Transformer Winding Resistance Testing Best Practices

Purpose of Winding Resistance Testing

Winding resistance testing detects loose connections, broken strands, contact problems in tap changers, and shorted turns within transformer coils. It is part of every commissioning and overhaul procedure for power transformers and is governed by IEEE C57.152 and IEC 60076-1.

Test Setup

Use a dedicated transformer winding resistance tester capable of injecting 1 A to 50 A DC depending on transformer size. Connect kelvin clamps directly to the bushing terminals. The tester applies current, waits for the inductive transient to settle, then records the stable resistance.

Recommended Test Currents

• Distribution transformers: 1-5 A on the LV side, 0.1-1 A on the HV side.
• Power transformers: 5-25 A typical.
• Large generator step-up units: up to 50 A.

Tap Changer Verification

For on-load tap changers (OLTC), step through every position from extreme to extreme. The resistance should change monotonically and predictably. A skip or sudden jump indicates a contact problem and warrants internal inspection.

Demagnetization After Test

After DC injection, the core retains residual magnetization that distorts subsequent SFRA, ratio, or excitation tests. Always run the demagnetization routine before the next test. Modern testers automate this with a programmed bipolar current sweep.

Reporting

Provide ambient temperature, winding temperature correction to 75 C reference, raw and corrected values, and tap-by-tap data tables. EK / ETCR transformer testers generate compliant PDF reports automatically.