Yoshitaka Nakao

Electron swarm development in SF6. I. Boltzmann equation analysis

The electron swarm behaviour in SF6 gas is studied for E/N values from 141 to 707 Td by a three-term Boltzmann equation method, in which the effect of generation and loss of electrons due to ionisation and attachment is considered properly. A consistent set of electron collision cross sections, which gives the swarm parameter values in agreement with previous measurements, is determined considering the latest cross section data. The calculation is performed mainly for the steady-state Townsend condition. The validity of the results obtained has been confirmed by a Monte Carlo simulation carried out parallel to the analysis. The present results are also compared with those of the usual two-term Boltzmann analysis. It is found that the two-term approximation is fully valid for deduction of the swarm parameters for E/N values as considered despite the fact that SF6 is a strongly electronegative gas.

Electron transport coefficients in SF6

The electron swarm behaviour in SF6 gas is re-analysed over the E/N range 141-7000 Td by a six-term Boltzmann equation method and by a Monte Carlo simulation considering the latest cross section data, in particular, that of the elastic momentum transfer cross section. The Boltzmann equation analysis shows that the present set of cross sections gives the values of swarm parameters such as ionization and electron attachment coefficients, drift velocity, longitudinal and transverse diffusion coefficients in excellent agreement with the respective measurements for a wide range of E/N. The swarm parameters calculated by the six-term approximation analysis agree well with those by Monte Carlo calculation. Furthermore, the Monte Carlo simulation confirms the results of a previous computer simulation study for correspondence between experimental and theoretical electron drift velocities; that is, the drift velocity deduced from Schlumbohms experiment (1965) assumes a value close to but slightly larger than Wm and the drift velocity deduced from Frommholds experiment (1959) assumes a value represented by (Wr+Wm)/2, where Wm and Wr are the mean arrival time and the centre-of-mass electron drift velocities, respectively.

Effects of additives on prebreakdown phenomena in n-hexane

Prebreakdown phenomena in n-hexane are observed in detail for positive and negative polarities by using simultaneously a high speed schlieren technique and an LED current measuring system, when an impulse voltage (1.1/225 /spl mu/s) is applied to a point-to-plane electrode gap. Furthermore, the effects of several additives on the streamer propagation are investigated. Especially the effects of electron-trapping additives on negative streamer propagation and of low ionization potential additives on the positive streamer propagation, are examined, as is a correlation between the shape and the propagation velocity of the streamers. >

Electron transport coefficients in SF6 and c-C4F8 mixtures

The ionization alpha , electron attachment eta and other electron transport coefficients for SF6 and perfluorocyclobutane (c-C4F8) gas mixtures have been calculated by a three-term Boltzmann equation method over the E/N range 283-707 Td. A set of electron collision cross-sections that gives transport parameters in agreement with measurements for the pure gases is used. The calculation is performed for important experimental conditions, i.e. the steady-state Townsend (SST), pulsed Townsend (PT) and time-of-flight (TOF) methods. It is found that the greater the reduction in E/N the more pronounced is the peak of eta values of mixtures, e.g. at E/N=283 Td the peak appears at the SF6 partial pressure k around 20%, though alpha values show monotonical variation with k, at a fixed E/N for the E/N range studied here. As a result, the E/N value at which alpha = eta , i.e. the limiting E/N of 20% SF6 and 80% c-C4F8, can exceed the values for the respective pure gases. This fact suggests that this combination may be one desirable synergistic mixture since the limiting E/N is an important parameter for predicting the dielectric characteristics in electronegative gases.

Influence of insulating barrier on the creepage discharge in transformer oil

The propagation aspects of a creepage discharge in transformer oil are observed simultaneously and in detail under a positive impulse voltage condition, by means of a high-speed Schlieren optical system, an LED current measurement system and a charge measurement system using a capacitor. In the present paper, the influence of the material type and the thickness of insulating barriers on the propagation of the impulse creepage discharge are discussed for a point to plane electrode geometry. In addition, the influence of a barrier surface to the electric field direction on the propagation of the impulse creepage discharge is discussed for a parallel plane electrode gap with a protruding point.

Electron energy distribution and transport coefficients of electron swarms in SF6 and nitrogen mixtures

Electron swarm behaviour in SF6 and nitrogen mixtures is analysed over the E/N range from 141 to 707 Td by a three-term Boltzmann equation method. A set of electron collision cross sections, which was determined consistently with measurements for the respective pure gases by the authors ((SF6, Itoh et al., 1988); N2(Ohmori et al., 1988)), is used. The calculation is carried out for the steady-state Townsend (SST), pulsed Townsend (PT) and time-of-flight (TOF) experiments. In particular the TOF parameters of these mixtures are deduced for the first time as far as the authors are aware. It is found that the calculated values of the effective ionisation coefficient alpha shows a monotonic variation with percentage of SF6 at a fixed E/N, the same as previous measurements. The E/N value at which alpha =0, i.e. the limiting E/N, is also deduced and in close agreement with measurements. The variations of the electron energy distribution and other transport coefficients with the partial SF6 pressure are calculated and discussed in detail.

Studies of impulse creepage discharge in transformer oil

The propagation process of a creepage discharge on the surface of a solid insulator in transformer oil was investigated in detail when impulse voltages with various rates of rise T/sub f/ of the wave front are applied to a point electrode. The shape and size of the Schlieren figures agree well with dust figures. The propagation of density changes (steamer-like density changes) corresponds to the current and the charge waveform. The effect of T/sub f/ on their propagation is also discussed. >

Electron swarm development in SF6. II. Monte Carlo simulation

For pt.I see ibid., vol.21, p.922 (1988). Electron swarm parameters for the steady-state Townsend, pulsed Townsend and time-of-flight experiments are calculated for SF6 gas by Monte Carlo simulation over the E/N range from 141 to 707 Td. The calculation is carried out using the same cross sections determined by a Boltzmann equation method in pt.I. The present results are in excellent agreement with those deduced by the Boltzmann equation analysis for steady-state and pulsed Townsend experiments. The swarm parameters such as the centre-of-mass drift velocity Wr, the mean-arrival-time drift velocity Wm, the longitudinal DL and transverse DT diffusion coefficients for the time of flight, which were not calculated in the previous paper, are discussed in detail.

Development of electron swarms in SF6

The electron swarm behaviour in SF6 is calculated and analysed for a wide range of E/N values from 71 to 5656 Td by both a Boltzmann equation method and a Monte Carlo simulation, using a set of electron collision cross sections determined by the authors. The Monte Carlo result shows clearly that the difficulty in deducing the electron energy distribution increases as the E/N values decrease due to a large electron attachment probability, while the result of the Boltzmann equation analysis shows that the set of cross sections determined by the authors is again fully valid for E/N>707 Td, since the effective ionisation coefficient agrees with measurements at very high E/N. The swarm parameters for important experimental conditions are deduced and the validity of the usual two-term method is discussed in detail.

Propagation characteristics of impulse creepage discharge in a parallel‐plane gap with a protruding point in transformer oil

The propagation characteristics of the creepage discharge in a parallel-plane electrode gap with a protruding point in transformer oil are observed and investigated in detail under an impulse voltage application by using a high-speed schlieren optical technique and an LED current measurement system. Streamer-like density changes appeared from the protruding point tip of one high-voltage plane electrode and propagated to the other grounded plane electrode. In the present paper, the effect of the arrangement of a barrier insulator and the electric field direction on the propagation of the creepage discharge is discussed.