Polyethylene/Dibenzylidene Sorbitol: effect of composition on space charge of polymeric insulation systems

This paper examines the effects of 1,3:2,4- dibenzylidene sorbitol (DBS), a low-molecular organic molecule, on the space charge distribution in polyethylene. Morphological studies of the blends show the presence of a DBS network in the host system. Avrami analysis shows an increase in the nucleation density of polyethylene with the nucleating agent. Accumulation of space charge is one of the main causes of failure for high voltage cables and it depends on many factors such as additives, materials and treatment. In the present study we investigate the effects of different quantities of DBS in a blend of polyethylene. The space charge response during a stressing period and decay after removal of the applied electric field were observed. The results show that space charge accumulations are dependent on the concentration of the nucleating agent. In particular, the decay in a sample with 0.3 % DBS indicates shallow traps, whilst samples containing 1 and 3 % DBS show evidence of deep trap charges

On the dielectric performance of modified epoxy networks

Epoxy resins represent a commonly used basis for insulation materials and have been used in many different electrical applications. The formation of these systems involves reactions between a hardener, such as an amine-curing system, and an epoxy terminated resin. Recent studies have reported that epoxy resin systems can exhibit enhanced physical properties when the stoichiometry of the resin is varied using reactive diluent. This has been attributed to the increased free volume within the molecular structures within the epoxy resin network. The work described here set out to investigate this hypothesis concerning the potential benefits of varying the network structure of epoxy resin through the inclusion of monofunctional reactive diluents within the epoxy formulation. This research is of potential significance because any modification of the epoxy resin network results in modified thermal, mechanical and electrical properties and, consequently, represents a potential mean of tailoring overall performance to suit particular applications

The effect of resin/hardener stoichiometry on the electrical properties of silicon nitride/epoxy nanocomposites

The effect of resin/hardener stoichiometry was investigated for both unfilled epoxy and nanocomposite samples. The results indicate that incorporating silicon nitride nanofiller, which contains amine groups on its surface, has a significant influence on the resin/hardener reaction. At 2 wt.% of nanofiller, it was estimated that the powder contains amine groups equivalent to around 5 wt.% of the hardener mass, which results in the displacement of the optimum resin/hardener mass ratio by the same amount (~5 wt.%). The dielectric spectra showed that the B relaxation is directly related to the hydroxyether groups that are generated by the reaction between the epoxy and the amine groups. Therefore, the relaxation strength is proportional to the crosslinking density and consequently related to the glass transition temperature. The DC conductivity increases considerably as a result of incorporating silicon nitride nanofiller when not compensating for its impact on the resin/hardener stoichiometry. This might be related to the increase in the amine content of the material caused by the amine groups existing on the surface of the nanoparticles. When the stoichiometry effect is taken into account, the DC conductivity decreased to a value that is comparable to that of the unfilled polymer

Effect of aging on the physical, chemical and dielectric properties of dodecylbenzene

High voltage transformers and cables form integral parts of the electrical power distribution network and therefore their reliable operation is crucial to preventing costly outages. A liquid dielectric, typically mineral oil, is used in transformers whilst dodecylbenzene is gradually replacing mineral oil in paper/oil cable systems. As such, the oil serves as a convenient medium for sampling to assess plant health. In the current investigation, samples of dodecylbenzene cable oil were aged at 135 ºC under conditions where the oxygen diffusion into the oil was deliberately limited. The resulting aged samples were subjected to a battery of tests to determine changes in their physical, chemical and dielectric properties. Aging resulted in yellowing, oxidation, increased water content, increased dielectric loss, increased electrical conductivity and reduced electrical breakdown strength. Copper accelerated the aging process resulting in the formation of an insoluble precipitate; however, the inclusion of this material did not affect the electrical properties of the oil indicating that it is non-polar in nature. Instead, the deterioration of the dielectric properties was found to be correlated with the production of small polar molecular compounds, which remain dissolved within the oil, such as water, alcohols, ketones, carboxylic acids and aldehydes

Improving Learning of Electronic Engineering Skills through e-Learning: a Case Study

