Mechanical degradation of hydro generator stator core: advanced diagnostics, on-site remediation, and engineering insights from a field case

Mechanical damage in stator cores is a multifaceted problem, often involving lamination stack distortion, slot geometry deformation, localized hot spots, and structural misalignments. These defects can lead to elevated core losses, non-uniform magnetic flux distribution, insulation stress, and even secondary electrical breakdowns. Hence, early-stage detection and root cause analysis are critical to prevent catastrophic failures. This paper emphasizes the integration of advanced condition assessment techniques—including Electromagnetic Core Imperfection

Detection (EL CID), flux density probing, infrared thermography, and visual inspection—to accurately characterize the extent of mechanical damage and formulate an evidence-based repair plan.

The discussion centers on a comprehensive case study involving a 122 MVA, 13.8 kV, power factor-0.9, 16 pole vertical hydro generator that suffered mechanical failure due to a displaced rotor V-block. This event resulted in a complex failure mechanism involving physical indentation of stator slots, lamination fusion, vent duct deformation, and axial misalignment of the stator core pack. EL CID testing revealed elevated inter-laminar circulating currents indicative of core shorting and insulation compromise. Conventional approaches would necessitate complete core replacement, typically involving extended downtimes and high costs.

However, given logistical constraints and asset criticality, an innovative in-situ repair methodology was adopted. Fig: -1 Stator top view The field repair process was executed within the generator housing, following a meticulously engineered sequence: localized disassembly, lamination flattening, targeted lamination replacement using laser-cut blanks, mechanical realignment, and reinforcement of vent spacers and clamping structures. The repair adhered to IEEE and IEC guidelines for core rebuilding, and was validated through post-repair EL CID scans, polarization index measurements, and thermal profiling during no-load and load runs. Subsequent operational monitoring over a twoyear period confirmed mechanical stability and thermal performance within nominal thresholds, validating the long-term effectiveness of the repair.

This paper offers technical insights into failure propagation from mechanical anomalies to electrical degradation, the diagnostic resolution of high-sensitivity tests such as EL CID, and the engineering pragmatism required for large-machine field repairs. Attendees will gain a practical understanding of the challenges associated with stator core damage, the decisionmaking process for repair versus replacement, and the importance of asset-specific maintenance strategies to extend service life and minimize unplanned outages.