Fast and slow charge packets in polymeric materials under DC stress

The presence of slow space charge packets crossing the insulation thickness from one electrode to the other and causing significant electrical field distortion has been reported already in several papers. They are activated in general by very high DC fields or, in highly polluted materials, by relatively low fields and constitute an important ageing factor, concerning DC electrical stress. It has been observed, in fact, that such packets can cause accelerated breakdown of insulation. The development of fast systems for space charge measurements has allowed the presence of almost instant heterocharge to be observed close to electrodes in certain field and temperature conditions, especially in cable models. This has been explained often by the separation of ionic charge populations, even though such heterocharge appears also in materials, such as Polyethylene or cross-linked Polyethylene that represent the best extra-clean technologies. The measurements reported here use a high speed technique to investigate the build up of heterocharge in model cables that have been treated to remove volatile chemical species. They show that in fact the hetero-charge is built up by many very small and very fast charge packets (i.e. charge packets having a high mobility), which are injected from both electrodes and cross the insulation in less than one second. Because the packet charge is unable to exit the counter-electrode at the same rate at which it arrives, hetero-charge is built up within just a few seconds from the beginning of the polarization. The mobility of these charges, depending significantly on temperature, is estimated through observation of charge packets as a function of time, and compared with that of the already-known slow packets, generally occurring at higher fields with respect to fast packets. The basis for the interpretation and modelling of such phenomena is discussed.