Takeshi Shinkai

Thomson scattering diagnostics of decay processes of Ar/SF6 gas-blast arcs confined by a nozzle

Because of its instability, it is difficult to measure precisely the electron density (ne) of a long-gap decaying arc discharge in a circuit breaker. However, it is well known that it is an essential parameter for the determination of success or failure of the current interruption in a circuit breaker. In this paper, the spatiotemporal evolutions of the electron density were successfully measured in decaying SF6 gas-blast arc discharges formed with a long gap (50 mm) in a confined nozzle using laser Thomson scattering. Pure Ar gas and an 80%Ar/20%SF6 mixture gas were used as the arc quenching media at atmospheric pressure. After reducing the current to zero, both the measured ne and arc radius in the Ar/SF6 gas arc clearly decayed more rapidly than in the pure Ar gas arc.

Thomson scattering diagnostics of SF6 gas-blasted arcs confined by a nozzle under free-recovery conditions

Collective Thomson scattering has been applied to gas-blasted arcs confined by a nozzle to measure their decay processes with respect to electron density (ne). Pure SF6 gas, an Ar/SF6 gas mixture (SF6 was 20, 40, 60, and 80% of the mixture), and pure Ar gas were used as arc-quenching media at atmospheric pressure. Copper–tungsten (40% copper) electrodes were installed inside a polytetrafluoroethylene (PTFE) nozzle and 50 mm-long gap arcs were generated between the electrodes. First, steady-state arcs were generated with 50 A current. Then, a semiconductor switch removed the arc current. It was found that after the current decreased to zero, ne exponentially decreased and the decay time constant of ne systematically decreased with an increasing SF6 gas ratio. Thomson scattering measured ne in the range 1021–1023 m−3. Self-emission measurements, which were performed with a high-speed camera at 200 000 frames per second, exhibited good agreement with the results of the Thomson scattering.

Thermal interruption capability of carbon dioxide in a puffer-type circuit breaker utilizing polymer ablation

When adopting an alternative arc quenching gas to SF/sub 6/ which has recently been recognized as a greenhouse gas, it is easily anticipated that the thermal interruption capability of the GCB will be lower than that in using SF/sub 6/. In this paper, adopting CO/sub 2/ as an alternative gas, the means utilizing ablation phenomenon of polymer materials as one of the breakthrough technologies compensating the drop in the interruption performance will be proposed and tested by a full-scaled GCB model. As a result, a change in the blasting pressure characteristics was observed, and also the peak pressure for the ablation application model was about 1.3 times higher than that of the conventional model without the ablation element. Furthermore, even if compared at the same blasting pressure condition at current zero, the thermal interruption capability of the CO/sub 2/ gas in the ablation application model was presumed to be improved with comparison to the conventional model without the ablation element. The thermal interruption capability of the CO/sub 2/ gas in the ablation application model could be estimated to be about 50 % of that of SF/sub 6/ gas in the conventional model in this interrupting test.

Gas density and temperature in thermal volume for self-blast interrupting chambers

Self-blast type interrupting chambers making use of pressure rise due to arc heating have been developed for gas,circuit breakers with lower driving energy. Blowing gas temperature is probably higher in such chambers. Although SF/sub 6/ gas is considered to have good arc extinguish capabilities at higher temperature, it is important to investigate blowing temperature to raise arc extinguish efficiency. This paper reports results of the temperature evaluation carried out in a self-blast interrupting chamber model with up to 25kA interrupting current. The peak of gas temperature is about 1000K in a thermal volume of the self-blast chamber. Higher temperature and lower density hot gas from arcing space hardly mixes sufficiently with low temperature gas that remains in the thermal chamber.

Evaluation on current interruption ability of CO2 and SF6 using current and voltage application highly controlled by power semiconductors

This paper reports a new simple test technique to evaluate current interruption ability of arc quenching gases. In the test, current and voltage applied to the arc was controlled using a insulated gate bipolar transistor (IGBT). Switching the IGBT enables us to produce free recovery conditions for a fundamental arc decay in nozzles. In addition to this, a voltage was intentionally applied to the free recovery arcs between the electrodes by switching-off IGBT again at the specified delay time td. This applied voltage is called quasi transient recovery voltage (quasi-TRV). We can evaluate successful interruption or interruption failure by measuring the current between the electrodes after quasi-TRV application. We compared the interruption ability of SF6 and CO2 through this developed technique. The experimental results show that a residual arc in SF6 gas flow decays four times more rapidly than that in CO2 gas flow. Influence of observation holes in the nozzles used in the experiments was also investigated, showing less influence of observation on the arc behavior.

Development of a thermally and chemically non-equilibrium model for decaying SF 6 arc plasmas

This paper describes the development of a model with both chemically and thermally non-equilibrium effects in an SF₆ arc plasma during decaying phase with transient recovery voltage (TRV) application. The SF₆ arc plasma in decaying phase can be seen in a high voltage SF₆ gas circuit breaker during a large current interruption process. The TRV is often applied to the arc plasma, which may elevate the electron temperature T_e than heavy particle temperature T_h. This developed model solves energy equations for electrons and heavy particles, accounting for totally 122 reactions due to 19 species in SF₆ arc plasmas. As a result, transient distributions of T_e and T_h were calculated for SF₆ arc plasma in a TRV application condition for a fundamental study.

A study on proximity effect of current distribution in conductors and improvement of GIS arrangement

Proximity effect is a phenomenon such that an electric current density distribution becomes asymmetric when an alternating current flows in conductors arranged at a short distance. This effect possibly raises problems in the case when cylindrical conductors are arranged at a highly short distance like in compact and large capacity GIS. For example, unexpected local concentration of an electric current may cause welding or arcing especially in contact joint areas. Therefore, detailed study on the proximity effect is important. In this paper, it is shown that this effect can be quantitatively evaluated with an experiment and two kind of computations, finite element method and approximated lumped parameter circuit. Finally, ideas for improvement of GIS arrangement are suggested.

Fundamental study on re-ignition process for CO 2 -blast arcs in a model circuit breaker using synthetic tests highly controlled by power semiconductors

This paper reports fundamental measurement results on re-ignition process in synthetic tests for CO2 arcs in a gas blast nozzle. The synthetic test system used comprises a DC current source for an arc ignition, a half cycle AC current source, a DC current source for artificial current zero point and an impulse-like voltage source for application of quasi-transient recovery voltage (quasi-TRV). The electric current and voltage were switched by power-semiconductor-switches to guarantee high time-accuracy of the arc current injection and voltage application. After current zero point, quasi-TRV with a peak of 7.5 kV was applied between the electrodes with a precise specified delay time td to judge arc re-ignition. The results indicated that td = 25 μs involves 60% probability of arc re-ignition. The re-ignition was inferred to originate in thermal mode from joule heating by a small current injection to the residual arc during the quasi-TRV application.

Fundamental studies on switching arcs — Experimental and numerical approaches

Both experimental and numerical approaches have been done on switching arc plasmas for fundamental studies. In the experimental approaches, power-semiconductor switching has been used to control intentional current injections and voltage applications to switching arcs with a high accuracy in time. The systematic experiments provided the interruption probability property for different gas kinds and gas flow rates, as well as the dielectric recovery properties between the electrodes. Time evolutions in electron density were derived by Laser Thomson Scattering (LTS) and Shack-Hartmann (SH) method for different gases. On the other hand, for this nozzle space, numerical models were developed to simulate arc dynamic behaviors with and without local thermodynamic equilibrium (LTE) assumptions for different gases. It was found that the time evolution in electron density derived by the chemically non-equilibrium model is in good agreement with the experimental results by LTS.