Comprehensive electrical and thermal analysis of the stress grading system of a large hydro generator

In the slot portion of a generator core the installed stator bars are covered with a semi-conductive coating to prevent partial discharge (PD) activity between the main insulation surface and the grounded laminated sheets. Typically, this outer corona protection (OCP) layer ends a few centimeters outside the slot portion. To suppress PD-activity at the end of the OCP, a field grading is mandatory for machines with a rated voltage greater than approximately 6 kV. Figure 1 shows the effect of a stress grading layer on stator bars energized to high voltage. It suppresses potential creepage discharges at the OCP ends caused by field enhancement.