The proposed work will be consecrated to the study of positive pre-breakdown currents triggered in mineral transformer oil under 50 Hz alternating overvoltage. Since negative currents are recorded in low rates and for higher voltage levels than positive ones, only the latter will be prior taken into consideration. Both streamer propagation and arc discharge current types are identified and are used in the training process of an artificial neural network and the multi-linear regression line of these currents in order to develop a complementary diagnosis tool that can serve as an on-line transformer protection. More successful results than those obtained by other developed techniques are expected.
Electric field stimulates formation of gas-filled cavities on sharp electrodes in insulating liquids which may initiate the dielectric breakdown of these liquids. Using an analytical model based on electrical and thermodynamic properties of fluids, the critical dimensions of gas-filled bubbles in ester fluid and transformer oils stressed with an impulse electric field were calculated. The results obtained in this paper could be important for the optimisation of the application of these liquids in high voltage and pulsed power systems.
Spontaneous current study has been carried out in 70 nm particle size of wheat starch nano powder. The starch nano powder was prepared by mechanical grinding method. The spontaneous current was found to be induced and flowing in external circuit under short circuit condition by heating a sandwitched system. Sandwitched system consist of a starch nano powder (i.e. powder in different weight ratio of distilled water) between similar (Al-Al) and dissimilar (Al-Zn, Al-Cu, Al-Ag) electrode combination. The Spontaneous current spectra show two peaks lies in 46 ± 4°C and 72 ± 4°C, respectively, in the first heating run. However, only single peak was observed in second heating run. In the dissimilar electrode combination the first peak shifts to lower temperature side, while second peak shifts to higher temperature side. Spontaneous current mechanism could be explained on the basis transitional changes of starch nano powder caused by influence of water. The temperature dependent open circuit voltage V_{oc} was recorded. A linear relationship was observed between dissimilar electrodes system due to difference in electrode work functions. It has been observed that spontaneous current is influenced by water and depends on the choice of electrode material.
Investigation results concerning acoustic emission signals coming from partial discharges generated by modeled sources are presented in the paper. These sources have been made using different bushing ends (without any extra elements and with a thread or a pike) situated directly in the oil, but without screens typical for partial discharge investigations in a bushing. Measurements have been carried out using own measuring acoustic emission system DEMA-COMP and - in a parallel way - computer-aided partial discharges measuring system TE 571 (produced by the firm Haefely Trench). Fundamental and advanced analysis of acoustic emission signals has been made. These signals were recorded in 20 measuring situations which need to multiple installation of acoustic emission sensors. Conclusions resultant from own originate advanced analysis of signals content description of properties revealed by defined acoustic emission descriptors.
Authors of the paper present investigation results concerning properties of ADP, ADC and ADNC descriptors which have been obtained during measurements made within three oil transformers and then tested also by other methods (electric and dissolved gas analysis ones). Methodology which makes easier an analysis and identification of acoustic emission signals generated by partial discharges is presented. Results obtained by acoustic emission method have been referred to results coming from other measuring methods.
Data processing algorithms are important parts of modern measurement systems. These algorithms are often delivered to the user as complex program and their numerical structure is not known. Therefore also influence of algorithm on processed data accuracy is not known. One of the methods to evaluate uncertainty propagation through algorithm is based on matrix form of algorithm. Coefficient matrix of algorithm represents its numerical operations and it is a basis to algorithm accuracy evaluation. The paper presents a method how to identify this coefficient matrix when algebraic form of the algorithm is known or is difficult to use. Identified matrix form of algorithm is then used to estimate uncertainty propagation through exemplary algorithms. Results are compared with experiments.
Applicability of thin HfO_2 films as gate dielectric for SiC MOSFET transistor is reported. Layers characterisation was done by means of atomic force microscopy and scanning electron microscopy, spectroscopic ellipsometry and C-V and I-V measurements of MIS structures. High permittivity dielectric layers were deposited using atomic layer deposition. Investigation showed high value of κ = 15 and existence of high density surface states (5 × 10^{12} eV^{-1} cm^{-2}) on HfO_2/SiC interface. High leakage current is caused probably due to low conduction band offset between hafnium oxide and silicon carbide.
