The effect of surface treatment of silica nanoparticles on the breakdown strength of mineral oil

In previous work, the results of AC breakdown tests showed that unmodified silica nanoparticles improve the breakdown strength of mineral oil based nanofluids, especially at a relatively high humidity level of around 25 ppm. It was proposed that, since the hydrophilic surface of unmodified silica nanoparticles can absorb water, this would lead to a reduction of free moisture in the bulk of the oil, which has a strong influence on the breakdown strength. In the present study this proposition is verified, by comparing the breakdown strength of two mineral oil based nanofluids: a reference with unmodified silica nanofluid and a nanofluid with Z-6011 modified silica. The silane coupling agent Z-6011 turns the surface of silica nanoparticles hydrophobic, thus preventing water adsorption

An investigation into the dynamics of partial discharge propagation in mineral oil based nanofluids

Recent studies present a model which assumes that conductive nanoparticles can reduce the speed of the positive streamer propagation in mineral oil due to electron trapping at the particle surface. Time resolved partial discharge measurements can be used to evaluate the discharge dynamics and to verify this hypothesis. A special measurement setup was built to enable the recording of the discharge dynamics. In this study, the effect of nanoparticles with different conductivities on the discharge dynamics of mineral oil is investigated. The time resolved current shapes of partial discharges in nanofluids and mineral oil are compared. To understand the effect of the conductivity of the nanoparticles on the partial discharge dynamics of mineral oil, nanoparticles with two different conductivities are synthesized with mineral oil. The two types of nanoparticles are silica and fullerene. The host fluid used in this study is Shell DialaS3ZXIG mineral oil

Dielectric properties and space charge behavior of MgO-epoxy nanocomposites

Epoxy resin with magnesium oxide filler has already shown a reduced space charge accumulation compared to neat epoxy and composites with other nanofillers. This paper addresses the changes in the structure due to the introduction of surface functionalized nano-MgO into polymers and possible explanations for the unique dielectric behavior. Short term DC breakdown tests were performed alongside space charge measurement and dielectric spectroscopy. The breakdown strength was measured for negative DC voltages with Rogowski shaped electrodes immersed in oil, in order to prevent surface flashover. The base polymer is a commercially available bisphenol A epoxy with anhydrite hardener. As filler material we use magnesium oxide powder with an average particle size of 22 nm and alumina filler with 50 nm average diameter as comparison. The particles were surface modified with a silane coupling agent, in order to achieve a uniform dispersion of particles in the host material. Neat epoxy samples were used as a reference. It is shown that the superior space charge behavior did not reflect on the short term DC breakdown strength however. MgO-epoxy exhibited better short term breakdown results than neat epoxy, but lower breakdown values than epoxy filled with nano alumina

Thermal and electrical behaviour of epoxy-based microcomposites filled with Al2O3 and SiO2 particles

Epoxy resin is a polar thermosetting polymer that is widely employed in different branches of industry and everyday life, due to their stable physical and chemical properties. Of all the polymer materials currently being used in the electrical insulation industry, epoxy resin is the most widely used kind, together with polyethylene and chosen as the base polymer material in the present study. As a common practice, in order to obtain materials of the desired thermal, mechanical and electrical properties, polymers are processed with different types of inorganic fillers. In this paper, the authors made an attempt to thoroughly analyze both the thermal and the electrical behaviour of the created epoxy microcomposites. Epoxy-based composites containing microparticles of aluminum oxide and silicon dioxide were prepared by high shear mechanical mixing and ultrasonic processing to obtain a fine dispersion of the fillers in the matrix. The incorporation of all filler types led to noticeable improvement in thermal conductivity compared to the pure epoxy resin. The thermal conductivity and the relative permittivity of the composites were greatly influenced by the filler loading of the inorganic particles. Rules of mixture are used to predict the thermal conductivity and the relative permittivity of two-phase composites have been applied to compare experimental results with theoretical models

Size effect and electrical ageing of PDMS dielectric elastomer with competing failure modes

