Electrical Power and Energy Systems 45 (2013) 223–228
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Electrical Power and Energy Systems
journal homepage: www.elsevier .com/locate / i jepes
Field tests: Performances of practical earthing systems under lightning impulses
N. Mohamad Nor ⇑, S. Abdullah, R. Rajab, K. RamarFaculty of Engineering, Multimedia University Malaysia, Jalan Multimedia, 63100 Selangor, Malaysia
a r t i c l e i n f o a b s t r a c t
Article history:Received 27 April 2012Received in revised form 7 August 2012Accepted 29 August 2012Available online 13 October 2012
Keywords:Earthing systemsSoil ionisationNon-linearVoltage and current shapesTime delaysEarth resistance values
0142-0615/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.ijepes.2012.08.077
⇑ Corresponding author. Tel.: +60 383125387; fax:E-mail address: [email protected] (N. Mo
In this paper, experimental test set and test results on the performance of practical earthing systemsunder high impulse currents are presented where the earthing systems consist of 3 earthing systems con-figurations, 2, 3 and 4 rod electrodes. Calibration tests using linear test load in order to validate the trans-ducers adopted for this study are also discussed in this paper. This paper also presents the selection of theremote earth and some data from previously published work of the same site and earthing systems con-figurations under low magnitude currents and at power frequency. Soil ionisation characteristics on thereal earthing systems were investigated. The performance of earthing systems are investigated based onits voltage and current shapes, earth pre- and post-ionisation resistance values and time delays. It wasfound that under impulse conditions, earthing systems behave differently where the earth resistancewas found to be non-linear which are due to thermal and ionisation processes.
Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction
Investigations on the performance of earthing systems underhigh impulse currents have been analysed and can be found inthe literature using laboratory [1–5] and field measurements [6–9] and also analytical and computer models [10–14]. However,the approaches using the field tests are not yet widely publishedas compared to other types of techniques (laboratory and compu-tational methods) due to its cost and difficulty. Furthermore, in re-cent decades, there have been significant developments incomputer technology, which have led to the development of verypowerful computational tools for earthing systems [13,14], andmake the computational techniques more popular. The laboratoryscaled models [1–5] are also popular due to its advantage of a con-trolled environment, thus allow the study of various conditions,such as controlled moisture contents of soil [1,2], different typesof soil [1], and different grain size of soils [1]. However, as gener-ally known, the laboratory and computational methods do not rep-resent the real scenario of operating systems, since it affects bothvoltage and current distributions. This makes them less suitablefor the validation and to provide the full picture of the earthingsystems under field conditions. Moreover, the field site tests havenon-uniform soil structures with both lateral and vertical varia-tions which are difficult to be presented in laboratory and compu-tational methods. Thus, this gives difficulties when relatinglaboratory and computational with experimental at real field sites.Due to these reasons, this paper presents the experimental set and
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+60 383183029.hamad Nor).
test results of impulse tests on rod bed electrodes at real field site.Practical earthing systems can be of various electrode configura-tions namely hemispherical, mesh grid, counterpoise, rod beds,etc. In this paper, rod bed electrodes are used. Rod beds consistof a few rods, and connected by the copper strips. These experi-ments at field sites and using typical configurations of practicalearthing systems can therefore represent the real scenario of earth-ing systems characteristics under lightning response.
In this present study, there are 3 types of configurations used: 2,3 and 4 rod electrodes to provide variations in the earth resistancevalues. For the same site and earthing systems, the calculation andmeasurements of earth resistance values under low voltage and atpower frequency of these rod bed electrodes have been presentedin previously published work [15,16], where the aim of the study[15] is to validate and compare the earth resistance values usingthe calculations (which include the resistivity interpretation tech-nique of 2 layers soil model) and measurements using a DC meggeror under steady state condition. Results from the impulse testswith the maximum output voltage of 4 kV on the same earthingsystems can also be found in the literature [16], where the purposeof the study is to observe any correlations between the steady stateand impulse earth resistance values. However, the limitations inboth papers [15,16] are that Reference [15] only demonstratedthe earth resistance value for low frequency and at low voltagelevel. On the other hand Reference [16] discussed the impulse testsof only up to 4 kV of which may not represent the real scenario ofearthing systems performance under lightning response due toits low voltage. The ionisation process in soil due to high impulsecurrent as seen in the literature [1–9] is not clearly seen in thestudy [16] which could be due to limitations in its voltage level,
ights reserved.
224 N. Mohamad Nor et al. / Electrical Power and Energy Systems 45 (2013) 223–228
of which the impulse generator which could generate only up to4 kV was used.
Due to these reasons, this paper is aimed to investigate the perfor-mance of earthing systems under lighting response at much highervoltage levels, up to 50 kV. With the funding from Ministry ofScience, Technology and Innovation (MOSTI) Malaysia, this projectis aimed to achieve a better understanding of the conduction mech-anisms in the real earthing systems. Due to limitations in the impulsegenerator which can generate up to 50 kV, the practical earthingsystems are built of a small scale, which can be seen in Section 2.
