How do we measure winding resistance, and what does phase imbalance mean?
The DC winding resistance test measures the ohmic resistance of each transformer winding using a low-resistance ohmmeter (micro-ohmmeter) or a DC-current source (10 A or more for power transformers).
Test Procedure:
1. Isolate the transformer and discharge all residual charges.
2. Connect test leads to the winding terminals (e.g., H1-H2 for HV, X1-X2 for LV).
3. Inject a stabilized DC current and measure the voltage drop once stable.
4. Record the oil temperature simultaneously for temperature correction.
5. Repeat for all phases and tap positions.
Interpreting Imbalance (per IEEE Std 62-1995):
• < 2% imbalance → Acceptable (1% for new units)
• > 2% imbalance suggests:
- Loose or oxidized tap-changer contacts
- Loose bolted connections at bushing terminals
- Broken strands in multi-strand conductors
- Internal winding deformation or damage
Temperature Correction (copper windings):
R₂ = R₁ × (235 + T₂) / (235 + T₁)where T₂ is the reference temperature (usually 75°C) and T₁ is the measured temperature.