FRA for Reactor Type Testing: Verifying Air Gap Uniformity Before Shipment
Shunt reactors and current-limiting reactors rely on precise air gap distribution within the core to achieve linear inductance and avoid saturation. Manufacturing variations—uneven spacers, incorrect core bolt torque, or misaligned gap segments—can create asymmetrical gaps that alter reactor performance and vibration. A Transformer Frequency Response Analyzer applied during factory acceptance testing detects these gap uniformity issues before the reactor leaves the factory.
FRA Sensitivity to Air Gap Variations
The air gap length directly determines inductance. FRA detects gap variations through:
Resonant frequency shifts: A 5% decrease in effective gap (tighter clamping) increases inductance and lowers the first resonant peak by 2–3%.
Phase asymmetry among limbs: In three-phase reactors, mismatched limb gaps cause different natural frequencies, visible as split peaks in the combined FRA.
Amplitude variation in low-frequency band: Uneven gaps affect magnetizing impedance, changing low-frequency gain by 1–3 dB.
Factory Test Protocol for Gapped Reactors
During type testing, perform FRA in three configurations:
End-to-end measurement of each phase individually
Comparison of all three phases (should have CC > 0.95 between phases)
Measurement at 80% and 110% of nominal clamping force to verify gap stability
Case Example: Uneven Gap Detected in Factory Test
A 50 MVAr shunt reactor failed factory FRA: phase-to-phase CC between A and B was 0.97, but A vs. C was 0.82. The first resonant peak for Phase C was 2.1 kHz, vs. 1.9 kHz for Phases A and B. Disassembly found that Phase C gap spacers were 0.3 mm thicker than specified due to a machining error. The spacers were replaced, and FRA passed with CC > 0.96 across all phases. Without FRA, the reactor would have been shipped, causing unbalanced reactive power and elevated vibration in service.
For reactor manufacturers, FRA type testing is a low-cost, high-value quality assurance step that prevents field failures and warranty claims.
