Pattern Recognition in Three-Phase FRA: What Phase Discrepancies Tell You About Fault Type

When a Transformer Frequency Response Analyzer is applied to a three-phase transformer, comparing the frequency responses across phases often provides diagnostic insights even without a historical baseline. The pattern of deviations—whether symmetrical across all three phases or isolated to one or two phases—directly points to different failure mechanisms. Mastering this pattern recognition skill transforms phase comparison from a fallback method into a proactive diagnostic tool.

Symmetrical Deviation: All Three Phases Affected Equally

A symmetrical deviation means the FRA signatures of all three phases have shifted in a similar manner relative to their expected or baseline shapes. Common causes include:

• Global core issues: A loose core clamping structure or a shifted yoke affects the magnetic circuit of all phases uniformly. This typically appears as low-frequency (

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• Temperature effects: When comparing a baseline taken at 20°C to a test at 50°C, all phases shift similarly, especially in the high-frequency band (>200 kHz).

• Tap changer position change: If the de-energized tap changer was moved from position 1 to position 3 between tests, all phases will show a consistent, predictable shift.

• Different grounding scheme: A change from tank ground to bushing ground affects the reference potential for all phases equally.

Symmetrical deviations rarely indicate winding damage because winding deformations are almost never identical across all three phases. Before concluding a fault, rule out temperature, tap position, and grounding differences.

Asymmetrical Deviation: Single or Two Phases Affected

When one phase deviates while others remain normal, or when two phases show similar changes but the third does not, mechanical damage is highly likely. Typical scenarios:

• Single-phase through-fault: A fault on Phase B of the low-voltage side subjects only that phase to high electromechanical forces, causing axial or radial displacement visible in mid-frequency band (2–200 kHz).

• Shipping impact: A dropped or jolted transformer often damages one outer phase (Phase A or Phase C) due to asymmetric mechanical stress during transport.

• Single-phase short-circuit in service: The affected phase may show broad damping across all frequencies, indicating shorted turns.

• Partial core damage: A shifted core limb on Phase B affects only that phase’s low-frequency response.

Quantifying Asymmetry with Statistical Indices

Beyond visual inspection, compute the correlation coefficient (CC) between Phase A and Phase B, Phase B and Phase C, and Phase A and Phase C. In a healthy transformer, phase-to-phase CC typically exceeds 0.95 in the mid-band for symmetrically designed units. If one phase-pair shows CC < 0.85 while the others remain >0.95, the deviating phase is the suspect. Similarly, compute the relative deviation index (RDI) by normalizing each phase’s trace to the average of all three phases; a phase with RDI > 3 standard deviations from the mean warrants investigation.

Case Example: Asymmetrical Pattern After a Lightning Strike

A 15 MVA, 69/13.8 kV transformer experienced a lightning strike on the high-voltage bushing of Phase C. Post-event FRA testing showed:

• Phase A vs. Phase B correlation: 0.97 (normal)

• Phase B vs. Phase C correlation: 0.68 (significant deviation)

• Phase A vs. Phase C correlation: 0.71

The deviation was concentrated in the 50–150 kHz range on Phase C. Internal inspection revealed that the high-voltage winding of Phase C had experienced radial buckling on the first five discs. Phase A and Phase B were undamaged. This asymmetrical pattern was the key indicator that ruled out global causes and triggered targeted internal inspection.

Special Case: Two-Phase Symmetrical Deviation

Occasionally, two phases show identical deviations while the third remains unchanged. This pattern suggests:

• Outer phases (A and C) may both be damaged due to mechanical resonance during transport, while the center phase (B) is protected by its position.

• A tap changer issue affecting only two phases if the tap changer has independent phase mechanisms (rare).

In such cases, treat the two affected phases as suspect and investigate further with DGA and turns ratio testing.

By systematically analyzing phase discrepancy patterns, engineers using a Transformer Frequency Response Analyzer can rapidly triage faults, focusing costly internal inspections only on transformers where asymmetrical deviations point to genuine mechanical damage.