Dry-band discharges on polluted silicone rubber insulation: control and characterization

Inclined-plane tests of silicone-rubber samples prepared with textured patterns of hemispherical protuberances are described. These are compared with the results obtained for conventional non-textured plane-surface samples of the same material. A remarkable improvement is achieved by such texturing: a material which normally fails the test is enabled to satisfy the standard test criteria, and the tracking and erosion of the samples caused by the tests are significantly reduced. These texturing techniques are currently being extended to prototype composite insulators. It is shown that this improvement for textured samples is a result of the formation of a stable, controlled dry band on the polluted surface. Although discharge inception occurs across the band, these discharges result in a fall, rather than an increase, of the leakage current. The transition to a higher current arc phase is thus inhibited, compared with non-textured samples for which stable dry bands did not occur. These observations of reproducible dry band breakdown also provide an opportunity to characterize experimentally the electrical properties of these discharges. A recent proposed model represents the dry-band discharge as a streamer-spark structure, in contrast to the standard model of an arc-like interdependence of current and voltage. These two models are tested against the present new data, and the discharge is shown to have streamer characteristics.