1. Safety Precautions
De-energize the Transformer: Disconnect from all power sources and ensure it's electrically isolated.
Discharge Stored Energy: Use a grounding stick to discharge capacitive/inductive energy in windings and core.
Lockout/Tagout (LOTO): Secure the system to prevent accidental re-energization.
Wear PPE: Insulated gloves, safety goggles, and appropriate workwear.
2. Tools Required
Micro-Ohmmeter or 4-Wire Kelvin Bridge (for low-resistance accuracy).
Thermometer (to measure winding temperature).
Calibrated Multimeter (for preliminary checks on small transformers).
Recording Tools (notebook, digital camera).
3. Preparation
Label Windings: Identify primary, secondary, and tertiary terminals.
Stabilize Temperature: Let the transformer cool to ambient temperature if recently energized. Record ambient temperature.
4. Measurement Steps
Connect Leads:
Use the 4-wire Kelvin method to eliminate lead resistance errors.
Ensure clean, tight connections to terminals.
Set Tap Positions:
Test all tap positions (if applicable) by adjusting the tap changer.
Measure Resistance:
Apply DC current and wait for stabilization (core saturation may delay readings).
Record resistance for each winding and tap position.
Check All Phases (for 3-phase transformers):
Compare line-to-line (delta) or line-to-neutral (star) resistances. Phases should be balanced (typically ±2% deviation).
5. Analysis
Compare with Specifications: Check against manufacturer data or historical records. Deviations >5% may indicate issues.
Identify Faults:
High Resistance: Loose connections, broken conductors, or corroded joints.
Low Resistance: Shorted turns or insulation failure.
Phase Imbalance: Faulty tap changer or winding damage.
6. Documentation
Record all measurements, temperatures, tap positions, and environmental conditions.
Include photos of connections for future reference.
7. Troubleshooting Tips
Unstable Readings: Ensure proper lead contact and wait for stabilization.
No Reading: Verify open circuits with a multimeter.
Inconsistent Data: Recheck connections and meter calibration.
8. Post-Test
Re-energize only after confirming all connections are restored and safety checks are complete.
By following these steps, you ensure accurate assessment of transformer health, aiding in predictive maintenance and fault prevention.
