Root Cause Analysis of Insulation Failure in Hydro-generator Stator Windings

Stator windings in hydro-generators undergo progressive degradation over long-term operation due to combined thermal, electrical, mechanical, and environmental stresses. These stresses facilitate the formation of internal voids within insulation materials, degradation of semi-conductive layers, and surface fouling of end-windings, which collectively induce phase-to-phase discharges and eventually result in catastrophic insulation failure. Recently, the rapid expansion of wind and solar power has intensified the intermittency of renewable energy sources. This shift has forced hydro-generators to undergo more frequent start-stop cycles, leading to the formation of defects at welded joints connecting the stator windings a phenomenon increasingly linked to recent insulation failures. This paper presents a root cause analysis (RCA) of stator winding insulation failures in two hydro-generators following long-term operation. A hydro-generator (26.76 MVA, 10.2 kV) in service for 14 years, experienced a phase-to-phase insulation breakdown between the U and V phases. Another hydro-generator (27.8 MVA, 11 kV) after 31 years of operation, exhibited abnormal thermal behavior, with U-phase temperatures peaking at 134°C, which necessitated an emergency shutdown. Based on the RCA, subsequent repairs and component replacements were performed. Finally, diagnostic insulation tests were conducted to verify the integrity and reliability of the restored stator windings.