The 19th International Symposium on High Voltage Engineering, Pilsen, Czech Republic, August, 23 – 28, 2015
ELECTRICAL BREAKDOWN OF SHORT SPARK GAPS WITH
SEVERAL ELECTRODE MATERIALS IN ATMOSPHERIC AIR AT AC
(50Hz) AND RAMP VOLTAGE (10 kV/µs)
1*
1
1
S. Gossel and C. Leu
Technische Universität Ilmenau, Interdepartmental Center for Energy Technologies,
Research Unit High-Voltage Techologies, Gustav-Kirchhoff-Straße 1,
98693 Ilmenau, Germany
*Email: < stefan.gossel@tu-ilmenau.de >
Abstract: Within this paper the investigation of electrical breakdown behaviour of short,
quasi-homogeneous spark gaps in atmospheric air with several electrode materials (e.g.
copper, aluminium, graphite and composite materials) at AC voltage (50Hz) and ramp
voltage with a steepness of 10 kV/µs will be presented. Investigations were carried out for
breakdown distances in the range of 0.5 mm up to 5 mm. The investigated electrode
arrangements are classified by a degree of homogeneity in the range of η ≈ 1 - 0.85.
Initially, the influence of electrode material on the breakdown behaviour for AC – 50 Hz
stress by using metallic, non-metallic and composite materials in atmospheric air will be
presented. The theoretical background is given by Paschen´s law. The measured
breakdown voltages will be compared with the calculated values from Paschen's law. For
the evaluation, a comparison between the work function of the studied materials and the
measured breakdown voltages will be made. The coefficients of ionization and electron
emission γ will be estimated for several materials. Furthermore the electrical breakdown
behaviour of selected electrode materials in atmospheric air under stress of AC – 50 Hz
will be compared with electrical breakdown behaviour at ramp voltage stress with an
approximated steepness of 10 kV/µs. The significant differences relating to the measured
breakdown voltages with several electrode materials will be presented and discussed.
1
INTRODUCTION
The controlled ignition as well as the prevention of
gas discharges requires the knowledge of the
dielectric strength for the used electrode arrangements. This is relevant for many technical applications. Precisely insulating clearance requirements
regarding dielectric strength, reproducibility of
ignition or choice of electrode materials are often
very versatile. The breakdown behaviour of several
materials is mostly investigated for spark length
more than 10 mm.
This paper wants to have a share to the
clarification of the breakdown behaviour of several
electrode materials with several attributes at short
gap length. Therefore measurement investigations
with different electrode materials took place with a
gap length less than 5 mm.
2
2.1
MEASURING ARRANGEMENT AND
VOLTAGE SOURCE
Electrode configuration
Sphere gaps with different electrode materials
have been used for the measuring of the electrical
breakdown voltage in nearly homogenous electric
field. The electrode configuration is schematically
shown in Figure 1.
Figure 1: Electrode configuration
The investigated electrode materials are shown in
Table 1.
Table 1: Used electrode materials
Electrode material
description
copper
E-Cu58
aluminium
AlCuMgPb (3.1645)
stainless steel
St 304 (1.4306)
copper / graphite
30 / 70
copper / tungsten
20 / 80
graphite 1
16% porosity, 600µm grain size,
block pressing
graphite 2
6% porosity, 10µm grain size,
isostatic pressing, pitch impregnated
graphite 3
10% porosity, 10µm grain size,
isostatic pressing, pitch impregnated
The sphere electrodes have a radius of r = 10 mm
and the investigated electrode arrangements are
classified by a degree of homogeneity in the range
of η ≈ 1 (d = 0.5 mm) to η ≈ 0.85 (d = 5 mm) [1].
From this follows that the electric field in the
sphere gap can be classified as quasi-homogeneous.
It is well known, that Pollution or oxide layers on
the electrode surface do affect the electrical
breakdown behaviour. Suchlike layers change the
potential barrier for the electron exit from the
electrode material (work function). Therefore, all
used electrodes of the spark gaps with the
exception of graphite electrodes were polished and
cleaned with isopropanol before each series of
measurements. This is done to obtain a clean,
non-oxidized electrode surface and thus not to
affect the dielectric behaviour.
The ramp voltage with an approximated steepness
of 10 kV/µs was generated by an impulse voltage
circuit according to Figure 2.
Figure 2: ramp voltage generator
An exemplary voltage curve according to this
generator is shown in Figure 3.
Table 2 shows a comparison of physical parameters for a selection of electrode materials.
