Frequency dependence of high field dissipation current waveform of LDPE film

Non-polar polymers, such as polyethylene (PE), polypropylene (PP) and so on, are widely used as insulators for power cable and capacitor. In ac high field region, it is known that various time dependent mechanisms, such as carrier injection from electrodes and space charge formation, are getting dominant. Here, the dissipation current waveform shape begins to show a nonlinear property. To study the conduction mechanism, especially about the space charge effect under ac high field, by using low density polyethylene (LDPE) film, frequency dependence of high field dissipation current waveform observations at room temperature are carried out in the frequency range from 50 to 200 Hz under certain fixed ac high field. These results shows nonlinearity depend on not only applied ac field but also frequency. These results indicate that the momentary time differential of applied ac field dE(t)/dt will be also the one important factor on considering the ac high field condition mechanisms. From the results of these experiments, it is found the two kinds of onset values for showing nonlinear properties. These are the momentary time differential of applied ac field dE(t)/dt and the current density. The values are around 12 kV/(mm - ms) and around 8-10 nA/mm², respectively. When they exceed these values, then, the dissipation current waveform deviates from the sinusoidal characteristic and starts to show nonlinear properties.