Electrical Power and Energy Systems 43 (2012) 156–161

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Electrical Power and Energy Systems

journal homepage: www.elsevier .com/locate / i jepes

Comparison between utility sub-station and imitative earthing systems whensubjected under lightning response

N. Mohamad Nor ⇑, S. Abdullah, R. Rajab, Z. OthmanFaculty of Engineering, Multimedia University, Jalan Multimedia, 63100 Cyberjaya, Selangor, Malaysia

a r t i c l e i n f o

Article history:Received 8 June 2010Received in revised form 1 February 2012Accepted 20 April 2012Available online 18 June 2012

Keywords:Utility sub-stationEarthing systemsLow magnitude currentsPower frequencyImpulse currents

0142-0615/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.ijepes.2012.04.042

⇑ Corresponding author.E-mail address: normiza.nor@mmu.edu.my (N. Mo

a b s t r a c t

A comparison study between the utility substation and imitative earthing systems under lightningresponse is investigated. The utility sub-station is a non-operational sub-station, and the imitative earth-ing system is the earthing system that is purposely built to emulate the real earthing systems. However,no equipment is placed or connected to the earthing systems. The imitative earthing system is of a smal-ler size and is built separately at a different site. Both sites were designed according to the IEEE standard80. Earth resistance measurements at low magnitude currents and at power frequency were also con-ducted for both earthing systems based on the IEEE standard 81. When impulse tests were conductedfor both configurations, surprisingly the post-ionisation earth resistance of the imitative earthing systemunder lightning response was found to be similar to those tested under low magnitude currents. How-ever, for the utility sub-station, the earth resistance under lightning response was found to be higher thanthat designed at steady-state conditions. The results in terms of time delays were also found to be differ-ent than that found in literature.

Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

It is necessary that earthing systems are designed with a low-magnitude earth impedance so that the overvoltage protection de-vices can divert high fault currents effectively to earth. A numberof publications [1–16] investigated the earthing system perfor-mance by experimental or computational methods. However,some authors [5–8] allow for non-linear behaviour and the others[1–4,9–16] did not incorporate the effect of non-linear behaviourinto their investigations.

Non-linear soil behaviour under high impulse currents has beenobserved by many researchers using the field test techniques[5–7]. Based on previous work [5–7], there are two observationsto describe the non-linear soil behaviour:

(i) Voltage and current peaks occurred at different time.(ii) Reduction of earth resistances with current magnitudes and

lower than the steady state conditions.

Generally, it was found in the literature [5–8] that if there is areduction of earth resistance, the voltage and current peaks arelikely to occur at different time due to non-linear behaviour of soilunder high impulse conditions.

012 Published by Elsevier Ltd. All r

hamad Nor).

However, not all studies on the performance of earthing sys-tems observed this reduction of earth resistances, and differentpeak times for voltage and current traces when subjected underhigh impulse currents. Some studies [9,13] found that the earthresistance values are almost equal to that measured at low magni-tude currents and at power frequency, and independent of currentmagnitudes. The voltage and current peaks are also found to occurat the same time.

On the other hand, some studies [14–16] found that the impulseimpedance values were higher than the power frequency imped-ance. Gupta and Thapar [15] and Gupta and Singh [16] howeverused scaled model in a tank filled with an electrolyte in the labora-tory to simulate for this analysis. They also found that the voltageand current traces have the same rise and decay times.

It has been presented in a few standards [17–20] on the earthresistance formulas for different sizes and configurations of earth-ing systems for uniform soil layer or resistivity at low magnitudecurrents and at power frequency or steady-state conditions. Gener-ally, it is known that the earth resistance can be reduced by havinglarger dimensions and more conductors in the earthing systems.However, previously published work [5–16] showed that underhigh impulse currents, the earth impedance of the earthing sys-tems can become lower, constant and higher than those obtainedfrom its steady state, where they are dependent on the size ofthe earthing systems. The larger the size of the earthing systems,the less dependent it becomes on the current magnitudes. Thiscan be explained by the simple relationship between electric field,

ights reserved.

