Field Case Study: Uncovering Axial Winding Displacement with FRA After a Through-Fault
This case study examines a 30 MVA, 115/13.8 kV power transformer that experienced a bolted three-phase fault on the low-voltage bushing side. The fault was cleared by the upstream breaker after 150 ms. Post-event inspections included dissolved gas analysis (DGA), turns ratio testing, and short-circuit impedance measurements—all returned within normal limits. However, the utility maintenance team remained concerned about possible mechanical damage to the windings and deployed a Transformer Frequency Response Analyzer for further investigation.
Test Setup and Baseline Comparison
The transformer had a baseline FRA fingerprint from its factory acceptance test two years prior. Measurements were performed using an end-to-end configuration (H1–H0, H2–H0, H3–H0) across a 20 Hz to 2 MHz sweep. The test setup strictly followed the original grounding scheme and lead routing documented in the baseline report. Ambient temperature and tap changer position were also matched to the original conditions.
Observed Deviations in the Mid-Frequency Band
When the post-fault FRA traces were overlaid on the baseline, phase B exhibited a distinct deviation in the 5 kHz to 80 kHz region. Specifically, two resonant peaks originally located at 12 kHz and 45 kHz had shifted downward by approximately 2 kHz and 4 kHz respectively, accompanied by a 1.5 dB amplitude reduction. No significant changes appeared below 2 kHz (core region) or above 200 kHz (lead region). Phase A and Phase C remained nearly identical to their baselines.
Statistical Quantification and Interpretation
Using the correlation coefficient (CC) and absolute sum of logarithmic error (ASLE) indices:
Phase A CC = 0.98 (no significant change)
Phase B CC = 0.73 (moderate deviation, actionable threshold below 0.85)
Phase C CC = 0.97
The ASLE for Phase B in the 5–80 kHz band exceeded the historical fleet alert level by 40%. According to IEEE C57.149 interpretation guidelines, mid-frequency deviations localized to a single phase strongly suggest axial or radial winding displacement on that phase, likely caused by the electromechanical forces during the through-fault.
Inspection and Repair Findings
The transformer was de-energized, drained, and internally inspected. On Phase B, multiple spacer blocks had shifted axially by approximately 8–12 mm, and two winding discs showed slight tilting. No shorted turns or conductor damage were present. This matched the FRA prediction precisely. The repair involved repositioning the spacers, re-clamping the winding assembly, and performing a light reconditioning. A post-repair FRA test confirmed that the frequency response returned to within 2% of the original baseline.
Lessons Learned for Asset Management
This case illustrates three critical takeaways:
Normal DGA and impedance results do not rule out mechanical damage after a through-fault.
Mid-frequency FRA deviations are the primary indicator of winding displacement.
Having a reliable baseline fingerprint is essential for confident diagnosis and repair validation.
The utility subsequently updated its maintenance protocol to mandate FRA testing on all transformers subjected to fault currents exceeding 80% of nameplate impedance capability. The avoided cost of a catastrophic failure was estimated at over $1.2 million, justifying the FRA investment many times over.
This real-world example confirms that the Transformer Frequency Response Analyzer is not merely a laboratory instrument but a field-proven diagnostic tool that delivers actionable intelligence, prevents unplanned outages, and guides cost-effective repairs.
