Lars E. Schmidt

Dielectric properties of epoxy based nanocomposites for high voltage insulation

Epoxy polymer with micro, nano and micro + nano silica fillers have been evaluated for their electrical performance in high voltage insulation applications. The dielectric strength of these samples was measured in accordance with the ASTM D-149 standard. Dielectric spectroscopy was used to understand the role of space charge and interfaces in these materials. The results of dielectric spectroscopy suggest that significant improvement in the electrical performance can be expected by using samples containing nanofillers and micro + nanofillers when compared to materials containing only microfillers. However, the dielectric strength measurement showed no statistically significant improvement for the nanofilled samples. Techniques other than dielectric breakdown may be required to adequately characterize differences in the electrical performance of the dielectrics. For example, a partial discharge test using a highly non-uniform field may be more useful as it would correspond to simulate actual service conditions.

Characterization of epoxy microcomposite and nanocomposite materials for power engineering applications

This article presents the results from round-robin tests performed on epoxy composite materials. These results show the potential of these materials for use as electrical insulation in some specific applications. A small section of the article addresses the health and safety issues related to the use of nanoparticles in the electrical power engineering industry. We define epoxy nanocomposites as epoxy-based materials containing exclusively nanosized filler particles. Epoxy microcomposites are defined as epoxy materials containing exclusively microsized filler particles, and epoxy micro+nano composites are materials containing both microsized and nanosized particles.

Tracking and erosion resistance of high temperature vulcanizing ATH-free silicone rubber

Traditionally aluminum trihydrate (ATH) is added to high-temperature vulcanized (HTV) silicone rubber for outdoor insulation applications to improve flame retardancy and the electrical tracking and erosion performance. In this work, the flammability of ATH-free HTV silicone rubber formulations were evaluated by the UL94 vertical burning test. The most promising candidates, as well as reference formulations filled with silica or silica/ATH only, were then evaluated by the inclined plane tracking and erosion test at 4.5 kV. The best result was obtained by silicone rubber containing 100 phr silica and 15 phr melamine cyanurate. It was suggested that this was a result of the arc quenching ability of the melamine hence extinguishing the electrical arc. If the filler level was too high, resulting in poor dispersion, the resistance to tracking and erosion was significantly reduced. Thus formulating a HTV silicone rubber with good tracking and erosion resistance is a balance between having a high enough filler content, while still maintaining a good dispersion of the particles in the polymer matrix, as well as good bonding between the particles and the matrix.

Epoxy based materials containing micro and nano sized fillers for improved electrical characteristics

In this paper the effect on dc and ac electrical properties of silica nano and micro particles dispersed in epoxy resins is discussed. In particular, space charge, conductivity, dielectric strength and partial discharge resistance is analyzed. The results show that nanostructured materials exhibit smaller space charge accumulation with respect to both base and microfilled materials. Regarding PD resistance, micro + nano filled materials display longer lifetimes with respect to base epoxy resin and materials including nanofillers or microfillers alone.

Inclined-Plane Tracking and Erosion Test according to the IEC 60587 Standard

The main goal of the paper is to summarise experiments using the tracking and erosion test according to the IEC 60587 standard. Various polymeric materials, e.g., high temperature vulcanised silicone rubber, liquid silicone rubber, epoxy resin and fibre reinforced materials were tested and differences in performance noted. A leakage current monitoring system has also been developed and the results of leakage current analysis are presented. Possible improvements of the present standard are also discussed.

Inclined-plane tracking and erosion test according to the IEC 60587 Standard

The present IEC 60587 IEC standard needs several improvements which would help manufacturers of power equipment and material suppliers to assess outdoor polymeric materials in a more reliable way.

