Influence of the surface conductivity of a single glass
barrier on the breakdown voltage in an air insulated rod
plane arrangement
1
Schueller M.1, Blaszczyk A.2, Krivda A.2, Smajic J.1
University of Applied Sciences Rapperswil, CAEM/HVL - Group, Switzerland
2
ABB Switzerland Ltd, Corporate Research, Switzerland
Abstract- The effect of barrier systems on electrical breakdown
is well known and of wide use in the field of high voltage
engineering. As shown in literature, barriers can increase the
breakdown voltage of an electrode configuration. The effect of the
surface conductivity of the barrier system on the breakdown
voltage has not yet been studied in full detail. The aim of this
contribution is to show this effect. For that purpose, we
investigated a rod plane arrangement with 80mm distance
between high voltage electrode (rod) and a grounded plate
electrode. A single barrier was placed in the arrangement and its
surface resistivity was decreased systematically. The decrease of
the breakdown voltage of the arrangement with decreasing surface
resistivity has been shown for positive and negative lightning
impulse and AC voltage as well.
I. INTRODUCTION
The barrier effect has been found by E. Marx and H. Roser
when they studied air gaps discharges in the 1930’s [1, 2]. In
these early works the barrier effect in gases was explained with
redistribution of the electric field in the gap because of the
space-charge field that was formed by the accumulated charges
on the barrier surface originating from impact ionization near
the needle tip [1-3].
Nowadays this model from Marx and Roser is still used by the
majority of authors to explain the barrier effect in gaseous
dielectrics. Some authors even use it as explanation for liquid
and solid dielectrics [4, 5]. At present day there are many other
experimental hypotheses trying to explain the barrier effect in
dielectrics such as inhomogeneous polarization or electrophysical characteristics of the main and barrier materials [6].
According to [6] the Marx-Roser model can only explain the
barrier effect in relatively short gas gaps. But it doesn’t hold for
large gas gaps, liquids or even solid arrangements as the drift
velocity and free path length of the charge carriers are too small
to form a significant surface charge layer on the boundary
within the for the Marx-Roser model required time [6].
Although this great number of conducted experimental work,
the exact mechanism of the barrier effect is still not known [6].
With the measurements presented here the yet not in detailed
studied effect of the surface resistance of the barrier on the
breakdown voltage was investigated.
II. THEORY
The Marx-Roser model explains the barrier effect with the
creation of space charge due to ionization near the electrode tip.
In case of positive tip fast electrons will move into the needle
tip and recombine there. The remaining positive space charges
drift in direction of the field and accumulate at the barrier. There
a quasi-uniform electric field is formed between the opposite
plane electrode and the barrier resulting in the increase of the
breakdown voltage [1, 2, 6].
In case of negative needle tip the faster electrons move away
from the tip also trough the barrier [1] and a positive space
charge remains at the tip. These positive charges decrease the
field strength at the electrode tip. So the authors of [1, 2]
concluded that this leads to a field equalization resulting in a
higher breakdown voltage of the arrangement [1, 2, 6].
III. METHODS
A. Experimental Set up
To measure the effect of decreasing surface resistance of a
barrier a vertical single barrier arrangement between a high
voltage rod and a grounded plate electrode was chosen, see
Fig 1. The grounded plate electrode was made of copper and
had the dimensions (height x width) 1100 x 1000 mm. As
barrier, a 6 mm thick float glass plate with dimensions 700 x
525 mm was used. The high voltage rod electrode was made of
alumina and had a diameter of 7 mm. It was 260 mm long and
the tip was rounded with a radius of 3.5 mm.
As gap distance between the tip of the high voltage electrode
and the ground electrode 80 mm was chosen. The barrier was
put in the arrangement at two different positions. Position 1 was
20 mm in front of the high voltage electrode and position 2 was
60 mm in front of the high voltage electrode.
The experiments were conducted with lightning impulse
voltage and alternating voltage. An eight stage 800 kV, 40 kJ
Marx generator was used as source of the positive and negative
impulse voltages. A 150 kV, 40 kVA high voltage test
transformer was used in the experiments with AC voltage.
Fig. 1 Dimensions of the experimental set up.
B. Barrier Material and barrier position
As barrier material glass was used because collected surface
charges are not bound so strong at the surface as it is the case
with PVC, acrylic glass or some other polymers. Thus at every
discharge the surface charges are more or less removed. This
was important as we wanted to have as much as possible the
same electric field conditions for every measurement.
The used glass for the barrier was a 6 mm float glass plate with
polished edges. It had the dimensions (height x width)
700 x 525 mm.
