Stator Insulation Performance: Six Decades of Experience

Measurements were performed phase-wise at multiple voltage levels using Schering bridge techniques under field conditions. Given the heterogeneous population, non-laboratory measurement environment, and uncontrolled temperature conditions, a semi-statistical approach was adopted. Ordinary least squares regression combined with median absolute deviation was applied to identify robust trends. The results indicate that older thermoplastic insulation systems generally exhibit higher absolute tan δ and capacitance values than modern thermoset designs. These differences are primarily attributed to material properties and manufacturing processes rather than in-service ageing. Further analysis demonstrates largely stable dielectric performance over several decades when production-related anomalies are excluded. Observed increases in low-voltage tan δ with service age are mainly linked to surface contamination effects rather than intrinsic insulation degradation. From a utility perspective, the findings suggest that both thermoplastic and thermoset stator insulation systems can exceed their nominal design lifetimes when supported by appropriate diagnostics, effective maintenance, and quality control.