J. Ramirez

Application of the SSA calorimetric technique to characterise an XLPE insulator aged under multiple stresses

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 cross-linking 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.

Calculation of thermal capacity of underground cables in multicircuit systems

An alternative for the calculation of thermal capacity of underground multicircuit transmission systems is proposed in this paper. The method used for this purpose is based on the calculation of the temperature developed in each component of the cable in the multicircuit system during a certain period of time. The load cycle associated to the cable system is known during this period of time. This method also takes into account the thermal transients suffered by power cables stressed to variable load cycles. The calculation of the temperature is done by means of thermal equivalent models of the phenomenon. In these models, different thermal parameters of the cables and trenches, thermal sources of each cable, and the effect between cables of the same circuit and cables of adjacent circuits located on the same trench are considered. The method mentioned considers the solving of numerical integration of differential equations associated to the thermal circuits and the multiconductor electric circuit, in order to determine the loses due to circular currents. In this manner, it is possible to include different types of cable (e.g. single-pole and three-pole cables. Pipe-Type cables, etc.) and grounding settings metallic shields and armors. The application of the method in a real life case is also exposed; the potential of the developed tool is also demonstrated.

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.

Effects of the PV-generator's terminals connection to ground on electromagnetic transients caused by lightning in utility scale PV-plants

The connection to ground of the negative or positive terminal of photovoltaic (PV) generators is a common practice to mitigate the effects of the potential induced degradation (PID), which is attributed to chemical reactions (ions exchange) between the materials that constitute the PV-module during operation. Furthermore, the complex topology of a utility scale PV-plant, its dimensions and the stored energy in the central PV-inverters DC-link open the hypothesis that overvoltages at the DC- and AC-side of the PV-plant may arise during transients caused by impulse currents. The objective of this publication is to explore the effects of this practice on the operation of utility scale MW-class PV-plants during transients caused by lightning.

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.

Evaluation of Lightning Impulse Test by Frequency Response Analysis

In this work are presented the basis for improving the interpretation of transformer lightning impulse test and the development of a graphical user interface system, which allows comparisons of time domain data and frequency response. The frequency response is obtained from deconvolution of voltage and neutral current records. A quantitative comparison of frequency response is performed using the techniques applied to displacement detection through frequency response analysis, such as correlation and spectral deviation. The system is implemented using 8 bit digitizers to acquire the voltage and neutral current records. The quantization error and reliability of the frequency response obtained is handled through the use of the coherence function and tolerance bands. The system is thoroughly tested applying a lightning impulse test to a dry type distribution transformer, simulating an inter-disc fault with a spark gap. Failure detection is confirmed