Toshiyuki Uchii

Effects of hot SF/sub 6/ on post-arc circuit breaker design

SF/sub 6/ is blown through the arc during interruption of an SF/sub 6/ puffer circuit breaker. This hot gas flows down an exhaust tube into the chamber of a dead tank circuit breaker where it can lower the dielectric withstand between the exhaust tube and the tank of the circuit breaker, leading to dielectric breakdown during the transient recovery voltage after clearing. This paper presents experimental evidence that any such breakdown is controlled by the hot gas near the exhaust tube, with cold gas in the remainder of gap having little effect on the breakdown voltage. This experimental conclusion is supported by a detailed theory for conditions under which breakdown can occur.

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.

Optimization of dead tank gas circuit breaker design based on quantification of hot gas flow and dielectric properties

This paper describes the technologies for developing a compact and highly reliable dead-tank gas circuit breaker (GCB) on the basis of extensive data on the behavior of hot gas flow and the mechanisms of ground fault under highly inhomogeneous thermal conditions. When predicting the dielectric capability of a GCB during a large current interruption, the temperature and gas density distributions caused by the hot gas must be taken into account in addition to the electrical field imposed across the gas. Hot gas behavior was investigated quantitatively and qualitatively, to provide a basis for determining optimum exhaust tube dimensions which minimize overall circuit breaker dimensions while avoiding ground faults.

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.

Analytical approach to SF/sub 6/ breakdown under transient conditions

Under many circumstances, the initiation of breakdown can be viewed as conversion of energy stored in the electric field (capacitive energy) to thermal energy which converts matter in a thin channel from a nonconducting state to a conducting plasma. This paper analyzes this process in the context of breakdown in SF/sub 6/ as a prelude to future papers which treat the subject of breakdown under highly inhomogeneous thermal field, quasihomogeneous electric field conditions.

Behavior of inhomogeneous high-temperature SF/sub 6/ gas in a gas circuit breaker

The purpose of the present paper is to show the dielectric characteristics of inhomogeneous hot SF/sub 6/ gas in a gas circuit breaker (GCB) experimentally. High-temperature and low-density SF/sub 6/ gas generated by a heavy current interruption is distributed inhomogeneously in the grounded tank, and can strongly threaten the dielectric capability of the GCB. Few studies, however, have been carried out on the dielectric characteristics of the inhomogeneous hot gas. Using small gap discharges, the hot gas behavior of a GCB model, having breakdown or no breakdown, was investigated. The temperature of the hot gas flowing through the exhaust tube was estimated from the sparking voltage of the small gap installed in the exhaust tube. It was also found that there was very little or no effect on breakdown voltages of the cool gas in the breakdown paths. This suggests that the breakdown voltage of inhomogeneous hot gas could be obtained as the applied voltage at which the electrical field strength equals E/sub crit/ of local hot gas at the location concerned.

Real-Time Numerical Analysis on Insulation Capability Improvement of Compact Gas Circuit Breaker

In order to avoid a ground fault during a large current interruption, the effect of roughness pattern inside the exhaust tube on the rapid cooling of high temperature SF6 exhaust gas has been clarified in detail. In this study, large-eddy simulation of compressible turbulent flow under the realistic inlet conditions related to the available experimental data has been carried out. It is shown that introducing roughness pattern on the inner wall of exhaust tube is very effective for the improvement of insulation capability due to the enhanced active mixing and the flowing in of cold ambient gas from the tube exit. Finally, the computed temperature on the inner wall of exhaust tube shows a good qualitative agreement with the experimental data especially for the rough surface just after the applied transient recovery voltage.

Observation of weakly ionized dusty plasma in exhaust gas in a compact-size SF/sub 6/ gas circuit breaker

Measurement of exhausted gas was carried out in an exhaust chamber of a practical compact-size gas circuit breaker. The result show that the gas is weakly ionized, ablated metallic materials exist as metal clusters of sizes from 10 to 100 /spl mu/m, and their major constituent is iron.

Investigation of hot gas flow in a dead-tank-type gas circuit breaker

This paper describes the behavior of hot gas flow during a large current interruption, which may cause a ground fault in a dead-tank-type gas circuit breaker even if hot gas is locally distributed between live part and grounded tank. By observations with high-speed-video and measurements of breakdown voltage with small gaps it became clear that gas temperature and electrical field strength at the mouth of exhaust tube are decisive to ground fault occurrence. How to avoid a ground fault and to efficiently select the optimal dimensions of an exhaust tube for a compact and reliable dead-tank-type GCB is shown.