GROUND’2008
&
3rd LPE
International Conference on Grounding and Earthing
&
3rd International Conference on
Lightning Physics and Effects
Florianopolis - Brazil
November, 2008
EXPERIMENTAL DEVELOPMENTS ON SOIL IONIZATION: NEW FINDINGS
José Luís Cerqueira Lima
SIlvério Visacro
LRC - Lightning Research Center – Federal University of Minas Gerais - Brazil
Abstract - Experimental tests were performed to evaluate
the soil ionization effect for different soils at several
humidity values. Impulsive currents were impressed to soil
samples to apply uniform and non-uniform electric field
distributions on them, according to the test recipient shape.
It was found that the field non-uniformity influences
significantly the value of the critical electrical field EC
required for ionization onset, decreasing its value in relation
to the condition of uniform field distribution. This parameter
shows a dynamic behavior. It begins with a small value,
around 0.3 MV/m when the process is onset in the highly
non-uniform field zone, close to the electrode. As the
ionization zone is being extended, EC increases fast,
reaching a value around 0.8 MV/m that is typically found for
a condition of uniform electric field distribution.
ionization process or to compute its effect on the
response of grounding electrodes, such as the traditional
work by Liew and Darveniza [4] or some other
approaches such as the ones that assumes an equivalent
increase of the electrode radius to compute the reduction
of impedance due to ionization [11] or recent proposals
that attributes a resistance value for the ionized zone
around the electrode [12].
1 - INTRODUCTION
2 - DEVELOPMENTS
The ionization effect has been investigated since a long
time ago [1 to11]. Concerning lightning currents, this
effect might be relevant to diminish the grounding
impedance when very high currents are impressed to
concentrate electrodes [13].
2.1 –METHODOLOGY
One fundamental parameter required for evaluation of the
intensity of this effect is the so-called Critical Electrical
Field EC. It corresponds to the field threshold, where the
effect is onset in the soil.
There is a lot of controversy about the appropriate value
to be adopted for this parameter in the numerical and
analytical evaluations of soil ionization. The problem is
that the intensity of this effect is extremely dependent of
this value.
Basically two main different proposals were presented by
the researchers who had investigated this effect. The first
one suggests a value around 0.9 MV/m for Ec. It is based
on the results of Oettlé [9], who evaluated this field
through the application of impulsive currents to soil
samples filling the gap between parallel conductive discs
(and also between hemispheres). The other school
suggests this value is much lower (around 0.3 MV/m). It
is based on the results of measurements performed by
several authors in two different conditions: (i) field tests
consisting on impressing impulsive currents to vertical
rods placed in natural soils or (ii) laboratorial tests
consisting on impressing impulsive currents to samples
filling the space between coaxial conductive cylinders.
The latter value is most commonly used in lightning
protection evaluations.
Also, different models were proposed to explain the
Definitely, the most important issues in this topic
concerns establishing reliable references for the critical
electric field and how to articulate this reference to
compute the effect of the process. This specific issue is
the focus of this paper.
The authors developed an experimental investigation
about the ionization process, applying a methodology that
is similar to laboratorial tests previously reported,
consisting on impressing impulsive currents to soil
samples from a voltage impulse generator in order to
obtain high current densities (and therefore intensity of
electric fields) in the soil. The applied voltage and then
current density is increased by steps until the associate
field is able to onset the ionization process. A similar
procedure was applied by Oettlé for an uniform field
condition in the gap between parallel discs [9]. In this
case, once the critical field is achieved, the soil sample is
broken down. This procedure was also applied by Loboda
[7] and Visacro [11] for non-uniform cylindrical fields to
observe the evolution of ionization process as a function
of applied electric field in the soil filling the gap between
two coaxial conductors. In this case, since the field is
larger close to the inner electrode and diminishes as the
radius is increased, it is possible to observe the
expansion of the ionized zone while the applied voltage
and resultant field are being increased.
The authors` experiments present, however, two
differential aspects. In all previous works the tests were
performed for only one of the field conditions (uniform or
non-uniform). Also, the effect of soil resistivity was
evaluated only considering different types of soil.
