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

366 Dielectric Gases

D. Voltage Waveform Effects
and Time to Breakdown
When a voltage of sufficient magnitude (≥V s for dc) is
suddenly applied to a gas-insulated electrode gap, or a
gas-insulated conductor, breakdown does not occur in-
stantaneously, but after a finite time t = t s + t f . The t s is
called the statistical time lag and is the time that elapses
between the application of the voltage V (≥V s ) and the
occurrence of a free electron in the stressed gas volume
which initiates the breakdown process. The t f is called
the formative time lag and is the time interval between
the occurrence of the free electron and the collapse of the
voltage (i.e., breakdown).
The statistical time lag t s can vary from nanoseconds
to milliseconds (or longer) depending on the time the ini-
tiatory electron becomes available when V ≥ V s . Initia-
FIGURE 8 The ac breakdown voltage versus pressure in a tory electrons can be produced by cosmic radiation, nat-
150 mm/250 mm coaxial geometry containing free copper wires ural radioactivity from materials, field emission from the
with 0.4 mm diameter and lengths ranging from 0.8 to 12.7 mm. cathode surface, or collisional detachment from negative
[From Cookson, A. H., et al. (1971). IEEE Trans. Power App. Syst. ions in the case of electronegative gases. In experimental
PAS-90, 871.]
apparatus they can be produced intentionally by, for in-
stance, an ultraviolet source, in which case t s is reduced
considerably.
Although the free conducting particles cause reductions
The formative time lag usually varies from nanosec-
in the V s of quasi-uniform field electrode geometries, the
onds to microseconds and is influenced by the overvolt-
V s is higher when the particles are fixed to an electrode.
age [ (V − V s )], the field distribution, and, for unsym-
The fixed-particle breakdown voltage/pressure character-
metrical fields, the polarity. This is apparent from the data
istic exhibits the familiar corona stabilization region dis-
in Fig. 9 on N 2 and SF 6 , which were obtained with a
cussed in the previous section, where V s > V i (see Fig. 7).
square impulse generator having ∼2 nsec rise time and
It also has been found that the V s for free particles with
∼150 kV amplitude. It is evident that the higher the over-
ac voltage corresponds closely to the impulse breakdown
voltage (voltage in excess of V s ), the shorter the t f at which
voltage with fixed particles. This indicates that the free-
breakdown occurs. It is also seen that t f is significantly
particle breakdown mechanism is similar to that of im-
higher for negative polarity and that it varies consider-
pulse breakdown. For impulse voltages, the corona stabi-
ably with field uniformity, especially for electronegative
lization process for the fixed particle fails and the impulse
gases (Fig. 9). The voltage–time characteristics of dielec-
breakdown voltage is considerably lower than for ac over
tric gases are of practical importance for the insulation
the pressure range where the corona stabilization mech-
coordination and overvoltage protection of gas-insulated
anism is dominant. Observations of particle breakdown
equipment.
have shown that as the particle approaches an electrode,
discharges occur between the particle and the electrode as
a result of their different potentials. The result is a sudden
E. Gases with Insulator/Conductor Properties
change in the electric field at the particle tip that faces
the main gap, which is equivalent to an impulse voltage In a number of technologies a gas is needed which is
applied to the particle. both a good conductor and a good insulator. For exam-
Particle-initiated breakdown is one of the most severe ple, in pulsed power technologies the key element is a
imperfections in gas-insulated apparatus, seriously reduc- fast repetitive switch. Among a number of switching de-
ing the dielectric strength of gases and the reliability of vices, the diffuse gas discharge switch is promising for
gas-insulated apparatus. Obviously, the best way to alle- a system such as inductive energy storage. The opera-
viate the effect of conducting particles is to remove them tion of a diffuse discharge switch for inductive storage is
from the equipment. Various techniques to remove them characterized by two distinct stages: (1) the conducting
2
and to promote particle motion or scavenging into low- (storing) stage, when E/N is small (∼3 × 10 −17 Vcm ),
field particle traps have been studied and are in use in and (2) the transferring stage (when the stored energy in
gas-insulated equipment. the inductor is transferred to the load), when E/N is large