Voltage–Current Characteristic of Free Burning Arcs in SF₆ Alternative Gas Mixtures

Abstract

Voltage-current characteristics of free burning arcs in SF6 and air have been known for decades. As the demand for an SF6-free solution is increasing, there is an accompanying need to determine arc parameters in the alternative gases. An unblown arc experiment has been established to determine the voltagecurrent characteristics of SF6 alternative gases, which have not yet been thoroughly studied. In this experiment free burning arc measurements were performed in a number of gases under consideration of SF6 alternatives, including CO2 and mixtures of CO2 / O2 with and without C4F7N or C5F10O additives at concentrations of up to 10 %. Measurements were also performed in air and SF6 for comparison. Arc voltage was measured in each gas at pressures ranging from 1 bar to 5 bar absolute, and electrode separations ranging from 20 mm to 95 mm. Voltagecurrent characteristic measurements for air and SF6 show good agreement with previously published results. A linear relationship of the arc voltage to the arc length is shown, as well as fourth root dependence of the arc voltage on the gas pressure. It was shown that neither the O2 nor the fluorinated additives to CO2 have any significant influence on the voltage-current characteristic. The minimum arc voltage in all measured gases was slightly higher than in SF6, but the arc in SF6 was the least stable and had the highest elongations resulting in high voltage peaks. The arc voltage in air had a similar minimum value to the CO2 based gases, but the arc was much more stable, resulting in lower effective voltage, especially at low currents.

Wear of the Arcing Contacts and Gas Under Free Burning Arc in SF$_{6}$ Alternatives

“…It is likely that many other factors are influencing the decomposition rates, like the arc temperature, usually linked to the current density. The performed experiment is however reproducing conditions closer to some switching duties in disconnectors [4] in terms of geometry. Applicability to bus-transfer switching, described in IEC 62271-102, is possible but dependent on the correct equivalences between the experiment and the type-tests.…”

“…As a result of the arc-driven decomposition of the gases, only an insignificant pressure increase was observed, which was not included in the analysis. A more detailed description of the experiment can be found in the article by Pietrzak et al [4].…”

“…The highest concentration for those two gases have been measured for arced gas mixtures with 10 % of C 4 F 7 N and were equal to 3.45 % mol and 3.19 % mol for CO and CF 4 respectively. In total, those two decomposition products make a similar fraction of the mixture as the C 4 F 7 N. It is expected that those decomposition byproducts will not influence the voltage of free burning arc [4] but can affect the insulation properties of C4 10 % C4 5% C5 10% C5 5% 0 0.5 the gas. A full list of measured decomposition byproducts can be found in the appendix.…”

“…This ratio is changing over the lifetime of the switching device as, unlike SF 6 , the novel fluorinated gases do not recombine after arcing [2] [3]. As it was previously shown, the low content of fluorinated gas does not significantly influence the arc voltage and the arc behavior in the current zero region for free burning arc condition [4]. Hence, the reduction of the fluorinated compound concentration would affect mostly the electric withstand of the gas mixture.…”

Trace Analysis of C4F7N Insulating Gas Mixtures by Spontaneous Raman Spectroscopy and Gas Chromatography

“…38 These parameters also affect the insulating ability of C 4 F 7 N. For example, as the distance between the arc contacts in a circuit breaker increases, the insulating efficiency of the gas is improved. 39 It was also found that the breakdown voltage for mixtures of 20% C 4 F 7 N and 80% CO 2 follows a nonlinear pattern as pressure increases. 40 When it comes to general-purpose chemical trace analysis, GC has been the default laboratory technique for separating and analyzing mixtures.…”