Effect of Impulse Polarity on a New Grounding Device with Spike Rods (GDSR)

The characterizations of grounding systems subjected to high impulse conditions are known to be dependent on the polarity, current to peak time and discharge time, as well as the electrical properties of the soil, and the grounding electrodes themselves. It is therefore important to investigate the behavior of grounding systems under high impulse conditions under negative impulse polarity, and compare it with positive impulse polarity results. Experimental test results for the same grounding systems installed with various earth electrodes at several sites under positive impulse polarity have previously been presented. In comparison, this paper presents the results of negative impulse polarity injected on the same grounding systems. It is found that a significant difference between positive and negative impulse polarities is observed for the grounding systems installed at high soil resistivity.


Introduction
Impulse polarity has been known to affect the performance of many dielectric materials, such as oil, gas and solid insulators [1][2][3][4][5], as well as other materials, namely conductive water [6][7][8], soil or grounding systems [9][10][11][12][13][14][15][16]. Among these, the typical observations on the differences due to impulse polarities are, in terms of streamer propagation, that such streamers exhibit a distinctive treelike shape for a positive impulse, as compared to resembling a bush under a negative impulse for all solid samples [5]. A leader streamer was found to propagate from the live terminal of the rod to the ground when a positive polarity was applied to the gaps of a rod-plane with barriers made of dielectric material, whereas a leader was found to propagate from the ground to the rod-plane when applied with negative impulse polarity [3]. There were higher breakdown voltages for negative polarity than for positive impulse polarity for a rod-rod gap in air [2], where there were larger differences between positive and negative breakdown voltages in larger gap spacing. Similarly, lower breakdown voltage in positive impulse polarity was seen for a CF 3 1/N 2 gas mixture in a highly non-uniform electric field, compared to negative impulse polarity [2].
Though most electrical equipment experiences breakdown at lower voltage under positive rather than negative impulse polarity [2,3,5], some impulse tests on high voltage applications show higher breakdown voltage in positive impulse polarity as compared to negative impulse polarity [7]. Zhao et al. [2] found that in a slightly non-uniform field, and with a shorter electrode gap of 5 mm, breakdown voltage under positive impulse polarity is higher than breakdown voltage under negative impulse polarity. Similarly, Darveniza [6] found lower breakdown voltage in negative impulse than positive impulse polarity for an air gap between the Cross-linked Polyethylene (XLPE) insulated conductors. polarity, higher Z impulse and slower discharge time are observed than for the Z impulse and discharge time of positive impulse polarity for earth electrodes installed at high soil resistivity (site 2).

Experimental Arrangement
Using the same earth electrodes for remote earth, voltage and current probes, impulse generating test circuits and test procedures as in [17], impulse tests under negative impulse polarity with increasing current magnitudes were performed right after the positive impulse polarity tests. Figure 1 shows the experimental arrangement used in this study for all three sites. An impulse generator was used, where a range of voltage levels from 30 to 210 kV were applied to the ground electrodes under test. A current transformer (CT) with the ratio of 0.01 VA −1 and a resistive divider with a 3890:1 ratio were used for current and voltage measurement respectively, which were captured in Digital Storage Oscilloscopes (DSOs). All the cables were placed above the ground, with an insulation rod of 1 m length, to provide enough clearance from the ground. The test set up and measurements were based on the patent filed in [19].

Experimental Arrangement
Using the same earth electrodes for remote earth, voltage and current probes, impulse generating test circuits and test procedures as in [17], impulse tests under negative impulse polarity with increasing current magnitudes were performed right after the positive impulse polarity tests. Figure 1 shows the experimental arrangement used in this study for all three sites. An impulse generator was used, where a range of voltage levels from 30 to 210 kV were applied to the ground electrodes under test. A current transformer (CT) with the ratio of 0.01 VA −1 and a resistive divider with a 3890:1 ratio were used for current and voltage measurement respectively, which were captured in Digital Storage Oscilloscopes (DSOs). All the cables were placed above the ground, with an insulation rod of 1 m length, to provide enough clearance from the ground. The test set up and measurements were based on the patent filed in [19].

