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The central goal of dielectric nanocomposite design is to create a large interfacial area between the matrix polymer and nanofillers and to use it to tailor the properties of the composite. The interface can create sites for trapping electrons leading to increased dielectric breakdown strength (DBS). Nanoparticles with a bimodal population of covalently anchored molecules were created using ligand engineering. Electrically active short molecules (oligothiophene or ferrocene) and matrix compatible long poly(glycidyl methacrylate) (PGMA) chains comprise the bimodal brush. The dielectric breakdown strength was evaluated from recessed samples and dielectric spectroscopy was used to study the dielectric constant and loss as a function of frequency. The dielectric breakdown strength and permittivity increased considerably with only 2 wt% filler loading while the dielectric loss remained comparable to the reference epoxy. [ABSTRACT FROM AUTHOR]

Crosslinked polyethylene (XLPE)/silica nanocomposites are promising candidates for future cable insulation. While a significant number of studies have demonstrated improved dielectric properties in nanocomposites compared to XLPE, the performance of polyolefin nanocomposites in humid environments has not received much attention. This paper presents and explains the dielectric behavior of XLPE/silica nanocomposites in humid environments such as the decrease in AC breakdown strength, increases in loss and space charge formation, and the significant reduction in water tree aging. XLPE/silica nanocomposites are found to have an increased moisture uptake compared to the XLPE base polymer due to inclusion of silica particles. It is hypothesized that the formation of a concentric shell surrounding the particle with a high concentration of water (water shell), and the change in the inter-particle/cluster distances are two major factors governing the dielectric behavior in wet XLPE/silica nanocomposites. The dispersion and distribution of the nanofillers were quantified using a new tool and a method to reconstruct the 3D structure was used to determine the size of the water shell required for percolation. It was found that a water layer thickness of tens of nanometers could initiate percolation in the XLPE/silica nanocomposites studied. Notwithstanding that, water tree growth was substantially reduced in the XLPE/silica nanocomposites, and some speculative explanations are provided on the basis of the characteristics observed. [ABSTRACT FROM PUBLISHER]

For high temperature wire enamel application, the dielectric properties of nanoscale silica and alumina filled polyamideimide were characterized as a function of filler loading, and temperature. It was found that although the dc breakdown strength improved for both composite systems, only the alumina also exhibited an increase in ac breakdown strength. In addition, the voltage endurance at elevated temperature in both a needle/plate and twisted wire geometry was improved over a narrow range of filler loading (5-7.5 wt%). At room temperature, pulsed electroacoustic analysis (PEA) data does not indicate any difference in space charge buildup in the composite for fields up to 25 kV/mm over a period of 2 h. This is in contrast to other nanocomposite systems, which tend to show mitigation of space charge buildup with the addition of nanoparticles. Dielectric spectroscopy shows that the addition of nanoparticles increases the real permittivity above that expected for volumetric mixing rules. This enhancement is associated with high effective permittivity interfacial areas. No systematic changes in losses in the frequency range between 10-4 and 105 Hz were seen. The results suggest that carrier scattering is the primary mechanism leading to the improved breakdown strength. [ABSTRACT FROM PUBLISHER]

Electron Paramagnetic Resonance (EPR) has been used to study the properties of trapped electrons or holes in XLPE and in 12.5% vinylsilane-treated SiO2 nano-particles in XLPE specimens. Both electrically unstressed and electrically stressed (up to 25 kV/mm) specimens were used. The EPR spectra of both materials indicated that the acceptors/donors were oxygen radicals in the polymer, probably originating during the cross-linking. It was found that the anisotropic oxygen environments were not randomly oriented, but were textured, presumably during the fabrication of the sheet specimens. It was found that under the action of an applied electric field, the population of an additional number of oxygen radicals occurred. This result is discussed in relation to the implied polymer structure and conductivity mechanisms. [ABSTRACT FROM AUTHOR]

Polymer nanocomposites with metal oxide nanoparticle fillers exhibit enhanced electrical breakdown strength and voltage endurance compared to their unfilled or micron filled counterparts. This paper presents the following hypothesis for the mechanisms leading to improved properties. The inclusion of nanoparticles provides myriad scattering obstacles and trap sites in the charge carriers' paths, effectively reducing carrier mobility and thus carrier energy. The result is homocharge buildup at the electrodes, which increases the voltage required for further charge injection due to blocking by the homocharge. The hypothesis is supported by electroluminescence, pulsed electro acoustic analysis, thermally stimulated current measurements, a comparison of AC, DC, and impulse breakdown, as well as absorption current measurements, in silica/crosslinked polyethylene matrix composites with supporting evidence from titania/epoxy composites. [ABSTRACT FROM AUTHOR]

