Causes of Resistance Decrease of Corona Protection Materials in Thermal Aging Process

During the operation of the high-voltage electrical machine partial discharge (PD) may occur in the slot of stator winding. A resistance conductive coating based on graphite is applied to the surface of wall insulation to eliminate PD. The coating resistance decreases rapidly when high temperature impact to insulation system. Low value of resistance can lead to the appearance of vibration sparking (VS). In our work, we consider a different factor affected to quality of conductive coating which include the chemical nature of the polymer matrix, the degree of curing of the binder, the degree of grinding of the filler (graphite) and the technological process of manufacturing enamel for a conducting coating. This paper shown that the unfinished process of forming the polymer matrix structure is the most probably cause of resistance instability.