S G Swingler

On the effects of morphology and molecular composition on the electrical strength of polyethylene blends

The effects of morphology and molecular composition on the electrical strength of blends of linear and branched polyethylenes were investigated. A range of blend systems were considered, in which both the molecular mass of the linear polymer and the comonomer in the branched component were varied. All the blends contained 10% linear polyethylene and 90% branched polymer and, in each system, three crystallization procedures were employed to modify the morphology. Isothermal crystallization at 124 °C generally resulted in compact linear inclusions within a branched matrix; isothermal crystallization at 115 °C produced a space-filling network of open, spherulitic structures; and quenching gave a banded spherulitic morphology. In these systems, the electrical strength, as measured by ramp testing, was dependent on the morphology of the material but was not influenced per se by significant changes in the molecular composition of the blend. The effect of crosslinking was also examined; the inclusion of a network did not, in itself, affect the breakdown strength or the morphology.

Electric field in polymeric cable due to space charge accumulation under DC and temperature gradient

When power cables are loaded under high voltage direct current (HVDC), an accumulation of space charge and a radial distribution of temperature gradient are developed across the insulation material. Such existence and accumulation of space charge within the insulating material poses a threat to the reliability of the operation of dc power cables. The electric field of a practical dc power cable is affected by the conductivity of the material, which is a function of both temperature and electric field. This causes difficulties in identifying the electric field distribution. In this paper, a method of determining the electric field distribution in dc power cables was proposed by considering the influence of space charge on the conductivity of the insulating material under different temperatures. Commercial 11 kV ac cross-linked polyethylene (XLPE) power cables were used and the space charge in these cables under dc conditions was measured using a modified pulsed electroacoustic (PEA) system with an attached current transformer. Therefore, a replica of a power cable under load conditions is obtained, which allows an investigation of the formation, migration and accumulation of space charge in a power cable with and without temperature gradients across the insulating material. COMSOL Multiphysics software package was used to accurately determine the electric field distribution in the dc power cable with consideration of the influence of electric field on the conductivity of the insulating material. The numerical modelling is based on the hopping conduction mechanism and its parameters were obtained from experiments carried out on the XLPE insulation material.

Aging of biodegradable oils and assessment of their suitability for high voltage applications

In many items of high voltage plant, a mineral or synthetic oil is used in conjunction with paper as the dielectric medium. However, increasing awareness of the environmental impact of human activity and increasing disposal costs have encouraged researchers to direct their attention to renewable and biodegradable alternatives. Originally used in capacitors, vegetable oils are now finding widespread use in some transformer installations, particularly in the United States. Therefore, it seems prudent to begin systematic investigations of the aging behavior of a number of vegetable based oils and assess their potential for application in high voltage systems. A total of five food grade vegetable oils, an oil specifically formulated for high voltage applications (Envirotemp FR3) and dodecylbenzene (DDB) were aged at various temperatures in air. Their aging behavior was assessed by ultra-violet/visible, infrared and dielectric spectroscopies supplemented with measurements of viscosity. It was found that olive oil offered excellent resistance to aging (comparable to Envirotemp oil), rapeseed oil offered intermediate properties whereas corn and sunflower oil oxidized appreciably after aging. Despite being more prone to oxidation, all the vegetable oils offered dielectric properties which were better than dodecylbenzene and similar to mineral oil.

On Additives, Morphological Evolution and Dielectric Breakdown in Low Density Polyethylene

A series of low density polyethylene systems has been studied with respect to structural evolution and short-term dielectric breakdown behaviour. All materials were based upon a single polymer, that is commonly used in high voltage applications, but with different additives. In all three of these systems, multiple melting transitions were observed, as a result of molecular fractionation effects during crystallization. In the virgin polymer, a space-filling banded spherulitic morphology was found to develop at low temperatures (102 °C and below) whereas, at higher temperatures, only a few isolated axialites were observed. Inclusion of the antioxidant resulted in greatly increased nucleation densities, such that, at low temperatures, no evidence of spherulitic organisation remained. At higher temperatures, sheaf-like lamellar aggregates developed, which were much smaller and much more numerous than in the case of the virgin polymer. Further addition of dicumyl peroxide (DCP) resulted in the rapid formation of a crosslinked network at 200 °C. Some crosslinking also occurred at 150 °C, but over a much longer timescale. Where extensive crosslinking occurred prior to crystallization, the resulting gel inhibited structural development, such that only a few small, isolated sheaves were able to form at 102 °C. In view of the principal application area of this material, the breakdown strength of each of the above systems was then measured and the whole data set was analysed statistically. When structural factors were considered alongside the statistics, no clear trends emerged to indicate that either the compositional or morphological variations were reflected in the short-term electrical failure processes.

