A F Holt

On the dielectric response of silica-based polyethylene nanocomposites

The dielectric response of silica-based polyethylene nanocomposites is studied by dielectric spectroscopy. The results indicate that nanocomposites absorb significantly more water than unfilled polyethylene, with the consequence that both permittivity and loss tangent increase with increasing duration of water immersion. However, appropriate surface treatment of nanosilica is found to reduce the water absorption effect and to modify the dielectric response of the nanocomposites compared with those containing untreated nanosilica. While water absorption may not be a technologically desirable characteristic, our results indicate that water molecules can act as effective dielectric probes of interfacial factors.

On the space charge and DC breakdown behavior of polyethylene/silica nanocomposites

Space charge occurs in a dielectric material when the rate of charge accumulation is different from the rate of removal, which arises due to moving or trapped charges. Inevitably, the local electric field is increased at some point within the material, which then leads to faster degradation and premature failure. The determination of space charge behavior has seen wide implementation in characterizing novel dielectric materials, especially in connection with the newly emerging field of nanocomposites. In this paper, we report on an investigation into space charge dynamics in silica-based polyethylene nanocomposites. The various systems differed with respect to the amount of filler and its surface chemistry; the pulsed electro-acoustic (PEA) technique was used to evaluate the space charge distribution in each. Experimental results indicate that the incorporation of nanosilica into polyethylene results in a significant amount of homocharge development near both electrodes. With appropriate surface treatment of the nanofiller, homocharge formation was successfully suppressed, indicating less severe space charge development in the nanocomposite materials. The mechanisms leading to the observed space charge development and direct current (DC) breakdown properties of the nanocomposites are discussed.

Absorption Current Behaviour of Polyethylene/Silica Nanocomposites

Absorption current is an important characteristic of polymers with regard to their time-domain response to a direct current (DC) poling field. This is because the results of absorption current measurements can be used to gain understanding of the relationship between space charge accumulation and movement. In semicrystalline polyethylene, for example, charge accumulation is likely to be influenced by the presence of charge trapping sites, which are associated with interfaces between the crystalline and amorphous phases. With the addition of a nanofiller, the charge transport mechanism will become more complicated than in the unfilled polymer, as the inclusion of the nanofiller will introduce nanofiller/polymer interfaces. The presence of such interfaces will affect the current flow due to the introduction or modification of the distribution of trapping sites within the system. In this paper, we report on an investigation into the absorption current behaviour of polyethylene nanocomposites containing 0 wt%, 2 wt%, 5 wt% and 10 wt% of silica nanofiller, either untreated or treated using trimethoxy(propyl)silane coupling agent. Our results indicate that the absorption current behaviour of the polyethylene was affected by the presence of the nanosilica. While the current behaviour through the unfilled polymer decreases with time in a conventional manner, all nanocomposites reveal an initial decrease followed by a period in which the current increases with increasing time of DC field application.

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.

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).

Impact of corrosive sulfur in transformer insulation paper

In recent years a significant volume of research has been undertaken in order to understand the latest failures in oil insulated power apparatus due to deposition of copper sulfide on the conductors and in the insulation paper. Dibenzyl Disulfide (DBDS) has been found to be the leading corrosive sulfur compound in the insulation oil. As there are currently no nondestructive techniques available to analyze the degree of damage from sulfur corrosion, electrical plant operators are faced with a choice of either replacing an asset which might still have several years of operational life or keeping a transformer operational with an increased risk of failure. This paper investigates permittivity and loss factor curves of transformer oil impregnated insulation paper, at different corrosive sulfur degradation stages. Three types of transformer conductors are investigated; new, artificially aged and service aged. To increase the understanding of the electrical behavior of Cu2S this paper demonstrates how the electrical properties change with moisture content. Dielectric spectroscopy is used to demonstrate some of the electrical changes across a wide frequency range, from 1mHz to 1MHz.

