Discrepancy Between Field Test and Nameplate on a Rebuilt Transformer
A 75 kVA, 12.47 kV–480Y/277 V three-phase pad-mounted transformer is being commissioned after being rebuilt. Nameplate shows a 5.75% impedance and a 65°C rise. You perform a turns-ratio test and get results within 0.5% of nameplate on all phases. Insulation resistance is acceptable. However, a low-voltage excitation/impedance check suggests the impedance is closer to 4.5%. The customer's protective device is set based on the nameplate 5.75%. The transformer will feed a building with significant motor load.
1. How would you interpret the impedance discrepancy and what risks does it create for protection coordination?
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A measured impedance lower than nameplate means the transformer will deliver more fault current than the protection was set for. If the upstream device is set on 5.75% but the unit is actually ~4.5%, the available short-circuit current at the secondary is higher, so:
- The primary protective device may operate slower relative to the fault or may not coordinate with downstream devices as designed.
- Downstream equipment may see fault current above its rating.
- Motor starting voltage dip will be somewhat less, but the protection margin is the concern.
Possible causes of the discrepancy: test method error (excitation test not equivalent to impedance test), incorrect nameplate, or a rebuild that changed the winding. Do not assume the field test is right or the nameplate is right - resolve the discrepancy before energizing.
2. What steps would you take to resolve the discrepancy before energizing, and what would you do if it cannot be resolved?
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Resolution steps:
- Repeat the test with a proper impedance/short-circuit test method, not just excitation, and confirm instrument calibration.
- Compare against the rebuild report and factory test data; request the original test sheet.
- If the unit truly is ~4.5%, recalculate available fault current and re-coordinate the primary and downstream protective devices to the actual impedance.
- Verify the 65°C rise rating and loading assumptions still hold.
If it cannot be resolved before the scheduled energization:
- Do not energize on nameplate-based settings that may be unsafe.
- Either obtain corrected settings, or hold the transformer out of service until the impedance is confirmed and protection updated.
- Document the discrepancy and the decision; do not release the unit on the assumption that the nameplate is correct.