It is known from literature that the position of the barrier in the
gap has a strong influence on the breakdown voltage of the
arrangement [7-6]. Placing the barrier in front of the high
voltage electrode at a distance of 20-30 % of the overall length
of the gap the positive effect of the barrier is maximal. If the
barrier is placed far away from the high voltage electrode the
effect of the barrier can even be negative, meaning the
breakdown voltage can be lower than without barrier [7-6]. To
take this effect into account the barrier was placed at two
positions in front of the high voltage electrode. Position 1 was
20 mm in front of the electrode, and Position 2 was 60 mm in
front of the high voltage electrode. Considering that the gap was
80 mm these two positions were at 25 % (20 mm) and 75 %
(60 mm) respectively of the gap distance. 8 9 10
C. Test Procedure
The surface resistance of the barrier was systematically
decreased with every experiment. This was realized with a
conductive graphite spray. At every series of measurement
more graphite spray was applied on the glass barrier and thus
the surface resistance was decreased. The surface resistance of
the barrier was measured with a Keithley source meter 2410
before every series of measurement. It can provide up to
UDC = 1.1 kV and measure a minimal current of 1 pA. Thus it
was possible to measure the surface resistance up to a value of
= 10 Ω.
After measuring the surface resistance of the barrier the
experiments were conducted. Like mentioned before the barrier
was set once 20 mm and once 60 mm in front of the high
voltage electrode.
In case of positive and negative impulse voltage the 50%
breakdown voltage was obtained using an up&down method
according to [11]. This method requires the normal probability
distribution of the breakdown voltage, which is fulfilled for
tests in gaseous insulation. Rise and decay time of the impulse
voltages was used according to the standardized lightning
impulse shape with 1.2/50 µs.
In case of alternating voltage the breakdown voltage was
obtained by applying a voltage ramp with a defined rise time of
9kV/sec. This was done until breakdown occurred. For better
comparability with the impulse voltages the peak value of the
AC voltage was recorded.
A minimum number of 30 measurements was performed for
every point and indicated together with the corresponding
standard deviation in Fig. 2.
All voltages were corrected for pressure, humidity and
temperature according to IEC 60060.
IV. RESULTS
In Fig. 2 the results of all the performed measurements are
shown.
As dashed-dotted lines the breakdown voltage of the pure air
gap without barrier is indicated for positive (blue), negative
(violet) impulse voltage and for AC (red).
Further positive and negative impulse tests with barrier are
indicated by a solid line. AC measurements with barrier are
shown by a dashed line.
In general a decrease of the breakdown voltage could be shown
when the conductivity of the barrier was increased. This effect
was measured irrespective of the voltage polarity (impulse) and
voltage form (AC and impulse).
For impulse tests the breakdown voltage of the system is higher
for negative voltage (-142 kV) than for positive voltage
(83 kV). The breakdown voltage of the gap for AC is with
92 kV slightly higher than for positive impulse voltage.
The polarity effect is reversed when the breakdown voltages of
the impulse experiments with 20 mm spacing of the barrier are
compared. In that case the breakdown voltage of the
arrangement is higher for positive (182 kV) than for negative
impulse voltage (-154 kV).
The breakdown voltage of the arrangement is higher when the
barrier is placed close (20 mm) to the high voltage electrode
than if it is placed further away (60 mm). This holds for all
measured voltage forms and polarities from a surface resistance
of 3.66 1011 Ω down to 3.2 kΩ. At the latter value this behavior
changes for all voltage forms and polarities as the breakdown
voltage is higher for the barrier further away.
For all voltage forms and both polarities the breakdown voltage
of the gap was increased when a nonconductive barrier was
placed in front of the high voltage electrode. Compared to the
values without barrier for positive impulse voltage the increase
was the largest as the breakdown voltage was increased by
119 % (20 mm position) and 36 % for the 60 mm position. In
case of negative impulse there was more or less no increase of
Fig. 2 Breakdown voltage depending on the resistance of the barrier. The dotted lines represent the breakdown voltage of the 80mm gap without barrier. The solid
lines are for lightning impulse (LIP) and the dashed lines represent measurements with AC. For each measurement point the standard deviation range is indicated.
the breakdown voltage measured. With barrier the breakdown
voltage was only 8 % higher than without barrier. For AC the
increase of the breakdown voltage was 50 % for the 20 mm
position but even slightly lower than the value of the pure gap
for the 60 mm position.
With decreasing surface resistance the breakdown voltage even
increase a little till a measured surface resistance of 11.9 106 Ω.
From this value on the breakdown voltage decreases and at a
value of 104 Ω the breakdown voltage was lower than without
barrier for all measured polarities and voltage forms.
V. DISCUSSION
If the derived breakdown voltage of the gap only is compared
with the value obtained with an insulating barrier (no carbon
spray Rsurface = 366 GΩ) in the experiment, the breakdown
voltage of the system could be increased in case of positive
impulse and AC voltage. This was expected and was already
shown in literature for many times [1-6].
In case of negative impulse voltage the barrier had no major
effect on the breakdown voltage when it is compared to the
value of the gap only stressed with negative impulse. One
explanation for that could be the transparency of glass for UV
light. So the UV light from the streamer head coming from the
high voltage electrode just passes through the glass plate
directly leading to further ionization on the other side of the
glass plate. Thus the high voltage rod electrode is not shielded
by accumulated charges on the barrier like the Marx-Roser
model suggests and both formed streamers (the one from the
high voltage electrode and the other formed on the other side of
the barrier by ionization due to the UV) meet at the corner of
the barrier leading to a value of the breakdown voltage close to
the value of the gap only. This hypothesis has to be confirmed
by conducting positive and negative impulse experiments with
a barrier not transparent for UV.