Table 2: List of different material parameters
[3],[4],[5]
Electrode
material
3
Wa [eV]
κ [1/Ω∙m]
ρ [g/cm ]
-
1,4∙106
7,90
tungsten
4,45…5,22
1,9∙107
19,3
copper /
tungsten 20/80
4,50…5,15
2,3∙107
15,5
copper
4,48…5,10
6,0∙107
8,96
graphite
4,80…5,00
104…3∙106
3,51
electro graphite
4,00…4,50
6,3∙104
aluminium
4,06…4,26
3,8∙107
stainless steel
2.2
Figure 3: ramp voltage curve
3
BREAKDOWN BEHAVIOUR FOR AC –
50 HZ STRESS
1,8…2,8
3.1
Measurement results
4,25
With the described electrode arrangements, the
breakdown voltages were measured for AC –
50 Hz stress. The results are shown in Figure 4.
Test voltage
The investigation of electrical breakdown behaviour of short, quasi-homogeneous spark gaps
described above were carried out with AC voltage
(50Hz) and ramp voltage with a steepness of 10
kV/µs.
The AC 50 Hz high voltage was generated with the
aid of a test transformer. The rate of rise of voltage
was 3 kV per second. In order to limit the
discharge current, a series resistor in the range of
several kΩ was installed in front of the test object
(the spark gap).
Figure 4 presents the arithmetic mean and the
standard deviation for each electrode parameter
configuration and sparking distance. In order to
achieve a certain confidence level, 10 replicate
measurements were carried out each having the
same parameter configuration and sparking
distance. All shown measured data of breakdown
voltages were normalized to atmospheric standard
conditions (20°C, 1.01325 bar) using the air
density correction factor.
3.2
Influence of electrode materials on the
dielectric behaviour
The measured breakdown values with copper and
aluminium electrodes are correlating very well with
the calculated values from the Paschen's law. This
shows that Paschen´s law is useable to evaluate
the breakdown voltages for these short sparking
distances even for the used AC voltage.
The breakdown voltages and the scattering of the
different electrode materials partly diverge from
each other significantly. There is obviously a
material dependence of the breakdown voltage of
the investigated small sparking distance. Starting
from Townsend's theory for the electron avalanche
generation mechanism it comes clear, that a
significant difference in the coefficient of ionization
and electron emission (γ) must exist between
some materials.
Figure 4: breakdown voltages for AC – 50 Hz
Corresponding to Paschen´s law:
U pd 
B pd
A p  d
ln
ln1  1 


(1)
where: p = air pressure (bar),
d = sparking distance (mm),
γ = coefficient of ionization and electron
emission,
A = gas constant (1/bar mm),
B = gas constant (kV/bar mm),
for
the
values
p·d = 500 bar µm
up
to
p·d = 5 bar mm
the
breakdown
can
be
characterized as a discharge on the right range of
Paschen minimum without any space charges.
According to equation (1) and with the values for
the coefficient of ionization and electron emission γ
as well as the gas constants A and B for copper or
aluminium electrodes in air [2] a theoretical
breakdown voltages can be calculated. This
according to equation (1) calculated breakdown
voltages for copper electrodes in air and aluminium
electrodes in air are also diagrammed in Figure 4
for the respective sparking distance (Upd).
The measured breakdown voltages of the graphite
electrode materials do almost not differ from each
other in face of very different material parameters.
So the different grain size and porosity as well as
the different pressing method (anisotropy) have no
influence of the breakdown voltage at AC stress.
This means, that the influence of the electrode
surface on the microscopic field strength has no
effect on the breakdown voltage.
Copper / graphite electrodes show an equivalent
behaviour to the graphite electrodes. Apparently
the graphite component is dominating the
breakdown behavior of the copper / graphite
electrodes.
Copper / tungsten electrodes and stainless steel
electrodes have significant higher breakdown
voltages and much larger scattering than any other
tested materials.
The measured breakdown voltages do not
correlate with the work functions of the different
materials (see Table 2).
From the measured breakdown voltages, the
coefficients of ionization and electron emission (γ)
(see equation (1)) can be estimated. The
coefficients of ionization and electron emission for
graphite and copper / graphite electrodes are in the
range of the values for copper and aluminum. The
coefficients of ionization and electron emission (γ)
for steel and copper / tungsten are in the range of
-6
-5
10 to 10 .
4
COMPARISON OF BREAKDOWN
BEHAVIOUR FOR AC 50 HZ AND
RAMP VOLTAGE
The electrical breakdown behaviour in atmospheric
air of selected electrode materials was also
investigated at ramp voltage stress with an
approximated steepness of 10 kV/µs (see
Figure 3). Therefore, the electrode materials listed
in Table 3 were selected. In order to achieve a
certain confidence level at least 10 replicate
measurements were carried for each electrode
arrangement.
Also for the investigations with ramp voltage the
electrode configuration shown in Figure 1 was
used. But in this case only a sparking distance of
d=0.5 mm was tested.