Differently, in the present work, for each soil sample and
humidity the evaluations were performed for both field
conditions to allow comparing their results. Also, the
change of resistivity was obtained by varying the water
content of the soil. This allowed evaluating the influence
of the resistivity separately without the interference of
other factors associated to differences on soil types.
Table 2 – Soil resistivity as a function of humidity
2.2 - THE TESTS AND MEASUREMENTS
The tests consisted basically in impressing impulsive
current waves to the soil samples through a high voltage
impulse generator, Figure 1. The voltage (and, therefore,
the current) was increased by steps until the gap
breakdown with a flashover between the electrodes. The
applied voltage and resultant current were measured for
each impulse shot and the data were registered and
stored in a microcomputer.
Type of
soil
I
II
III
IV
Resistivity ( ρ )
[Ω.m]
600
350
250
1090
600
240
80
10000
7000
2000
10000
5000
3000
Humidity: water
added to the soil
[% of weight]
0
5
10
0
3
6
12
5
10
15
20
25
30
3 - RESULTS
3.1 – PRIMARY RESULTS
Figure 1 - Experimental set-up - Impulsive currents with two
different front-times (around 1 and 3 µs) provided by a Haefley
high voltage impulse generator were impressed to the soil
samples in each test.
The process was developed applying a uniform electric
field distribution to each soil sample through a parallel
dish configuration (Figure 2.a) and also applying a
cylindrical field distribution through a coaxial
configuration, with a an inner electrode consisting in
0.72-cm-radius rod, 19.5 cm long (Figure 2.b).
Two primary results were obtained according to the test
recipient configuration.
For the parallel discs responsible for a uniform field
distribution in the soil sample, voltage waves were
registered for increasing applied voltage amplitudes from
5 kV to the breakdown voltage. Figure 3 illustrates such
result for soil type I at 600 Ω.m. It depicts the breakdown
voltage for two current front-times, 1 and 3 µs.
d
D
L
(a) parallel discs:
(b) coaxial conductors: L= 19.5cm
D = 19.3 cm, d = 2 cm
R0=0.72 cm , Rext = 22.25 cm ,
Figure 2 –Recipients and electrode configurations used to
impress uniform (a) and cylindrical (b) electric field distribution
over the soil sample.
Four types of soils, described in Table 1, were tested. To
prepare the samples, the soil was first dried. Then deionized water was added to it. After mixing it, the soil was
let to rest in closed plastic recipients to obtain uniform
samples with different resistivity values according to the
weight of water added, Table 2.
Figure 3 – Typical primary result: voltage waves impressed to
the soil sample placed between two parallel disc conductors.
For the coaxial electrode configuration, responsible for
impressing a cylindrical field distribution to the soil
sample, the voltage and related current waves were
obtained to monitor the ionization process evolution while
the applied field intensity was increased, as illustrated in
Figure 4 for the same soil sample of Figure 3. Though
this figure shows only the result for a 1 µs current fronttime, measurements were done also for a
3 µs fronttime.
Table 1 – Composition of the tested soils.
Soil
Type
I
II
III
IV
Sand
Sand
large grains
Small grains
(%)
(%)
3.80
9.20
26.80
29.20
63.74
21.76
32.12
17.26
Silt
Clay
(%)
(%)
31.16
67.84
38.92
21.54
1.30
1.20
2.16
32.00
Figure 4 – Typical primary result: voltage waves impressed to
the soil sample placed between two coaxial conductors.
3.2 – ELABORATE RESULTS
From the primary results, some elaborate results were
derived to allow analyzing the phenomenon.
First, the critical electrical field in the uniform field
distribution was determined for each soil sample. This
result is shown in Table 3 and was calculated directly
from the breakdown voltage.
Another elaborate result consists in curves, presenting
the instantaneous value of the ratio v(t)/i(t) , along the
time scale, calculated for each corresponding pair of
voltage and current waves. This ratio is here designated
the transient impedance of soil sample Z(t). Figure 6
illustrates this type of result for the specific soil sample of
Figure 4.