Another explanation could be that at a negative tip the electrons
“…will penetrate through the barrier into the gap between the
barrier and plane as the barrier is transparent for electrons…”
[1]. So a positive space charge is left behind at the negative tip
due to the slower mobility of the positive ions. This space
charge reduces the field strength at the negative tip. This is
known as polarity effect. According to [6] this effect is minor
than the positive charge accumulation at the barrier in case of
positive impulse. So this could explain the difference between
positive and negative impulse experiments as well.
In general a decrease of the breakdown voltage with decreasing
surface resistance could be shown.
First compared to the breakdown voltage of the insulating
barrier (no carbon spray Rsurface = 366 GΩ) the breakdown
voltage increases even a little with decreasing surface
resistance. This is very interesting as it leads to the conclusion
that the barrier performance may be improved by applying a
conductive layer. An improvement up to 10 % could be
achieved in our measurements. This can be significant for
insulation design. This effect seems to be present for negative,
positive impulse and AC.
The most interesting region seems to be between a surface
resistance of 107 to 104 Ω as a transition of the behavior occurs
there and the breakdown voltage of the system is smaller than
for gap only. Following the Marx-Roser model [1, 2, 6] this
could be because the charge at the barrier is distributed faster at
the surface due to the smaller resistance and the barrier behaves
as a floating electrode. Based on the stability field for the
positive streamer we can estimate that the potential of the
floating barrier is during the positive impulse just 10 kV lower
≈ 0.5kV/mm ⋅ 20mm = 10
than the applied voltage (
[12]). Due to low barrier resistance this potential will be
transferred to the barrier edge, which creates many inception
points around the large barrier circumference. This
"enlargement" of inception can lead to the lower withstand
voltage than in the no-barrier case where the inception is limited
to the small area at the tip of the rod.
At a low surface resistance of the barrier the breakdown voltage
of the negative impulse voltage is higher than the breakdown
voltage for positive impulse. For example for the 20 mm
distance at a resistance of 104 Ω the breakdown voltage of
positive impulse is 81 kV and for the negative impulse it’s
-111 kV. Like explained above with low surface resistance of
the barrier the edge of the barrier behaves as electrode and
starting point for the streamer. In this case it seems that the
polarity effect works like it is known as the breakdown voltage
of the negative impulse becomes higher as it is measured for the
positive impulse voltage.
Following the Marx-Roser model [1, 2, 6] this would also mean
that more positive charges are accumulated at the barrier than
negative charge. This can be seen as at a high surface resistance
of the barrier the breakdown voltage for the positive impulse
voltages is much higher than for the negative impulse voltage.
Comparing positive impulse voltage and AC the AC breakdown
voltage of the gap without barrier is higher than for the impulse
voltage. This was expected as with AC in every half circle all
the charges are removed from the gap. Thus the probability of
a start electron is reduced resulting in a higher breakdown
voltage as it was measured for impulse voltage. It was believed
that this effect is so explicit because the 9 kV/sec risetime of the
AC voltage ramp was rather fast. Repeating the AC
measurements of the 80 mm gap with a slower rise time of
1 kV/sec did result in the same breakdown voltage as it was
measured for the 9 kV/sec ramp measurements.
But if the values for the measurements with barrier are
compared this behavior changes as the breakdown voltage of
positive impulse is much higher than for AC. Following the
Marx-Roser model [1, 2, 6] in case of positive impulse positive
charges are accumulated at the barrier resulting in a quasiuniform electric field between the barrier and the plate electrode
[1, 6]. Thus the breakdown voltage is increased for a positive
needle [1, 6]. Like discussed before one theory [6] is that the
positive charging of the barrier guards a positive tip more than
a positive space charge guards a negative tip. As with AC the
polarity changes at every half circle and also all charges are
removed every half circle it could be that the breakdown at AC
always happens when the wave is negative. Thus a lower
breakdown voltage of AC with barrier was measured as it was
measured for positive impulse. But this theory has to be
confirmed in further experiments.
VI. CONCLUSIONS
A decrease of the breakdown voltage of a rod plane
arrangement with a single barrier could be shown when the
resistance of the barrier was decreased.
Interestingly an increase of the breakdown voltage by 10 %
could be shown when the barrier was slightly conductive
compared to a totally insulating barrier. This might be a
significant finding for insulation design.
Still many questions of the exact physics in these arrangements
remain to be investigated and answered with further
experiments and computational models.
ACKNOWLEDGEMENTS
The authors want to thank Jonas Ekeberg and Sergey
Pancheshnyi at ABB, CRC Switzerland, for valuable
discussions during this work.
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