Test Results
The results are presented in Tables 1 and 2, showing the soil resistivity values and steady-state earth resistance value (Rdc) respectively, as presented in [17]. Voltage and current traces for negative impulse polarity similar to those captured in [17] for positive polarity were observed (see Figures 2 and 3). Comparative results between the positive and negative impulse polarities were studied based on current rise time, discharge time and impulse resistance values, as in [17].

Test Results
The results are presented in Tables 1 and 2, showing the soil resistivity values and steady-state earth resistance value (Rdc) respectively, as presented in [17]. Voltage and current traces for negative impulse polarity similar to those captured in [17] for positive polarity were observed (see Figures 2 and 3). Comparative results between the positive and negative impulse polarities were studied based on current rise time, discharge time and impulse resistance values, as in [17].

Time to Peak Current
Time to peak current versus peak current plots for all tested grounding systems under positive impulse polarity have been presented in [17]. In this section, time to peak current versus peak current is compared between the positive and negative impulse polarities for all grounding systems. Figures  4-6 show the plots of time to peak current of negative impulse polarities for sites 1, 2 and 3 respectively. It can be seen from the figures that the growth rate of conduction is slower at low current magnitudes, and current magnitudes are almost constant for all earth electrodes at higher current magnitudes, with current magnitudes above 2, 3 and 9 kA for sites 1, 2 and 3 respectively. The trend of slower time to peak current at low current magnitudes is similar to the values obtained under positive impulse polarity, as presented in [17]. It was also observed that earth electrodes installed at site 1 had the highest time to peak current, which can be seen in Figures 7-12 for earth electrodes 1, 2, 3, 4, 5 and 6 respectively. It was also observed that there was no specific pattern per se on a relation of impulse polarity on the time to peak current, as can be seen in Figures 7-12 for earth electrodes 1, 2, 3, 4, 5 and 6 respectively. For some earth electrodes installed at various sites, negative polarity was found to have slower time to peak current and vice versa for the same earth electrode at different

Time to Peak Current
Time to peak current versus peak current plots for all tested grounding systems under positive impulse polarity have been presented in [17]. In this section, time to peak current versus peak current is compared between the positive and negative impulse polarities for all grounding systems. Figures  4-6 show the plots of time to peak current of negative impulse polarities for sites 1, 2 and 3 respectively. It can be seen from the figures that the growth rate of conduction is slower at low current magnitudes, and current magnitudes are almost constant for all earth electrodes at higher current magnitudes, with current magnitudes above 2, 3 and 9 kA for sites 1, 2 and 3 respectively. The trend of slower time to peak current at low current magnitudes is similar to the values obtained under positive impulse polarity, as presented in [17]. It was also observed that earth electrodes installed at site 1 had the highest time to peak current, which can be seen in Figures 7-12 for earth electrodes 1, 2, 3, 4, 5 and 6 respectively. It was also observed that there was no specific pattern per se on a relation of impulse polarity on the time to peak current, as can be seen in Figures 7-12 for earth electrodes 1, 2, 3, 4, 5 and 6 respectively. For some earth electrodes installed at various sites, negative polarity was found to have slower time to peak current and vice versa for the same earth electrode at different

Time to Peak Current
Time to peak current versus peak current plots for all tested grounding systems under positive impulse polarity have been presented in [17]. In this section, time to peak current versus peak current is compared between the positive and negative impulse polarities for all grounding systems. Figures 4-6 show the plots of time to peak current of negative impulse polarities for sites 1, 2 and 3 respectively. It can be seen from the figures that the growth rate of conduction is slower at low current magnitudes, and current magnitudes are almost constant for all earth electrodes at higher current magnitudes, with current magnitudes above 2, 3 and 9 kA for sites 1, 2 and 3 respectively. The trend of slower time to peak current at low current magnitudes is similar to the values obtained under positive impulse polarity, as presented in [17]. It was also observed that earth electrodes installed at site 1 had the highest time to peak current, which can be seen in Figures 7-12 for earth electrodes 1, 2, 3, 4, 5 and 6 respectively. It was also observed that there was no specific pattern per se on a relation of impulse polarity on the time to peak current, as can be seen in Figures 7-12 for earth electrodes 1, 2, 3, 4, 5 and 6 respectively. For some earth electrodes installed at various sites, negative polarity was found to have slower time to peak current and vice versa for the same earth electrode at different sites. Tables 3-6 summarize the time to peak current for various earth electrodes and soil resistivities under negative impulse polarity, and under both impulse polarities. sites. Tables 3-6 summarize the time to peak current for various earth electrodes and soil resistivities under negative impulse polarity, and under both impulse polarities.