In this paper, various aspects of the experimental set up are pre-sented. The experimental work which includes the calibration testusing the linear resistive liquid, the selection of the remote earthfor the safety purposes are also presented. Some results from pre-viously published work [15,16] of which using the same site andearth electrode configurations are also presented, as comparisonto this present paper. The voltage and current traces, impulse resis-tances and breakdown characteristics of the earthing systems testcell were also presented.
2. Experimental arrangement
2.1. Test set-up
A commercial Marx generator with voltage levels from 17.5 kVto 50 kV and produces a standard lightning response of 1.2/50 lswas adopted. Voltage measurement was obtained with a commer-cially voltage divider with a ratio of 1000:1 and a response time of12 ns. Current measurement was achieved with a commerciallyavailable current transformer of sensitivity 0.1 V/A and a responsetime of 20 ns. A commercially, 300 MHz Digital Storage Oscillo-scope (DSO) which has in-built facilities for data acquisition andanalysis was used to capture the voltage and current signals.Fig. 1 shows the circuit arrangement for the impulse testing. Thetriggering of the spark gap is achieved with a pulsed system. Thecompressed air is supplied from a gas bottle with a suitable pres-sure through a tube into the sealed spark gap. When the right pres-sure of the gas in the spark gap reaches the appropriate level,
CompressedAir Cylinder
DC-ChargingUnit
AC Diesel Generator
Marx G
DSO
High Voltage Probe
Curren
Elec
Fig. 1. Impulse
triggering can be achieved over a wide range of charging voltage.A DC charging unit was used along with this Marx generator tocontrol the operation of the impulse generator.
2.2. Earthing systems
Extensive work has been done on the calculation and measure-ments of earth resistance values under low voltage and at powerfrequency and under impulse voltages up to 4 kV for these 2, 3and 4 rod bed electrodes (see Fig. 2), which have been presentedin previously published work [15,16]. Each rod is of 14 mm diame-ter and with the length of 180 cm, and separated from each other by200 cm. However, Refs. [15,16] were conducted in year 2008. Whenthe Fall-of-Potential (FOP) tests were conducted again on the samesites, the readings are found to be slightly higher, which could bedue to many factors such as seasonal variation of soil resistivity,and loose contact of earth rod over the years. For the convenienceand comparison with the results of this study, the results from pre-vious work [15,16] and new results are shown in Table 1. In Refs.[15,16], the soil resistivity values were also conducted, which laterthe earth resistance values were obtained using the mathematicalcalculation. The results of the resistivity values obtained using theWenner method, which were interpreted using the Master Curvesand the earth resistance values obtained with mathematical calcu-lations are shown in Table 2. The results have been compared anddiscussed in detail in Refs. [15,16]. It was found from visual inspec-tion when the soil nearby is excavated that the upper layer, which isnear to the surface of the earth, has more like a mixture of clay,sandy and rock. Since it is closer to the surface, the soil is hotterthan the lower surface of soil, and most moisture content driedout. As for the second layer, which is more than 1 m underneaththe surface is more moist and of clay terra type. Due to this soil type,it is rather expected to have lower soil resistivity on the secondlayer.
2.3. Remote earth
ANSI/IEEE Std 81–2000 [17] discusses and outlines manyimportant aspects of earthing systems including measurement
enerator
t Transformer
trode under Test
Remote Earth
test circuit.
Fig. 2. Rod bed electrodes for (i) 2-rods, (ii) 3-rods and (iii) 4 rods configurations.
Table 1Earth resistivity and resistance values under steady state.
Rod-bedsconfigurations
Steady stateearth resistancevalues (X) bycalculations[15]
Steady state earthresistance values(X) bymeasurements in2008 [15]
New steady stateearth resistancevalues (X) bymeasurements in2010
2 rods 126.34 122.23 130.533 rods 94.58 95.27 105.574 rods 76.2 72.67 80.33
Table 2Soil resistivity profile.
Upper resistivity (Om) 780Lower resistivity (Om) 195Thickness, h (m) 1.3
N. Mohamad Nor et al. / Electrical Power and Energy Systems 45 (2013) 223–228 225
and testing of earthing system impedance and soil resistivity.However, this standard [17] is essentially based on the site inves-tigations at power frequency or steady state condition. Only asmall part of the chapter in the standard is contributed to theguideline on the method and instrumentation used for the testingof earthing systems under transients. It is stated in the standard[17] that the impedance of an auxiliary ground or remote earthwhich carries the return current from the impulse generator mustbe significantly lower than that of the measured ground. In thisstudy, the data from previous work [16] showed that the steady-state earth resistance values for all the configurations are in arange of 73–122 X. Since there is a nearby 4-floor apartment,about 200 m from the earthing systems under test, the earthingsystems of the apartment is used as the remote earth. In Fig. 1,the remote earth is placed on the right side of the diagram. Thisremote earth would be expected to be lower than the electrodeunder tests. It would be expected that the earthing systems ofthe apartment is lower, since its earthing systems is of mesh types,and have many more copper strips and rods. When the earth resis-tance value of the apartment is measured using the Fall-of–Potential(FOP) method, as expected the remote earth was found to be lower
than that of earthing systems under test (2, 3 and 4 rod electrodes),which is 25.2 X.