Dissolved Gas Analysis (DGA) is a well established method to monitor and predict the condition of devices with oil-cellulose insulation, especially power transformers. However, the effect of the migration of theses gases between the liquid and solid insulation is often ignored. The migration phenomenon is temperature dependent and, if ignored, can potentially lead to erroneous condition assessment by the DGA method. This paper addresses the dynamic nature of propagation of these gases in such systems based on temperature variations. Diffusion times, and therefore diffusivity of the gases, in oil-impregnated cellulose are calculated by identifying the diffusion process as a linear system. The diffusion time-constants, and steady-state condition at the working concentrations, are estimated based on the identified model. A sensitivity study is performed to show the dependency of the diffusion time-constants to the temperature and the oil-cellulose ratio in the system. The equilibrium curves are obtained in the case of carbon dioxide by using solubility information of this gas in the oil and paper pulp, similar to the case of moisture equilibrium. The results are compared with the experimental data. An example of the implication of the obtained data is shown. [ABSTRACT FROM AUTHOR]

Transformer oil/cellulose structures are often designed based on a cumulative stress criterion derived from experimental tests at power frequency. However, such structures must also meet stringent impulse requirements defined by a Basic Insulation Level (BIL). The industry has tried to establish an equivalence factor to permit power frequency cumulative stress methods to be used to estimate impulse withstand strength. Since the mechanisms of failure differ substantially under surge conditions, there would seem no good reason to suppose that a universal equivalence factor is appropriate. Tests are reported using a 2.3 MV generator to document impulse failure of a number of bulk, creep and hybrid structures to establish the nature of this relationship through statistical comparisons with the established 50/60 Hz methods. Factors varied from 1.94 to 3.34, depending on the configuration. The methodology is described and the results discussed in the context of the design of oil-cellulose structures, having regard to complicating factors such as waveshape and electrode covering. The study permits some speculation about impulse design under hybrid situations (i.e. failure paths involving both creep and bulk liquid). [ABSTRACT FROM AUTHOR]

The incorporation of silica nanoparticles into polyethylene increased the breakdown strength and voltage endurance significantly compared to the incorporation of micron scale fillers. In addition, dielectric spectroscopy showed a decrease in dielectric permittivity for the nanocomposite over the base polymer, and changes in the space charge distribution and dynamics have been documented. The most significant difference between micron scale and nanoscale fillers is the tremendous increase in interfacial area in nanocomposites. Because the interfacial region (interaction zone) is likely to be pivotal in controlling properties, the bonding between the silica and polyethylene was characterized using Fourier Transformed Infra-red (FTIR) spectroscopy, Electron Paramagnetic Resonance (EPR), and X-ray Photoelectron Spectroscopy (XPS). The picture which is emerging suggests that the enhanced interfacial zone, in addition to particle-polymer bonding, plays a very important role in determining the dielectric behavior of nanocomposites. [ABSTRACT FROM AUTHOR]

The utility of condition monitoring of the stator insulation structure of large machines has been enhanced substantially by the use of computer-based analysis of partial discharge signatures. A number of indices are examined through tests in a multi-stress aging facility to assess their promise in describing insulation degradation in machine insulation. In transitioning such tests to the field environment, an operational test is also described which offers some promise in identifying degradation without prior baseline data for a machine. On the basis that most electrical failures have mechanical precursors, acoustic methods are described for assessing stator bar mechanical integrity. This includes both using the conductors as wave-guides and also employing acoustic emission for the location of large discharges. [ABSTRACT FROM AUTHOR]

The incidence of streaming electrification in large transformers (and other industrial equipment) has generated the need for both an understanding of the underlying physics and a mathematical description of the phenomenon. Modeling is needed since the mechanisms do not readily scale and small-scale laboratory experiments are limited in predicting performance. The development of both steady-state and transient descriptions of the streaming electrification process is described for a rectangular duct structure. The nature of the phenomenon is such that appropriate assumptions and approximations need to be made. As a consequence, some experimental verification of the outcomes, at least in qualitative form, is also included. The entrainment of charge from the interface charge layers also intimately involves the flow of the liquid. A description of the flow regimes and a transient solution of the charge development through a duct are thus an integral part of the modeling effort. The utility of the steady-state model developed is demonstrated by application to an oil-cellulose system typical of a large core-form transformer. The method advocated takes advantage of the repeat duct arrangement in such units. The results presented allow the internal behavior of such a unit to be studied in considerable depth, and provide insight into the complex interplay between generation and relaxation in circumstances when flow rates, temperature, external fields and influent charge are changed. [ABSTRACT FROM AUTHOR]

Presents an experimental study which quantified the voltage endurance of electrical grade epoxy resin insulation when subjected to pulse voltages in a divergent field. Results; Discussion.

