AC charge injection investigated by means of space charge measurements: threshold and frequency dependence

The object of this paper is to clarify some of the items that regard space charge injection and accumulation in AC in polyethylene-based materials. In particular, the dependence of space charge injection threshold on supply frequency (from DC to 1000 Hz), as well as of space charge accumulation on electrical stress, frequency and time are dealt with. It is shown that AC charge injection is asymmetric, from positive to negative half waveforms, and that space charge can be accumulated with time, reaching steady conditions in times from several hours to days depending on supply voltage magnitude. However, the amount of charge accumulated in AC is always smaller that under DC field, decreasing as frequency increases and tending to zero above 50 Hz.