Table 3 - Critical electric field: Breakdown Field
Type of
soil
Resistivity ( ρ )
-------
-------
I
II
III
IV
[Ω.m]
600
350
250
1090
600
240
80
10000
7000
2000
10000
5000
3000
Breakdown field (ER)
[kV/cm]
Front-time
1 µs
3 µs
9.3
9.0
6.0
6.3
5.5
6.1
9.5
6.0
5.8
5.7
5.2
5.3
4.2
5.0
5.3
5.5
5.0
5.0
3.7
3.7
14.2
14.4
12.0
11.6
10.0
9.0
For the non-uniform field distribution two kind of
alternative results were developed from the voltage and
current waves.
First, to support a qualitative analysis in respect to the
evolution of the ionization process, VxI curves were
constructed from each pair of voltage and current waves
corresponding to a single shot. The typical result is
shown in Figure 5 for a specific soil sample.
Figure 6 – Transient impedance of soil sample Z(t) = v(t)/i(t): For
each sample, this ratio was calculated along time scale from
voltage and current waves (example: soil I, 600 Ω.m)
Results similar to those of figures 3 to 6 were obtained for
each soil sample and resistivity, considering the two
values of current front-time.
5 - ANALYSIS OF THE RESULTS
Several types of analysis were developed and provided
very interesting conclusions.
5.1 - UNIFORM FIELD TESTS
Concerning the results for the uniform field distribution, it
was observed a wide dispersion of the breakdown field
within the 3.9-to-14.4-kV/cm range, according to the soil
type and resistivity.
However, the average value is large, around 7.4 and
7.1 kV/m respectively for 1 and 3 µs current front-times,
and the lowest values are larger than 5 kV/cm. Values
below this threshold are found only in conditions where
the soil is completely wet and are attributed to the
establishment of continuous water path across the gap
between the parallel discs, a condition that is not
expected in natural environment while lightning current is
impressed to the grounding electrode.
In this respect the range of values are consistent with the
finds of Oettlé [9], suggesting large values of critical
electric fields, around 8 kV/cm.
Another result for the uniform field tests is the clear
dependence of the critical electric field on the soil
resistivity value. Figure 7 summarizes this result for all
tests and soils and shows the growth of critical field with
increasing resistivity.
Figure 5 – Curves VxI: For each test, the pair of instantaneous
values of corresponding voltage and current waves (such as
those of figure 4) were taken and placed together in a graph
(example: soil I, 600 Ω.m)
Oettlé also found similar results [9], though for different
soils. The results found in this work are a little more
significant since they were obtained for the same soil at
different resistivity values obtained only changing the
water content of soil. The other soil properties such as
porosity, grains etc were preserved in the evaluation.
Anyway, the trend observed by Oettlé for different soils
holds also for the different soils of this work.
non-existence of ionization process or its very weak
effect. As the peak field is being increased the effect of
the process becomes noticeable, with the upper extremity
of the loop becoming sharper. The ascending and
descending curves tend to be overlapped in this region,
denoting the effect of a moderate ionization. Finally, when
the process becomes intense with a pronounced effect,
the loop shape tends to an inclined "8". This qualitative
results agrees quite well with those presented by Visacro
in [11], where the variation of the curve slope due to
ionization was employed to modeling the effect as an
equivalent increase of the inner electrode radius and then
to calculate the decrease of impulsive impedance of
grounding electrodes.
A second analysis was developed based on the
interpretation of the curves of transient impedance along
the time scale such as those of Figure 6.
The ionization effect is modeled as an expansion of the
inner electrode. The "expanded" radius Rexp was
calculated from the decrease of the coaxial sample
impedance from its original value Zo (for the inner radius
Ro and external radius Rext) corresponding to the first
curve (in blue) in Figure 9 to a decreased impedance Zd
given by an increased inner radius Rexp corresponding to
other impedance curve such as the one in red,
expression (1).
Figure 7 – Critical Electric Field as a function of soil resistivity
One more analysis is depicted in Figure 8. It denotes that
the breakdown time presents a wide dispersion in a range
from less than 1 µs to more than 5 µs. Considering that
only front-times larger than 1µs were tested, it is clear
that the on-set of the ionization process does not require
a large inception time. In general a similar behavior was
verified for all the tested soil samples.
