A.E. Davies

Electrodes and charge injection in low-density polyethylene using the pulsed electroacoustic technique

The effects of electrode materials on space charge formation in low-density polyethylene (LDPE) have been investigated experimentally using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either the laboratory or power cable industry were selected, i.e. aluminum, gold and carbon loaded crosslinked polyethylene (XLPE), and space charge accumulation after the application of high electric stress was monitored. Experimental results demonstrated that charge injection processes take place in all cases once the applied stress has exceeded a threshold. However the amount of charge, and the polarity of the dominant injected charges showed a significant dependence on the electrode materials under the same applied electric stress.

Comparison of on-line partial discharge detection methods for HV cable joints

The capacitive coupler, acoustic emission (AE) sensor and radio frequency current transducer (RFCT) have been used to detect partial discharge (PD) activity within a 132 kV prefabricated cable joint containing a known defect. Although each of the three methods has been applied individually under different situations, a comprehensive investigation and comparison of these three methods has not yet been made. Results obtained were compared to measurements made using the conventional electrical detection method. The quantification of the capacitive coupler measured signal in mV with a discharge apparent quantity in pC has been investigated and an on-line quantification method proposed. The capacitive coupler has good detection sensitivity and PD location can be realised by studying the time of Right between signals from two sensors. The difference in pulse shape, spectrum and time of flight between an internal discharge and external interference has been investigated. The AE approach has the advantage of being free from electrical interference. However, investigation indicated that AE signals were significantly attenuated within the cable joint. RFCTs were used to detect the discharge current flowing through the cable sheath. Where the detection sensitivity was low, a wavelet de-noising method was applied to process the RFCT signals and proved to tie effective in increasing the measurement signal to noise ratio.

Effect of additives on morphology and space charge accumulation in low density polyethylene

The effects of an antioxidant additive on polymer morphology and space charge formation have been investigated, as a function of temperature, in melt-quenched films of low-density polyethylene. On quenching, the additive-free polymer crystallizes to give extensive spherulitic structures; similar objects are not seen in the system containing the antioxidant. The pulsed electro-acoustic method was then used dynamically to follow both the formation of space charge at high voltage and its subsequent decay under short circuit conditions in both material systems. Data were obtained at room temperature, 50 and 70/spl deg/C. Comparing the additive free material with identical polymer containing a standard anti-oxidant package indicates that the addition of the antioxidant markedly changes both space charge formation and decay processes. At room temperature, both materials were found to behave similarly, suggesting that the changes in gross morphology described above are of secondary importance. At higher temperatures, the addition of the antioxidant enhances negative charge accumulation in the material, implying its association with relatively deep traps for the negative charges within the bulk.

The influence of defects on the short-term breakdown characteristics and long-term dc performance of LDPE insulation

Extruded polyethylene is used as the bulk insulation for ac HV cables because of its high electric resistivity and breakdown strength. Although the material at present has limited use in dc power cables, it is used extensively in submarine optical communication cable systems. This paper reports on the study of the short-term characteristics and long-term performance of low-density polyethylene (LDPE) insulation under dc electric stress. The results are presented in which controlled defects as found in practical systems (voids, metallic and non-metallic particles) were introduced into well-characterized polymer material so that their influence on electrical strength and breakdown mechanisms could be determined. Samples were compression-molded under laboratory conditions and subjected to ramp-to-failure and electrical aging tests at various stresses. Weibull statistics are used to analyze the results. Examination of electrically aged samples indicates an oxidation degradation around the defects due to stress enhancement. From the failed samples a value of n in the inverse power model was estimated. The residual life of aged samples was also examined.

Partial discharge detection in cables using VHF capacitive couplers

In order to detect and locate partial discharge (PD) activity within HV cables and in particular cable joints, the feasibility of using a very high frequency WHO capacitive coupler technique has been investigated. In addition, an alternative PD offline test method for cable joints or cable sections within long cable systems has been developed. By energising the joint or a short section of the cable screen only this method has the advantage of reducing the power supply requirement of the test source. The application of capacitive couplers for this PD offline test method has been investigated. In order to use this approach there must be an insulation gap between the coupler and the outer sheath of the test cable. The effect of this gap on measured sensitivity has been investigated experimentally and through the use of a simulation model. Obtained results are in good agreement. Cross-correlation techniques have been implemented to automatically estimate the signal time of flight from different-coupler outputs. Methods of calibrating the coupler applied to the alternative PD offline test method have also been investigated.