N. Mohamad Nor et al. / Electrical Power and Energy Systems 43 (2012) 156–161 157

E, and current density, J, E = qJ, where q is the soil resistivity. Dueto a large dimension of the earthing systems, thus gives lower J,since J = I/A where I is current and A is the cross-sectional area. Thisreduces the electric field to be less than critical electric field, Ec, theparameter which initiated the non-linearity in soil, and contrib-uted to the knowledge of the degree of resistance reduction in soil.This shows that it is not necessarily correct to assume that an in-crease in the dimensions of the earthing systems, can give a betterperformance of earthing systems. Also, large dimensions of earth-ing systems can contribute to the inductive effects which can wor-sen the performance of earthing systems when subjected underhigh impulse currents.

This paper is therefore aimed to investigate the performance oftwo combined grid-rod earthing systems in order to better under-stand on the non-linearity effect which can lead to the optimumdesign of earthing systems.

2. Experimental arrangement

2.1. Test set-up

A commercially available Marx generator with voltage levelsfrom 17.5 kV to 50 kV and produces a standard lightning responseof 1.2/50 ls was adopted. Voltage measurement was obtained witha commercial voltage divider with a ratio of 1000:1 and a responsetime of 12 ns. Current measurement was achieved with a currenttransformer of sensitivity 0.1 V/A and a response time of 20 ns. Acommercially, 300 MHz Digital Storage Oscilloscope (DSO) whichhas in-built facilities for data acquisition and analysis was usedto capture the voltage and current signals.

Fig. 1 shows the circuit arrangement for the impulse test. Trig-gering of the spark gap is achieved when the right pressure of thecompressed gas in the spark gap reaches the appropriate level.

Fig. 1. Impulse

Minimum output voltage for this Marx generator is 17.5 kV. A DCcharging unit was used along with this Marx generator to controlthe operation of the impulse generator.

2.2. Construction of earthing systems

As stated in IEEE/ANSI standard 80 [18], mesh grids which con-sists of combination vertical and horizontal conductors are typi-cally used by utility in their sub-stations, particularly GasInsulated Substation (GIS) for the reasons of:

(i) Horizontal grids are only buried not more that 0.5 m, whichcaused the performance of earthing systems to be influencedby the upper layer of soil only. Thus, long ground rods whichcan reach a few more layers under the surface can stabilisehigh potentials.

(ii) It is observed in many publications [17–19] that lower layerssoil are more effective in dissipating high fault currents dueto its lower soil resistivity (which is thought to contain morewater and it is not influenced by the temperature andweather like in the upper layer soil). Thus, extra long groundrods are necessary to be added to the horizontal earth gridsto reach for lower soil layers.

(iii) Based on the earth resistance formulas at power frequencygiven in the standard [14] , it shows that at power frequency,having more conductors, it would be expected that the cur-rents dissipate faster into the ground, thus more effective

In this study, the earth resistance measurements are conductedon the combined mesh grid-rod earthing systems using a Fall-of-Potential (FOP) method. It was found that the earth resistance va-lue for the utility substation is 0.3 X. This sub-station is a not yetoperational, and the lay-out of the sub-station earthing system ispresented in Fig. 2. This Gas Insulated Switchgear (GIS) substation

test circuit.

Fig. 2. Combined grid-multiple rods electrode (utility sub-station earthingsystems).

Fig. 3. Combined grid-multiple rods electrode (imitative earthing systems).

158 N. Mohamad Nor et al. / Electrical Power and Energy Systems 43 (2012) 156–161

is used for the 132/275 kV systems. The size of this substation isaround 64 m � 76 m. The burial depth of the grid is 0.3 cm andthe multiple rods depth is between 3 m and 5.4 m under the sur-face of the soil.

