Application of the SSA Calorimetric Technique to Characterise an XLPE Insulator Aged under Multiple Stresses

Abstract

In this work the thermal behaviour of crosslinked low density polyethylene (XLPE) used as an insulator for commercial underground high tension (15 kV) cables was studied. Three types of materials were selected: an uncrosslinked low density polyethylene (NXLPE) used as a control sample, an XLPE and an aged XLPE sample. The ageing conditions involved the application of multiple stresses: temperature, voltage and voltage impulses during 60 d under time and temperature cycles that are the most representative load of daily operation in Caracas, Venezuela. The effect of morphology segregation or thermal fractionation under multiple stresses conditions was analysed by measuring the percentage of crosslinking before and after the ageing tests were performed, and by investigating the thermal response of the material by conventional DSC and by the application of the successive self‐nucleation and annealing (SSA) thermal fractionation technique. The degree of crosslinking was found to vary in the material depending on the distance from the conductor because a thermal gradient is generated radially during the curing reaction. Such differences did not significantly affect the usual DSC heating scans of the samples. However, when SSA was applied, a difference in the distribution of thermal fractions was detected as a function of the distance towards the conductor that could be correlated to the variations in the crosslinking degree. After the accelerated ageing the thermal response of XLPE changes as evidenced by the presence of multiple melting peaks in subsequent DSC heating scans. This multiple melting was interpreted, as a first approximation, as arising from thermal fractionation during ageing (ignoring the possible effects of the other stresses applied) and SSA was able to simulate a similar fractionation that was very accurate in the prediction of the exact temperatures of the melting peaks produced.

Space Charge Profile of TR-XLPE Cables Aged Under Multiple Stresses

“…In consequence, some important characteristics are modified by their thermal history [1,2,3]. TR-XLPE can operate at 90°C continuously and in emergency situations it could stand temperatures up to 130 'C.…”

“…TR-XLPE can operate at 90°C continuously and in emergency situations it could stand temperatures up to 130 'C. The temperatures associated with the thermal gradients under which XLPE is degraded play an important role on the electrical, mechanical and physicochemical properties [3].…”

Thermal Properties and SSA Fractionation of Metallocene Ethylene‐Oct‐1‐ene Copolymers with High Comonomer Content Cross‐linked by Dicumyl Peroxide or β‐Radiation

“…[8][9][10] Some of them focus on thermal fractionation techniques such as successive selfnucleation and annealing (SSA). [11][12][13][14][15][16][17] But there is a lack of studies on the thermal properties of these copolymers after being cross-linked either by peroxides or b-radiation.…”

Comparison of the accelerated aging under different thermal cycles and voltages impulses in XLPE cables

“…When the cable reaches the temperature associated to the thermal gradient under which the cable is degraded, the polymeric material may be partially molten and self-nucleation, crystallisation and annealing processes may take place [2,3,4]. On the other hand electrical stress associated to voltage impulse contribute extensively to the deterioration of XLPE, fundamentally in regards to the development of the treeing phenomenon from inclusions or protuberances of the material [5,6].…”

“…T'his fact explains more that the outermost points of the conductor (denominated points 2 and 3 of the cross section, figure 6), undergo the greater deterioration and substantial changes in endothermic of fusion are observed there (figure 7 and 8) [2,3,11 The history of the thermnoelectrical aging undergone by the total of the polymeric samples with the number of temperature -voltage -impulses cycles increase.…”