Evaluation of high-field dielectric properties of polymers with superposition of AC and DC field

High-field dielectric properties of polymeric insulating materials under DC voltage application have been studied extensively, however, about their AC characteristics only a few papers have been presented. Although the waveform of high-field AC dissipation current has valuable information on the high-field dielectric loss property, it has not been studied because the separation of loss current from extremely large capacitive current is usually very difficult. Therefore, we have developed a new method for the detection of AC dissipation current and its waveform for dielectric materials around the power frequency by use of the unbalanced operation of the current comparator type high-field tanδ-capacitance bridge and the de-convolution technique in signal processing. Using this new technique and applying AC ramp voltage, electric field dependence of tanδ just after AC field application can be evaluated. The time variation of tanδ can also be evaluated for each cycle of AC applied field. In this paper, we apply this technique to the measurement of high-field dielectric properties with superposition of AC and DC field. The relation between AC loss property and superposed DC field immediately after applying both fields can be evaluated for each one cycle of AC field. Finally, this technique is applied to the measurements of high-field dielectric properties of an oxidized polypropylene (OPP) film and their relation to the superposed DC field. The relation between AC loss behavior and space charge formation of polymeric material can be clarified by using this measurement system