P.G. Halaris, A.D. Polykrati, P.T. Tsarabaris, J.S. Katsanis, and E.I Koufakis (Greece)
HVDC stress, dielectric aging, solid insulator, partial discharges, impact ionization.
In this work, capacitance change (ΔCM) measurements during HVDC stress of the model solid-liquid insulators, are given. Samples of these materials (e.g. industrial bakelite specimens) with thickness 1 mm were electrically stressed with high DC voltage (half-wave rectified AC) in an insulating oil environment. The electrode set-up used was metal tip-sample-metal plane in insulating transformer oil environment. Measurements were taken by increasing the imposed voltage in steps of 1 kV until the ultimate breakdown of the insulators. At each voltage level, the electrical stress duration was 2 minutes, 5 minutes, and 10 minutes. The charge which flows through the sample, as a result of partial discharges, and the capacitance change of the solid dielectric, was measured. The partial discharges appeared in the oscillographs in the form of potential steps in the measuring capacitor voltage, which was placed in series with the electrode set-up. From these measurements the exponential increase of the capacitance change (ΔCM) of solid insulators in relation with the value of the imposed electric field was noted. The satisfactory agreement of theoretical data and experimental measurements show that impact ionisation takes place during the dielectric ageing stages of solid insulators.
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