The paper presents investigation on the magnetic fluids that are stable colloidal suspensions of single-domain magnetic particles in a liquid carrier of dielectrics nature. Studies were made on the electric field vs. current density, e.i. E-J characterization commonly observed in insulating liquids under uniform low electric or magnetic fields. High performance oil was used as the dielectric carrier. The experiments were carried out at different volume concentrations of magnetite nanoparticles up to 4%.
In this paper the experimental study of the breakdown field strength in a transformer oil based ferrofluid is reported. The experiments are conducted on five ferrofluid samples with different magnetic volume fraction. The influence of external magnetic field on the breakdown field strength is investigated, when a quasi-homogenous magnetic field was applied in parallel and perpendicular configuration in regard to the electric field. The obtained results are analysed in accordance to the electron charging of ferrofluid nanoparticles theory.
In this paper an evaluation of a number of different insulating materials, under pulse breakdown conditions, is described. The experimental setup used an 8 stage Marx generator in order to generate a high potential difference (in the range 80-220 kV) between two spherical electrodes which were spaced 1.5-3.5 mm apart. The breakdown voltage of each of the materials was recorded and the data was then post-processed in order to determine the breakdown strength of each of the samples.
Emission acoustic signals, recorded in investigated power oil transformers, have been analyzed in the time, frequency and time-frequency domain. Analysis of each signal has been started by filtration within selected frequency band and subsequently the following quantities have been calculated: spectral power density, phase-time characteristic, averaging phase-time characteristic, short-time Fourier transform spectrograms, signal amplitude distributions, descriptors with acronyms ADC and ADP and thereafter maps of descriptors on lateral walls of transformers can be carried out. Frequency bands applied in order to filtration have been chosen in such a way so that signals coming from different sources (among other things from partial discharges, Barkhausen's effect, oil circulation and outer acoustic disturbances) can be differentiated. The sources have been localized using maps of descriptors calculated for selected frequency bands. The fundamental properties of obtained signals have been determined. Such properties describe: partial discharges, Barkhausen's acoustic effect and other acoustic interferences.
The original system useful for analysis of signals recorded during investigations of partial discharges within power oil transformers by means of acoustic method is presented. This method includes the basic and advanced analysis of recorded data. In the frame of basic analysis of data recorded signals undergo filtration in chosen frequency bands and next the analysis is made - in domain of time, frequency, time-frequency and discrimination threshold. In the frame of advanced analysis of data the amplitude distributions of acoustic emission signals and the acoustic emission descriptors (defined by the authors) are calculated in order to outline maps of acoustic emission descriptors on lateral walls of a transformer; it is a base for location of sources of partial discharges by means of the original method consisted in determination of advance degree of a signal. Results of this analysis, for signals recorded in two chosen transformers with identical construction (partial discharge occurred only within one of them), are presented in the paper. The source of partial discharge, situated within oil near transformer tank, was localized; the revision confirmed this result. Properties of recorded emission acoustic signals at chosen measuring points situated on the tank, in function of distance between the partial discharge source and measuring points, are presented.
Investigation results of properties characteristic for acoustic emission signals recorded in two selected power oil transformers are presented. Signals were put to the filtration, whereas components coming from partial discharges have been left. The calculations concerned: phase-time characteristics, averaging phase characteristics, averaging short time Fourier transform spectrograms, amplitude distributions of signals, values of acoustic emission descriptor with acronym ADC. On the ground of calculated basic characteristics and maps of ADC descriptor three areas have been selected on lateral walls of transformer tanks. Acoustic emission signals recorded in these areas were analyzed from the point of view how is influence of propagation path on these properties.
