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Surface potential decay dynamic characteristics of negative-corona-charged fibrous dielectric materials

The dielectric behavior of an insulating material is commonly characterized by exposing its surface to a DC corona discharge for relatively short duration and then recording the decay of the electric potential Vs at its surface, Vs = f(t). The representation of the surface potential decay (SPD) by using tdVs/dt = f(log t) plots has already been used to perform a finer analysis of the dielectric response of film insulators. The present paper takes advantage of this peculiar way of representing the SPD in view of characterizing the dielectric behavior of fibrous electrets negatively-charged from a triode-type (wire-gridplate) electrode system. The experiments were performed in ambient air (temperature: 20 ± 1 °C), at various values of the relative humidity (range 25 % to 65%). This paper reports experimental data collected on a polypropylene (PP) film (sheet thickness: 250 μm) and two types of PP non-woven media (fabric thickness: 300 μm), characterized by different fiber diameters (20 μm and 4 μm). The initial surface potential and the dynamics of SPD of fibrous dielectric materials were found to be affected by the corona charging conditions (relative humidity of the ambient air, grid voltage and grid current), as well as by the properties of the fibers and the structure of the non-woven media. An appropriate model of SPD should take into account charge injection from the media-electrode interface, charge transport controlled by bulk mechanisms and local high electric field effects due to the porous structure of the media.