1
For the current magnitudes below 2 kA, time to peak current has no direct relation to Rdc values. However, the time to peak current is around 10 µs, independent of earth electrodes, and constant with increasing currents for higher current magnitudes above 2 kA. 2 The time to peak current is dependent on Rdc values; conf. 1 with the highest Rdc takes the slowest time to reach peak current, at lower current magnitudes below 1.8 kA.
The times to peak current are close, around 8 µs, independent of earth electrodes, constant with increasing currents at higher current magnitudes; i.e., above 1.8 kA. 3 For the current magnitudes below 6 kA, the time to peak current is around 9 µs, independent of Rdc. Between 6 and 9 kA, a drop of time to peak current is seen for all earth electrodes. For the current magnitudes above 9 kA, the time to peak current is around 7 µs, independent of the earth electrodes, constant with increasing currents.

Site Comparison between Negative and Positive Impulse Polarities
1 Times to peak current for positive impulse polarity are directly dependent on the Rdc of grounding systems, whereas times to peak current for negative impulse polarity are found to occur randomly, and are not directly dependent on Rdc values observed for currents below 2 kA. Similar time to peak current is seen for both impulse polarities, of 10 µs for current magnitudes above 2 kA. 2 The time to peak current is dependent on Rdc values for both impulse polarities for current magnitudes below 2.5 kA for positive polarity, and 1.8 kA for negative impulse polarity. Time to peak current for the current magnitudes higher than 2.5 and 1.8 kA for positive and negative impulse polarity respectively is constant at 8 µs. 3 For high Rdc values, slower time to peak current is seen, and this trend is noted for currents below 9 and 6 kA for positive and negative impulse polarity respectively. At higher current magnitudes, time to peak current is constant at 7 µs.

Earth Electrode Time to Peak Current at Various Sites
1 Time to peak current for earth electrode 1 installed at sites 1 and 2 is close. As the current magnitude increases, faster time to peak current is seen for earth electrode 1 installed at all sites.

2
Earth electrodes 2 to 6 installed at site 1 have the slowest time to peak current. As the current magnitude increases, faster time to peak current is seen for earth electrodes 2 to 6 installed at all sites.   Figures 7-12).

Earth Electrode Comparison between Negative and Positive Impulse Polarities
1 Earth electrode 1 installed at site 2 has the slowest time to peak current when subjected to positive impulse. When subjected to negative impulse polarity, the slowest time to peak current is seen for earth electrode 1 installed at site 1. Faster time to peak current is seen when the current magnitudes increase, for all earth electrodes subjected to both impulse polarities.

