Milos Vujisic

Mechanism of electrical breakdown of gases for pressures from 10−9 to 1 bar and inter-electrode gaps from 0.1 to 0.5 mm

This paper discusses the mechanisms of gas breakdown at low values of pressure and inter-electrode gap, i.e. in the vicinity of the Paschen minimum. In this area of pressure and inter-electrode gap values, breakdown occurs either through gas or vacuum mechanisms, and also the so called anomalous Paschen effect appears. Electrical breakdown of electropositive, electronegative and noble gases has been investigated theoretically, experimentally and numerically. Based on the results obtained, regions in which particular breakdown mechanisms appear have been demarcated. Special attention has been devoted to the anomalous Paschen effect as well as to the avalanche vacuum breakdown mechanism.

Calculation of impulse characteristics for gas-insulated systems with homogenous electric field

The possibility of generating a statistical sample of the pulse breakdown voltage random variable numerically is examined for arbitrary shaped pulses. Impulse characteristics are then determined on the basis of the generated statistical sample. Numerically generated statistical samples of the pulse breakdown voltage random variable are compared to the corresponding experimentally obtained statistical samples. Impulse characteristics obtained from the numerically generated statistical samples are compared to the corresponding impulse characteristics derived from the semi-empirical Area Law and the Time Enlargement Law. The set of impulse characteristics obtained in this way is compared to the results obtained experimentally for different shapes of the pulse voltage load. Gases used in the experimental and numerical models include SF6, N2 and Ar. Gas pressures range from 1 × 102 Pa to 6 × 102 Pa, and inter-electrode gaps from 0.1 to 10 mm. A homogenous electric field is considered.

Simulated Effects of Proton and Ion Beam Irradiation on Titanium Dioxide Memristors

Effects of titanium dioxide memristor exposure to proton and ion beams are investigated. A memristor model assuming ohmic electronic conduction and linear ionic drift is used for the analysis. Simulations of particle transport suggest that radiation induced oxygen ion/oxygen vacancy pairs can influence the devices operation by lowering both the mobility of the vacancies and the resistance of the stoichiometric oxide region. These radiation induced changes affect the current-voltage characteristic and state retention ability of the memristor.

Conditions for the applicability of the geometrical similarity law to impulse breakdown in gases

This paper investigates the conditions for the applicability of the geometrical similarity law to impulse breakdown in gases, using statistical methods necessitated by the stochastic nature of the impulse breakdown voltage. Theoretical analysis in the paper is accompanied by experimental results obtained for geometrically similar systems, i.e. for systems with equal shapes of the macroscopic and microscopic electrical field. The experiments were conducted in controlled laboratory conditions, for a wide range of gas pressure and inter-electrode gap values. Two-electrode systems were used, with both homogeneous and non-homogeneous electrical fields, utilizing SF6, N2 and He gases as insulators. Standard lightning and switching voltage impulses were applied, as well as ramp-shaped impulses with different slopes. On the basis of the statistical processing of the obtained experimental results, conclusions regarding the conditions for the applicability of the geometrical similarity law to impulse breakdown in gases are drawn.

Surface-time enlargement law for gas breakdown

This paper investigates, through theory and experiment, the applicability of the results obtained in laboratory tests of relatively short duration performed on model structures, as a part of the process of designing high-voltage equipment intended for lasting exploitation. Possibilities and limitations of applying these results to practical structures are examined using the methods of mathematical statistics. Special attention is devoted to the influence of electrode surface enlargement and pulse load (overvoltage) prolongation on the statistical behaviour of the pulse breakdown voltage random variable, expressed in the form of the enlargement law. In the theoretical part of the paper, the general four-dimensional (space-time) enlargement law is derived, along with its simplified three-dimensional (surface-time) variant. In the part of the paper related to the experiment, performed with the aim of testing the applicability of the derived surface-time enlargement law to SF6 gas-insulated two-electrode systems, a description of the experimental equipment and procedure is provided, along with the details of measurement data processing. Comparison of experimental results with those predicted by the surface-time enlargement law proved its validity for a two-electrode configuration with a homogenous and radial electric field, insulated by SF6 gas under pressure (with gas pressure as a parameter).

