Fast and slow charge packets in polymeric materials under DC stress

Abstract

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 heterocharge 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.

Fast soliton-like charge pulses in insulating polymers

“…These experiments were performed on 0.3 mm flat slabs in a PEA system with a carbon-loaded polyethylene as the high voltage electrode but aluminum as the earth electrode above the piezo-electric sensor. Preliminary results were presented in Fabiani et al 47 and more details are given in Montanari et al 48 Soliton pulses of both polarity were observed in all of the Bohemite loaded samples at E ¼ 20MV/m and 30MV/m and temperatures of 293 -313 K with a magnitude (1À2 Â 10 À6 Cm À2 ) that was much smaller than those observed in the XLPE-insulated mini-cable, but with similar repetition rates. In these materials also we confirmed the correlation between the charge solitons measured via the PEA system and current pulses.…”

“…Their repetition rate was found to be dependent only upon temperature and electric field, as will be shown in the following, see Tables I-IV. Measurements made every 25 ms for the first minute of voltage application allowed between 100 and 200 such pulses of each polarity to be observed. It was noted that: (a) the shape of each pulse was retained during its transit of the insulating layer, (b) the amplitude of the pulses exhibited a polarity dependence though all pulses of the same polarity had the same amplitude and showed little dependence upon field and temperature, 23 (c) the speed of transit was independent of radial position (within our time resolution) but depended upon temperature and field and polarity, (d) the pulse repetition rate was polarity dependent at the same temperature and field, resulting in situations where both a positive and negative pulse moving in opposite directions were present in the XLPE layer at the same time, Fig. 3.…”

“…Charge pulses have been observed previously in similar materials at E ! 80 MV/m 18,24-27 however in those cases the pulses often show variations of amplitude both during transit and over a sequence of pulses, and more importantly a mobility (l ¼ 10 À16 -10 À15 m 2 V À1 s À1 ), and activation energy (1-1.2 eV) 13,27 close to that of the carriers in steady state currents. Explanations for these pulses have been proffered in terms of a negative differential resistance, 28 imbalance between injection/extraction and transport 29 or a dc-conductivity discontinuity.…”

“…3 For the last 20 years it has been possible to quantitatively measure the space charge concentration within an insulator by means of pressure wave propagation (PWP) and pulsed electroacoustic (PEA) techniques [8][9][10][11][12] as a function of the time of application of the potential difference (or subsequent to its removal) and hence to determine directly the charge dynamics, thereby allowing a direct comparison of theoretical expectations with experimental data. Recently the time resolution of the PEA technique has been improved such that a complete profile of the space charge density can be obtained every 25 ms. 11,13,14 Application of the technique to miniature cables possessing a thickness of 1.5 mm of Cross-Linked-Polyethylene (XLPE) (Fig. 1) have revealed that at moderate fields (30 MV/m to 50 MV/m) the injection of charge instead of being continuous, 15 takes the form of small pulses (containing approximately 3-6 Â 10 9 electronic charges) that transit the insulation at speeds several orders of magnitude greater than those estimated from steady state currents in XLPE.…”

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“…If the applied field exceeds the threshold for space charge accumulation, charge injected from the electrodes can accumulate in traps located at the interface with electrodes and in the insulation bulk [1][2][3][4][5].The existence of such a threshold would have considerable impact on the design of insulation systems. Designing an insulation system for an electrical stress below the threshold would assure very long life and high reliability, provided that there are no other significant stresses acting on the insulation.…”