Figure 9 – Set of transient impedance curves of a soil sample as
function of applied voltage – soil I , 600 Ω.m, 1 µs front-time
Zd
=
Zo
R

ln ext

Rexp 
ln Rext − ln Rexp
Z

⇒ d =
ln Rext − ln Ro
Zo
Rext


ln
Ro 

(1)
In order to illustrate this procedure, Table 4 shows the
calculated expanded radius for the sample of Figure 9. A
similar procedure was applied to determine this expanded
radius for all the tested soil samples.
Figure 8 - Time to Breakdown
5.1 – NON-UNIFORM FIELD TESTS
A qualitative analysis allows identifying the intensity of
ionization process along the soil gap between the coaxial
cylinders from the loops closed in the curves of figure 5.
Initially, the loops corresponding to low peak values of
electric field intensity (associated to low current peaks)
present a VxI profile that is typical for a constantparameters-parallel RC circuit. This behavior denotes the
Table 4 – Expanded radius Rexp calculated from (1)
Soil I , 600 Ω.m and 1-µs-front-time current – Zo=.1.7 kΩ
Zd [k Ω]
1.65 1.61 1.58 1.56 1.55 1.49 1.40 1.22 1.05
Rexp [cm] 0.80 0.86 0.92 0.95 0.97 1.10 1.31 1.90 2.67
One additional analysis was developed from the
calculated expanded radius. The tests allowed verifying
that the ionization has a very non-uniform distribution
around the cylinder, being mainly composed by a large
number of ionized channels that develops from the inner
cylindrical conductor. Some of this channel extends far
beyond the most ionized region. This process is quite
different from the corona effect in the air that develops a
uniform and well defined sheath around the electrode.
twice the inner conductor radius, the value of Ec approach
0.8 MV/m. Then it oscillates, tending to a value around
the critical electric field measured in the uniform field
distribution that is in the range suggested by Oettlé (~ 0.8
to 0.9 MV/m) [9].
From a macroscopic perspective, it seems reasonable to
assume that the expanded radius corresponds to the
average border of the high ionized region in the soil and,
therefore, the electric field calculated in this border would
correspond to the critical electric field there. With such
assumption, using the calculated expanded radius, the
critical electric field was determined as a function of this
radius for all the tested soil samples. Table 5 shows the
results for the data of Table 4.
Exceptions to this rule are found only in the case of very
wet soils, which correspond to the lowest value of
resistivity obtained for any of the tested soils. The critical
fields determined for such soils in uniform field are very
low, even smaller than the value found in non-uniform
field distribution. This is attributed to the establishment of
continuous thin paths of water connecting the parallel
dishes, whose distance is very reduced in comparison to
the distance between the coaxial cylinders. The existence
of such paths, that is not expected in real soil conditions
related to grounding applications, is responsible for an
earlier breakdown of the soil in the gap and, therefore, for
a reduced value of critical field.
Table 5 - Critical Electric Field EC as a function of
the expanded radius Rexp - Inner radius: 0.72 cm
Soil I , 600 Ω.m and 1-µs-front-time current – Zo=.1.7 kΩ
Rexp [cm] 0.80 0.86 0.92 0.95 0.97 1.10 1.31 1.90 2.67
Ec [kV/cm] 3.12 4.22 5.38 6.63 7.87 8.66 8.82 7.80 7.23
5.3 – GENERAL ANALYSES
The most impressive analysis was developed from curves
depicting the dynamics of the calculated critical electric
field in the non-uniform field distribution tests with the
increasing expanded radius in the presence of the critical
electric field determined for the same sample for uniform
field distribution condition. Figure 10 illustrates this
dynamics and represents approximately a typical profile
observed in all tested samples.
EC-UF
9.3 kV/cm
According to the typical behavior illustrated in Figure 10,
the influence of soil non-uniformities seems clear to
reduce the critical field. The smaller the expanded radius
is, the lower the critical field is. This result is consistent
with the findings of Loboda [7], who reported that the
critical field is rised with increasing inner electrode radius.
