Transformers: Ratio, Winding Resistance, and Insulation Testing
Transformers: Ratio, Winding Resistance, and Insulation Testing verifies turns ratio, winding integrity, and insulation condition. Ratio testing detects tap errors and shorted turns; winding resistance finds loose connections and unbalanced windings; insulation resistance checks dryness and cleanliness.
Core rules
- Perform ratio tests at all specified taps and compare with nameplate ratio. A transformer rated 13.8 kV to 480 V has a nominal ratio of 28.75:1.
- Winding resistance is measured with a DC low-resistance ohmmeter. Record temperature and correct to a common base when comparing phases.
- Insulation tests are made HV-to-LV, HV-to-ground, and LV-to-ground as applicable; guard and discharge properly.
- Compare phase resistance balance and prior trends. A large phase imbalance suggests a bad connection, tap changer issue, or winding problem.
- Do not assume a good ratio proves good insulation, or a good insulation test proves correct turns.
Worked example
A 13.8 kV/480 V transformer is ratio-tested by applying 100 V to the HV winding. Expected LV voltage is 100 / 28.75 = 3.48 V. The measured value is 3.42 V, about 1.7% low. The technician checks the tap position, lead placement, and nameplate ratio before judging. The reasoning is that ratio error must be compared on the correct tap and with metering error considered; a ratio test alone does not prove insulation health.
Traps
Testing the wrong tap; including lead resistance in low winding-resistance readings; not correcting for temperature; forgetting to discharge after insulation tests; and using ratio results to clear a transformer without insulation and winding-resistance data.