Thermally stimulated depolarization currents of crosslinked polyethylene relaxations in the fusion range of temperatures

Abstract

With thermally stimulated depolarization currents, we researched the relaxations of crosslinked polyethylene as it is used in medium-voltage cable insulation. Through conventional polarization two heteropolar peaks stand up in the spectra, at 80 and 105°C. As the sample is annealed, a homopolar peak is developed at about 99°C. With window polarization, our results indicated that the 80°C peak is a structured peak related to polar crosslinking subproducts and impurities. The 105 and 99°C peaks are fitted to the general kinetic-order model because the 105°C peak is related to free-charge detrapping at the crystalline phase, in the bulk and maybe at the amorphous-crystal interphases, and the peak that is observed at 99°C is due to injected charge. Annealing at high temperatures promotes the creation of traps in the material. Charge trapping at T Ͻ 70°C seems to be related to the increased insulator resistivity with annealing time.

Effect of annealing on conductivity in XLPE mid-voltage cable insulation

“…This process is clearly prominent at temperatures above the critical temperature, T c , approximately equal to 80 1C, because TSDC current reverses its polarity (from heteropolar to homopolar). Further annealing results in new current polarity inversion, and the current regains its heteropolar character [24]. These changes are in agreement with the fact that diffused components continue to penetrate the insulation volume.…”

“…The spectrum obtained (intensity of the depolarization current versus temperature) is complex and it is formed by several peaks associated with the relaxation processes of the conduction mechanisms that are present in the material. A broad current peak can be observed at 105 1C, which has been associated with charge in the crystalline volume [24]. The combination of TSDC and infrared spectroscopy (IR) studies [25] proved that annealing procedures in MV cables lead to the diffusion of components from the SC screens into the XLPE insulation bulk.…”

“…Thermally stimulated depolarization current technique (TSDC) has been applied to XLPE cable electrets [24,25]. The spectrum obtained (intensity of the depolarization current versus temperature) is complex and it is formed by several peaks associated with the relaxation processes of the conduction mechanisms that are present in the material.…”

Annealing effect on the conductivity of XLPE insulation in power cable

“…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.…”

“…Annealing at these temperatures reverses the polarity of TSDC spectra (from heteropolar to homopolar). Further annealing results in new current polarity inversion, and the current regains its heteropolar character [25].…”