This electric field profile brings new insights into the issue
of critical electric field. It is no more a question of "what
value to recommend for it" but to compute a dynamic
behavior of the electric field. This critical field has a very
low value around 200 to 300 kV/m in the region close to
electrode (expanded radius smaller than 0.8 cm) that
evolves fast to a larger value around 0.8 to 0.9 MV/m in
the region determined by an expanded radius larger than
1.2 cm.
This dynamic profile has significant impact on the
reduction of grounding impedance due to ionization effect
in relation to the expected reduction on the assumption of
a constant value for the critical electric field, as commonly
processed in procedures recommended by literature.
Computing such dynamics for the critical field leads to a
smaller ionization effect, meaning that the grounding
impedance is reduced less than commonly expected.
EC-UF
9.0 kV/cm
6 – SUMMARY OF CONCLUSIONS
Figure 10 – Typical profile of the critical electric field with
increasing expanded radius of ionized zone - EC-UF: critical
field for uniform field tests - soil I, 600 Ω.m, 1 µs front-time
Though, the results show naturally some dispersion, the
general trend observed in this figure for soil I occurs for
all the soil samples.
Typically, the calculated value of Ec is very low for the
radius of a ionized zone border close to the inner
conductor radius, around 3 kV/cm, a value that is
suggested by several authors as the recommended value
for the critical electric field [10,17,18]. However, as the
radius of the ionized zone border is being increased there
is a substantial rise of Ec. Before this radius becomes
The experimental results of this work showed that, though
in general there is a large dispersion on the values
measured or calculated according to the soil type and
other parameters, clear trends are observed leading to
some relevant conclusions that are summarized next.
First, the typical values found for the critical electric field
in the uniform and non-uniform field distribution are
consistent respectively with those presented in the most
important references in literature, respectively around 0.8
to 0.9 MV/m (Oettlé [9]) and 0.2 to 0.3 MV/m (several
authors [10,17,18]). As it is clear, the critical fields in
uniform fields are much higher, around 3 to 4 times more.
For the uniform field distribution, it is clear that increasing
the soil resistivity leads to a higher critical electric field
though this effect is not observed for the lower values of
this parameter found in the non-uniform field distribution.
This suggests that the mean factor responsible for
decreasing the electric field in the non-uniform field
distribution is the enhancement of the field strength in
most non-uniform-field region that is close to the inner
electrode. The authors attribute this enhancement to the
presence of soil grains in this region. This effect makes
the onset of ionization process earlier. Though the same
grains are present in the region of uniform field
distribution, the enhancement of field there is only
moderate and not capable to reduce the critical field.
Another relevant conclusion is that the time-tobreakdown that is a parameter referred for measuring the
inception time of ionization effect is a random variable,
being smaller than 1 µs in several cases. This finding
contradicts the usual assumption that the ionization
inception time is larger than 5 µs. It seems that this
assumption is due to an erroneously interpretation of a
practice adopted by Oettlé [9] who arbitrated a minimum
5 µs time-to-breakdown as a reference to consider the
electric field that was able to cause the breakdown of soil
samples, disregarding those cases with shorter time.
The qualitative behavior of soil due to ionization effect as
described by VxI curves is consistent with the findings of
previous works [11,7].
The most relevant conclusion of this work is the finding of
a dynamic behavior for the critical electric field as a
function of the border of the ionized zone while this zone
is being extended due to increasing impressed current. It
is no more a question of "what value to employ" for the
critical field but to compute a dynamic behavior of the
electric field. This critical field has a very low value
around 200 to 300 kV/m in the region close to electrode
(expanded radius smaller than 0.8 cm) that evolves fast
to a larger value around 0.8 to 0.9 MV/m in the region
determined by an expanded radius larger than 1.2 cm.
The practical implication of computing such dynamic
behavior is a significant decrease of the reduction of
grounding impedance due to ionization effect in relation
to the reduction expected with the common assumption of
a constant value of critical electric field around
300 MV/m, as recommended in literature [10,17,18 ].
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