17-11-2012, 03:04 PM
DC Corona Discharge of a Metal Filament Particle Within Parallel-Plate Electrodes
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Abstract
For the prevention of a particle-initiated breakdown
within gas-insulated switchgear, fundamental characteristics of
particle motion were investigated. It was found that the particle
motion depends not only on Coulombic force, but on the corona
discharge from the tip of a particle. Thus, the corona current from
a fixed particle on the lower electrode was measured, for either
positive or negative corona case. The measured current showed
different characteristics of current depending on the polarity.
Corona onset voltage of negative corona is somewhat lower than
positive, but the breakdown voltage of negative corona is much
higher. Although the current characteristics follow the square
law, the applicable region of this law for positive corona is very
small. Positive corona depends on the tip configuration contrary
to negative corona.
INTRODUCTION
IN ORDER to develop prevention and protection methods
for particle-initiated breakdown within gas-insulated
switchgear (GIS), the fundamental characteristics of particle
behavior have to be known. Thus, we have investigated the
particle motion by using a parallel electrode system extensively
[1]–[3].
According to our previous observations, either the particle
will stand still on the lower electrode, or it shows precession
motion and wandering around on the electrode. When the particle
shows the precession motion, it usually spins around its
longitudinal axis. Depending on the condition, it jumps up and
down between two electrodes. Occasionally, the particle hangs
down from the upper electrode, or shows precession motion, according
to the polarity of the applied voltage on the upper electrode.
It was presumed that the motion of a metal particle is not
only due to a Coulombic force, but the corona discharge from
a particle has significant influence on the motion. During the
standing motion or spinning motion, the stable corona discharge
from the top end of a particle was observed.
EXPERIMENTAL RESULTS
Corona Discharge with Right Angle Cut
The negative corona discharge occurs from the top end of the
filament particle fixed on the lower electrode, when the positive
high voltage is applied to the upper electrode. On the contrary,
if the negative high voltage is applied to the upper electrode,
positive corona discharge is obtained. The current–voltage
characteristics for negative and positive corona are shown in
Fig. 3(a) and (b), respectively. The gap between the electrodes
was set to 20 mm. Increasing the applied voltage, the corona
current increases gradually after the threshold voltage (corona
onset voltage) and, finally, flashover occurs. The corona onset
voltage decreases when the length of the filament particle
becomes large.
Corona Onset and Flashover Voltages
The dependence of the corona onset voltage on the length
of the particle for the right angle case is shown in Fig. 6(a)
and (b). The gap between two electrodes is usually set to 20
mm. However, the data obtained from 10- and 15-mm gaps are
also included in the figures. As expected, the longer the gap is,
the higher the corona onset voltage becomes. For both negative
and positive corona discharges, the onset voltage gradually decreases
inversely proportional to the particle length. The value
of the onset voltage of the negative corona is somewhat lower
than that of the positive corona.
The flashover characteristics are indicated in Fig. 7(a) and
(b). While the flashover voltage linearly decreases with the particle
length for the negative discharge, its dependency is more
complicated for the positive discharge. Generally speaking, the
flashover voltage in the positive discharge is somewhat higher
than that in the negative one. However, the flashover voltage
with the particle shorter than 4 mm tends to become small compared
with the value presumed from the characteristics with the
longer ones. As a result, it shows a peak around the length of 4
mm.
Current Waveform
For the negative corona, current waveforms can be recorded
nicely. Typical waveforms are shown in Fig. 10, where the particle
length is 10 mm and the electrode separation is 20 mm.
These waveforms consist of Trichel pulses and dc components
[13]. The intervals of Trichel pulses become shortened when the
applied voltage is increased. As seen in Fig. 10(a) and (b), pulse
heights are almost constant if the current is not so high as to
approach the breakdown. However, when the current increases
steeply near breakdown voltage, pulse heights fluctuate considerably,
as shown in Fig. 10©, where the dc component is also
large. Thus, it might be said that corona becomes abnormal near
the breakdown region.
CONCLUSIONS
Corona current from a tip of a filament electrode was measured
by changing various parameters. From these measurements,
several interesting results were obtained.
1) Corona onset voltage of negative corona is somewhat
lower than positive.
2) Breakdown voltage of negative corona is much higher
than positive.
3) Corona current versus voltage could be approximated by
the square law away from the flashover region.
4) Positive corona has peculiar characteristics compared to
negative corona, including the effect of tip configuration.
5) For the negative corona, average breakdown field strength
might be predicted by knowing the gap ratio.