Prevention of slot discharge and on-line condition monitoring of high voltage machine insulation

The various factors responsible for slot discharge damage are discussed. The lowering of surface resistance in combination with lamination edges is shown to be an important factor in establishing the conditions of the initial discharge. Space charges on the free surface on the coil can cause the development of a large voltage when the coil vibrates in the slot. This leads to discharges to the coating. A magnetic field effect in conjunction with a thermal effect can create a free surface or an isolated painted area responsible for the onset of discharges. A novel insulation technique has been developed which practically eliminates the slot discharge phenomena. The technique provides a reliable and stable electrical path for allowing the capacitive current of the main insulation to ground. An additional feature of this development is the provision of a simple online condition monitoring method. Information can be obtained about any discharging condition, ground fault in an isolated neutral system, and interturn fault that could lead to ground insulation failure