Temperature dependence of charge packet velocity in XLPE cable peelings

Abstract

Abstract-The generation and transit of charge packets in 150m thick peelings from the insulation of Medium Voltage Cables manufactured using the same XLPE batch have been investigated at a number of different temperatures. Charge packet motion was investigated in peelings taken from cables that have been electro-thermally stressed at T= 90C for 5000 hours with a Laplacian field E  20kV/mm at the location of the samples. It was found that charge packets were generated by an applied field of 120kV/mm when the internal space charge field reached 140-150kV/mm. Measurements were made in three controlled temperature environments and it was found that the transit time fitted an Arrhenius behaviour with an activation energy of 1.2 eV. This value is similar to estimates made for the deepest traps (>1.1eV) made from the decay of space charge accumulated at lower fields of 46.67kVmm. It is also close to the activation energy for the conductivity (1.25 eV) obtained from the dielectric spectroscopy of unpeeled cable sections at E rms  0.41 V/mm, and 1.15eV from the dielectric spectroscopy of the peelings (E = 40V/mm). It is therefore concluded that the transit of the charge packet is associated with the trap-to-trap transport of charge carriers located in the deepest traps available. The implications of these results for the mechanism of charge packet generation is discussed.

Fast and slow charge packets in polymeric materials under DC stress

“…The mobility of slow charge packets has been estimated from data at a single temperature to have large activation energies of the order of 1 eV or more [27]. Measurement of the temperature-dependence of the transit time of slow packets [18] in XLPE material of the same origin as that used here yields an activation energy of 1.2 eV for the mobility of positive packets, see Fig. 7.…”

“…Anyway, the nature of charge transport in packets as well as the reason why packets remain as packets and do not spread out, particularly in the case of the slow packets whose transit may take hours even in thin 150 m samples, requires a detailed analysis of the transport mechanism that will be reported in a subsequent paper. What does seem clear is that the explanation of charge packets as the manifestation of a peak in the dependence of the charge carrier speed as a function of increasing electric field, [15], cannot be accepted as the packet does not slow down when the field at the packet front increases, either during transit of the sample or in a packet sequence [10,18]. Nor do the packets speed up when the packetfront field diminishes in a slow charge packet sequence.…”

“…Such a procedure was justified in [18] for slow charge packets where it was pointed out that the packet speed did not change with changes in field generated at the packet front, either as the packet crossed the insulation or in a packet sequence generated under the same applied field. The values of average mobility calculated according to eq.…”

“…The mobility of the slow packets reported in [18] was found to be 1.5x10 -16 m 2 V -1 s -1 at T=293 K, and was associated with charge residing in the deepest traps accessible in the narrow range of trap energies estimated from charge decay during depolarization at lower applied fields, see [28]. Simulations using an exponential trap energy distribution, [8], were able to reproduce features of the low-field charge distribution in similar XLPE peelings at 298 K with a charge mobility that goes from 10 -14 to 10 -13 m 2 V -1 s -1…”

“…It has been observed, in fact, that such packets can cause accelerated breakdown of insulation [15,16], particularly when they are associated with a continual charge accumulation [17]. Various theories have been proposed to explain the generation of slow packets, from hysteresis of the fielddependence of the injection current, or fluctuations in the current injected from electrodes, to field-induced ionization of impurities or neutral sites [15,[18][19][20][21]. Only one of these theories has reached the stage of a successful simulation [21], where the creation and transport of large slow negative packets was described by assuming field ionization in a three-level system comprising an ionizable state, trap state, and valence band.…”