How are SFRA curves read, and which part of the winding corresponds to each frequency range?
An SFRA plot shows magnitude ratio (dB) vs. frequency on a log scale. The trace is divided into three main regions:
|
Frequency Range |
Sensitive To |
Typical Features |
|
Low (10 Hz – 2 kHz) |
Core condition |
Dominated by core inductance; changes indicate residual magnetism, core grounding faults |
|
Mid (2 kHz – 200 kHz) |
Winding geometry (axial/radial) |
Resonant peaks from LC interactions; shifts indicate disc buckling, clamping pressure loss |
|
High (200 kHz – 2 MHz) |
Lead configuration & local structure |
Standing waves on leads; changes indicate displaced tap-changer or bushing lead movement |
Key interpretation rules:
1. A frequency shift of resonant peaks (left/right) → change in inductance or capacitance (deformation).
2. A magnitude change (> 3 dB) → change in damping resistance (loose connections).
3. New or missing resonant peaks → structural modification or internal damage.
4. Always compare phase-to-phase traces - a healthy unit shows near-identical traces across all three phases.
Quantitative criteria (per IEEE C57.149):
1. Correlation coefficient (CC) > 0.98 → windings in good condition.
2. 0.90 < CC < 0.98 → marginal, may require additional investigation.
3. CC < 0.90 → significant deformation, likely requires further inspection.