Mechanisms Responsible for Arc Cooling in Different Gases in Turbulent Nozzle Flow

Abstract

Abstract.A high voltage gas blast circuit breaker relies on the high speed gas flow in a nozzle to remove the energy due to Ohmic heating at high current and to provide strong arc cooling during the current zero period to interrupt a fault current. The physical mechanisms that are responsible for the hugely different arc cooling capabilities of two gases (SF 6 and air) are studied in the present work and important gas material properties controlling the cooling strength identified.

A Method for Analysing and Characterizing the Arc Cooling Effect of Different Gases in Strong Axial Flow With SF6 and Air as Examples

“…In the present work, a method to quantify the arc cooling ability of different gases, with SF6 and air as examples, is established. Its main novelty is the clear accountability of the role of different energy transport processes in influencing the change of arc resistance, which is a significant step forward from the work in [32] where the relative strength of each energy exchange process towards energy removal from the arc column is studied, but no link to the change of arc resistance was established.…”

“…The approximate radiation model [36] based on the net emission coefficient (NEC) is adopted to calculate the radiative energy source of q in Table Ⅰ. NEC is defined for an isothermal sphere of hot gas with a radius of RNEC. When this concept is used for switching arcs, an equivalent RNEC needs to be defined at each axial position in the arc column [32], and this equivalent radius is called the emission radius in this work. It depends on the radial temperature profile at the axial position.…”

“…The arc cooling behaviour of a gas is determined by its material properties. A qualitative explanation can be constructed by comparing the energy density, which is related to the product of specific heat capacity at constant pressure and mass density (ρCp), in the electrically conducting arc column and the surrounding gas [32], as well as the relative largeness of arc radius between the two gases as shown in Fig. 7, which again is related to ρCp through turbulent viscosity [32].…”

“…A qualitative explanation can be constructed by comparing the energy density, which is related to the product of specific heat capacity at constant pressure and mass density (ρCp), in the electrically conducting arc column and the surrounding gas [32], as well as the relative largeness of arc radius between the two gases as shown in Fig. 7, which again is related to ρCp through turbulent viscosity [32]. A thin arc column and low energy density inside the arc would benefit the thermal interruption process through faster arc cooling.…”

Self-Blast Current Interruption and Adaption to Medium-Voltage Load Current Switching

“…In high voltage circuit breakers, the flows around the instance of current interruption are typically sonic or supersonic, and the nozzle shape and exhaust area are designed to take advantage of this in the interruption and post-arc phases. For example, turbulence in the layer between arc and gas flow is an important cooling mechanism in high-velocity SF6 [11] but has proven much less important in other gasses [14]. It is also shown that the formation of shock waves affects the dielectric breakdown strength [15].…”