Large-scale dielectric elastomer generators dielectric elastomer generators (DEGs) such as those employed in wave energy converter projects require a significant volume of electrically stressed materials. Meanwhile, predictions of energy output from such systems are generally extrapolated from electrical and mechanical breakdown measurements performed on small scale samples, where the presence of small defects can be extremely small. This can lead to overly optimistic upscaled predictions for the performance and reliability of full-scale devices. In this study, multilayer DEGs were prepared to evaluate the dielectric breakdown strength of thin polydimethylsiloxane PDMS elastomer at different values of active areas. The results indicated the presence of two separate breakdown mechanisms resulting in an enhanced size effect and a reduced reliability for the larger samples. Electrical ageing tests were performed on three different sample geometries and the dielectric breakdown strength was found to be marginally affected by the time under stress. A Weibull competing failure model was applied to the distribution of experimental breakdowns and electrical reliability was accurately modeled over more than four decades of variation in the electrode area

Dielectric breakdown strength of PDMS elastomers after mechanical cycling

PDMS-based composites such as silicone elastomers are commonly found in high-voltage engineering, especially in outdoor insulation as coatings or structural elements or at interfaces between network elements, such as cable sealing ends (CSE). They are also promising prospects for dielectric elastomer generators (DEG), which are retrieving electrostatic energy from large strain amplitudes. The upper limit of energy conversion from these transducers is determined by the dielectric breakdown strength (DBS). Therefore, developing reliable systems that operate under high electric fields and variable repeated strains requires a thorough understanding of the mechanisms behind electrical breakdown and its coupling to mechanical cycling. In this study, the effect of Mullins damage and mechanical fatigue on silicone elastomers has been investigated. An electro-mechanical instability model that considers cyclic softening allows for predicting the evolution of the breakdown strength depending on the loading history. The results highlight the importance of the “first cycle,” where up to a 30% reduction in the mean DBS was measured. However, subsequent mechanical fatigue only marginally contributes to the degradation, which is a promising perspective for the long-term performance of any silicone elastomer as long as the precise impact of the first cycle is known

Space charge behavior of magnesium oxide filled epoxy nanocomposites at different temperatures and electric field strengths

Space charge accumulation is a phenomenon, which is typical for high voltage dc insulation. It is of special importance for polymers, since they do not possess self-healing properties. Thus, the accumulation of space charges, which is linked to ageing, proved to be a limiting factor for HVDC applications. The focus of this paper is the behavior of epoxy based nanocomposites with magnesium oxide filler material. Nanoscale magnesium oxide has already been shown to decrease the space charge density for high field strengths. Additionally, MgO-nanocomposites showed an increase in the short term dc breakdown strength for low filler concentrations. Base material for all samples is commercially available bisphenol-A epoxy resin. Transmission electron microscopy was performed to validate the particle size and dispersion and showed that MgO has an average particle size of 22 nm. Space Charge profiles were obtained with the PEA-method and compared to neat epoxy. The profiles were taken under dc field strengths between 10 and 18 kV/mm. To see the influence of temperature on the charge distribution, the measurement was performed both at room temperature and at 60°C. The field enhancement factor of both the neat epoxy and the nanocomposite for different field strengths and temperatures has been calculated. It turned out that MgO nanocomposites show overall better space charge behavior at higher field strengths and at higher temperatures, compared to the unfilled epoxy. Both the space charge accumulation and the field enhancement factor are reduced, when going to higher electric field strengths or temperatures. Possible explanations for the observed space charge behavior are given

Short term DC breakdown strength in epoxy based BN nano- and microcomposites

Nanocomposites have been known in the field of electrical engineering for more than a decade, albeit there are still uncertainties of how the filler properties influence the properties of the composite material. Filler size, shape, aspect ratio and the surface modification, which consequently leads to even dispersion or clusters of particles, are all suspected to influence the dielectric behaviour of the insulation. But it is disputed what influences the properties to what extent. The focus of this paper is the dependence of the short term breakdown (BD) strength of polymer based composites filled with boron nitride (BN) on the filler size. Base polymer is commercially available bisphenol A epoxy with anhydrite hardener. As filler material we use boron nitride powder with different average particle sizes. These are ranging from 70 to 5000 nm. The particles were not surface modified in order to see solely the influence of the filler size on the BD strength. The short term BD strength was measured for negative DC voltages with Rogowski shaped electrodes. Dielectric spectroscopy was done complementarily to see how the filler size influences the relative permittivity of the composites