Polymer insulation operated at voltages below critical inception voltage (CIV, corona inception voltage, partial discharge inception voltage) sometimes fail in a few months to a few years, showing the same physical appearance as partial discharge induced failures. Morphologic examination of polyethylene (PE) suggests that inhomogeneities may provide electrical conducting paths to polymer cavities. Hence, the authors performed calculations and experiments applying voltage to a test cell simulating a high-resistance conducting path in series with a cavity in PE. These experiments generated chemical changes on the surface wall of the cavities for both sub-corona currents (currents produced by voltage below CIV) and corona currents in different cavities. X-ray photoelectron spectroscopy (XPS) spectra of the cavity surface chemical species indicate similar moieties (functional groups), implying that sub-corona current or discharge is causing failures similar to corona insulation failures. The longer time for the sub-corona species to develop is consistent with field failure occurrence within a few months to a few years. Finally, the authors develop electric field conditions favorable to the existence of sub-corona currents at relatively low voltage.
