Electric discharges in air-gaps facing solid insulation in high-voltage equipment

The performance of high-voltage alternators under conditions of over-voltage is materially improved by the application of an insulating sheathing to exposed parts of the bare earthed metal. The authors first derive the physical explanation for this observation from experiments using auto-photographic recording methods. These experiments are then expanded to cover other phenomena observed with discharges between insulated electrodes and lead to problems of discharges in voids. In particular, two hitherto unexplained phenomena are considered: the increase of discharge current with rising voltage, and the origin of the long discharge-free range in the decaying part of alternating voltages. Both these observations can be explained by the loss of some of the surface charges left from earlier discharges due to local reversals of the electric field. Some of the explanations of the probable mechanism by which charges are lost must still be considered as tentative, and more experimental evidence will be needed for a complete clarification of the processes involved. The results give some information on the effect of harmonics on internal discharges, which have been observed to lead to accelerated ageing of dielectrics.