Showing papers by "John C. Fothergill published in 2008"

The effect of water absorption on the dielectric properties of epoxy nanocomposites

Abstract: In this research, the influence of water absorption on the dielectric properties of epoxy resin and epoxy micro-composites and nano-composites filled with silica has been studied. Nanocomposites were found to absorb significantly more water than unfilled epoxy. However, the microcomposite absorbed less water than unfilled epoxy: corresponding to the reduced proportion of the epoxy in this composite. The glass transition temperatures (Tg) of all the samples were measured by both differential scanning calorimetry and dielectric spectroscopy. The Tg decreased as the water absorption increased and, in all cases, corresponded to a drop of approximately 20 K as the humidity was increased from 0% to 100%. This implied that for all the samples, the amount of water in the resin component of the composites was almost identical. It was concluded that the extra water found in the nanocomposites was located around the surface of the nanoparticles. This was confirmed by measuring the water uptake, and the swelling and density change, as a function of humidity as water was absorbed. The water shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. This has been discussed in terms of a percolation model. At 100% relative humidity, water is believed to surround the nanoparticles with a thickness of approximately 5 monolayers. A second layer of water is proposed that is dispersed but sufficiently concentrated to be conductive; this may extend for approximately 25 nm. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 3 to 4 nm thick at 100%. This "characteristic thickness" of water surrounding a given size of nanoparticle appeared to be independent of the concentration of nanoparticles but approximately proportional to water uptake. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. Comments are made on the possible effect on electrical aging.

Space charge formation and its modified electric field under applied voltage reversal and temperature gradient in XLPE cable

Abstract: The results of space charge evolution in cross-linked polyethylene power cables under dc electrical field at a uniform temperature and during external voltage polarity reversal are presented in the paper. A mirror image charge distribution was observed in the steady state, but the pre-existing field altered the way in which the steady state charge distribution was formed from that obtaining when the cable was first polarized. Polarity reversing charge was generated in the middle of the insulation and moved towards the appropriate electrodes under the influence of a field in excess of the maximum applied field. Our results show that the mirror effect is a steady state effect that is due to cross-interface currents that depend only on the interface field and not its polarity. Measurements on cable sections with an elevated mean temperature and temperature gradient show that the interface currents are temperature dependent, and that differences between the activation energies of the interface and bulk currents can eliminate, and possibly even invert the polarity of the space charge distribution.

Statistical Analysis of Partial Discharges from Electrical Trees Grown in a Flexible Epoxy Resin

Abstract: Electrical treeing is a long-term degradation mechanism in polymeric insulation, which can lead to electrical failure of HV insulation systems. The rate at which trees grow across the insulation depends on the PD activity occurring within them and hence the detection of the onset of electrical treeing could be established by PD monitoring. In this paper, a statistical analysis of the partial discharges detected during the growth of trees in an epoxy resin will be reported. The aim of this work was to provide additional insight into the physical mechanisms that lead to the observed fluctuations in the partial discharge activity. The results demonstrate interesting correlations between a number of statistical parameters, such as average discharge magnitude and standard deviation in the partial discharge amplitudes. These correlations could also be related to physical parameters such as the applied voltage magnitude and the measured power dissipation due to the partial discharges occurring during tree growth. The implications of this work for deterministic methodologies for the simulation of tree growth as well as for condition monitoring using feature recognition strategies for the early detection of tree growth will be discussed.

PD Pattern Recognition Using ANFIS

Abstract: An application of an adaptive neuro-fuzzy inference system (ANFIS) has been investigated for partial discharge (PD) pattern recognition. The proposed classifier was used to discriminate between PD patterns occurring in internal voids. Three different void shapes were considered in this work, namely flat, square and narrow. Initially, the input feature vector used for classification was based on 15 statistical parameters. The discrimination capabilities of each feature were assessed by applying discriminant analysis. This analysis suggested that some of the features possess much higher discriminatory power than the others. As a result, a simplified classifier with reduced feature vector has been obtained. The results demonstrate the importance in identifying and removing redundancy in the input feature vector for reliable PD identification.

Optimizing trading capabilities and reliability of power transmission networks

Abstract: The European Commission's Energy Green Paper 2006 identifies the need to develop a flexible single European grid as one of the most important priorities of the European Union energy policy for the next few years. This fits a background of increased renewables generation and an increasing mix of generation and transmission technologies. In this paper we will present a scheme that addresses an approach of increasing the trading capacity, controllability, stability and reliability of such a future power transmission network. Further, the scheme seeks to achieve this with reduced life cycle costs and enhanced environmental benefits.

Direct analysis of heterogeneous insulators with the PWP and the PEA methods

Abstract: In this paper a simple numerical signal analysis applicable to PWP and PEA methods is presented in order to directly extract the space charge distribution from measurements.

Effect of temperature and voltage application time on space charge decay of HDPE

Abstract: Polyethylene is a material that is widely used as insulation for power cables. In this paper, the effect of temperature and voltage application time on space charge decay in HDPE is described. Negative charge was found to be retained by the samples when a field of 50kV/mm was applied at temperatures of 25 and 40 degC, with an amount that increased with increasing voltage application time. The same field applied for 4800s at 60degC gave both negative and positive space charge, but only positive charge at 90 degC The decay time of the negative charge increased as the amount of negative space charge retained after voltage removal increased. This indicates that much of the trapped negative charge could not be de-trapped easily. In contrast positive space charge was observed following the application of 180 kV/mm and 300kV/mm. The speed of positive charge decay in these cases was much faster than that of the negative charge accumulated under 50kV/mm. It was also observed that the decay time of positive charge decreased with increases in the cathode field measured just before short-circuiting at 25 and 40degC, but not at 60 and 90degC.

Space-charge measurement in composite dielectrics

Abstract: To improve the manufacturing of multi- composites of solid insulation materials, this paper has studied the space charge distribution in pure, micro- and nano-filled composites of solid dielectric materials at various high impulse voltages. The pulsed electro-acoustic (PEA) method has been used as one of the most effective techniques for the measurement of the space charge distribution in insulating materials. This can measure the space charge profiles every 10 musec so, the effect of stressing on impulse breakdown voltage has been investigated. Results of PEA measurements performed on various kinds of multi-dielectrics are presented and discussed.