Discharge Time
In [17], the time taken for the current trace to discharge to the ground indicates how effective the grounding systems are; the faster the discharge time, the more effective the grounding systems would be. In this paper, impulse test results are plotted for negative impulse polarities in Figures 13-15 for earth electrodes installed at sites 1, 2 and 3 respectively. Similar to those found under positive impulse polarity [17], faster discharge time to zero for a current trace with increasing current shows better conduction at high magnitudes of current. A distinctive difference can be seen from all these figures, where the ranges of discharge times noted for sites 1, 2 and 3 are from 200 to 800 µs, 200 to 1200 µs and 120 to 200 µs respectively. This indicates that the lower the soil resistivity, the lower the Rdc is, hence the more effective the grounding systems are in discharging the current to the ground. In this paper, the discharge time versus applied voltage for each earth electrode installed at various sites under both positive and negative impulse polarities is also plotted, as shown in Figures 16-21, for earth electrodes Energies 2020, 13, 4672 9 of 19 1, 2, 3, 4, 5 and 6 respectively. The highest discharge time occurred for earth electrodes installed at site 2, followed by sites 1 and 3 for negative impulse polarity. It can be seen that for the same applied voltage and earth electrode, large differences were observed between the discharge times of site 2 and site 1, and site 1 and site 3, where for a certain applied voltage the difference was more than 50%. Results revealed that for site 1, the discharge time was found to be lower for all earth electrodes subjected to negative rather than positive impulse polarity. This was found to be contradictory for the earth electrodes installed at site 2, where it was observed that all earth electrodes installed had slower discharge times when subjected to negative rather than positive impulse polarity. Slower discharge times for grounding systems subjected to negative impulse polarity were also observed for most earth electrodes installed at site 3. However, for earth electrodes 1 and 4 installed at site 3, there was no impulse polarity effect seen in terms of discharge time. In several publications [9][10][11][12][13], lower conduction currents and higher voltage magnitudes are needed to cause the breakdown in grounding systems subjected to negative impulse polarity.
Cabrera et al. [11] observed that for sand grains of a size above 1 mm, and above 10 MΩ, higher voltage causing breakdown was seen when the sand was subjected to negative rather than positive impulse polarity. Contradictorily, for the soil of similar resistivity above 10 MΩ, but with a fine grain size, they [11] found higher breakdown voltage in positive impulse polarity than in negative impulse polarity. No effect was seen in their study of impulse polarity for soil with soil resistivity lower than 10 MΩ [11]. In this study, it was observed that the highest soil resistivity occurred at site 2 and as can be seen in some publications [15,16], the coarser the soil, the higher the soil resistivity. Though no measurement was taken on the soil composition and coarseness of the soil, it is presumed that site 2 may have coarser types of soil due to its high soil resistivity. A rather more significant impulse polarity effect was seen for earth electrodes installed at site 2, in which the grounding systems experienced lower conduction; hence, a slower time for the current to discharge to the ground when subjected to negative impulse polarity.
From the Figures, the differences between the positive and negative impulse polarity were observed for grounding systems with high Rdc (all earth electrodes installed at site 2). This trend could be related to a high electric field in high resistivity soil, hence its relative Rdc, as seen in [17], where the earth electrodes at site 2 had electric field values 70% times higher than the electric field values for sites 1 and 3. Furthermore, this shows that for electrodes with a high electric field, a significant difference can be expected in impulse polarity. Tables 7-10 summarize the results of discharge times when subjected to a negative impulse, and to both impulse polarities.
Energies 2020, 13, x FOR PEER REVIEW 10 of 19 significant difference can be expected in impulse polarity. Tables 7-10 summarize the results of discharge times when subjected to a negative impulse, and to both impulse polarities.                              Figures 13-15, and [17]).

1
Faster discharge time as the applied voltage increases, and depending on the values of Rdc is seen for both impulse polarities; i.e., faster discharge time for earth electrodes with low Rdc, and slower discharge time for earth electrodes with high Rdc for both positive and negative polarities. 2 3 Faster discharge time with increasing applied voltage for negative impulse polarity, whereas it is found to be constant for positive impulse polarity. Discharge time is dependent on Rdc values for negative impulse, but discharge time is hardly dependent on Rdc values for positive impulse polarity. Earth electrode 4 has a significant reduction in discharge time for both impulse polarities. Table 9. Summary of the Effect of Soil Resistivity on Discharge Time for Negative Impulse Polarity (based on Figures 16-21).

1
Discharge time decreases with increasing applied voltage for all earth electrodes installed at all sites. It is observed that discharge time is the highest for earth electrodes installed at site 2, followed by sites 1 and 3. Large differences between discharge times for sites 1, 2 and 3; i.e., difference between discharge times for earth electrodes 1 and 2 at site 1 is as large as 400 µs for the same peak current magnitudes.   Figures 16-21).

