A. Boubakeur

Influence of barriers on the lightning and switching impulse strength of mean air gaps in point/plane arrangements

The influence of clean and polluted barriers on the breakdown voltage of point/plane mean air gaps (30 to 200 cm) under lightning and switching voltages is addressed. The conditions of existence of flashover around the barrier without breakdown and without surface discharge are determined. It is shown that a clean insulating barrier situated near the electrode point, particularly at about 20% of the gap, leads to a significant increase in the breakdown voltage of the air gap. This increase is especially due to channel elongation of the disruptive discharge. A polluted barrier with surface conductivity higher than 1.6 mu S leads, similar to a metallic barrier, to a decrease in the breakdown voltage as compared with that without a barrier. A simplified model to estimate the optimal position of the barrier, considering the surface charge densities on both of its sides, is presented. It gives results in good agreement with experimental ones. >

Dielectric and mechanical behavior of cross-linked polyethylene under thermal aging

This paper deals with the influence of continuous thermal aging on the dielectric and mechanical properties of Cross-Linked Polyethylene (XLPE) used as electrical insulation in high voltage cables manufactured by ENICAB (Algeria). In order to characterize the degree of the material degradation under thermal aging and to prevent its failure, continuous accelerated thermal aging tests during 5000 hours on dumb-bells and circular shaped probes of Cross-Linked Polyethylene XLPE UNION CARBIDE 4201 were performed at four levels of temperature ranging from 80/spl deg/C to 140/spl deg/C. Some results showing the changes in dielectric losses factor, dielectric constant, resistivity, elongation at fracture and tensile strength with respect to the aging time at the different used aging temperatures are presented. The obtained results show that the thermal aging affects considerably the properties of the material.

Influence of Artificial Thermal Aging on Transformer Oil Properties

Abstract This article presents research results on electrical and physic-chemical properties of insulating oil used for high-voltage applications under accelerated artificial thermal aging. Unlike in previously published work, this article gives an extensive set of test data of artificially aged transformer oil under various temperatures. In addition, the aging effect of test temperature on various properties of the oil was quantified. Continuous thermal aging tests were made on transformer oil (Borak 22, Sonelgaz, Nynas, Austria) using a 5000-hr test at four different temperatures of 80, 100, 120, and 140°C. First, the electrical properties of transformer oil, such as the dielectric dissipation factor and the breakdown voltage, are measured after every 500 hr of aging. Next, the physicochemical parameters, including the acidity degree, water content, color factor, kinematic viscosity, density, and flash point, are quantified in order to find the changes in the chemical composition of the insulating oil. The results show that thermal aging significantly affects the properties of transformer oil by the formation of acidic products and the formation of water in oil, which lead to the degradation of the oils electrical and physicochemical properties.

DSC Study of Artificial Thermal Aging of XLPE Insulation Cables

This communication describes work and gives results focused on the application of Differential Scanning Calorimetry (DSC) to reveal the effect of thermal aging on the crystallinity variation of XLPE insulation cables. The changes in the crystalline phase of XLPE during aging at four different temperatures have been analyzed. This study shows also the effect of aging on the melting peak temperature Tm and melting enthalpy ¿Hm. The obtained results pointed out that thermal aging at temperatures above the melting temperature of XLPE have a great effect on the crystallinity variations.

Prediction of mechanical properties of XLPE cable insulation under thermal aging: neural network approach

The widespread use of Cross-linked Polyethylene (XLPE) as insulation in the manufacturing of medium and high voltage cables may be attributed to its outstanding mechanical and electrical properties. However, it is well known that degradation under service conditions is the major problem in the use of XLPE as insulation in cables. In order to reduce the aging experiments time, we have used Artificial Neural Networks (ANN) to predict the insulation properties. The proposed networks are supervised and non supervised neural networks. The supervised neural network was based on Radial Basis Function Gaussian (RBFG) and was trained with two algorithms: Backpropagation (BP) and Random Optimization Method (ROM). The non supervised neural network was based on the use of Kohonen Map. All these neural networks present good quality of prediction.

Influence of discontinuous pollution width on the surface conduction. Frequency characteristics of the leakage current

In this report, we have examined the influence of the discontinuous pollution layer width on the behavior of a laboratory plan model on which we reproduce the surface state of the 1512L insulator greatly used in Algerian desert areas. The investigations were carried out by analyzing the leakage current magnitude, the phase angle current-voltage, the flashover process and leakage current characteristics. Because the recorded current waveforms include too many distortions, we have elaborated a numerical tool treating the leakage current signal allowing one to determine its frequency spectra using the discrete Fourier transform. The phase angle is determined from voltage and current phase spectra or also from the angles of their fundamentals.

Influence of thermal ageing on transformer oil properties

In this paper, we present the electrical and physic-chemical behaviours of oil in high voltage transformers, under thermal ageing. Continuous thermal ageing tests were made on transformer oil BORAK22 during 5000 hours at two different temperatures 80degC and 120degC. Here, every 500 hours, the dielectric losses factor, the relative permittivity and the resistivity were measured. Other properties such as the breakdown voltage, the acidity degree, the water content, the colour and the cinematic viscosity are measured before and after ageing. The obtained results of this investigation show that the thermal ageing affects greatly the properties of transformer oil.

Effect of artificial thermal aging on the crystallinity of XLPE insulation cables: X-ray study

X-ray diffraction has been applied to reveal the effect of thermal aging on the crystallinity variation of XLPE insulation cables. The changes in the crystalline phase of XLPE during aging at different temperatures have been analyzed. The carried out investigations pointed out that thermal aging at temperatures above the melting temperature of XLPE have a great effect on the crystallinity variations.

Numerical model describing the effect(s) of a barrier and the space charge fields on the electrical strength of a point-plane air gap using the finite elements

In this paper, we have computed fields of the positive ions and electrons created at the head of avalanche in a positive point-barrier-plane air gap. The polarisation field effect of the dielectric is taken into account. The general problem will be formulated with finite elements and it is focussed on the polarisation field formulation. It was found that the effect of the two space charge fields is to reduce the breakdown voltage.

Comportement diélectrique et mécanique du polyéthylène réticulé chimiquement soumis à un vieillissement thermique continu

Abstract In this paper, we present the electrical and mechanical behaviour of Cross-Linked Polyethylene (XLPE), used as electrical insulation in high voltage cables, under thermal ageing. Continuous accelerated thermal ageing tests on 18/30kV XLPE cable samples were performed during 2000 hours at four temperatures ranging from 80 °C to 140 °C. Every 200 hours, we measured the partial discharge threshold voltage, the dielectric loss factor and the dielectric constant. The mechanical properties (elongation at fracture and tensile strength) were measured before and after thermal ageing. The obtained results show that the thermal ageing affects greatly the properties of the material.