Ageing of corona protection material on rotating machines

The outer corona protection (OCP) is a field grading part of an insulation system, which is located in the stator slot area and the step iron area of an electrical machine. Recent publications show increasing numbers of ageing phenomena regarding the OCP, unrelated to a specific manufacturer. Although no direct correlation could be determined between the phenomenon and essential damage of machine insulation, it is necessary to establish a basic understanding of its main cause and the contributing parameters in order to estimate possible consequences for the machine operation and future designs.