Table 3: Selected electrode materials
Electrode material
description
copper
E-Cu58
aluminium
AlCuMgPb (3.1645)
copper / graphite
30 / 70
graphite 1
16% porosity, 600µm grain size,
block pressing
graphite 2
6% porosity, 10µm grain size,
isostatic pressing, pitch impregnated
Figure 5 shows the arithmetic mean and the
standard deviation of breakdown voltages for the
selected electrode materials for ramp voltage with
an approximated steepness of 10 kV/µs in comparison with the measured breakdown voltages for
AC – 50 Hz.
By viewing Figure 5 it is obvious that the measured
breakdown voltages of the selected electrode
materials at ramp voltage stress at 0.5 mm
sparking distance drastically differ from the values
at AC – 50 Hz voltage.
These differences cannot be solely attributed to the
impulse volt-time characteristic. There are several
reasons for this. Initially the investigated electrode
arrangement can be classified as quasi-homogeneous and in this case, the impulse volt-time
characteristic is almost a straight line [6].
Furthermore, it would be expected that for all
electrode materials the ramp voltage breakdown
voltages are higher than the corresponding AC
breakdown values. But this is not the case. The
measured breakdown voltages at ramp voltage
stress of copper graphite electrodes are even
lower than for AC load. But the values for copper
and aluminum are almost twice as high.
Figure 5: breakdown
voltages
of
selected
electrode materials for ramp voltage
and AC – 50 Hz at d=0.5 mm
There is apparently a drastic dependence of the
breakdown voltage on the electrode material. But
also at ramp voltage stress the measured
breakdown voltages do not correlate with the work
functions of the electrode materials (see Table 2).
The values for the coefficients of ionization and
electron emission (γ) for AC stress are apparently
no longer valid for ramp voltage at the investigated
sparking distance.
It is known [6] that the surface condition of the
electrodes (e.g. oxide layers) has an influence on
the carrier storage time. This means, with clean
oxide-free electrodes should be no significant
difference in the breakdown voltage between AC
and ramp voltage stress. All used electrodes of the
spark gaps with the exception of graphite
electrodes were polished and cleaned with
isopropanol before each series of measurements.
Nevertheless, the measured values for the metal
electrodes (copper and aluminium) differ
significantly between AC and ramp voltage stress.
Although the different material parameters, the
breakdown voltages of graphite electrodes at AC
stress differ only marginally (see Figure 4). But the
breakdown voltages at ramp voltage stress are
substantially different. The graphite electrode
arrangement with the significant larger grain size
and larger porosity (see Table 3, graphite1) has
the much lower breakdown voltage.
5
CONCLUSION
Short, spark gaps with several electrode materials
(see Table 1) have been used for the measuring of
the electrical breakdown voltage in nearly
homogenous electric field in atmospheric air at AC
voltage (50Hz) and ramp voltage with a approximated steepness of 10 kV/µs. Investigations were
carried out for breakdown distances in the range of
0.5 mm up to 5 mm. The sphere radius was
r = 10 mm (Figure 1).
For the measured breakdown voltages for AC –
50 Hz (see Figure 4) several things become
apparent: The measured breakdown values with
copper and aluminium electrodes are correlating
very well with the calculated values from the
Paschen's law (equation 1). The different investigated graphite electrode materials have very
different material parameters. The measured
breakdown voltages do not differ from each other.
So the different grain size and porosity as well as
the different pressing method have no influence of
the breakdown voltage at AC – 50 Hz stress. The
measured breakdown voltages do not correlate
with the work functions of the different materials.
The comparison of the breakdown voltages of
selected electrode materials for ramp voltage with
an approximated steepness of 10 kV/µs and AC –
50 Hz shows, that there is obviously a drastic
dependence of the breakdown voltage on the
electrode material for ramp voltage breakdown
(see Figure 5). The measured values for the metal
electrodes (copper and aluminium) differ
significantly between AC and ramp voltage stress.
But also at ramp voltage stress the measured
breakdown voltages do not correlate with the work
functions of the electrode materials. The different
material parameters of the graphite electrodes do
have a significant influence the breakdown
voltages. The graphite electrode arrangement with
the significant larger grain size and larger porosity
has the much lower breakdown voltage.
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[3] D. R. Lide, W. M. Haynes, T. J. Bruno.:
Handbook of Chemistry and Physics. 90.
edition: CRC Press Inc., USA 2009
[4] A. Aretz: Experimentell und theoretische
Untersuchungen zur Elektronenaustrittsarbeit
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Düsseldorf, 1994
[5] E. Lasser, W.-D. Schubert: Tungsten –
Properties, Chemistry, Technology of the
Element, Alloys and chemical Compounds.
Springer-Verlag, Berlin, 1999
[6] R. Strigel, Elektrische Stoßfestigkeit, SpringerVerlag, Berlin Götingen Heidelberg, 1955