Site Assessment of Multiple-Sensor Approaches for Buried Utility Detection

The successful operation of buried infrastructure within urban environments is fundamental to the conservation of modern living standards. Open-cut methods are predominantly used, in preference to trenchless technology, to effect a repair, replace or install a new section of the network. This is, in part, due to the inability to determine the position of all utilities below the carriageway, making open-cut methods desirable in terms of dealing with uncertainty since the buried infrastructure is progressively exposed during excavation. However, open-cut methods damage the carriageway and disrupt societys functions. This paper describes the progress of a research project that aims to develop a multi-sensor geophysical platform that can improve the probability of complete detection of the infrastructure buried beneath the carriageway. The multi-sensor platform is being developed in conjunction with a knowledge-based system that aims to provide information on how the properties of the ground might affect the sensing technologies being deployed. The fusion of data sources (sensor data and utilities record data) is also being researched to maximize the probability of location. This paper describes the outcome of the initial phase of testing along with the development of the knowledge-based system and the fusing of data to produce utility maps.

On the crystallization, morphology and physical properties of a clarified propylene/ethylene copolymer

The crystallization behaviour, morphology and electrical and mechanical properties of a propylene/ethylene copolymer containing a clarifying additive have been studied. The addition of the clarifier generally results in enhanced nucleation and consequent formation of a more uniform structure. The melting behaviour is analysed and it is shown that, only following quenching, are the observed multiple melting peaks associated with dynamic reorganization effects. Otherwise, the various endotherms are associated with the initial crystallization process itself. The morphology of the copolymer is shown to change abruptly from a fine to a coarse structure when the crystallization temperature exceeds 128°C. This is associated with a marked reduction in the nucleating efficiency of the additive, which permits the growth of distinct, spatially separated morphological features that are best termed quadrites. Through a combination of the increased crystallization temperature and the growth of these relatively large structural units, appreciable molecular fractionation is also observed. These changes in crystallization behaviour should have important consequences for the macroscopic physical properties; dielectric breakdown and mechanical failure have been investigated. Although the electric strength of the material exhibits a clear dependence on sample morphology, most of the mechanical parameters do not vary in the same discontinuous manner. The only significant correlations seen between electrical and mechanical parameters involve the ultimate failure stress and strain, as recorded at a relatively high strain rate of 100 mm/min. Consequently, although this suggests that dielectric breakdown and mechanical failure may be weakly related, no agreement with existing theoretical predictions has been found.

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 wavelet-based de-noising to online measurement of partial discharges

For online detection of partial discharge (PD) activity within power cables and cable accessories, one of the non-conventional PD detection techniques, VHF capacitive coupling has been proved to be suitable. However, the existence of excessive interference can significantly influence the measurement sensitivity and reliability. This paper investigates the application of wavelet transforms for de-noising signals obtained using capacitive couplers. The experimental work has been carried out on a 132 kV cable loop with a known defect within the cable joint. Obtained results indicate that the wavelet analysis technique can effectively discriminate internal PD pulses among corona discharges and pulse-like interference. Further, removal of interference has been achieved by applying level dependent threshold values.

Improving detection sensitivity for partial discharge monitoring of high voltage equipment

Partial discharge (PD) measurements are an important technique for assessing the health of power apparatus. Previous published research by the authors has shown that an electro-optic system can be used for PD measurement of oil-filled power transformers. A PD signal generated within an oil-filled power transformer may reach a winding and then travel along the winding to the bushing core bar. The bushing, acting like a capacitor, can transfer the high frequency components of the partial discharge signal to its earthed tap point. Therefore, an effective PD current measurement can be implemented at the bushing tap by using a radio frequency current transducer around the bushing-tap earth connection. In addition, the use of an optical transmission technique not only improves the electrical noise immunity and provides the possibility of remote measurement but also realizes electrical isolation and enhances safety for operators. However, the bushing core bar can act as an aerial and in addition noise induced by the electro-optic modulation system may influence overall measurement sensitivity. This paper reports on a machine learning technique, namely the use of a support vector machine (SVM), to improve the detection sensitivity of the system. Comparison between the signal extraction performances of a passive hardware filter and the SVM technique has been assessed. The results obtained from the laboratory-based experiment have been analysed and indicate that the SVM approach provides better performance than the passive hardware filter and it can reliably detect discharge signals with apparent charge greater than 30 pC

Morphology and electrical conductivity in polyaniline/polyolefin blends

The effects of processing and molecular composition on the morphology and electrical conductivity of melt pressed polyaniline (PANI)/polyolefin blends have been investigated. In all systems, the morphology was found to consist of two distinct phases, a conducting network of PANI embedded within the thermoplastic host. Low field conductivities ranging from 10/sup -14/ to 10/sup -4/ S/cm could be accessed, through a suitable choice of PANI content and processing conditions. To 200 V/mm, the conductivity of the material was found to increase exponentially with applied field, thereafter remaining close to constant until the onset of breakdown. On further analysis, the applied field E dependence of conduction current i prior to breakdown was found to obey a relationship of the form i=A sinh(bE), as reported in the literature for insulators in the so-called low-field regime. Despite the dramatic variations in electrical properties that result from incorporation of the conducting polymer, the low strain mechanical properties were not degraded to any great degree.