EXPERIMENTAL DEVELOPMENTS ON SOIL IONIZATION: NEW FINDINGS

Transcription

1 GROUND 2008 & 3 rd LPE International Conference on Grounding and Earthing & 3 rd 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 E C 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, E C increases fast, reaching a value around 0.8 MV/m that is typically found for a condition of uniform electric field distribution. 1 - INTRODUCTION 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]. One fundamental parameter required for evaluation of the intensity of this effect is the so-called Critical Electrical Field E C. 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 E c. 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 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]. 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. 2 - DEVELOPMENTS 2.1 METHODOLOGY 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.

2 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 Table 2 Soil resistivity as a function of humidity Resistivity ( ρ ) Humidity: water added to the soil [Ω.m] [% of weight] 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). 3.1 PRIMARY RESULTS 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: D = 19.3 cm, d = 2 cm (b) coaxial conductors: L= 19.5cm R 0 =0.72 cm, R ext = 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 Sand large grains Sand Small grains Silt Clay I II III IV Figure 4 Typical primary result: voltage waves impressed to the soil sample placed between two coaxial conductors.

3 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 ) [Ω.m] I II III IV Breakdown field (E R ) [kv/cm] Front-time 1 µs 3 µs 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 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.

4 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 R exp was calculated from the decrease of the coaxial sample impedance from its original value Z o (for the inner radius R o and external radius R ext ) corresponding to the first curve (in blue) in Figure 9 to a decreased impedance Z d given by an increased inner radius R exp 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 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 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 Rext ln Z d Rexp Z d ln Rext ln Rexp = = Zo Rext Z o ln Rext ln R ln o R o 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. (1) Table 4 Expanded radius R exp calculated from (1) Soil I, 600 Ω.m and 1-µs-front-time current Z o =.1.7 kω Z d [k Ω] R exp [cm] 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

5 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. 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. Table 5 - Critical Electric Field E C as a function of the expanded radius R exp - Inner radius: 0.72 cm Soil I, 600 Ω.m and 1-µs-front-time current Z o =.1.7 kω R exp [cm] E c [kv/cm] 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. E C-UF 9.3 kv/cm E C-UF 9.0 kv/cm Figure 10 Typical profile of the critical electric field with increasing expanded radius of ionized zone - E C-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 E c 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 E c. Before this radius becomes twice the inner conductor radius, the value of E c 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]. 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. 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. 6 SUMMARY OF CONCLUSIONS 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.

6 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 ]. 7 - REFERENCES [1] BELLASCHI, P.L., Impulse and 60-cyle Characteristics of Driven Grounds, AIEE Transactions, Vol.60, p , março, [2] BELLASCHI, P. L., ARMINGTON, R. E., SNOWDEN, A. E., Impulse and 60-Cyle Characteristics of Driven Grounds - II, AIEE Transactions,Vol.61, p , [3] BELLASCHI, P. L., ARMINGTON, R. 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