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Encyclopedia of Physical Science and Technology En004F-171 June 8, 2001 17:11

368 Dielectric Gases

FIGURE 11 A view of Oak Ridge National Laboratory’s electrostatic tandemtype heavy ion accelerator, which is
insulated with SF 6 . (Courtesy of Oak Ridge National Laboratory.)

lines for 1200 kV and higher voltages are under con- B. Circuit Breakers
sideration for the future. Early uses of gas dielectrics
Prior to the 1950s, high-voltage power circuit breakers
included the use of N 2 and CO 2 (at pressures up to
mainly relied on oil and compressed air for insulation
∼20 atm) in high-voltage standard capacitors. The distinct
and current interruption. In 1956 the first high-voltage
advantages of gaseous dielectrics (e.g., low weight, excel-
power circuit breaker was put into service at 115 kV, us-
lent recovery characteristics, low dielectric losses, com-
ing SF 6 as the insulating and interrupting medium. Since
pactness of equipment, and environmental advantages)
then, circuit breakers with SF 6 gas have been put into ser-
resulted in their deployment for electrical insulation and
vice in all distribution and transmission voltage classes
arc interruption purposes in a variety of electrical equip-
from 34.5 through 800 kV with interrupting capabilities
ment. In this section we briefly mention some of their main
in excess of 80 kA. Virtually all new circuit breakers to-
uses.
day use a single-pressure chamber (usually at ∼6 atm) in
which the gas is further compressed with a piston attached
to the moving contact and provides an axial gas blast for
A. Research Equipment
the are interruption (see Fig. 12). Earlier versions of SF 6
High-voltage power supplies and Van de Graaff-type ac- interrupters were of the two-pressure design, where the
celerators are examples of laboratory equipment that em- gas blast for the arc interruption was provided to the arc
ploy gas dielectrics for electrical insulation. Most such interruption chamber from a high-pressure (∼14.5 atm)
equipment uses SF 6 . Other gases or gas mixtures (e.g., reservoir.
N 2 /CO 2 and N 2 /SF 6 ) have occasionally been used. A basic requirement for an arc-interrupting medium,
One of the largest Van de Graaff-type accelerators is the besides a high dielectric strength, is a high rate of recovery
Holifield Heavy Ion Accelerator at Oak Ridge National of its dielectric strength. The ability of SF 6 to recover its
Laboratory in Tennessee. It uses 0.7 MPa of SF 6 as an dielectric strength quickly after arc interruption along with
insulating gas. With this machine a dc voltage of 34 MV its rapid thermal recovery and high degree of molecular
was achieved. A view of this electrostatic tandem-type reconstitution after arcing make it the most attractive arc-
heavy ion accelerator is shown in Fig. 11. interruption gaseous medium.