Corona discharges in d-c and partially rectified a-c insulation systems

During the past several decades of rather intensive study of corona (partial) discharges and their effects on insulation systems, most of the attention has been focused on a-c discharge behavior. This has been rightly motivated by the much more serious effects of corona in most a-c insulation applications, where usually many more discharges can occur, during an average time, than with d-c. Because discharges in d-c insulation systems are much more intermittent, their rate of erosion or penetration of insulation barriers is correspondingly reduced, and it has been practical to apply much higher d-c stresses, without completely preventing corona, than with a-c cases. This characteristic of d-c stresses has been utilized effectiyely in d-c over-voltage testing of insulation systems, particularly of rotating machines.