TR-XLPE medium voltage cables subject to accelerated aging cycle under multiple stresses

In this article we report the most important results about the evaluation of TR-XLPE cables after an accelerated aging process under multiple stresses: temperature, AC voltage, impulse voltages and humidity. This work was developed at the Simon Bolivar University High Voltage Laboratory, Venezuela. 15 kV cables were subjected to thermal cycles between 55 - 130°C, 26 kV AC voltage and (+/-) 88 kV impulse voltages applied periodically. The cables were aged in a duct filled with water. Dissipation factor and permittivity were evaluated periodically. Differential scanning calorimetry (DSC), crosslinked percentage, crystallinity and tree formations were evaluated periodically in a radial direction across the thickness of the samples and longitudinally along the cable. The most important observation was the increment in the dissipation factor after impulse applications and their corresponding reduction after thermal cycles. Important changes in DSC and considerable loss of crosslinked reticulation percentage, mainly in the region near the conductor and the termination zone, were detected. This research makes evident the nonuniformity of the properties according to the radial and longitudinal position. This is a result of different thermal-electric stress across the thickness and in the termination area. In consequence, at these points multiple stresses lead to premature degradation.