S.G. Swingler

Structure-property relationships in polyethylene blends: the effect of morphology on electrical breakdown strength

Blends of linear and branched polyethylene were prepared covering the composition range 1–20% linear polyethylene, and three thermal treatments were subsequently chosen to produce a range of different morphologies. Isothermal crystallization at 124 °C gives rise to compact linear inclusions within a matrix of branched polyethylene, isothermal crystallization at 115 °C produces an open, banded spherulitic morphology and, finally, quenching leads to a continuous spherulitic texture. Ramp testing was then employed to investigate the effect of morphology on electrical strength. It was found that the electrical strength of the blend depends primarily on the morphology and that, by optimizing thermal treatment and linear polyethylene content, substantial improvements in properties can be obtained.

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.

The effects of measuring technique and sample preparation on the breakdown strength of polyethylene

The effects of specimen thickness, testing conditions and preparation technique on the measured ramp-breakdown strength of 2 polyethylene systems have been investigated. The obtained data have been found to be highly reproducible, enabling results under different test conditions to be correlated with one another. In all cases, an increase in ramp rate results in an increase in the measured breakdown strength, as does a reduction in sample thickness. Irrespective of the test parameters, the short-term performance of a polyethylene blend containing a designed microstructure is always improved compared to that of the base low density polyethylene alone. Impulse testing leads to an identical conclusion. The effect of the blending method has been shown to have a negligible effect on the breakdown strength, despite consequent changes in nucleation density, implying that it is the local lamellar texture that is responsible for the enhanced properties of the blend.

Effect of water absorption on dielectric properties of nano-silica/polyethylene composites

The effect of moisture content on the dielectric properties of polymer/nano-silica blends was investigated. It was found that the DC breakdown strength, electrical conductivity and complex permittivity were all strongly influenced by absorbed water. However, a control sample without nano-silica was largely unaffected by changes in moisture content. This has important implications for researchers and cable designers.

Continuous on-line monitoring of partial discharges in high voltage cables

This work reviews data acquisition, transmission and processing methodologies for continuous on-line monitoring of partial discharges in power cable systems. A PD continuous online monitoring system for underground cable circuits using capacitive couplers, LiNbO/sub 3/ electro-optic modulators, laser, optical switch and optical fibers has been proposed. Data processing methods include pulse shape and spectrum analysis, time of flight analysis, phase-resolved two-dimensional histograms, three-dimensional /spl phi/-q-n patterns, trend analysis, wavelet-based denoising, alarm strategy and telecommunication function to enable remote control and data download.

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.

Do semicons affect space charge

This paper reports on space charge measurements on /spl sim/1.5 mm thick XLPE and polyolefinic plaques with two different treatments and semicon electrodes, namely, the same XLPE and polyolefin loaded with carbon. Samples were subjected to both DC and 50 Hz AC electric stresses in the region of 25 kV/mm (at room temperature). Measurements of charge profiles and stress distributions were made using the modified laser induced pressure propagation system. Emphasis has been placed on comparing the space charge characteristics of the two insulation systems with the different sernicon electrodes. The effects of sample treatment (i.e. degassing) on the space charge dynamics are also presented.

Space charge measurements in polymeric HV insulation materials

This paper reports on a study using the laser induced pressure propagation (LIPP) method to measure the space charge characteristics in bulk crosslinked polyethylene (XLPE) insulation under dc electric stress. Particular attention has been given to the quantitative appraisal of calibration parameters, and the resultant estimations of space charge and electric stress distributions. A method is described for analyzing the measured raw data from ramp voltage and aging tests to give an estimate of charge and stress without resorting to a complicated mathematical exercise. The consequence of stress enhancement due to the trapped charge on the life of the insulation is estimated using the empirical inverse law. Using analysis of variance and a multivariate analysis technique, the effects of treatment and type of XLPE on the space charge characteristics are also discussed.

The influence of survival mechanical stress and voltage frequency on electrical tree in XLPE

The initiation and growth of electrical trees under high electric stress have been studied in XLPE to explore the effects of the internal mechanical stress. Treeing tests were carried out under an applied voltage of 7 kV rms and both tree initiation and growth were monitored. The needle with a tip radius around 5 /spl mu/m was inserted into an XLPE block at elevated temperature that was cut from a XLPE power cable. Two types of samples have been prepared i.e. one with a significant internal mechanical stress around the tip of the needle and the other without. It was found that in the presence of the mechanical stress around the tip the tree initiation time is significantly shorter and growth rate is faster compared with those without. The combining effects of the internal mechanical stress and the frequency of the applied voltage on the structure of electrical tree during the growth stage have also been investigated. For example, in the presence of the internal mechanical stress, the results showed that the tree has a mixture of branch-like and bush-like structures at low frequency. In the absence of the mechanical stress, the results indicated that the tree at the early stage has many thicker branches at high frequency and thinner branches at low frequency.

On the effect of functionalizer chain length and water content in polyethylene/silica nanocomposites: Part I — Dielectric properties and breakdown strength

A series of nanoparticles was prepared by functionalizing a commercial nanosilica with alkylsilanes of varying alkyl tail length, from propyl to octadecyl. By using a constant molar concentration of silane, the density of alkyl groups attached to each system should be comparable. The effect of chain length on the structure of the resulting nanosilica/polyethylene nanocomposites was examined and comparison with an unfilled reference system revealed that, other than through a weak nucleating effect, the inclusion of the nanosilica does not affect the matrix structure. Since water interacts strongly with applied electric fields, water was used as a dielectric probe in conjunction with dielectric spectroscopy to examine the effect of the nanofiller and its surface chemistry on the system. Sets of samples were prepared through equilibrating under ambient conditions, vacuum drying and water immersion. While the water content of the unfilled polymer was not greatly affected, the water content of the nanocomposites varied over a wide range as a result of water accumulation, in a range of states, at nanoparticle interfaces. The effect of water content on breakdown behavior was also explored and, in the unfilled polymer, the breakdown strength was found to depend little on exposure to water (∼13% reduction). In all the nanocomposites, the increased propensity for these systems to absorb water meant that the breakdown strength was dramatically affected (>66% reduction).