Evaluation of Insulation Characteristics in High-Voltage Motor Stator Windings Under Varying Environmental Conditions

When a high-voltage (HV) induction motor is disassembled for stator winding maintenance, moisture ingress frequently occurs on the winding surfaces due to rapid ambient humidity changes. Although operational practices involve hot-air drying followed by insulation resistance and diagnostic tests up to the rated AC voltage, a detailed comparative analysis regarding the degradation stage is required. This paper presents a comparative analysis of insulation diagnostic parameters for two distinct stator windings under both dry and moisture-absorbed conditions : an aged 18-year-old HV motor (336 kW, 4.0 kV) and a newly manufactured HV motor (1,000 kW, 6.6 kV). AC current, dissipation factor (tanδ), capacitance, and partial discharge (PD) magnitudes were measured with increasing applied voltage. The experimental results reveal that while both motors exhibit elevated values under moisture-absorbed conditions compared to their dry baselines, the aged motor shows significantly higher sensitivity and distinct PD phase patterns. These findings demonstrate that the combined analysis of tanδ and PD characteristics can effectively distinguish between transient environmental moisture absorption and permanent insulation degradation, providing critical indicators for HV motor asset management.