Hydrogen-Cooled Generator FRA: Diagnosing Stator Winding Integrity Under Pressure
Large turbine generators (200–1500 MVA) use hydrogen cooling for superior heat dissipation. The stator windings operate in a pressurized hydrogen atmosphere (3–6 bar). Over time, winding vibrations, thermal cycling, and hydrogen pressure variations can cause conductor deformation, spacer displacement, or core movement. A Transformer Frequency Response Analyzer, adapted for generator stator windings, detects these mechanical changes before they lead to short-circuits or ground faults.
Generator Stator vs. Transformer Windings: FRA Differences
Stator windings differ from transformer windings in ways that affect FRA:
Bar windings (Roebel bars) with transpositions, not continuously wound coils
Slot-embedded conductors with magnetic wedges
End-winding support structures (glass-epoxy rings, blocking brackets)
Hydrogen atmosphere (εr ≈ 1.0003) vs. oil (εr ≈ 2.2), raising resonant frequencies by 30–50%
Detecting End-Winding Displacement
End-winding movement (common in generators with high vibration) produces:
Mid-band (5–100 kHz) amplitude reduction of 3–8 dB due to changed mutual coupling between bars
Additional resonant notches from spacer or blocking bracket loosening
Phase asymmetry between phases (for three-phase generators)
Case Example: End-Winding Blocking Failure
A 300 MVA hydrogen-cooled generator experienced increased vibration during operation. FRA performed during a maintenance outage (stator terminals de-energized, hydrogen replaced with air) showed:
Phase A: CC = 0.97 vs. baseline (normal)
Phase B: CC = 0.82, with a 5 kHz downward shift of the 45 kHz resonant peak
Phase C: CC = 0.96
Internal inspection found a cracked glass-epoxy end-winding blocking ring on Phase B, allowing bar movement. The ring was repaired, and post-repair FRA returned to CC > 0.96. Without FRA, the cracked ring would have progressed, causing bar-to-bar contact and a costly stator rewinding.
For hydrogen-cooled generators, FRA provides non-invasive stator winding integrity assessment without rotor removal.