Characterization and Modelling of the Mechanical Properties of Pressboard

Cellulose-based components constitute the bulk of the current insulation for transformers. Cellulose is an organic polymer material which combines excellent electrical properties and good mechanical performance. As a polymeric material, cellulose is very sensitive to moisture and temperature. These factors can influence the electrical and mechanical performance of a transformer throughout its lifetime. In order to ensure the quality of the product during transformer manufacturing, as well as during transformer life-time services, adequate models for predicting the physical properties of its constituents are needed. The present investigation tackles the mechanical description of pressboard. For this purpose, a three dimensional mechanical model is developed for simulating the in-plane and out-of-plane behavior of the pressboard material. The model is based on an anisotropic viscoelastic-viscoplastic constitutive law, which includes features that are particular for cellulose-based materials, e.g. the peculiar double nature of fiber-network-based and porous material. The material is orthotropic by nature, i.e. the in-plane mechanical properties markedly differ from the out-of-plane ones. Particular regard is taken when considering the effect of out-of-plane stresses which both cause viscous deformation and permanent compaction. The analyses on the mechanical behavior of pressboard are performed by comparing the experimental data on pressboard and the results of model simulations.

Space charge behaviour on epoxy based dielectrics filled with micro and nano silica

Epoxy based compounds containing micrometric and nanometric silica were prepared. These were utilized in a comparative study to establish their dielectric behaviour. In this study, the weight %-content of the silica filler was varied as a parameter. Some filler distributions were characterized using TEM. In general, the micrographs illustrate a satisfactory dispersion, showing a nanostructured morphology. The polarizability of the various materials is established and compared using dielectric spectroscopy. Indeed, the polar nature of the dielectric can affect the space charge accumulation. Space charge was studied using the Thermal Step Methop (TSM). We observed the ability of the different materials to accumulate space charges after an electrical and thermal poling close to practical applications. The influence of the combination of micro and nano filler is highlighted. A classification of the studied materials regarding their space charge accumulation is done.

Investigation of hydrophilic pollution layers on silicone rubber outdoor insulation

Polluted silicone rubber test plates were investigated before and after one year ageing at the Koeberg Insulator pollution Test Station (KIPTS), South Africa. The hydrophobicity of the rubbers was lost during the first two weeks after installation. The plates showed no sign of degradation after testing: no significant surface hardening or change of amount and molar mass distribution of the extractable silicones was found. The salinity and chemical composition of the hydrophilic pollution layers were similar a hydrophobic pollution layer from another geographical location. Additionally, it was concluded that the loss of hydrophobicity was not caused by biological growth. If the hydrophilic pollution layers were removed, hydrophobicity of the cleaned rubber surfaces recovered within a day. Optical microscope and scanning electron microscopy showed that the hydrophilic pollution from KIPTS formed a continuous, porous layer with a thickness of ∼20 µm. The layer also contained high-aspect ratio structures. In comparison, the hydrophobic pollution layer was thinner, <10 µm, and not homogenously distributed. It is suggested that the loss of hydrophobicity at KIPTS was caused primarily due to the structure of the pollution layer. In addition it was suggested this kind of structured pollution layer was a result of a fast deposition of moist salt in combination with other pollutants, both in form of particles and fibers.

Dielectric properties and resistance to corona and ozone of epoxy compositions filled with micro- and nano-fillers

This paper describes activities aiming to evaluate and compare the resistance to a prolonged corona and ozone exposure for a range of epoxy based compositions filled with micro- and nano-fillers of silica at different proportions. It has been earlier shown for this system that substitution of 5 wt% of micro-filler by nano-filler slightly improved mechanical properties, yielding an increase in Youngs modulus, tensile strength and elongation at break. At the same time, the toughness of the systems decreased with the addition of nano-fillers indicating a poor particle-matrix interaction, which was in accordance with the prior art claiming that this interaction is weaker in case of SiO2 nano-particles, as for example compared to A12O3 nano-particles. For the exposure to corona and ozone a methodology employing a multiple-needle electrode system was used, as recommended by CIGRE working group WG D1.14. Measurements of surface resistivity, bulk resistivity as well as dielectric response (DR) at broad frequency range (10 4-103 Hz) were performed on new samples of the investigated compositions and after each of two sequences of the corona-ozone treatment, lasting 100 hours each. It was found that the long-term corona-ozone exposure had obvious effect on surface resistivity for all the compositions investigated and the contents of the micro- and nano-fillers played a significant role in the observed changes. On the other hand, bulk resisitivity, dielectric permittivity as well as dissipation factor, all being the properties of material bulk, did not exhibit high sensitivity to the exposure.