Effect of Nozzle Parameters on SF6 ARC Interruption

Experimental measurements of dielectric recovery following 400 *#x003BC;sec duration square current pulses of 350 A and 1000 A have been conducted on ten nozzle configurations with upstream and downstream pressures of 4 atm. and 1 atm. respectively. The data have been analyzed to determine the dependence of recovery, or current interruption capability, upon nozzle geometrical parameters. The analysis indicates that recovery is insensitive to nozzle divergence angle (ø) between 9°and 15°, but a cylindrical nozzle severely reduces arc interruption ability. Whereas recovery time t
_R
decreases as the ratio of outlet area A
_c
to throat area A
_t
increases for low recovery voltages V
_R
, a nozzle with a ratio of A
_C
/ A
_t
equal to 2 minimizes t
_R
at high V
_R
because the nozzle expansion matches the expansion of the arc. The optimum value of upstream arc length ℓ
_u
is about one half of the effective nozzle throat diameter. An increase in ℓ
_u
causes a significant increase in ultimate dielectric strength, but if ℓ is greater than optimum, there is a tendency towards upstream energy accumulation, delaying recovery. Arc channel recovery is most rapid if the downstream arc length is either very short, thus limiting arc power input, or very long, thus reducing the voltage stress per unit length on the column when V
_R
is applied.