Hüseyin R. Hiziroglu

The behavior of N/sub 2/+SF/sub 6/ gas mixtures subjected to orthogonal electric and magnetic fields

The limiting equivalent electric fields in N/sub 2/+SF/sub 6/ binary gas mixtures due to Townsend discharges are evaluated directly from a Monte-Carlo simulation when the mixture is subjected to orthogonal electric and magnetic fields. Along with the limiting equivalent electric fields, transverse and perpendicular drift velocities, electron mean energies and collision frequencies are also determined within the scope of the Monte-Carlo simulation. The equivalent reduced electric field (EREF) concept for the corresponding limiting electric fields is also investigated for the calculated mean energy levels and collision frequencies. The EREF values are found to be in good agreement with the previously published limiting electric field data.

Statistical characteristic in time-domain of direct current corona-generated audible noise from conductor in corona cage

The corona-generated audible noise (AN) has become one of decisive factors in the design of high voltage direct current (HVDC) transmission lines. The AN from transmission lines can be attributed to sound pressure pulses which are generated by the multiple corona sources formed on the conductor, i.e., transmission lines. In this paper, a detailed time-domain characteristics of the sound pressure pulses, which are generated by the DC corona discharges formed over the surfaces of a stranded conductors, are investigated systematically in a laboratory settings using a corona cage structure. The amplitude of sound pressure pulse and its time intervals are extracted by observing a direct correlation between corona current pulses and corona-generated sound pressure pulses. Based on the statistical characteristics, a stochastic model is presented for simulating the sound pressure pulses due to DC corona discharges occurring on conductors. The proposed stochastic model is validated by comparing the calculated and ...

Boltzmann analysis of electron swarm parameters in binary CF 4 + Ar mixtures

The electron swarm parameters, namely electron mean energy, drift velocity, effective ionization coefficient and limiting values of number density reduced electric fields, E/N, are calculated in CF4+Ar mixtures for various CF4 concentrations that vary from 2 to 100% over a range of E/N from 50Td to 600Td by solving Boltzmanns equation. In this study Boltzmanns equation was solved using finite difference method under steady-state Townsend condition. One of the most important results of this study indicated that at higher E/N values inelastic processes due to argon began to control the swarm energy thus leading this binary mixture act essentially like pure argon.

Electromagnetic Field Theory Fundamentals: Waveguides and cavity resonators

Introduction In our discussion of transmission lines, we pointed out that the resistance of a conductor increases with an increase in the signal frequency, leading to an increase in power loss along the line. This power loss becomes intolerable at microwave frequencies (in the GHz range) and makes the transmission line almost impractical. At such high frequencies hollow conductors, known as waveguides, are employed to guide electrical signals efficiently. Figure 10.1 shows a typical waveguide assembly. In the study of transmission lines with at least two conductors, we found that the propagating wave has field components in the transverse direction and is referred to as the transverse electromagnetic (TEM) wave. However, as a waveguide consists of only one hollow conductor, we do not expect it to support the TEM wave. In this chapter, we show that a waveguide can support the other two types of waves, the transverse magnetic (TM) and the transverse electric (TE) waves. These waves can exist inside a hollow conductor under certain conditions. TM and TE waves can also propagate in a region bounded by a parallel-plate transmission line, in which case the two conducting plates are said to form a parallel-plate waveguide . The propagation of an electromagnetic wave inside a waveguide is quite different than the propagation of a TEM wave. When a wave is introduced at one end of the waveguide, it is reflected from the wall of the waveguide whenever it strikes it.

Calculation of breakdown voltages of short air gaps

In this study, breakdown voltages in air are calculated based on the streamer criterion and compared with experimental data. The calculations are done under standard atmospheric conditions, and the computed breakdown voltages are in good agreement with the experimental findings.

