Corona screen effectiveness in large machines under high voltage, high frequency transient conditions

In a class ´F´ insulated winding, the development of high energy discharge activity is almost always associated with discharge sites on the coil surfaces, rather than within the insulation tape structure. The function of the corona screen applied to coil surfaces within the slot section of the winding is to short out the air gap between coil side and slot wall and thus to prevent slot discharge across this gap. The effectiveness of this technique is well established and significant slot discharge under normal power frequency conditions is only possible if the screen has been damaged in some way. However, in certain circumstances, high energy discharge may be possible from the surface of a corona screen under high frequency transient conditions even if the screen is undamaged. Whether or not this presents a threat to the integrity of insulation depends upon the frequency of occurrence of the transients, as well as the discharge energy. However, machines operating in hazardous areas, where all potentially incendive effects are a matter for concern, are designed and built to standards which are intended to minimise the risk of ignition of flammable gas mixtures. This paper reports on a detailed study of high energy discharge from a corona screen under high frequency transient conditions, and particular attention is paid to the nature of the follow current, which is a characteristic of discharge under high frequency conditions and which may significantly increase the incendive potential of the discharge