Electrical Characterization of Stress Grading Coating in High Voltage Stator Bars and Coils

High-voltage rotating machines rely on a complex insulation system whose reliability strongly depends on the behavior of stress control coatings (SCCs). These coatings, applied at the ends of stator bars, are essential for controlling electric fields and preventing partial discharges (PDs). This study presents an in-depth experimental investigation of the electrical behavior of SCCs made from silicon carbide (SiC) and iron oxide (FeO). Surface potential, surface resistivity, and corona PD measurements were conducted on stator bars with different manufacturers, levels of aging, and year of manufacture. High-resistivity coatings localize the electric field and raise PD activity, while optimal resistivity smooths the field. Surface potential measurement and corona PD measurement are effective tools to assess the quality of the SCC. However, humidity is identified as a significant inhibitor to corona PDs and may obscure anomalies during quality assessments.