Winding resistance testing is a critical diagnostic test for transformers, helping detect:

• Loose connections

• Broken strands in windings

• Contact issues in tap changers

• Imbalance between phases

1. Test Equipment

• Micro-ohmmeter (preferred, with 1µΩ resolution)

• Kelvin Bridge (DC resistance bridge)

• Temperature sensor (for correction to 20°C)

• Test leads with high-current clamps

2. Test Procedure

Step 1: Safety Precautions

✔ De-energize the transformer and ground windings.
✔ Discharge stored energy (inductive kickback).
✔ Verify lockout/tagout (LOTO).

Step 2: Connections

• Star (Wye) Windings:

• Measure phase-to-neutral (if accessible) or phase-to-phase and calculate phase resistance.

• Delta Windings:

• Measure phase-to-phase and use formulas to derive individual phase resistances.

Connection Diagram

       R1                          R2
       ▲                            ▲
       │                            │
       └────[Transformer]────┘
       ▲                            ▲
       │                            │
       R3                          (Neutral, if Wye)

Step 3: Apply DC Current & Measure

• Inject 10–50% of rated current (to avoid heating).

• Wait for stabilization (typically 1–5 minutes).

• Record resistance (R) + temperature (T).

Step 4: Temperature Correction

• Convert readings to 20°C for comparison:

  $${\mathrm{<span\; style="font-size:\; 18px;">R</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">20</span>}\mathrm{<span\; style="font-size:\; 18px;">°</span>}\mathrm{<span\; style="font-size:\; 18px;">C</span>}}<span\; style="font-size:\; 18px;">=</span>{\mathrm{<span\; style="font-size:\; 18px;">R</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">m</span>}\mathrm{<span\; style="font-size:\; 18px;">e</span>}\mathrm{<span\; style="font-size:\; 18px;">a</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">u</span>}\mathrm{<span\; style="font-size:\; 18px;">r</span>}\mathrm{<span\; style="font-size:\; 18px;">e</span>}\mathrm{<span\; style="font-size:\; 18px;">d</span>}}<span\; style="font-size:\; 18px;">\times </span>\frac{\mathrm{<span\; style="font-size:\; 18px;">234.5</span>}<span\; style="font-size:\; 18px;">+</span>\mathrm{<span\; style="font-size:\; 18px;">20</span>}}{\mathrm{<span\; style="font-size:\; 18px;">234.5</span>}<span\; style="font-size:\; 18px;">+</span>\mathrm{<span\; style="font-size:\; 18px;">T</span>}}$$R20°C=Rmeasured×234.5+T234.5+20

  (For copper windings; use 225 for aluminum)

Step 5: Compare Results

• Acceptance Criteria (IEEE C57.12.90):

• Deviation ≤ ±2% between phases.

• Match factory data within ±5%.

3. Common Issues & Troubleshooting

4. Standards & Best Practices

• IEEE C57.12.90 (Transformer Testing)

• IEC 60076-1 (Power Transformers)

• Test at multiple tap positions (for OLTC transformers).

• Use 4-wire (Kelvin) method to eliminate lead resistance errors.

5. Example Measurement (11kV/433V Transformer)

Key Takeaways

✅ Detects winding defects before failure.
✅ Must be temperature-corrected for accuracy.
✅ Unbalance >2% indicates a problem.

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