Influence of radiation environments on space charge formation in /spl gamma/-irradiated LDPE

In the present paper the formation of space charge in irradiated low-density polyethylene (LDPE) was investigated using the pulsed electroacoustic (PEA) technique. Specimens of LDPE, /spl sim/100 /spl mu/m thick, were irradiated to various doses at room temperature in a /sup 60/Co /spl gamma/ source in one of three different environments: ambient room air, oxygen-free nitrogen and vacuum. The results obtained indicate that the space charge distribution is dependent on the irradiation environment. There is a large concentration of positive charge evident in the sample irradiated in air compared to very little for material irradiated in either vacuum or nitrogen under similar conditions. This indicates that the presence of oxygen during irradiation, and thereby oxidation itself, has a major influence on the mechanism by which space charge evolves. Thus the monitoring of space charge may in itself be an indicator of the degree of oxidation and of aging in a material. The evolution of space charge and its decay are discussed.

Morphology and the growth of electrical trees in a propylene/ethylene copolymer

Growth of electrical trees under 50 Hz high electric stress has been studied in a clarified propylene/ethylene copolymer, to explore the effects of the applied field and the material microstructure. Crystallization of the copolymer at low temperatures (<128/spl deg/C) produces a continuous lamellar texture and the material consequently is optically transparent. At higher crystallization temperatures (134/spl deg/C), more sporadic nucleation occurs and, as a result of the larger scale structural features that develop, the material becomes optically scattering. Nevertheless, CCD images of evolving tree structures could be obtained in both systems. Electrical treeing was found to occur reproducibly, but in a markedly different manner in the two morphologically different but chemically identical materials. In the low temperature crystallized copolymer, electrically conducting tree structures were found to develop with a growth rate that increased monotonically with increasing applied voltage. Conversely non-conducting tree structures formed in the 134/spl deg/C crystallized copolymer that mimic the well documented decreasing tree growth rate with increasing applied voltage behavior of both low density polyethylene and a flexible epoxy resin.

Application of acoustic emission techniques and artificial neural networks to partial discharge classification

Partial discharge (PD) detection, signal analysis and pattern identification, using acoustic emission measurements and the back-propagation (BP) artificial neural network (ANN) have been investigated. The measured signals were processed using three-dimensional patterns and short duration Fourier transforms (SDFT). Investigation indicates that using BP ANN with the SDFT components for classifying different PD patterns provides very good overall results.

Electrical reliability of dc-operated submarine telecommunications cables

The results of a research program carried out to quantify the reliability of joint moldings for dc operation in submarine telecommunications cables are reported. The region of the bulk insulation most at risk of electrical failure has been identified as the amalgamation zone of the molding, which is believed to be associated with additional electrical stress-enhancing defects present at the conductor-insulator interface. In addition to factory tests implemented to eliminate moldings with severely hazardous defects, a quality assurance sampling test procedure is proposed to optimize the manufacturing quality.

Space charge measurements in cables using the PEA method:-signal data processing considerations

Acoustic characteristics in the new cable PEA system have been analysed and it has been confirmed theoretically that the new system can provide the same sensitivity and resolution as the curved electrode system. The new system offers several advantages such as easy assembling sample, intimate contacts at the interfaces and its suitability for different sized cables. Several factors that influence the quantitative charge distribution in cable geometry such as divergence (both in pulsed electric stress and acoustic pressure) and attenuation/dispersion of the acoustic wave in thick insulation are discussed. A data processing technique has been developed which considers all of the above factors in order to improve the measurement accuracy both in sensitivity and spatial resolution. The results from the two XPLE power cables with different thermal treatments have demonstrated that the new system, together with data processing algorithms is capable of measuring space charge distribution and its change with the treatments.