In the paper the experimental study of magnetic nanoparticle aggregation in a transformer oil based ferrofluid driven by an external electric field is reported. The studied ferrofluid was composed of the magnetite nanoparticles, oleic acid surfactant, and transformer oil. Generally, it is considered that superparamagnetic nanoparticles do not interact in the absence of external magnetic field. In the paper we present an experimental observation of the particle assembly formation in a direct current external electric field by optical microscopy. During the observation no external magnetic field was applied. A diluted low-polarity ferrofluid drop on a glass surface was exposed to the external static inhomogeneous electric field. It is assumed that induced dipole-dipole interaction and subsequent dielectrophoretic motion give rise to the electrohydrodynamic flow in the fluid after a certain time period. As a result, a visible particle chain was formed at the drop electrical equator. This demonstration is expected to contribute to the understanding of the streamer formation and electrical breakdown in transformer oil based ferrofluids.
Investigation results of acoustic emission signals coming from partial discharges within chosen generator coil bar, in relation to measurement results of apparent charge introduced by partial discharges sources, are presented in the article. Analysis is based on amplitude distributions calculated for acoustic emission signals, ordered at different measuring points for different values of the supply voltage. In order to assign acoustic emission signal to its advance degree a descriptor named by an acronym ADP (defined by authors) is calculated for each amplitude distribution. Descriptor families describing acoustic emission signals measured at particular measuring points (for different supply voltages) are presented in relation to apparent charge introduced by acoustic emission sources. These families concerning acoustic emission signals, measured at different measuring points for identical supply voltage, enable us to locate acoustic emission sources with maximum activity.
Dielectric-spectroscopic and ac conductivity studies on 0.01 and 1.0 molar percentage manganese doped layered Na_{1.86}Li_{0.10}K_{0.04}Ti_3O_7 ceramics have been reported. The dependence of loss tangent (tan δ) and relative permittivity (ε_{r}) on temperature in the range 350-775 K and on frequency in the range 10 kHz-1 MHz have been undertaken. The losses are the characteristics of dipole mechanism, electrical conduction and space charge polarization. The obtained conductivity plots between log(σ_{ac}T) versus 1000/T have been divided into four regions namely region I, II, III, and IV. The mechanism of conduction in region I is acknowledged to electronic hopping conduction. The less frequency and more temperature dependent region II is ascribed as a mixed mechanism "associated interlayer ionic conduction, electron hopping, and alkali ion hopping conduction". The unassociated interlayer ionic conduction along with alkali ion hopping conduction mechanisms are contributing to the transport process in the mid temperature region III. The mechanism of conduction in the highest temperature region IV may be recognized as the modified interlayer ionic conduction. The conductivity versus frequency curves lead to conclude that the electronic hopping conduction diminishes with the rise of temperature.
Methods to modify gate dielectrics of MIS structures by irradiation treatments and high-field electron injection into dielectric are considered. In addition, distinctive features of these methods used to correct parameters of MIS devices are studied. It was found out that negative charge, accumulating in the thin film of phosphosilicate glass (PSG) of the MIS structure having a two-layer gate dielectric SiO_2-PSG under the high-field injection or during the irradiation treatment can be used to correct the threshold voltage to improve the charge stability and raise the voltage of breakdown for the MIS devices. It is proved that the density of electron traps rises with the increasing thickness of the PSG film. In this paper a method to modify electrophysical characteristics of MIS structures by the high-field tunnel injection of electrons into the gate dielectric under the mode of controlled current stress is proposed. The method allows to monitor changing of MIS structure parameters directly during the modification process.
The paper presents identification results of deformation processes within power oil transformers where - according to dissolved gas analysis investigation results - partial discharges took place. The original method worked out for analysis of acoustic emission signals recorded within tested transformer and particularly maps of ADC descriptors have been applied. Analysis has been carried out within chosen frequency bands in order to distinguish signals coming from different sources (among other things partial discharges, Barkhausen's effect, circulation of the oil, and outer acoustic sources). One source of partial discharges has been identified within the tested transformer as a result of analysis of acoustic emission signals.
One challenge in high power switching is to have a compact switch, which can hold off high voltages and close rapidly at the proper time. Most high power switches are large or complicated, such as triggered spark gaps. Typical opening switches are also not compact and often have too long switching time. It has been shown previously that certain insulating materials undergo a drastic change in conductivity under shock loading. Using such a material could greatly reduce the size of a switch. We will report on our continuing studies of different candidate materials for shock wave switches.
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