Jozef Kúdelčík - Academia.edu (original) (raw)

Papers by Jozef Kúdelčík

Communications - Scientific letters of the University of Zilina, 2010

Acta Physica Polonica A, 2020

In the application of a magnetic field to a magnetic fluid, magnetic nanoparticles are created wi... more In the application of a magnetic field to a magnetic fluid, magnetic nanoparticles are created with various structures. In this contribution, we compare the structural changes in three types of transformer oil-based (TECH-NOL, MOGUL, and MOL) magnetic fluids upon different developments of magnetic fields and temperatures by the acoustic spectroscopy. The change of the acoustic attenuation was slow (> 10 min) or fast (< 4 min) depending on the step change of the magnetic field. After switching off the magnetic field, a fast decrease of the acoustic attenuation was observed only for MOL-based magnetic fluid. For linear changes of the magnetic field, the acoustic attenuation increased in steps and a hysteresis effect was observed. The effect of anisotropy of the acoustic attenuation was observed and analysed.

ICERI proceedings, Oct 31, 2023

Communications - Scientific letters of the University of Zilina, 2014

Journal of Physics D, Apr 1, 2003

Current waveforms of first negative corona pulses have been measured in CO2-SF6 mixtures over a p... more Current waveforms of first negative corona pulses have been measured in CO2-SF6 mixtures over a pressure range extending from 6.65 to 50 kPa and various overvoltages. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. For a given set of experimental conditions it is concluded that in the mixtures containing up to 30% of SF6 the negative corona pulse is associated with the formation of a cathode-directed streamer-like ionizing wave in the immediate vicinity of the cathode. This is in contrast to the discharge behaviour in air-SF6 and N2-SF6 mixtures, where in similar conditions the discharge develops according to a multi-avalanche Townsend mechanism.

2022 ELEKTRO (ELEKTRO), May 23, 2022

Przegląd Elektrotechniczny, 2009

The initial state of breakdown development is explained on the basis of bubble theory. Applicatio... more The initial state of breakdown development is explained on the basis of bubble theory. Application of HV-DC voltage to electrodes immersed in oil results to creation of small channels, in which streamers can develop. In the next phase a plasma channel between the electrodes can be formed. The electrical resistance of plasma channel changes from a few ohms to a few hundred milliohms due to Joule heating caused by high arc current which flows through the plasma. The dynamics of the arc current depends on the parameters of outer circuit and is represented by RLC circuit.

Experimental analysis and diagnostics of insulating system oil-paper for traction transformer by ... more Experimental analysis and diagnostics of insulating system oil-paper for traction transformer by frequency method is presented in the paper. In the first part of the paper, the base theory about measurement and diagnostics insulating part (oil and paper) of transformers is described. In the other part of the paper, experimental results of the diagnostic measurement for the traction transformer at a constant temperature using to diagnostic method - frequency domain spectroscopy (FDS) are presented. This method is used for the analysis of insulating conditions of high-voltage equipment with insulating oil paper. Quality of paper moisture and oil conductivity of the transformer - before and after its drying was compared.

Acta Physica Polonica A, May 1, 2020

The study of the changes of dielectric parameters leads to a better understanding of polarization... more The study of the changes of dielectric parameters leads to a better understanding of polarization and relaxation phenomena in ferrofluids. The impacts of the electric field and magnetic flux density cause the changes in dielectric parameters and structural arrangement of magnetic nanoparticles in ferrofluid and this effect has been studied by using dielectric spectroscopy. The frequency dependence of tan(δ) was measured within the frequency range from 1 mHz to 10 kHz and in a wide range of temperatures from −25 • C to 25 • C. Magnetic flux density and electric field were applied by a capacitance method. The object of our study was the ferrofluid with two different concentrations of magnetic nanoparticles in transformer oil MOL. The dielectric parameters were measured at parallel and perpendicular orientations of the magnetic flux density with value of 200 mT to the electric field as a function of temperature. The Cole-Cole relaxation model was used for analysing measured data. From this model, basic dielectric parameters as a function of temperature were determined for different fields.

The degradation processes of pressboard, paper or oil used in transformer are influenced by many ... more The degradation processes of pressboard, paper or oil used in transformer are influenced by many factors. One of them is the moisture which is usually inhomogeneous distributed in oil-paper insulation, especially for power transformer. The dielectric properties of mineral oil impregnated pressboard with various moisture concentrations were measured. These measurements were made at different temperatures. The measured dielectric parameters were modeled by Debye model. Two different instruments: IDAX and LCR Meter with various frequency ranges for the measurements were used.

Surface partial discharges may degrade the electrical insulation in time, so they are destructive... more Surface partial discharges may degrade the electrical insulation in time, so they are destructive to material. For the measurement of surface partial discharges (PD) it was used a needle-cylinder electrode configuration and the analyses were done using the phase-resolved partial measurement and pulse-sequence. The surface PDs were spread between the zero cross and the peak of applied voltage. The effect of surface charge and the surface discharge events for various insulation materials were analyzed by PDs patterns, too. The corona discharge was observed when the needle tip was distant from surface insulation.

