Bystrík Dolník

Dielectric response of transformer oil based ferrofluid in low frequency range

In this article, our experimental study of the dynamic dielectric behaviour of transformer oil-based ferrofluid with magnetite nanoparticles is presented. Frequency-dependent dielectric permittivity and dissipation factor were measured within the frequency range from 20 Hz to 2 MHz by a capacitance method. The ferrofluid samples were placed in a liquid crystal cell, and experiments were carried out in an electromagnetically anechoic chamber. Two polarization processes and corresponding relaxations were revealed within the applied frequency range. Schwarz theory of electric double layer polarization is used to explain the low frequency relaxation maximum. Moreover, the shift of the maximum position towards higher frequencies is observed as the magnetic volume fraction in the ferrofluid increases. The related decrease in relaxation time due to higher counterion mobility is analysed. Reduced electric field intensity due to depolarization field, which is dependent on the particle concentration, is proposed as...

Direct observation of electric field induced pattern formation and particle aggregation in ferrofluids

Ferrofluids typically respond to magnetic fields and can be manipulated by external magnetic fields. Here, we report on formation of visually observable patterns in a diluted low-polarity ferrofluid exposed to external electric fields. This presents a specific type of ferrofluid structure driven by a combined effect of electrohydrodynamics and electrical body forces. The free charge and permittivity variation are considered to play a key role in the observed phenomenon. The corresponding changes in the ferrofluid structure have been found at nanoscale as well. By small-angle neutron scattering (SANS), we show that the magnetic nanoparticles aggregate in direct current (dc) electric field with a strong dependence on the field intensity. The anisotropic aggregates preferably orient in the direction of the applied electric field. Conducting SANS experiments with alternating current (ac) electric fields of various frequencies, we found a critical frequency triggering the aggregation process. Our experimental study could open future applications of ferrofluids based on insulating liquids.

Contribution to analysis of daily diagram of supply voltage in low voltage network: Working days versus non-working days

Supply voltage variations may have a significant impact on the service life and the operation of certain electrical equipments. Even relatively small deviations from the rated value can cause sub-optimal operation of equipment. The paper is focused on the voltage characteristics in low voltage systems, especially voltage variations, in order to characterize the point of consumption. A continuous long-term record of r.m.s value of supply voltage was realized. As a basis for analyzing of the voltage variations in low voltage network the comparative template of voltage for whole month, working days and non-working days was used. Selected statistics of the voltage record variations in the low voltage network were computed and the factor analysis was used to analyze the measured data. The results obtained from the experiments pointed to the fact that differences in daily diagram of supply voltage in working days and non-working days exists. These differences can be identified using the tools of statistical analysis on a monthly comparative template of voltage.

Electrode polarization and unusual magnetodielectric effect in a transformer oil-based magnetic nanofluid thin layer

In the present paper, we provide low-frequency dielectric spectra for a thin layer of a nanofluid based on transformer oil and iron oxide nanoparticles stabilized by oleic acid. The complex dielectric permittivity measured in the frequency range from 1 mHz to 200 kHz shows an obvious electrode polarization effect and a Debye-like dielectric relaxation process. Both effects stem from the presence of space charge in the oil due to impurity ions, and in the nanofluid represented predominantly by a residual surfactant and uncompensated particle surface charge. It is shown that the spectra, which were measured in the temperature range from 298 K to 358 K, can be well represented by a fitting function consisted of one Havriliak-Negami term and the Jonschers power law. In the investigated magnetic nanofluid layer, we found that the onset of the electrode polarization is suppressed to lower frequencies by the application of an external magnetic field (300 mT). This phenomenon is explained by a slowed-down migration of the space charge due to the Lorentz force and by a hindering effect of the formed magnetic nanoparticle aggregates. Surprisingly, a moderate decrease in the whole permittivity spectrum was observed for both parallel and perpendicular orientations of the electric and magnetic fields. This is in contradiction with the usual magnetodielectric anisotropy effect. Based on our qualitative analysis, we discuss potential reasons accountable for the observed effect.

Investigation of electrical properties of ZnO varistors stressed by current pulses

ZnO varistors produced by current technology have excellent non-linear voltage dependent characteristics. Therefore, they are suitable for use as a surge protection device of electric and electronic equipment. The operating life and the stability of installed arrester is affected by several factors, such as overvoltages having different duration and amplitude and increased ambient temperature. Moreover, the above factors act simultaneously and mutually influence each other. The article deals with the changes of electrical properties of low-voltage ZnO based varistor caused by injection of current pulses with a defined shape and energy. Comparison of changes in electrical properties is based on the observation of shift in dependence of the DC voltage and the corresponding current. Moreover, changes of nonlinearity coefficient and the absorbed energy have been observed. It was found that repeated injection of current pulses results in a significant change in electrical characteristics of the ZnO varistor. Total destruction of the varistor is demonstrated.

Monitoring of leakage current on HV surge arrester as pollution indicator of external insulation

The pollution of external insulation of power equipment has a significant impact on the lifetime of electrical equipment. The paper is focused on the possibility of using monitoring leakage current through the external insulation of hv surge arresters as indicator of pollution. In the experiment the modified procedure for the preparation of the polluting layer was used. The leakage current on the surface of the external insulation of the hv surge arrester and the internal leakage current of the hv surge arrester was monitored. It was found, that the surface resistivity of a sample with polluting layer in wet conditions, decreased by almost two orders of magnitude. The experimental results show that monitoring of leakage current along the surface of the external insulation is a useful indicator of pollution. In addition, measurement of the leakage current on the surge arrester with pollution layer during wet weather can lead to significantly erroneous results.

Dielectric Spectroscopy of Ferronematics Based on 6CHBT Liquid Crystal

In our study, the dielectric behaviour of the rod-like liquid crystal (6CHBT) doped with magnetic nanoparticles of spherical shape was investigated by means of dielectric spectroscopy in the frequency range from 20 Hz to 2 MHz. A low frequency dielectric dispersion in the nematic and isotropic phases of the pure liquid crystal (LC) has been assigned to the space charge polarization. After doping the host LC with the magnetic nano particles, a nearly Debye-like relaxation process was observed with the temperature dependence obeying the Arrhenius law. Considering a possible electric double layer formation on the particle surfaces, the detected relaxation process in the doped LC can be associated with the electric double layer polarization. The experimental results point out that in the measured frequency range the space charge and interfacial effects constitute the main dielectric response. Any anchoring effects were not observed and are therefore expected to appear in higher frequencies.