Study about conduction mechanism for polymer insulating material under ac high field application

Nonpolar polymers, such as low density polyethylene (LDPE), polypropylene (PP) and so on, are widely used as a dielectric insulation of power cable or power capacitor. It is considered that the charge injection from the electrode will become dominant under ac high field for such kind of non-polar polymeric materials. Under ac high field, it is well known that the EL pulses mainly appear at the former part of each half cycle of full ac waveform due to the space charge formation at the vicinity of electrode. For the same reason, the shape of dissipation current waveform also shows a peak at the former part of each half cycle as well. In order to grasp the charge behavior under ac high field, the simultaneous observations of the dissipation current waveform and the EL under the ac high field have been carried out the electric field from 1 to 25 kV/mm with 1 kV/mm step at the frequency 50 Hz for LDPE and PP films. As the results of experiments, it was found that though the nonlinear property which mentioned above was observed for LDPE film from the electric field over 14 kV/mm, in the case of PP film, it was not observed and the magnitude of dissipation current and the EL pulse´s number for the PP film. It seems to indicate that only the charge injection does not make the peaks appear at the former part of each half cycle and it requires the space charge formation at the vicinity of electrode.