Time enlargement law for gas pulse breakdown

This paper investigates the possibility of applying the time enlargement law for predicting how gas-insulated systems would behave when exposed to pulse voltage loads of different shapes. For this purpose, the validity of the time enlargement law in this case has first been tested and the most suitable theoretical distribution function of the breakdown time random variable established. Pulse characteristics of the investigated insulating system have subsequently been determined, by applying the time enlargement law to experimental values of the breakdown time random variable, obtained in measurements with predefined shapes of the voltage load. Pulse characteristics thus obtained were compared with the corresponding pulse characteristics derived from the area law. The results demonstrate the advantages of the time enlargement law method, especially in the case of a non-homogeneous electric field. The experiments were conducted with SF6 gas, at different values of the pd product (pressure × inter-electrode gap), in a wide frequency range of applied pulse voltages, for a homogeneous, radial and point–plane electrode configuration.

Determination of Pulse Tolerable Voltage in Gas-Insulated Systems

In this paper we expound on the procedure of determining the pulse tolerable voltage characteristic in the voltage-versus-time frame, by applying the time enlargement law to the breakdown time random variable, and using a single statistical sample for this variable, obtained through experiments with a predefined shape of the voltage load. The suggested algorithm has been experimentally tested for Ar, N2, and SF6 gases, in the pd product (pressure × interelectrode gap) range from 10-4 to 300 bar mm. The testing was performed by comparing the pulse tolerable voltage characteristic of a two-electrode configuration obtained by applying a particular shape of the pulse voltage load with the values corresponding to other pulse voltage shapes, covering a wide range of frequencies. Satisfactory results have been obtained concerning the applicability of the procedure, with certain minor limitations, which are pointed out.

Reliability of three-electrode spark gaps

Two types of three-electrode gas-insulated spark gaps have been investigated in this paper: one with the third electrode located inside the main electrode, and the other with a separate third electrode. Three types of insulating media have been used: vacuum, SF6 and N2. Additionally, three different electrode materials have been implemented: copper, steel and tungsten. The following characteristics have been tested experimentally: the influence of insulating gas parameters on spark gap operation, the influence of the polarity of working and trigger voltages on spark gap operation, and the influence of the rate of rise of the trigger voltage on spark gap operation and the degree of spark gap irreversibility. A theoretical model of the spark gap is presented in the paper, which provides a basis for explaining experimental results depending on specific characteristics of the spark gap.

Synergistic effect of SF 6 and N 2 gas mixtures on the dynamics of electrical breakdown

This paper discusses the efficiency of a moderation effect in a mixture of SF6 and N2 gases, in case of the application of impulse voltages with varying rise times. The moderation in question pertains to the slowing down of free electrons through nonelastic collisions with molecules of N2 gas, which increases the probability, i.e. the effective cross-section, for the formation of negative ions of SF6. Variable parameters included relative mixture composition, pressure, inter-electrode gap, shape of the electric field, and the manner of electrode surface processing. By applying statistical analysis to experimentally obtained values of the impulse breakdown voltage random variable, and presenting the results as impulse characteristics, it was established that a positive synergetic effect of adding N2 gas into the mixture was achieved at smaller impulse speeds. This effect was lost at greater impulse speeds. The obtained results were explained by examining the relative ratio of the time constant characterizing the process of moderating the spectrum (statistical distribution of energy) of free electrons, to the characteristic time of elementary electric discharge processes in gases.

Initiation and progress of breakdown in the range to the left of the paschen minimum

Mechanisms of gas breakdown initiation and development in the range to the left of the Paschen minimum are investigated in this paper. Experiments were conducted for gas pressures from 10-9 to 10-1 bar and inter-electrode gaps from 0.1 to 1 mm. The obtained results were subjected to the Chauvenets criterion for discarding spurious values, the U-test for checking if they belonged to a common statistical distribution, and the chisquared test for testing their adherence to one of the considered theoretical distributions (normal, Weibull or double-exponential). The statistical analysis shows that the region left of the Paschen minimum can be divided into three subregions. It is established that the so called anomalous Paschen effect refers to breakdown occuring by way of the edge mechanisms in the first subregion, just next to the Paschen minimum. In the second subregion, breakdown emerges through the vacuum avalanche mechanism, and in the third subregion it occurs through the vacuum cathode emission mechanism.