dissipation Factor (Tan Delta) Test

• Assesses insulation quality (moisture or contamination).

• Test Equipment: PF/Tan Delta Test Kit.

• Acceptance Criteria: Tan δ < 0.5% (new transformer).

G. Vector Group Test (Dyn11, YNd1, etc.)

• Confirms correct phase displacement (30° steps).

• Test Equipment: Vector Group Tester.

2. Routine Maintenance Tests

Conducted periodically to detect aging or faults.

A. Oil Quality Tests (For Oil-Filled Transformers)

• Dielectric Strength Test (BDV > 30 kV for new oil).

• Acidity Test (Neutralization value < 0.1 mg KOH/g).

• Dissolved Gas Analysis (DGA) (Detects internal faults like arcing, overheating).

B. Partial Discharge Test

• Identifies insulation defects.

• Test Equipment: PD Detector.

C. Sweep Frequency Response Analysis (SFRA)

• Detects mechanical deformations (winding displacement, core issues).

3. Type Tests (Manufacturer’s Design Validation)

• Impulse Test (Lightning surge withstand).

• Short-Circuit Test (Mechanical strength under fault).

• Temperature Rise Test (Verifies cooling efficiency).

Safety Precautions During Testing

✔ De-energize & ground before testing.
✔ Use proper PPE (gloves, goggles).
✔ Follow lockout/tagout (LOTO) procedures.

Conclusion

Three-phase transformer testing ensures safe, efficient, and reliable operation. Routine checks prevent failures, while pre-commissioning tests verify installation correctness.

The above content is published by demiks, please specify, demiks is a substation equipment manufacturer, specializing in the production of high-voltage tester, transformer test equipment, circuit breaker test equipment, relay tester, SF6 gas analyser, cable fault tester and other products, if you have power test testing needs, please feel free to contact demiks power science and technology limited company or send an email! Give us: contact@demikspower.com