Presents a study which introduced a concept of dynamic aging theory in machine insulation. Discussion; Conclusion.

Presents an introduction to the August 1, 2005 issue of the journal "IEEE Transactions on Dielectrics and Electrical Insulation."

Engineering dielectrics on the nanoscale is a 21st century phenomenon. The term "Nanodielectrics" typically refers to an insulating "Polymer Nanocomposite" and represents a class of materials with a polymer matrix, and a filler that is (often substantially) less than 100 nm in one dimension. Nanodielectrics have been under development only for about the last 10 years and are beginning to find their way into important commercial products based on the enhanced properties that can be demonstrated. This is particularly so for applications which can benefit from the substantial improvements in voltage endurance, which typically can be achieved with this technology. Indeed, this burgeoning interest is reflected in the choice of this topic to form 3 special issues of these Transactions. The previous two were in 2004 and 2008 [1, 2]. [ABSTRACT FROM AUTHOR]

High aspect ratio fillers are predicted to increase the dielectric constant of polymer composites more efficiently than spherical fillers according to the rule of mixtures. Using high aspect ratio fillers is a promising route for creating high dielectric constant, low loss materials at a low filler volume fraction, for use as capacitor and electric field grading materials. In this work, two high aspect ratio fillers were mixed into a polymer matrix, and the dielectric properties of composites were studied. Barium titanate fibers were synthesized by electrospinning a sol-gel, followed by a heat treatment to obtain a perovskite crystal structure. The heat treatment conditions were found to be crucial for obtaining tetragonal barium titanate fibers with high dielectric constant. Graphene platelets were prepared by a thermal shock method, which was found to result in a larger dielectric constant. A combination of barium titanate and graphene platelets yielded the highest dielectric constant when used in a polydimethyl siloxane matrix. The increase in dielectric loss over the pure matrix was small when the volume fraction was below the percolation threshold of graphene platelets. Electric flux density-electric field (D-E) measurements showed a linear dielectric constant in barium titanate filled composites and higher loss when graphene was added. The ac breakdown strength was reduced compared to the neat polymer and was affected by filler aspect ratio. The mechanisms that lead to the observed phenomena are discussed. [ABSTRACT FROM PUBLISHER]

Study of the experimental literature regarding nanodielectrics indicates numerous inconsistencies in the results obtained. In many cases, the likely cause is due to a lack of quality control during nanocomposite processing. By examining examples from the literature along with an alumina/polyamideimide nanocomposite and silica/crosslinked polyethylene nanocomposite, this contribution seeks to shed light on some of the likely causes for these inconsistencies. Measurements of the dielectric breakdown strength and voltage endurance confirm that poor dispersion can lead to poor material performance and the use of quantitative techniques is highlighted. Good dispersion alone is not sufficient to achieve improved properties. The addition of nanoparticles can alter the resulting structure of the nanocomposites, can introduce water into the system resulting in cavity formation and can also result in degradation of the polymer if the processing parameters are not carefully selected. In this review, understanding the effects of nanoparticle addition requires not only characterization of relevant dielectric properties, but also careful control of the processing parameters and characterization of changes in polymer structure, particle dispersion, and water content. [ABSTRACT FROM AUTHOR]

It is well known that hopping of charge carriers via spatially and energetically distributed localized states is a primary transport mechanism in many disordered semiconductors and polymeric dielectrics. In this contribution, the nonlinear I-V physics of a 25vol% 50nm p-SiC/silicone rubber composite for high voltage field grading application was investigated, and a composite bulk hopping mechanism proposed. It is hypothesized that nearest-neighbor hole hopping occurs through thin rubbery layers between the SiC particles, and is the mechanism governing the nonlinear electric response of SiC/silicone rubber nanocomposites. [ABSTRACT FROM AUTHOR]

Results and assessment are provided from a nearly 10 year study of the application of a variety of diagnostic methods applied to the condition monitoring of generator stator insulation. The methods studied included various forms of Partial Discharge (PD) techniques and Electromagnetic Interference (EMI) in the frequency domain. The condition of a number of operating machines was tracked by the various techniques and, in most cases, the findings compared with the actual condition determined by subsequent inspection after the rotors had been removed. This has permitted the effectiveness of the various methods to be gauged and allowed an appraisal of the attendant best practices and pitfalls. [ABSTRACT FROM PUBLISHER]

An introduction is presented to the articles about nanodielectrics published in the February 2008 issue of the "IEEE Transactions on Dielectrics and Electrical Insulation" periodical.