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NEGATIVE CORONA DISCHARGE IN THE MIXTURE N2 + SF6 AT
THE PRESSURE 26.6 kPa
J. Kúdelčík, [email protected], M. Kúdelčíková, [email protected], FPV ŽU Hurbanova 15
Žilina
INTRODUCTION
Electronegative gases, as SF6 and mixture N2+SF6,
are widely used in many industrial devices such as
circuit breaker or gases insulating systems. The
transmission from negative corona discharge to
breakdown in these systems is unwelcome and
elimination of this process depends on the understanding
of the mechanism of the initial stage of discharge. Initial
stage of negative corona discharge is in regime of
Trichel pulses. Trichel mechanism presents wide and
evident formation of cathode region at high pressure by
streamer mechanism.
The basic aspect of the corona discharge in a pointto-plane configuration at atmospherical pressures may
be explained on the basis of the streamer theory. At
a certain stage of the development of a single avalanche
the space charge generated can partially shield itself
from the applied field, creating a streamer initiating
plasma. The condition of its generation is defined by
Meek conditions [1] in the form:

N e  exp   x dx   108 [cm -3 ]


waveforms at the lowest gap voltage values (i.e., at the
corona onset voltage values) were measured with UV
illumination. At higher gap voltages the discharge
started spontaneously. It can be seen for the lowest gap
voltage values the current pulse had the form of a simple
hump and its rise was rather slow with no sign of
stepwise pulse on the leading edge. At higher gap
voltages it was observed that the formation of a step on
the pulse leading edge. At even higher gap voltage this
step was found to develop into a new peak, i.e., the
waveform took the form of a double peak.
(1)
If few electrons are just in front of the plasma,
avalanching in the locally enhanced field can cause
primary cathode- and anode-directed streamers to
propagate. Thus, the feedback-to-cathode Towsend
ionization mechanism is supplanted by faster feedforward-to-gas streamer mechanism. The streamers can
be viewed as ionizing-potential waves characterized by
the fact that the background gas is initially not ionized.
After the initial acceleration lasting for about 1 ns,
velocity of the cathode-directed streamer increases
exponentially reaching the order of 108 cm/s. The
streamer front propagation is really a phase movement,
and can easily be much faster than the charged particles
it contains.
Choosing of used mixture was motivated by the fact,
that when the concentration of SF6 is changed, at the
same conditions it is possible to find characteristic
aspects of streamer mechanism and we can also verify
the physical model of streamer generation [2]. Presented
results refer to the previous measurements for N2+SF6
[3] and O2 +SF6 [4].
EXPERIMENTAL PROCEDURE AND RESULTS
General scheme of the experimental set-up and
procedure used at the study of negative corona discharge
in the mixture of nitrogen with SF6 are identical with
these
described
in
the
related
papers
[3, 5].
The results in the Fig. 1 illustrate development of
the first negative corona current pulse with increasing
gap voltage at the pressure of 26.6 kPa for 5%
admixture of SF6 in the buffer nitrogen gas. The
Fig. 1. Current pulse waveforms taken for various gap
voltages in mixture N2+5 %SF6 at pressure 26.6 kPa (distance
point-to-plane 12 mm a radius of the point 0.1 mm)
COMPUTATIONS
The goal of computations was to find out, if the
condition for streamer (eq. (1)) is fulfilled for measured
values. So we dealt with the calculating of distance of
the space charge center from the cathode surface, and its
number. On the basis of these results it will be possible
to explain forms of measured current pulses shown in
the Fig. 1., i.e., if the current maximum responds to the
contact of streamer with the cathode, and step on the
leading edge to the i emission.
If it is assumed that before the onset of the cathode
directed streamer the discharge develops according to
Townsend ionization mechanism, it is reasonable to
suppose [6] that the maximum number of positive ions
(a center of the positive ion space charge) is generated at
a distance xc from the cathode, where the expression:
xc
 ( E x c ). exp  ( E z )dz

