Influence of paper insulation on the prebreakdown phenomena in mineral oil under lightning impulse

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This paper presents the results of experimental studies about the prebreakdown phenomena in mineral oil under lightning impulse. Experiments were carried out in a non-uniform field setup with HV electrode covered by crepe paper. The results of the experiments were compared with the results of research performed for bare HV electrodes. The onset voltage, propagation velocity, time to initiation, shadowgraph photos of discharges and light emission were respectively measured or recorded during the experiments. Analysis of the research outcomes indicates that the fundamental role of paper insulation is to raise the onset voltage by shoving away the onset sites out of the region of high field stress. The insulation wrapping does not have an influence on the shape of discharge patterns and on the propagation velocity. Some of the results connected with the times to initiation that the mechanism of discharge initiation at the HV electrode surface does not depend on the fact it is insulated. The source of weak points of the paper-oil insulation system is the oil, not the surface of insulation wrapping or metal of electrode. Observation of the oscillograms of light emitted by discharge channels indicates the difference in the mechanism of discharge propagation. The surface and space charge carriers left in the interelectrode space after discharge flash play an important role. In the case of insulated electrodes, the surface charge remains longer on the surface of electrode, reducing the electric field stress which is responsible for reignition. Simultaneously, the space charge reduces the degree of channel ramification. Both the observations are clearly visible on the oscillograms of light as differences in the frequency of light impulses and their peak values.