Comparative Study on Different Outer Corona Protection Materials for Rotating High-Voltage Machines

The insulation system of rotating high-voltage machines faces high requirements regarding longevity and functionality. A part of the insulation system is the outer corona protection (OCP), which serves as a field grading element within the slot of the grounded stator core. When eroded, the OCP loses its field grading ability and thus enables further deteriorating processes. Material components, manufacturing, thermal, electrical, environmental and mechanical stress influence the partial discharge driven erosion of the OCP. This paper presents a temperature dependent investigation of several OCP materials on a model geometry equipped with a typical 11kV insulation thickness. The investigated parameters are the erosion induction time as well as the erosion rate under combined electrical and thermal stress. To evaluate the influence of different OCP materials, model specimens with OCP consisting of different carrier materials are evaluated. Comparative measurements at room temperature, 90°C and 130°C are conducted. The results show no correlation between the thickness of the OCP and the erosion process. Nevertheless, the material can have an influence on the erosion rate and elevated temperatures accelerate this process significantly. For the manufacturing process appears to have a considerable impact on the erosion process, it is advised to investigate the manufacturing process as well as to confirm the compatibility of the individual insulation components (e.g. resin, binder, OCP, main insulation) to ensure longevity of the insulation system.