Influence of AC voltage on the positive DC corona current pulses in a wire-cylinder gap

In the development of hybrid HVDC and HVAC transmission lines, the study of radio interference is an important issue. Positive corona current pulses from high voltage transmission lines are the main source of radio interference. In this paper, the design of a wire-cylinder gap electrode system is presented to study the influence of AC voltage on the characteristics of positive corona current pulses. The study shows that the mode of the current pulses is different from that of either DC or AC corona discharge. Waveform parameters of the pulses, such as rise time, half wave time, duration time, repetition rates, average amplitude, and time intervals of secondary pulses are all statistically analyzed in this study. The empirical formulas for the repetition rates with different AC voltages are presented. A theoretical explanation based on an ion cloud model is given to reveal the mechanism behind the influence of AC voltage on positive corona discharge. The experimental results could provide some references for the prediction of radio interference from hybrid AC/DC transmission lines.

Prediction of breakdown voltages in N2 + SF6 gas mixtures

This study proposes artificial neural networks (ANN) to predict the breakdown voltages in N2 + SF6 gas mixtures. The proposed ANN consists of one input layer, two hidden layers and one output layer, which is essentially the predicted breakdown voltage. In order to train the ANN, the experimental data available in literature for N2 + SF6 have been used. When compared with the experimental data the average relative errors on predicted breakdown voltages are found to be less than plusmn5% for training as well as for testing in all cases using the proposed ANNs. Since the average errors are less than 5%, it is recommended to use the proposed ANNs to predict the breakdown voltages.

Assessment of electron swarm parameters and limiting electric fields in SF6+CF4+Ar gas mixtures

The electron swarm parameters, namely electron drift velocity, mean energy, effective ionization coefficient and limiting values of number-density-reduced electric fields, E/N, are calculated and assessed in SF6+CF4+Ar ternary mixtures for SF6 concentrations that vary from 1% to 20% over a range of E/N from 50Td to 600Td by solving Boltzmanns equation. The limiting electric field values for the ternary mixtures are deduced from the graphs of the calculated effective ionization coefficients as a function of E/N. Furthermore, breakdown voltage measurements are conducted for the ternary mixtures. The limiting electric field values obtained from the measured breakdown voltages are compared with those of the Boltzmann analysis and found to be in good agreement. The insulation strength of the ternary mixtures with constant Ar concentrations indicated an increase with increasing contents of SF6 while the share of CF4 in the mixture reduced in the investigated E/N range up to 600 Td with SF6 concentrations varying from 1% to 20%. Moreover, a significant synergy is observed due to the non-linear nature of the mixture, and the maximum synergy appears to be around 10% SF6 in the mixtures of this study with fixed Ar concentration. Furthermore, theoretical calculations of breakdown voltages from Boltzmann analysis are found to be in good correlation with the experimental values.

Electrical breakdown of polypropylene filled with natural clay as nanomaterial

This study presents the breakdown behavior of polypropylene filled with a various concentrations of nano-size natural clay. The concentrations of natural clay by weight were 0 %, 2 %and 6 % in polypropylene. For different concentrations of natural clay each specimen was subjected to a sinusoidal voltage waveform at power frequency with a certain ramp rate to a level where a breakdown occurred. The rms voltage at the breakdown was considered as the breakdown voltage of the sample. The mean dielectric strength improved about 3.5 % for the nanocomposite with 2 % of natural clay concentration in the host polypropylene. However, the improvement on the mean dielectric strength of the nanocomposite with 6 %concentration of natural clay appeared to be little more than 1 % when compared with 2 % concentration of natural clay in the host. This implies that there exits an optimum concentration of nanosize natural clay in polypropylene. Also observed was the increase of the dielectric strength of the materials with the increase of the ramp rate of the applied voltage.

Breakdown and partial discharge behavior of enamel-insulated copper conductors

During the manufacturing of the windings of an electric motor, enamel-insulated copper conductors are subjected to mechanical strain. This strain may adversely affect the electrical performance of the enamel insulation. Partial discharge inception voltage is useful for comparing the performance of insulations. Therefore, experiments were done in evaluating partial discharge inception voltages in order to determine the effect of different levels of tensile-strain on the electrical properties of the insulation system. Within the scope of this study, unstrained and tensile-strained, enamel-insulated, copper conductors with rectangular cross-section were investigated and compared for their breakdown voltages and partial discharge behavior.