Transformer Testing and Inspection
Transformer Testing and Inspection covers ratio, winding resistance, insulation, oil analysis, and condition assessment. Level 4 questions often ask you to normalize a measurement to a tap or temperature before judging pass/fail.
Core rules
- Turns ratio test: measured ratio should match nameplate within about ±0.5% on the rated tap; normalize to the tap before judging.
- Winding resistance should balance between phases; convert to a common temperature before comparison. A significant difference suggests a loose connection, broken strands, or tap-changer contact problems.
- DGA interprets dissolved gases: acetylene indicates arcing, ethylene indicates thermal overheating, and moisture lowers dielectric strength.
- Oil tests include dielectric breakdown voltage, moisture, acidity, interfacial tension, and furans.
- Sweep frequency response compares fingerprints to detect winding deformation.
Worked example
A transformer is rated 13.8 kV/4.16 kV. Nameplate ratio = 13.8/4.16 = 3.317. Measured ratio on the same tap is 3.30. Deviation = (3.30 − 3.317)/3.317 × 100 ≈ −0.52%. This is just outside a ±0.5% acceptance band, so check tap position, test lead connections, and repeat before condemning the winding. If the repeat is 3.315, the unit passes. A ratio error alone does not prove a fault; it flags a need for winding resistance and excitation current tests.
Traps
Testing on the wrong tap; not correcting winding resistance to a reference temperature; taking DGA samples without flushing the valve; interpreting a single gas without ratios and history; confusing excitation current with load current.