Recognition of Partial Discharge Trends and Patterns in 11 kV Motor Stator Windings

As the stator windings of weather protected type II(WPII) high-voltage(HV) motors undergo prolonged operation, degradation occurs through interfacial delamination and void formation within the groundwall insulation(GWI). Additionally, the outer corona protection (OCP) conductive layers deteriorate within the slot and slot-exit regions, while environmental contaminants accumulate on the core and endwinding surfaces. These factors not only amplify partial discharge(PD) magnitudes but also induce significant shifts in phase-resolved PD(PRPD) patterns. This paper presents insulation condition assessments performed on five 11 kV WPII motors operated for seven years at a 1,000 MW coal-fired power plant. For motors No. 1 and No. 2(2,700 kW), the AC current increment (△I), dissipation factor increment(Δtanδ), and PD magnitudes remained within nominal thresholds at the rated phase voltage(6.35 kV). However, critical PD activity was detected at 1.25 times the phase voltage(7.94 kV), necessitating a proactive maintenance recommendation. Furthermore, for motors No. 3 through No. 5(1,500 kW), a longitudinal analysis of PD characteristics was conducted after a two-year post-maintenance interval to verify the long-term efficacy and dielectric stability of the applied insulation reinforcement.