Measurement of spatial charge distribution in thick dielectrics by using pulsed electroacoustic technique

摘要

When high dc voltage is supplied to a power cable, space charges are formed in the polyethylene insulating material [1]. These space charges are induced by the injection of carriers, from electrodes, into the insulating material, or by the ionic polarization generated by impurities. The remaining charge may also distort the normal electric field distribution in the materials. Thus, for designing an electrical insulating system of high dc voltage apparatus, it is important to measure the space charge distribution within the insulator. The direct measurement of space charge distribution has previously been unsuccessful. Recently, Lewiner [2], Sessler [3], and Krutz [4], have succeeded in the direct observation of space charge distribution in thin dielectric materials using a laser-pressure-pulse technique. For measuring accumulated charges in thick insulating materials, the authors developed a new pulsed electroacoustic technique [5,6]. This technique was applied to enable the observation of space charge distribution in polymethylmethacrylate (PMMA) during and after electron beam irradiation. The spatial charge distribution profile as a function of time was also analyzed by means of a deconvolution procedure. Further to this, the process of charge decay after irradiation was discussed by using a model of shallow trap level in the irradiated region of PMMA.