Space charge studies of crosslinked polyethylene midvoltage cable insulation by thermally stimulated depolarization current, infrared/Fourier transform infrared, and scanning electron microscopy

Abstract

We studied the space charge behavior in crosslinked polyethylene cable insulation with thermally stimulated depolarization current (TSDC). The role of the semiconducting layers of the cable in space charge formation was determined with several experiments carried out with two different cables. TSDC spectra were obtained through the exchange of the semiconducting layers of each cable with aluminum or silver coatings. Heteropolar current peaks at 80 and 105 °C were observed in the TSDC spectrum. As a sample was annealed at high temperatures, the creation of traps in the material was promoted, and a homopolar contribution at 98 °C developed. This homopolar mechanism was linked to the diffusion, with annealing, of ionic components from the semiconducting layers into the insulation bulk. The penetration depth of these components was studied with Fourier transform infrared in sections obtained at several depths starting at the outer surface of the cable insulation. These diffused components remained on the crystal surfaces, and they became trapping centers for free charge. Charge injection from the semiconducting electrodes and homopolar trapping of injected charges on these components took place afterwards during polarization. This process tended to saturation. Besides this mechanism, the continuous dissociation of molecules took place in the material with thermal annealing. This ionic charge generation resulted in an increase in the heteropolar contribution of the TSDC spectrum. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4164–4174, 2004

Annealing effect on the conductivity of XLPE insulation in power cable

“…In those measurements, annealing up to 400 h at 90 ºC with the SC shields resulted in increments in σ up to five times the value obtained when annealed without the SC shield. These results suggest that the presence and diffusion of croslinking by-products is not the main reason of the observed maximum in σ, however, long term diffusion of acrylate species and other components from the SC shields into the XLPE [15,24] can explain this behavior. { TC "5 Weight loss in XLPE insulation versus annealing time at 90 EQ \s\up5(o)" \l 1 } Assuming that conductivity in XLPE is associated with a hopping mechanism, as the Mott plot suggests (figures 3 and 4), we can adequately explain the observed change in σ after long annealing times as follows; In the hopping mechanism, the energy difference and the distance between traps should modulate the whole process.…”

“…{ TC "5 Weight loss in XLPE insulation versus annealing time at 90 EQ \s\up5(o)" \l 1 } Assuming that conductivity in XLPE is associated with a hopping mechanism, as the Mott plot suggests (figures 3 and 4), we can adequately explain the observed change in σ after long annealing times as follows; In the hopping mechanism, the energy difference and the distance between traps should modulate the whole process. If we take into account a possible diffusion of components from the SC shields into the XLPE bulk (as previous works suggest [24], annealing should promote an increase in conductivity because of the increase in traps density. This phenomena can explain the increase in σ observed for annealing times up to 30-40 days.…”

“…Bamji et al [22] also attributes to acetophenone some contribution to the 1305 cm -1 absorption peak, usually associated with the amorphous fraction of XLPE [23]. Acrylate species and other components are shown to diffuse from the SC shields into the XLPE with thermal aging [15,24]. These components can be identified as well by IR measurements as they show absorbency peaks at 1736 cm -1 [23].…”

“…A broad current peak can be observed at 105 ºC, which has been associated with charge in the crystalline volume [25]. The combination of TSDC and infrared spectroscopy (IR) studies [24] proved that annealing procedures in mid voltage cables lead to the diffusion of components from the SC screens into the XLPE insulation bulk. Moreover, the defects associated with diffused particles act as trapping centers of charge injected from the electrodes.…”

Dielectric characterization and conduction modelling of a water tree degraded LDPE

“…These mechanisms should take into account the processes of space charge accumulation, charge transport and the distributions and concentrations of trapping centers [39]. Recent investigations ("ARTEMIS" European project [14,40]) have already characterized PE insulation by using "ageing markers".…”