Ablation-Dominated Arcs in CO2 Atmosphere - Part I: Temperature Determination near Current Zero

Abstract

Wall–stabilized arcs dominated by nozzle–ablation are key elements of self–blast circuit breakers. In the present study, high–current arcs were investigated using a model circuit breaker (MCB) in CO2 as gas alternative to SF6 and in addition a long polytetrafluoroethylene nozzle under ambient conditions for stronger ablation. The assets of different methods for optical investigation were demonstrated, e.g. high-speed imaging with channel filters and optical emission spectroscopy. Particularly the phase near current zero (CZ) crossing was studied in two steps. In the first step using high-speed cameras, radial temperature profiles have been determined until 0.4 ms before CZ in the nozzle. Broad temperature profiles with a maximum of 9400 K have been obtained from analysis of fluorine lines. In the second step, the spectroscopic sensitivity was increased using an intensified CCD camera, allowing single-shot measurements until few microseconds before CZ in the MCB. Ionic carbon and atomic oxygen emission were analyzed using absolute intensities and normal maximum. The arc was constricted and the maximum temperature decreased from >18000 K at 0.3 ms to about 11000 K at 0.010 ms before CZ. The arc plasma needs about 0.5-1.0 ms after both the ignition phase and the current zero crossing to be completely dominated by the ablated wall material.

Ablation-Dominated Arcs in CO2 Atmosphere—Part II: Molecule Emission and Absorption

“…The arc discharge was dominated by wall-ablation at that point in time; no emission from copper or oxygen but lines from atomic fluorine F I and atomic and ionic carbon lines C I, C II could be observed. This radiation was mainly emitted in a broad distribution over the arc cross-section with the highest intensities in central positions, as it is typical for the wall-stabilized arcs with broad and flat temperature profile [9]. However, an additional structure can be recognized with a different lateral distribution: A dense pattern of lines with increasing intensities and numbers towards higher wavelengths with abrupt breaks at positions near 516 and 564 nm, spread over the whole nozzle diameter and partly even with maxima close to the wall.…”

“…Two setups of electrodes and nozzles were used. They are described in detail in an accompanying paper [9]; basic features are sketched in Figure 1. Actually, the majority of experiments described in this paper were carried out with setup (b) and only a few with setup (a).…”

“…The investigation of the phase around current zero by optical emission spectroscopy (OES) and the determination of arc temperatures during the arc quenching as close as possible to CZ was a topic of our accompanying paper [9]. An MCB using CO 2 as a filling gas and a PTFE-nozzle experiment under ambient air were used for the analysis of line radiation of oxygen and fluorine atoms as well as of carbon ions.…”

“…During the OES study of an MCB and a nozzle experiment described in our first paper [9], molecule radiation of C 2 and CuF was recorded under different conditions and to some extent in unexpected ranges of the arc. The occurrence of strong temperature gradients in the arc are already known as well as a number of molecules that might be expected to appear favorably at lower temperatures, i.e., either in the vicinity to the nozzle walls or at low currents.…”

“…However, the determination of quantities like rotational temperatures and densities is out of the scope of the present paper. The MCB and nozzle experiment setups will be presented shortly in Section 2 together with the setup for spectroscopic measurements because details can be found in [9]. Results are given in Section 3 followed by a discussion in Section 4.…”