Huifei Jin

AC breakdown voltage and viscosity of mineral oil based SiO 2 nanofluids

A nanofluid is a fluid containing nano-sized materials. It exhibits more efficient heat transfer abilities compared to the host fluid. However the precise effect on the electrical properties of the host fluid is still uncertain. The purpose of the present study is to analyze the AC breakdown strength and viscosity of mineral oil based SiO2 nanofluids. The host fluid used in this paper is Shell Diala S3ZXIG. This type of mineral oil is widely used in HV transformers and in the power supplies of X-ray systems. The size of the SiO2 nanoparticles is ranging between 10 and 20 nm. The mass fractions of the investigated nanofluids are between 0.005% and 0.02%. The influence of nanoparticle concentration and moisture content on the breakdown strength is studied. The AC breakdown test results are analyzed with Weibull statistical distributions. Viscosity is an important physical property of mineral oil, which has an effect on both the electrical and the thermal performance of mineral oil. Viscosity tests were performed with a rheology meter.

Use of dissipation factor for life consumption assessment and future life modeling of oil-filled high-voltage power cables

Tan δ measurements can be used to assess the condition of OF insulation in HV power cables. Data showing the behavior of tan δ as a function of temperature at constant electrical stress, and as a function of electrical stress at constant temperature, at different stages of degradation (or aging) are essential. By combining on-site and laboratory measurements, the remaining service lifetime of a cable can be estimated for various future loading levels using a model incorporating ground temperature, the age of the cable, and its loading history.

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.

Partial discharge behavior of mineral oil based nanofluids

A previous study showed that both mineral oil based nanofluids with 0.01% silica mass fraction and with 0.1% fullerene mass fraction have a higher AC breakdown strength than mineral oil. Breakdown occurs following discharge initiation and propagation in the oil. The breakdown strength value alone provides little information on the discharge process. Therefore, it is important to investigate the details of the discharge mechanisms in mineral oil and in nanofluids. Hence, this study focuses on the partial discharge (PD) behavior of mineral oil, silica and fullerene nanofluids. The total charge, voltage and pulse shape were recorded with the help of a high bandwidth PD measuring system. The discharge mechanism in mineral oil appeared to depend strongly on the polarity of the applied DC voltage. Under positive DC voltage, the silica and the fullerene nanofluids show increased inception voltage and a reduction of the total discharge magnitude compared to the reference mineral oil. Under negative polarity, inception voltage and discharge magnitude of the nanofluids and the reference mineral oil are virtually the same.

Effect of Interfacial Polarization and Water Absorption on the Dielectric Properties of Epoxy-Nanocomposites

Five types of nanofillers, namely, silica, surface-silylated silica, alumina, surface-silylated alumina, and boron nitride, were tested in this study. Nanocomposites composed of an epoxy/amine resin and one of the five types of nanoparticles were tested as dielectrics with a focus on (i) the surface functionalization of the nanoparticles and (ii) the water absorption by the materials. The dispersability of the nanoparticles in the resin correlated with the composition (OH content) of their surfaces. The interfacial polarization of the thoroughly dried samples was found to increase at lowered frequencies and increased temperatures. The β relaxation, unlike the interfacial polarization, was not significantly increased at elevated temperatures (below the glass-transition temperature). Upon the absorption of water under ambient conditions, the interfacial polarization increased significantly, and the insulating properties decreased or even deteriorated. This effect was most pronounced in the nanocomposite containing silica, and occurred as well in the nanocomposites containing silylated silica or non-functionalized alumina. The alternating current (AC) breakdown strength of all specimens was in the range of 30 to 35 kV·mm−1. In direct current (DC) breakdown tests, the epoxy resin exhibited the lowest strength of 110 kV·mm−1; the nanocomposite containing surface-silylated alumina had a strength of 170 kV·mm−1. In summary, water absorption had the most relevant impact on the dielectric properties of nanocomposites containing nanoparticles, the surfaces of which interacted with the water molecules. Nanocomposites containing silylated alumina particles or boron nitride showed the best dielectric properties in this study.

AC breakdown voltage and viscosity of mineral oil based fullerene nanofluids

A nanofluid is a two-phase mixture composed of a liquid phase and solid nanoparticles in suspension. These nanoparticles exhibit unique properties, compared to those of the same material at bulk scale. In this study, mineral oil based fullerene nanofluids with different concentrations were prepared. Of main interest was the AC breakdown voltage of the nanofluids. The AC breakdown voltage of the nanofluids was tested at around 24ppm humidity level. Since viscosity has such a large impart on properties of liquids, viscosity tests of fullerene nanofluids were performed with a rheology meter, in order to see if observed changes in the AC breakdown strength could be related to a change of viscosity.

Thermal conductivity of fullerene and TiO 2 nanofluids

The term “nanofluids” was first proposed by researchers at Argonne National Laboratory and refers to a two-phase mixture, composed of a liquid medium and solid nanoparticles in suspension. One method of enhancing the thermal conductivity of a fluid is to add nanoparticles to the fluid, thus creating such a nanofluid. In this study, fullerene and titania nanoparticles were dispersed into mineral oil in order to investigate the effect on the thermal conductivity of the host fluid. The thermal conductivity of nanofluids was measured by means of the hot-wire method. A model was devised to calculate the thermal conductivity in mineral oil based nanofluids and then calculated with the help of COMSOL Multiphysics and compared to measurement results.

The effect of frequency on the dielectric strength of epoxy resin and epoxy resin based nanocomposites

In this study, the relative permittivity, dielectric loss and breakdown strength of epoxy resin and epoxy-hBN (hexagonal Boron Nitride) nanocomposites are studied under two different frequencies: 50 Hz and 1500 Hz. The effect of fill grade of the nanoparticles on the dielectric properties of the nanocomposites is also included. The following fill grades are investigated: 0.2 vol.%, 1 vol.% and 5 vol.%. The results indicate that the higher frequency (1500 Hz) affects the breakdown strength of the samples. The influence changes with the change of the filler concentration. The dielectric loss values of epoxy resin and epoxy-hBN nanocomposites show a significant increase with the increase of frequency. The decrease of the breakdown strength is in line with the increase of the tanô.

The effect of frequency on the dielectric breakdown of insulation materials in HV cable systems

In this study, the effect of frequency on the breakdown strength of polymeric insulating materials is investigated. Two types of polymeric insulating materials that are used in extruded high voltage (HV) cable systems have been investigated: cross-linked polyethylene (XLPE) and epoxy resin. Breakdown tests are performed under DC voltage and AC voltage with frequencies ranging from 50 Hz to 2500 Hz. Weibull statistical techniques are applied to analyse the breakdown results. The results show that, for both materials, the breakdown strength is significantly higher under DC voltage than AC voltage. For AC voltage, there is a statistically significant dependency between breakdown strength and frequency: the higher the frequency, the lower the breakdown strength.

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.