The study on the combined mesh grid of the imitative earthingsystems has been presented before in previously published work[18]. Fig. 3 shows the schematic for the imitative earth electrode.The area of the grid is 10 m � 10 m. The burial depth of the grid is0.5 m, and the multiple rods depth is 3 m. The measured earth resis-tance was found to be 6.23 X. However, the measurement was con-ducted in year 2008. It is stated in the standard [1,15–16] that theearth resistance measurement should be conducted periodically,since the earth resistance value could be influenced by the weather,close contact of the electrode with the earth, moisture content in soiletc. When the FOP method was conducted again, it was found thatthe value is 6.41 X, which is slightly higher than that in Ref. [18].This shows that in practice, the earthing systems need to be period-ically checked and measured. Both earthing grids and rods are madeof copper. From the FOP, it is found and rather expected that theearth resistance for utility substation is lower than that of imitativeearthing systems, due to its larger size of utility sub-station.

2.3. Remote earth

It is stated in the standard [17,18] that for impulse tests on theearthing systems, the impedance of an auxiliary ground or remote

earth which carries the return current from the impulse generatormust be significantly lower than that of the measured earthing sys-tems. In this study, however, since the earthing system is of meshgrid type, the earth resistance at steady-state would be expectedto be low. Thus, it would be a challenging task to build anotherearthing system with lower earth resistance value for the remoteearth. Due to that, a nearby earthing system to each site is used asthe remote earth, where for utility sub-station, since there is a near-by tower, the tower footing is used as remote earth, and for the imi-tative grid, the earthing systems of a nearby 4-floor apartment, andabout 200 m from the site is used. When the earth resistance mea-surements are conducted using the Fall-of-Potential method on theremote earth, it was found that the tower footing resistance is2.13 X. Since there are two towers close together, they are con-nected together with a copper strip, in order to reduce the earthresistance value, which is reduced to 1.2 X. On the other hand, theearth resistance for the apartment is 25.2 X, which is at 100 m awayfrom the electrode under test is used as the remote earth when con-ducted impulse tests on the imitative earthing systems.

Even though these remote earth resistance values were found tobe higher than the electrode under test for both impulse tests,throughout the test, no sparking or any danger or irregularities oc-curred. Thus, this remote earth can be considerably safe to be usedfor this study. Perhaps for higher level of applied voltage, a safer re-mote earth is required.

3. Investigations of earthing systems under impulse conditions

Impulse tests were conducted on both earthing systems. Differ-ences between these two sites can be seen in its voltage and cur-rent impulse shape, impulse resistance and time delays.

3.1. Voltage and Current Traces

Fig. 4a and b shows the voltage and current traces obtained fortests on imitative and utility’s sub-station earthing systems whensubjected at charging voltage of 30 kV at 2.5 ls. For both earthingsystem, impulse tests were conducted at different voltage levels.The voltage and current traces at different voltage levels of imita-tive earthing systems are similar to Fig. 4a. Similarly, when im-pulse tests were conducted on utility sub-station at differentvoltage levels, similar voltage and current traces were observedas in Fig. 4b at different voltage levels. Impulse tests were con-ducted at various points of injected points. The results in termsof voltage and current traces, and its resistance values were foundto be the same for all injected points’.

As can be seen in both figures, two current peaks are more sig-nificant for the utility sub-stations, even though utility sub-stationhas much lower earth resistance values. This result is contradict towhat has been found in the literature [19] where in previouslypublished work [19], the lower the earth resistance value, the lesssignificant the first current peak.

However, the two current peaks are not so observable for theimitative earthing systems. When the voltage and current tracesare plotted with a shorter time scale, 1 ls (see Fig. 5), only thenthe two current peaks become more observable. This shows thatthe time to peak current for both peak currents for imitative earth-ing system is shorter than that in utility sub-station. The fastertime delays for the imitative earthing systems as compared to util-ity sub-stations even though the latter has lower earth resistancevalue could be due to the inductive effects in the utility sub-sta-tion. This time delay will be further discussed in Section 3.2.

