Different partial discharge (PD) detection and measurement procedures suitable for use on cables, capacitors, transformers and rotating machines are examined and compared. Both narrow and wide bandwidth PD detectors are considered; particular attention is given in regard to their suitability to different types of electrical apparatus and cable specimens under test as well as their applicability to discharge site location and their capability to detect different forms of PD. A rather substantial portion of the discussion is devoted to the use of intelligent machines as applied to PD pattern recognition in terms of either PD pulse-height/discharge epoch (phase) distributions or discharge pulse shape attributes.

Partial discharges. Their mechanism, detection and measurement

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.

Breakdown of Solid Dielectrics in Divergent Fields

Water tree growth in polyethylene cable insulation is discussed. The characteristics of water trees, the effect of aging parameters on water tree growth, and the possible mechanisms of growth are considered, emphasizing vented tree development in polyethylene insulating materials. The morphology of water trees, the characteristics of the tree-infested dielectric cable, and test methods and measures to reduce water treeing are discussed. >

Water treeing in polyethylene cables

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

The deterioration of polythene and other dielectrics, when exposed to internal discharges, has been examined under controlled conditions. The discharge-inception voltage and the magnitude and energy of individual discharges have been determined, and the location and number of discharges in voids of different dimensions have been studied, by electrophotography. The discharge sequence is explained by the difference in the distribution of the residual charges on the positive and negative surfaces of a void after a discharge. Accelerated deterioration tests using a frequency of 150 kc/s show that, initially, discharges cause slow erosion at the surfaces of the void and the formation of a transparent film of resin which fluoresces under ultra-violet irradiation. About 10
 -15
 cm
 3
 of polythene is eroded by each discharge, probably as a result of thermal degradation. The rate of deterioration increases rapidly with increasing voltage; at twice the discharge-inception voltage, the discharges concentrate and form several deep uncarbonized pits near the periphery of the void. When the pits attain a critical length, the mechanism of deterioration is believed to change; narrow semi-carbonized channels are found and, generally, breakdown follows immediately. Deterioration is greater when the void is adjacent to an electrode than it is when the void is enclosed in polythene. Tests show that, under equivalent conditions, polytetrafluorethylene and perspex are less resistant to discharges than is polythene.

J.H. Mason

Papers

The deterioration and breakdown of dielectrics resulting from internal discharges