In this paper, we report on changes made to a module that is taken by all first-year electronic engineering students, and which covers transferable and engineering skills. To improve students’ perception of and learning experience on the module, half of the taught material was migrated from a traditional ‘physical’ lecture format to specially recorded ‘online’ lectures. The changes were evaluated through 1) looking at how regularly students were accessing material, 2) online questionnaires, 3) focus groups, and 4) the standard module evaluation surveys. The changes were very well received by the students and, in a single year, the overall module rating improved from 60% (static over the preceding three years) to 75%. The changes showed evidence of giving students greater freedom in the way in which they learn, for example allowing them to watch lectures when it best suited their learning style (even if this was at night or on a weekend), pause the lecture while they looked up more information, and rewind sections of the lecture to watch difficult topics again. This has also proved beneficial to international students, as the majority found the online lectures easier to understand, and were able to pause the lecture while they translated unknown words

Aging behaviour of dodecylbenzene in the presence of copper and dibenzyl disulfide (DBDS)

Copper sulfide deposition remains a serious issue in high voltage power transformers and can result in equipment failure. In order to understand the chemistry of this process, dodecylbenzene was used as a model oil system. Copper and dibenzyl disulfide (DBDS) were added in controlled amounts and accelerated aging was then performed. It was found that copper sulfide (CuxS) could be formed (i.e. the included copper sheets were blackened) even under non-oxidative conditions but only when DBDS was present in the oil. The reaction was accelerated in the presence of oxygen and competes for available copper with the parallel copper carboxylate producing reaction. The AC breakdown strength was reduced after aging, but exhibited a more catastrophic reduction in oils containing DBDS, confirming the detrimental effects of CuxS on breakdown performanc

Effect of aging on the physical, chemical and dielectric properties of dodecylbenzene cable oil

In high voltage transformers a liquid dielectric,such as mineral oil, serves both as an electrical insulator (in conjunction with paper) and as a coolant. Similarly, in paper/oil cables mineral oil or dodecylbenzene in conjunction with paper serves as the electrical insulator. In such systems, the oil serves as a convenient medium for sampling to indicate plant health. In the current study dodecylbenzene was aged at elevated temperatures in the presence of air and copper. A battery of tests was then performed to assess the changes in physical, chemical and electrical properties. After aging the oils were yellowed and oils aged with copper showed oxidation, increased water content and the formation of a precipitate, whilst those aged in the absence of copper showed much less aging. Changes in electrical properties were noted such as increased dielectric loss, increased electrical conductivity and decreased dielectric breakdown strength. It was found that the presence of the precipitate did not affect the electrical properties of the oi

Dielectric breakdown strength and electrical conductivity of low density polyethylene octylnanosilica composite

One challenge in studying nanodielectric composites is to produce reliable, reproducible samples. A common strategy to suppress aggregation and make the particles more compatible with the polymer matrix is to modify the nanoparticle surface chemistry but, often, evaluation of the effectiveness of the chosen surface functionalization process can prove difficult. In this paper the emphasis is on feasible ways to monitor the production of silane coupled nanosilica low density polyethylene (LDPE) composites, using Fourier transform infrared spectroscopy (FTIR) and thermal gravimetric analysis (TGA). The AC-breakdown properties of the resulting composites is studied and the field dependency of the DC-conductivity is measured and also calculated using a space charge limited conduction (SCLC) model together with densities of states obtained from ab initio calculations. For composites containing 13 wt% of nanosilica, breakdown strengths some 18 % higher than that of the unfilled LDPE were obtained. However, the results are not stable over time. This appears to be related to how extensively the composite is dried at elevated temperatures under vacuum

Comparative analysis of air-gap PD characteristics: vegetable oil/pressboard and mineral Oil/pressboard

Partial Discharge (PD) is considered to be one of the main reasons for aging and degradation of the oil/pressboard insulation system in power transformers. Vegetable oils which own excellent dielectric performance are introduced as potential insulation liquids substituting traditional mineral oil. In this paper, an air-gap PD model was adopted to investigate the PD characteristics of refined rapeseed oil and Karamay 25# mineral oil that are both currently employed in liquid filled power transformers. The PD current pulse waveform analysis (PD-CPWA) method was used to investigate PD mechanisms of two different insulation systems: mineral oil/pressboard and vegetable oil/pressboard. For both insulation systems, phase resolved partial discharge (PRPD) patterns throughout the accelerated deterioration experiments were compared. The extracted ?-q-n plots with respect to various PD times were analyzed. It is found that, the air-gap PD stage characteristics of vegetable oil/pressboard are more notable. It has fewer double-peak pulses, smaller inception phase angle, lower charge amplitude, higher repetition rate, and more remarkable ‘rabbit-ear’ patterns than mineral oil/pressboard. The PD development process of both insulation systems can be characterized by four stages: initial discharge stage, weak developing stage, discharge burst stage and pre-breakdown stage