It can be seen from Figs. 4 and 5 that that the initial oscillationson the current trace that were observed in the previous literature[12] which were thought to be due to capacitive effects of small

Fig. 4. Voltage and current records at charging voltage of 30 kV, at 2.5 ls/div.

Fig. 5. Voltage and current records for imitative earthing systems at chargingvoltage of 30 kV, at 1 ls/div.

Fig. 6. Impulse resistance as the ratio instantaneous voltage to instantaneouscurrent vs. time.

N. Mohamad Nor et al. / Electrical Power and Energy Systems 43 (2012) 156–161 159

air spaces between the sand grains and at the interface betweenthe soil particles and the earth electrodes are not seen in these re-sults. This could be due to low steady-state earth resistance valuesof both earthing systems which are below 10 X, thus the earthingsystems become more resistive. A similar behaviour is also seen inthe previous literature [20] where for highly water-saturated sand(sand mixed with 15% or more), which has higher conductivity, theinitial oscillations were not observed. This could be attributed tothe high conductivity of the test medium and was also explainedby the high resistive current that dominates in this high conductiv-ity medium. The same authors also found in other publication [21]that for low resistivity media (for sand mixed with water contentand salt), no oscillation was observed.

From the voltage and current traces shown in Figs. 4 and 5, itwas observed that both voltage and current have fast rise times.At tail times, as the current rises to its second peak, the voltage de-cays faster than the current, which could be due to lower earthresistance values for both earthing systems which are below 10 X.

In some publications [22–24] the earth resistance/impedanceunder high impulse currents is defined as the ratio between

instantaneous voltage and current, against the current, voltage ortime. In their analysis [22–24], they found that the earth resis-tance/impedance values under impulse are higher at the front time,and higher than that of steady-state values. However, at tail time, asthe voltage and current are discharged to the ground, the earthresistance was found to decrease significantly. Similar observationswere found in this present study when the ratio of instantaneousvoltage to current are plotted against time for both utility sub-sta-tion and imitative earthing systems (see Fig. 6a and b respectively).High resistance values are seen at the front time of the voltage andcurrent traces. The earth resistance was found to be higher for utilitysub-station as compared to imitative earthing systems.

3.2. Resistance of earthing systems

In this study, since two current peaks were observed, thus tworesistances were defined, pre- and post-ionisation resistances (R1

and R2 respectively) as proposed in Refs. [12,20,21] where:

R1 ¼V@Ipeak1

Ipeak1ð1Þ

and

R2 ¼V@Ipeak2

Ipeak2ð2Þ

This measurement is selected so that to eliminate any inductiveeffect in the test sample, since at the instant of peak current, di/dt = 0.

Fig. 7a and b shows the earth resistances for R1 and R2 respec-tively for both earthing systems. It would be expected that the R1

and R2 for utility sub-station is lower than that of imitative earth-ing systems since it has lower steady-state earth resistance value.However, as can be seen in both Fig. 7a and b, the resistance values

Fig. 7. Resistances vs. current magnitudes for both utility and imitative earthingsystems.

Fig. 8. Time to current peak vs. the peak current magnitudes.

160 N. Mohamad Nor et al. / Electrical Power and Energy Systems 43 (2012) 156–161

for utility sub-station are much higher than that of imitativeearthing systems. Both resistances R1 and R2 are also found to beindependent of current magnitudes. The average of R1 for utilitysub-station is 92 X, and for imitative earthing system is 42 X,which are higher than the steady-state earth resistance values.These results are similar to that found by Refs. [20,21]. This showsthat the earth resistance value at steady-state has differentcharacteristic than that under impulse conditions. Much higherR1 as compared to its steady state values could be due to the heat-ing process in soil as described in Ref. [20]. If excessive heating insoil occurs, water vaporization in soil usually takes place. Thisreduces the conductivity, and, hence, increases the resistivity ofthe soil, thus increasing the resistance instead of reducing it asthe current magnitude is increased. It could also be explained bythe relation E = qJ as given in Section 1 of this paper, where dueto a large dimension of the earthing systems, thus gives lower J,since J = I/A. This reduces the electric field to be less than criticalelectric field, Ec, hence no non-linearity in soil occurred. Thus high-er resistance values, R1 than its steady state values. This simpleequation could also explain the reason of higher R1 in utility sub-station as compared to imitative earthing systems. This could bedue to larger cross-sectional area of the utility sub-station thanthat of imitative earthing systems. Thus lower electric field inutility sub-station as compared to imitative earthing systems,hence gives higher R1 in utility sub-station as compared to imita-tive earthing systems.