On the effect of functionalizer chain length and water content in polyethylene/silica nanocomposites: Part II — Charge transport

The effects of functionalizer chain length and water content were explored in a series of polyethylene/silica nanocomposites. Silane molecules with differing chain lengths (propyl, octyl and octadecyl) were used to vary the nanoparticle surface chemistry, while vacuum drying and water immersion were used to extract water from or add water to samples previously equilibrated under ambient conditions. Electrical conductivity was found to be highly dependent upon water content as were the space charge distributions. Both interfacial conditions and the overall charge transport through the sample were strongly dependent on absorbed water and, as such, were found to vary with time through exchange of water with the samples environment. Changes to charge transport dynamics due to the functionalizer chain length were, however, subtle. The removal of surface hydroxyl groups appears to be the primary mechanism by which functionalization influences electrical behavior; this reduces water uptake and, as a consequence, modifies charge transport behavior.

X-ray fluorescence as a condition monitoring tool for copper and corrosive sulphur species in insulating oil

Currently there are no standard techniques to detect copper sulphide deposits in transformers to act as an early warning system. This paper develops and tests a procedure using X-Ray Fluorescence (XRF) spectroscopy to tackle this problem and to monitor dissolved copper in oil, which can have a major impact on the dielectric properties of insulating oil. Initially calibration curves for sulphur and copper demonstrate the XRFs limits of quantitation. Subsequently, a laboratory scale sample preparation procedure is developed to demonstrate that corrosive sulphur depletion trends may indirectly monitor copper sulphide formation. To confirm depleting sulphur species are corroding copper samples are analysed with Energy Dispersive XRay spectroscopy. Finally XRF demonstrates that corrosive oil has a low impact on the long term concentration of copper in oil. An XRF condition monitoring technique feasibility study is conducted and recommendations are given in how to implement it in the field.

On nanosilica surface functionalization using different aliphatic chain length silane coupling agents

The use of polymer nanocomposites in electrical insulation is claimed to be capable of enhancing the dielectric performance of insulation systems. This is believed to be related to the much smaller size of the filler particles, which leads to the presence of extensive interfacial areas. In this regard, nanocomposites are expected to possess unique dielectric properties that reflect the resulting interphase regions. Since the surface state of the nanofiller is closely related to the interphase regions, surface functionalization of the nanofiller has been shown by many workers to be an important factor in determining the nanocomposites interphase structure and, hence, the macroscopic physical properties of the system. While many comparisons of nanocomposites containing functionalized and unfunctionalized fillers have been reported, few systematic studies of this area have been undertaken with a view to understanding the mechanisms underpinning the concept of filler functionalization in nanodielectrics. In this paper, we begin to address this by using a series of functionalizing agents, which differ with respect to their aliphatic chain length; all systems were based upon trimethoxysilane and, in all cases, the nature of the thermodynamic interactions with the polyethylene matrix should be comparable. This paper reports on the effect of aliphatic chain length on the structure, breakdown and space charge behavior in the resulting nanocomposites. The possible mechanisms leading to the observed property changes are discussed.

Silver corrosion in transformers

The impact of corrosive sulfur on high value assets such as transformers is widely reported and can be considered a global problem. A great deal of research and development is focused on the remediation of corrosive transformers and the mitigation of future corrosion. Regeneration of aged and or corrosive oil by filtration through bauxite clays is a common method for removal of polar molecules and many sulfurous compounds such as dibenzyl-disulfide (DBDS). This process is largely successful, although over the last several years, increased corrosion and even total failure of transformers has been linked with the reclamation process. In some cases, the corrosion associated with a recent reclamation is selective for silver rather than copper. By thoroughly understanding the chemistry occurring during aged oil reclamation, it will be possible to take appropriate measures to limit the formation of corrosive species during future reclamation and regeneration processes. There is a requirement to minimize the amount of oil wastage during reclamation for both environmental and commercial reasons. It is also important that any changes made to the reclamation process do not have a detrimental effect on the health and life-expectancy of the transformer. This paper describes studies carried out to track the total sulfur content of oil samples taken from two different locations (transformer main tank and reclamation rig buffer tank), at multiple time intervals during an oil reclamation procedure. By tracking the total sulfur content, it is possible to determine when and where maximum sulfur accumulation occurs, possibly indicating the best point at which a portion of oil should be disposed. Once a fuller understanding of the sulfur content is obtained, GC-MS will be used to gain a detailed understanding of the types of sulfurous species present in the oil.