1
Faster discharge time as applied voltage is increased for all earth electrodes installed at all sites for negative impulse polarity, but for positive impulse polarity, only earth electrodes installed at sites 1 and 2 have a similar relationship, that there is faster discharge time at higher applied voltage. For earth electrodes installed at site 3, discharge time is mostly not dependent on applied voltage. For the same earth electrode, distinctive differences are seen between the discharge times for earth electrodes installed at sites 1 and 2, or between sites 2 and 3 for negative impulse polarity, reaching differences of hundreds of microseconds, while for positive impulse polarity, large differences in discharge time are only observed for earth electrodes installed at sites 2 and 3. Small differences, or close results for time for current to discharge to zero between earth electrodes installed at sites 1 and 2 when subjected to positive impulse.

Impulse Impedances, Z impulse
Impulse impedance (Z impulse ) values have been used in several studies [15][16][17] to indicate the degree of reduction of Z impulse when subjected to high impulse currents. In [17], impulse impedance values for earth electrodes with high Rdc, when subjected to positive impulse polarity, were found to decrease with increasing current magnitudes. Similar trends are seen for negative impulse polarity as shown in Figures 22-24 for sites 1, 2 and 3 respectively, where the higher the Rdc, the higher the dependence of Z impulse to currents. When Z impulse versus peak current is plotted for each earth electrode at all sites, it is observed that earth electrodes installed at site 2 had the highest Z impulse , followed by earth electrodes installed at sites 1 and 2 (see Figures 25-30 for earth electrodes 1, 2, 3, 4, 5 and 6 respectively). It is also observed that for the same peak current and earth electrodes, large differences in Z impulse values are seen between earth electrodes installed at sites 2 and 1, and sites 1 to 3; i.e., percentage differences between Z impulse of sites 1 and 2 for earth electrodes 1, 2, 3, 4, 5 and 6 at a peak current of 2 kA are around 55, 20, 28, 25, 22 and 24% respectively for negative impulse polarity. It is observed that the percentage differences between earth electrodes installed at sites 1 and 2 of Z impulse are not dependent on the percentage differences of their Rdc values, as presented in Table 2, where the percentage differences between Rdc values for sites 1 and 2 for earth electrodes 1, 2, 3, 4, 5 and 6 are 36, 44, 42, 21, 38 and 39% respectively.
Results of Z impulse for both positive and negative impulse polarities for site 3 are presented in [16], where it was found that earth electrodes subjected to negative impulses had higher Z impulse than Z impulse subjected to positive impulses for earth electrodes with high Rdc (earth electrodes 1, 2, 3 and 4). In this paper, it is observed that grounding systems with high Rdc experienced impulse polarity effects in terms of their Z impulse . It is found that Z impulse values under a negative impulse are higher than for a positive impulse, for earth electrodes 1, 2 and 4. For earth electrode 3, only site 2, which had a higher Rdc than that of site 1, experienced an impulse polarity effect. The finding that impulse polarity effects are seen in high resistivity soil is similar to those obtained in [11,15]. The impulse polarity effect trend seen in high resistivity soil could be caused by a high electric field in high resistivity, as seen in an earlier paper [17], where an electric value for earth electrodes installed in site 2 can be up to 70% higher than the electric field of the same earth electrodes installed at sites 1 and 3. Tables 11-14 summarize the Z impulse values for various earth electrodes and soil resistivity, subjected to negative and positive impulse polarities.
at all sites, it is observed that earth electrodes installed at site 2 had the highest Zimpulse, followed by earth electrodes installed at sites 1 and 2 (see Figures 25-30 for earth electrodes 1, 2, 3, 4, 5 and 6 respectively). It is also observed that for the same peak current and earth electrodes, large differences in Zimpulse values are seen between earth electrodes installed at sites 2 and 1, and sites 1 to 3; i.e., percentage differences between Zimpulse of sites 1 and 2 for earth electrodes 1, 2, 3, 4, 5 and 6 at a peak current of 2 kA are around 55, 20, 28, 25, 22 and 24% respectively for negative impulse polarity. It is observed that the percentage differences between earth electrodes installed at sites 1 and 2 of Zimpulse are not dependent on the percentage differences of their Rdc values, as presented in Table 2, where the percentage differences between Rdc values for sites 1 and 2 for earth electrodes 1, 2, 3, 4, 5 and 6 are 36, 44, 42, 21, 38 and 39% respectively.
Results of Zimpulse for both positive and negative impulse polarities for site 3 are presented in [16], where it was found that earth electrodes subjected to negative impulses had higher Zimpulse than Zimpulse subjected to positive impulses for earth electrodes with high Rdc (earth electrodes 1, 2, 3 and 4). In this paper, it is observed that grounding systems with high Rdc experienced impulse polarity effects in terms of their Zimpulse. It is found that Zimpulse values under a negative impulse are higher than for a positive impulse, for earth electrodes 1, 2 and 4. For earth electrode 3, only site 2, which had a higher Rdc than that of site 1, experienced an impulse polarity effect. The finding that impulse polarity effects are seen in high resistivity soil is similar to those obtained in [11,15]. The impulse polarity effect trend seen in high resistivity soil could be caused by a high electric field in high resistivity, as seen in an earlier paper [17], where an electric value for earth electrodes installed in site 2 can be up to 70% higher than the electric field of the same earth electrodes installed at sites 1 and 3. Tables 11-14 summarize the Zimpulse values for various earth electrodes and soil resistivity, subjected to negative and positive impulse polarities.                       [17]).

