Determination of the Permissible Range for Surface Resistance of the Outer Corona Protection Systems in High Voltage Electric Machines

Outer Corona Protection (OCP) is one of the main components of the insulation system in high voltage machines. The main function of OCP is to eliminate high potential differences between the outer surface of the insulated bars (coils) and the grounded stator core in order to avoid surface discharge. To achieve this goal, the surface resistance of the OCP should be lower than a maximum value. However, there are some unpleasant consequences, such as vibration sparking and shorted core laminations, when the OCP is employed. To limit these side effects, other constraints on the maximum and minimum values of the OCP resistance should be considered. This paper tackles this issue from different aspects to provide the acceptable range for the OCP resistance. For this purpose, voltage and current at different positions on the OCP are calculated by analytical formula, taking into account the effects of capacitive currents of the bar insulation and magnetic flux of the stator yoke. Assumptions on the type and number of contact points between the OCP and stator core are made to consider the worst-case scenario. It is shown that the distance of these contact points changes the proper resistance range. Furthermore, the maximum allowed value of this distance is determined. The OCP resistance of a large turbine generator has been calculated by the proposed method. The satisfactory operation of this generator validates the calculation results.