Production Engineering Archives, Mar 1, 2017

Austenitic stainless steels are materials, that are widely used in various fields of industry, ar... more Austenitic stainless steels are materials, that are widely used in various fields of industry, architecture and biomedicine. Their specific composition of alloying elements has got influence on their deformation behavior. The main goal of this study was evaluation of magnetic properties of selected steels, caused by plastic deformation. The samples were heat treated in different intervals of temperature before measuring. Then the magnetic properties were measured on device designed for measuring of magnetism. From tested specimens, only AISI 304 confirmed effect of plastic deformation on the magnetic properties. Magnetic properties changed with increasing temperature.

The influence of 1 wt. % nanoparticles (ZnO, TiO2, SiO2) as fillers in an epoxy resin, known as n... more The influence of 1 wt. % nanoparticles (ZnO, TiO2, SiO2) as fillers in an epoxy resin, known as nanocomposites, on the complex permittivity have been measured. The real and imaginary parts of the complex permittivity and dissipation factor within the frequency ranges from 1 mHz to 1 MHz were measured by a capacitance method. Two mechanisms of polarization processes to an explanation for changes of the complex permittivity were used. α- and β-polarizations processes at the measurement of the temperature dependence of the imaginary permittivity and dissipation factor were observed.

ICERI proceedings, Nov 1, 2022

The acoustic spectroscopy is used to study properties and changes in structural arrangement in si... more The acoustic spectroscopy is used to study properties and changes in structural arrangement in silicone oil based magneto-rheological fluids with carbonyl iron particles upon the effect of an external magnetic field. Attenuation spectra at three temperatures for various concentrations of magnetic particles are presented. The attenuation of acoustic waves was measured for a jump change of the magnetic field to 200 mT as a function of the temperature. The relaxation effects for the acoustic attenuation after switching off the magnetic field and its decrease to the similar value as for clean silicone oil were observed. The change of acoustic attenuation in magneto-rheological fluid versus angle between the wave vector of acoustic waves and direction of the applied magnetic field was measured, too. For the anisotropy measurement are characteristic two local maxima from which results chain orientation in direction of the magnetic field.

Dielectric materials and systems are an inseparable part of a considerable amount in high-voltage... more Dielectric materials and systems are an inseparable part of a considerable amount in high-voltage applications in the field of electricity generation or distribution. The main aim of this paper is to describe possible techniques for evaluation of inhomogeneous insulation materials in interaction with DC electric field. In this research, the two commonly used insulation systems (with and insulation system based Epoxy + surface treated MgO nanofiller + PEN reinforcement, have been used for this investigation. The current density dependence on the applied field, the electron mobility, and analyses of resorption currents are presented in this paper. The possible reasons for main differences between materials are discussed.

Advances in Electrical and Electronic Engineering, Jun 28, 2011

... Vladimir Kasik VSB - Technical University of Ostrava, Czech Republic. Matej Kavacky Slovak Un... more ... Vladimir Kasik VSB - Technical University of Ostrava, Czech Republic. Matej Kavacky Slovak University of Technology, Slovakia. ... Vladimir Vasinek VSB - Technical University of Ostrava, Czech Republic. Frantisek Vizda University of Defence, Czech Republic. ...

DOAJ (DOAJ: Directory of Open Access Journals), 2006

Power transformers are key equipment for transfer and distribution of the electric power. Conside... more Power transformers are key equipment for transfer and distribution of the electric power. Considering the significance of the power transformers in the electric system, their price and possible damages occurred by accidents, it is necessary to pay attention to their higher prevention. To prevent failure states of transformers, we perform different types of<br />measurements. They shall illustrate a momentary state of the measured equipment and if necessary to draw attention in advance to changes of parameters, which have specific relationship to no-failure operation of the equipment. The conditions under which breakdown of composite liquid/ solid insulation can occur, e.g. in transformer, play an important role in designing such insulation. The liquid, mainly mineral oil, generally constitutes the weakest part of insulation and a great amount of work has been devoted to the study of streamers, which appear in the gaseous phase, and most often are triggering the failure of insulation.

DOAJ (DOAJ: Directory of Open Access Journals), 2008

Acquisition of basic characteristic of defects has become possible through a wide class of measur... more Acquisition of basic characteristic of defects has become possible through a wide class of measurement techniques which probe the interface, the near interface, as well as the bulk of semiconductor. Results presented here are based essentially on the acoustic version of Deep Level Transient Spectroscopy (A-DLTS) measurements. This method is based on the acoustoelectric response effect observed at the interface. The A-DLTS uses the acoustoelectric response signal (ARS) produced by MOS structure interface when a longitudal acoustic wave propagates through a structure. The ARS is extremely sensitive to external conditions of the structure and reflects any changes in the charge distribution connected with charged traps. The temperature dependence of ARS after bias voltage step application is investigated and the activation energies and some other parameters of traps at the insulator-semiconductor interface are determined. The results obtained form Arrhenius plots of A-DLTS spectra of selected MOS structures are compared with results obtained from modeling of A-DLTS spectra using theoretical model.