Site
Comparison between Negative and Positive Impulse Polarities 1 Similar trend for both positive and negative impulse polarities are seen, higher Z impulse for high Rdc, and reduction of Z impulse is seen most significant for earth electrodes 1 and 4 (high Rdc). 2 3   Table 13. Summary of the Effect of Soil Resistivity on Impulse Impedance, Z impulse , under Negative Impulse Polarity (based on Figures 25-30).

1
Higher Z impulse values for all earth electrodes installed at site 2 than for earth electrodes installed at site 1. Z impulse values decrease significantly with increasing magnitudes of current for earth electrode 1 installed at sites 1 and 2, while Z impulse values are almost linear with increasing current magnitudes for earth electrode 1 installed at site 3 (low soil resistivity).

Earth Electrodes Comparison between Negative and Positive Impulse Polarities
1 For earth electrode 1, installed at site 1, Z impulse values are higher than for that installed at site 2 when subjected to positive impulse polarity, but for negative impulse polarity, Z impulse values are higher for earth electrodes installed at site 2, than for those at site 1 for all earth electrodes. For both impulse polarities, Z impulse decreases significantly as the current increases for all earth electrodes installed at sites 1 and 2, while Z impulse values are almost constant as the current magnitudes increase for all earth electrodes installed at site 3. A significant difference in Z impulse between positive and negative polarities is seen for earth electrodes 1 and 4, installed at both sites 1 and 2, which could be due to high Rdc (see Figures 25 and 28).

Conclusions
Impulse polarity effects were investigated for various earth electrodes, installed at three sites, where the analysis was done based on their time to peak current, discharge time and Z impulse . Current dependence of time to peak current was observed, which was higher at low current magnitudes and time to peak current became constant, and was independent of current magnitudes at high magnitudes of current. No specific trend pertaining to impulse polarity on the time to peak current was observed. Discharge time was found to be dependent on current magnitudes, which decreased with increasing currents. However, different discharge times under impulse polarity effects were observed. A significant impulse polarity effect was seen in the discharge time for earth electrodes installed at site 2 (high resistivity soil), where it was found to be slower in negative impulse polarity than in positive impulse polarity. Potentially, this was due to a coarser type of soil with high soil resistivity, providing more air voids than the other two sites (sites 2 and 3), which had no obvious time dependence for current to discharge to zero for different impulse polarities. Different relationships for Z impulse were also observed, in that Z impulse values had a high degree of non-linearity for high Rdc, in comparison to lower Rdc. Differences in impulse polarity were also seen in earth electrodes with high Rdc; higher Z impulse values were seen in negative impulse polarity in high Rdc than positive impulse polarity, with no obvious dependence on impulse polarity seen for earth electrodes with low Rdc. These differences in impulse polarity seen in high Rdc are due to high resistivity. The higher electric field in high resistivity, as seen in earlier publications, leads to higher negative impulse polarity than that of positive impulse polarity in high Rdc. However, there is no observable impulse polarity effect for earth electrodes installed in low resistivity soil (low Rdc).

Conflicts of Interest:
The authors declare no conflict of interest.