0
(2)
reaches the maximum value (  is effective Townsend
ionization coefficient). The value  for the mixture N2
+ 5% SF6 used in this relation was calculated using the
program SIGLO based on the analysis of Boltzman
equation [7]. To compute  values in dependence from
the distance from the cathode, the axial component of
Laplacian’s field was calculated using hyperboloid
approximation [8]. All computations and also finding
the maximum of (2) were realized using the program
MATHEMATICA. Our results were calculated for
experimental data set shown in the Fig.1 and written in
the Tab. 1. The value of xc and corresponding reduce
field intensity E(Xc)/N, where expression (2) is
the cathode [9]. All this process is quite long what
influences the form of current pulse.
xc
a maximum,

VZ1 : exp[   y dy ]
is
corresponding
0
number of electrons generated in the distance of xc, xcrit
and corresponding reduce field intensity E(Xcrit)/N,
where   0 .
Results of computation of the characteristic distances and
fields for measured experimental conditions
TAB. 1.
Voltage [V]
3300
3700
3900
Xc [μm]
199
237
257
(E(Xc)/N) [Td]
373
323
318
Xe[μm]
334
380
6
8.51710
403
7
4.621108
VZ1
3.17310
Xcrit [μm]
334
380
403
(E/N)crit [Td]
215
214
214
In the Fig. 2 there are shown results of computations for
N2 + 5% SF6 at the pressure 26.6 kPa and gap voltage
3900 V. There is displayed behavior of  , the
dependence of number of positive ions and electrons
from the axial distance from the cathode in arbitrary
units. In the figure it is very well seen the separation of
positive ions from electrons before streamer initiating
plasma, which corresponds to the ionizating trajectory
 1 (146 μm). Maximum number of generated
electrons is reached when   0 .
RESULTS AND DISCUSSION
On the basis of measured results and
computations it can be proclaimed, that at the gap
voltage of more than 3600 V, for given experimental
conditions, the form of current pulse is determined by
streamer ionization mechanism. In the Tab.1 it can be
seen, that the number of electrons developed of a single
avalanche at the gap voltage of more than 3600 V, is
sufficient to fulfill the criterion (1) for streamer
initiating. This proclamation is also supported by ultra
fast increase of the current to maximum of order ns,
what would not be able in the case of Towsend
ionization mechanism, because time of arrival of
positive ions in mixture N2+5%SF6 exceeds hundreds of
ns [2]. Also the total charge of observed current pulses
of the negative corona discharge, i.e., cathode-oriented
streamer, is typically of order ≈10-11 C, what
corresponds to about 108 ions. In the case of gap voltage
3300 V, for the generation of streamer there are needed
several generations of avalanches which are connected
through the photoemission of secondary electrons from
Fig. 2. Development  for the mixture N2 + 5% SF6,
expressions VZ1 and (2) in the dependence from axial
distance from the cathode (p = 26.6 kPa, U = 3900 V,
S = 12 mm, r0 = 0.1 mm).
SUMMARY
According to the streamer-based theory [2] the
stepped pulse leading edge, the current pulses in mixture
N2 + 5% SF6 (Fig. 1), forms as follows: The initial
current rises to the step (denoted by A), it is due to a
Towsend ionization mechanism fed by cathode
secondary process (γp – emission). After decay of the
current rises due to the rapidly shrinking cathode fall
region, the current begins to rise again because of the
development of cathode-direct streamer-like ionization
wave. The pulse maximum is attained just as the
streamer reaches the cathode. Next quick current fall in
mixture N2+SF6 is due to negative space charge.
This information about changing the mechanism of
discharge is very important when isolating and switching
apparatus are designed. By choosing the suitable
equipment and electrode geometry required conditions
for practice can be achieved.
ACKNOWLEDGMENT: This work was supported by
the institutional Grant no. 15/705/04 of FPV ŽU.
REFERENCES
1. D. Meek, in Električeskij proboj v gazach (1960)
2. M. Černák, J. Phys. D 26, 607 (1993).
3. A. Zahoranová, J. Phys. D: Appl. Phys. 35, 762
(2002).
4. J. Kúdelčík, Contrib. Plasma Phys. 42, 546 (2002).
5. J. Kúdlečík, PhD. work (2003)
6. T. Ushita, Elec. Eng. in Japan 88, 45 (1968)
7. www.siglo-kinema.com
8. P. Espel, J. Phys. D: Appl. Phys. 35, 318 (2002)
9. R. Hodges et all, Phys. Rev. A 31, 2610 (1985)