On the other hand, the average value for R2 is 14 X for utilitysub-station and 6.5 X for imitative earthing systems. Again, theformer has higher R2 value despite it has lower steady-state earthresistance value than that of imitative earthing systems. This againcould be explained from the simple relationship of E = qJ, wherethe E, which determines its non-linearity, is lower than Ec due toits large cross-sectional area, thus no ionisation is expected tooccur. Thus, increasing the earth resistance value to be greater thanits steady-state value. It should also be noted here that the averageR2 for imitative earthing systems is close to its steady-state value.This shows that in designing the earthing systems, it is not

necessarily to have a low earth resistance value, since its steadystate may have different characteristics than that under impulseconditions. In utility sub-station, a large combined rod and meshdesign is required for the reasons mentioned in Section 2.2.

This paper shows that the earthing systems should not be over-designed, which can introduce larger areas, which contribute tohigher inductive effect and lower electric field than critical electricfield, which can produce higher earth resistance value under highimpulse conditions than that of steady-state earth resistance.However, in the studies conducted by Poljak and Dorik [25,26]found that the higher the length of the earth electrode, the lowerthe input impedance.

3.3. Time delay

In many studies [8,20,21], time delays are generally defined asthe time to peak currents. In previous work [8,20,21], time delaysare used to estimate for thermal and ionisation processes occur-ring in soil under high impulse current conditions. Since two cur-rent peaks were observed for both sites, two time delays weredefined in this study where t1 for the first peak current, and t2

for the second peak current. Fig. 8a and b shows the time delays,t1 and t2 for both earthing systems. It was observed in previousstudy [20] of which also observed two current peaks that both t1

and t2 were found to be lower for higher conductivity soil (lowerDC earth resistances), and to decrease with voltage magnitudes.These are similar to the findings obtained by Snowden and Erler[27]. However, in this study, it was found that for lower steady-state earth resistance value (utility sub-station), higher timedelays were observed. This could be due to larger dimension ofutility sub-station, thus contributes to higher inductive effects,hence slower time delays as compared to imitative earthingsystems. It was also observed that both time delays for bothconfigurations are independent of voltage/current magnitudes.This shows that these large earthing systems are not influencedby ionisation process. Similar findings were also observed in otherstudies [10–12] where for large earthing systems, no ionisationwas found to occur.

N. Mohamad Nor et al. / Electrical Power and Energy Systems 43 (2012) 156–161 161

4. Conclusions

In this study, the characteristics of two combined grid-multiplerods electrode which are imitative and utility sub-station earthingsystems are investigated under low- magnitude low-frequencycurrents and under high impulse current conditions. It was foundthat the earth resistance value under steady state of utility sub-sta-tion is lower than that of imitative earthing systems. However, un-der impulse conditions, the resistance values are vice versa.

As for the time delays, it was found that time delays in utilitysub-station are higher than that imitative earthing systems, de-spite the former has lower steady-state earth resistance values.These findings are also found to be different than that publishedin previous work. This shows that further studies are required inorder to obtain better understanding on the characteristics ofearthing systems under high impulse conditions.

Acknowledgements

The authors of this paper wish to thank Malaysian Utility Com-pany (Tenaga Nasional Berhad), especially Mr. Halil Haron and MrZulkifli Othman for their valuable time and effort in arranging forthe field tests in the sub-station. Also, the authors wish to thankMinistry of Science, Technology and Innovation (MOSTI) of Malay-sia for the financial support.

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