Matheus Andrade Martinez

Evaluation of 400 kV Silicone Insulators Aged under Natural Tropical Conditions

Experience of polymer insulators used in transmission lines has now existed for almost 50 years and are widely used by many electricity companies around the world. Their superior performance under high pollution levels, as a consequence of their hydrophobic surface property, constitutes one of most important advantages. Lightweight, low maintenance cost and resistance to vandalism are also additional advantages of the composite insulators use. A first diagnosis, which includes a complete scheme of electrical, mechanical and physicochemical tests, was performed on 400 kV silicone rubber insulators removed after five years in service in the North/East region of Venezuela. The purpose of this paper is to present a summary of the visual inspection and main electric tests results. Having a short time in operation the insulator surface analysis gives information about the early aging period characteristics where the hydrophobicity can be still recovered. New studies would be planned in the future to find the degradation rate of the polymer insulators at this transmission line.

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/spl deg/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.

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

In this paper the space charge distribution profile along the thickness of tree-retardant crosslinked polyethylene (TR-XLPE) cables, commonly used in medium voltage system and degraded under multiple factors as temperature, AC and impulse voltages are presented. A comparison of the space charge distribution along the insulation thickness (radial direction) of samples extracted from 15 kV TR-XLPE cables, 2 AWG Cu conductor, manufactured under dry curing process was done. Two 7 m long cable samples were submitted to accelerated ageing; one sample was under thermal stress and the other was under electrothermal factors combining thermal cycles with 25 kV AC voltage applied continuously and impulse voltage transients. Looking at the results it can be concluded that after the first thermal cycle the space charge is reduced in comparison with the profile of the new sample. This confirms that crosslink by-products are expelled under thermal treatments. The space charge density is higher in the central region of the insulator thickness of the new sample and aged samples as well. It is also evident that the electrothermal ageing cycle is more severe because it releases more quantity of space charge along the insulation thickness than the thermal one.