High field dielectric properties of nonpolar polymeric films

Nonpolar polymeric materials, such as LDPE (Low Density Polyethylene), PP (Polypropylene), and so on, are widely used for an insulator of power cable, a dielectric material of power capacitance, etc. due to their high insulating performance. In high field region, it is known that the dissipation current waveform shows nonlinearity and the electroluminescence (EL) is also observed at the initial stage of degradation, that is, just after the high field application [1]. These phenomena are considered that it is relating with the charge injection from the electrode and the instantaneous space charge formation at the vicinity of metal electrode. Thus the investigation of the relation between the dissipation current waveform and EL is important on to clarify the conduction mechanisms under ac high field, which is connecting to the developing the further high performance insulation material and the studying the degradation phenomena [2]. As the space charge formation is a time dependence phenomena, in this study, the electric field dependences of dissipation current waveform and EL have been investigated at the frequencies 50, 200, 400 and 1,000 Hz, respectively, by using LDPE and PP film. As the results of experiments, the inception field of EL between LDPE and PP were examined. The inception field for PP was slightly higher than that for LDPE. Similar trend for these samples was also confirmed on the electric field dependence of the third harmonic component of dissipation current waveform. However, the relation between the dissipation current and EL pulse number between LDPE and PP were similar at each frequency. When the frequency varied from 50 to 1,000 Hz, as the dissipation current increase with the frequency rise was large compared with the increase of the number of EL pulse, the properties themselves were shifted to the large dissipation current in the higher frequency. It is indicated that the frequency rise makes the quantity of space charge layer small, and it - ay cause the internal field small compared with the applied field. Then it is considered that the contribution to the EL decreases as the results of frequency rise and most of injected charges contribute the conduction. In this study, the conduction mechanisms of nonpolar polymer under ac high field are discussed from the results of dissipation current waveform and the number of EL pulse distribution.