There has been speculation in the past about the extent to which space charge formed by the flow of prebreakdown current affects the electric strength of polythene In the present work, conduction and absorption-current measurements up to about half the breakdown field have given clear evidence of space-charge effects Further evidence was obtained from measurements of the apparent ‘intrinsic’ electric strength under various modes of stressing It was found that the electric strength of polythene measured with direct voltage below about 30°C exceeded the corresponding value measured with impulse voltages by up to about 25% The impulse electric strength could be increased to equal the direct voltage by prestressing each specimen for 4min with direct voltage of the same polarity as the impulse voltage However, when the impulse was of opposite polarity to the direct prestressing voltage, the electric strength decreased with increase in the prestressing field The effect was dramatic, for the impulse electric strength changed from the normal value of about 6MV cm−1 to less than 2MV cm−1 as the prestressing field was increased to about 75% of the electric stength measured with direct voltage Experiments in which the breakdown impulse was applied some time after removal of the prestressing field showed that the impulse electric strength at 20°C recovered in about 10−2s to the value obtained with impulses without prestressing Thus the prestressing caused no permanent damage The observed effects are attributed to space-charge injection from the electrodes The observations are not consistent with the concept that breakdown occurs when the field at a point within the specimen exceeds a critical value, the intrinsic electric strength of polythene

Conduction in polythene with strong electric fields and the effect of prestressing on the electric strength

This paper describes the effects of frequency, voltage, temperature and mechanical stress on the times to breakdown of XLPE cable insulation subjected to highly divergent fields. Temperature and mechanical stress reduce the times to breakdown but tests with increasing voltage and high frequency produced unexpected results which could not be explained by the existing theory of electrical tree growth. Results of partial discharge measurements indicate that space charges are dominant in determining the shape of the tree and the time to breakdown of the insulation.

An Investigation into the Growth of Electrical Trees in XLPE Cable Insulation

Mechanisms of water treeing are reviewed. A definition of water trees is proposed on the basis of permanent the hydrophilic character and the reduced breakdown strength. Properties are discussed that are relevant to the water tree definition and to the inception and propagation mechanism. This includes the water content, the (sub)micrometer structure and the dielectric properties. The discussed propagation mechanisms include electro-mechanical forces, diffusion of hydrophilic species and two types of oxidation mechanisms. Also attention is paid to the correlations between processes and aging conditions. Inception mechanisms include injection of hydrophilic species and/or oxidation in damaged regions, electro-mechanical damage due to high electrical fields.

Inception and propagation mechanisms of water treeing

The results of an investigation into electrical tree growth in XLPE cable insulation using an embedded needle electrode are reported for a range of voltages from 9 kV rms to 27 kV rms. The partial discharge (PD) activity and tree structures were measured simultaneously throughout the tree growth and the trees were recorded from initiation up to and including the final runaway stage. A multifractal analysis was also performed on the tree structures as they propagated, and it was found that their fractal dimension increased and the distribution of embedded structures changed as small side channels were added to the tree as it grew. At 11 kV rms only branch trees were found and only bush (bush-branch) trees at higher voltages, but at 9 kV rms trees of three different shapes were formed. Observation of the tree shapes at 9 kV rms under reflected light followed by a detailed analysis using con-focal Raman spectroscopy, showed that the stagnated and branch-pine (monkey puzzle) tree shapes were due to the formation of a conducting graphitic deposit upon the walls on tree branches in the region of the needle electrode. This was not present in the branch trees produced at 9 kV rms. A simple scheme is presented for the formation of branch-pine trees and their corresponding PD activity based on the concept of conducting branch generation. The trees produced at 13 kV rms and above have a bush shape, which converts into a bush branch shape when a runaway branch grows from their periphery. This is shown to happen when the field at the bush tree periphery exceeded a voltage independent critical value, which was estimated to be 100 MV/m. The consequence of this result for the initiation of the runaway stage in branch trees is commented upon.

Effect of tree channel conductivity on electrical tree shape and breakdown in XLPE cable insulation samples

Electrical treeing is one of the main reasons for long term degradation of polymeric materials used in high voltage AC applications. In this paper we report on an investigation of electrical tree growth characteristics in XLPE samples from a commercial XLPE power cable. Electrical trees have been grown over a frequency range from 20 Hz to 500 Hz and images of trees were taken using CCD camera without interrupting the application of voltage. The fractal dimension of electric tree is obtained using a simple box-counting technique. Contrary to our expectation it has been found that the fractal dimension prior to the breakdown shows no significant change when frequency of the applied voltage increases. Instead, the frequency accelerates tree growth rate and reduces the time to breakdown. A new approach for investigating the frequency effect on trees has been devised. In addition to looking into the fractal analysis of tree as a whole, regions of growth are being sectioned to reveal differences in terms of growth rate, accumulated damage and fractal dimension.

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Electrical treeing characteristics in XLPE power cable insulation in frequency range between 20 and 500 Hz

Breakdown channels are shown to propagate from steel needle-point electrodes embedded in polythene and polyisobutylene when the maximum effective stress at the end of the point reaches the intrinsic electric strength of the material at the test temperature. The average breakdown stress shows no significant variation with the radius of curvature of the point (for radii of 1?20 micron), but, at room temperature, about 25% greater stress is required with a negative than with a positive point. These results are explained by the effect of enhanced conductivity and space-charge accumulation increasing the effective point radius to some 25 microns for positive and about 45 microns for negative points. The effect of polarity decreases with increasing temperature and is negligible at 100°C. The decrease of the average stress required for breakdown with increasing electrode separation is explained, and the same concept is used to predict the variation of the industrial electric strength of materials with specimen thickness. Factors affecting the industrial electric strength of materials are discussed and illustrated by tests on polythene, polystyrene and cellulose acetate, using British Standard electrodes in air and in clean and contaminated transformer oil.

J.H. Mason

Papers

Breakdown of Solid Dielectrics in Divergent Fields