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It has been well recognized in the past that one of the most common causes for insulation system failure occurs from void inclusions, which are usually introduced during the various manufacturing steps associated with the formation of the insulating materials themselves or the composite insulation systems used. For example, in the case of polyethylene cables, voids are generally introduced during the extrusion process. They may also occur due to the maintenance of a poor vacuum during the impregnation cycle as is often the case with epoxy cast instrument transformers or the oil-impregnated-paper insulating systems used in cables, capacitors, and transformers. Whether or not the presence of certain voids in a given insulation system can eventually lead to breakdown will depend primarily on whether or not the occluded voids undergo corona discharge under the operating voltage stress and on the intensity of the corona discharge process itself as well as on the degradation characteristics of the insulating materials involved. Obviously, voids, which do not discharge under the normal operating voltage conditions, are quite innocuous. In fact, discharge-free voids represent the near-ideal case of a loss-free dielectric.
Hydro-Quebec Institute of Research, Varennes, Quebec