Transformer and Reactor Testing
Transformer and Reactor Testing
This domain tests turns ratio, polarity, excitation, winding resistance, insulation, impedance, oil, and sweep frequency response, plus reactor resistance and inductance checks.
Core rules
- Turns-ratio deviation is typically limited to +/-0.5% of nameplate or adjacent phases; check all taps and compare phase-to-phase.
- Polarity must match nameplate (subtractive or additive); reversed polarity can parallel transformers dangerously.
- Excitation current should be compared phase-to-phase and to previous tests; a high leg suggests a shorted turn.
- Winding resistance, IR/PI, and oil tests (DGA, moisture, acidity) support the electrical tests.
- Reactors: measure DC resistance, inductance, and IR; compare to nameplate and phase data.
Worked example
A 13.8 kV/480 V transformer has a nameplate ratio of 28.75:1. Measured ratio is 28.60:1. Deviation = (28.60 - 28.75)/28.75 x 100 = -0.52%, slightly outside +/-0.5%. Before condemning the transformer, verify the tap position, test lead connections, and nameplate voltage; a wrong tap or lead error is more likely than a 0.5% winding defect.
Traps
- Testing ratio without recording the tap position.
- Confusing polarity with phase rotation.
- Not correcting winding resistance for temperature.
- Treating one oil sample as conclusive without DGA trends.