As for the tests on liquid resistor and surge arrester, due to highexpected currents, the remote earth used has lower earth resis-tance value, which is 6.23 X has been designed and investigatedin Ref. [18].
2.4. Calibration impulse tests
A commercially available high voltage (HV) probe and currenttransformer were mainly used for voltage and current measure-ments in this research. Both HV probe and current transformersadopted in this work were first calibrated in order to validate itssuitability for the test circuit and the characterisations of the earth-ing systems. In this test, a liquid resistor made of Copper Sulphate(Cu2SO4) was used. This resistive test load is suitable for testsunder high impulse currents due to its low inductance and highcurrent ratings. Figs. 3a and b show typical voltage and currenttraces on the resistive load at short time and long time scalesrespectively. As can be seen and as expected, there is only littleinitial overshoot on the voltage and current traces, and the currenttrace was found to coincide with the voltage trace at both front andtail impulse. This proved that the HV probe and current trans-former adopted in this study were capable of measuring fast charg-ing voltages and currents.
In this study, the resistance was measured as the ratio of volt-age at Ipeak (in this case it is equal to Vpeak), to the peak current Ipeak,as adopted in Refs. [1,2]. This measurement is used throughout thisstudy because at peak current, the rate of change of current is zero,thus the errors associated with inductive effects can be minimised.As expected, since it is a linear load, the resistance was found to befairly constant for different levels of voltages/currents, and equal to760 X (see Fig. 4). The results proved that the voltage and currenttransducers used provide an accurate and reliable measurementfor high magnitude fast impulse tests.
3. Investigations of earthing systems under impulse conditions
Impulse tests were conducted on three sets of earthing systems;2, 3 and 4 rod electrodes with different current magnitudes in
0
10
20
30
0 1 2 3 4 5 6 7 8
Vol
tage
(kV
)
0
30
60
90
Cur
rent
(A)
V
I
(a)
0
10
20
30
0 10 20 30 40 50 60 70 80 90Time (μs)
Time (μs)
Vol
tage
(kV
)
0
30
60
90
Cur
rent
(A)
V
I
(b)
Fig. 3. Voltage and current records for a resistive liquid resistor at an appliedvoltage of 30 kV captured with (a) short and (b) long time scales.
0
250
500
750
1000
0 10 20 30 40 50I peak (A)
R (Ω
)
Fig. 4. Resistances vs. current magnitudes for a resistive liquid load.0
10
20
30
-1 0 1 2 3 4 5 6 7 8 9Time (µs)
Volta
ge (k
V)
0
100
200
300
Cur
rent
(A)
I
V
(a)
(b)
20
30
V) 200
300
)
I
226 N. Mohamad Nor et al. / Electrical Power and Energy Systems 45 (2013) 223–228
order to determine the effect of soil ionisation process under high-impulse currents. For the purpose of a better understanding of soilbehaviour under high-impulse currents, the results are discussedaccording to current impulse shape, impulse resistance and timedelays of these earthing systems.
0
10
-10 0 10 20 30 40 50 60 70 80 90
Time (µs)
Volta
ge (k
0
100 Cur
rent
(A
V
Fig. 5. Voltage and current records for 3-rod electrodes at an applied voltage of24 kV captured with (a) 1 ls/div (a) and (b) 10 ls/div.
3.1. Voltage and current traces
For the impulse generator used in this study, the desired outputvoltage is obtained by controlling the pressure of the compressedair. From the manufacturer’s data, for the lowest pressure (bargauge) of the compressed air, the minimum output voltage is17.5 kV. Hence, the lowest voltage level applied to the earthingsystems is 17.5 kV.
Fig. 5a and b shows the voltage and current traces obtainedwith tests on earthing systems consisting of 3 rod electrodes whensubjected at an applied voltage of 30 kV for short and long time
scales respectively. Similar voltage and current traces were ob-served for the earthing systems with 2 and 3 rod beds, and at dif-ferent voltage levels. As can be seen in Fig. 5a and b, both voltageand current traces have fast rise times. However, when capturedwith short time scale, some initial oscillations were observed onthe current trace (see Fig. 5a). These initial oscillations are also ob-served in previously published work on the soil ionisation phe-nomenon in soil under high impulse currents [1,2]. As explainedin previous publications [1,2], these initial oscillations on thecurrent traces are thought to be caused by the capacitive effectsof small air spaces between the sand grains and at the interfacebetween the soil particles and the earth electrodes. These oscilla-tions were found to appear in all current traces, independent ofneither applied voltages nor the types of configurations used.
Fig. 5a also shows that at front time, it was observed that thecurrent trace leads the applied voltage for all configurations andcharging voltages. These observations are not seen when testedfor liquid resistive (see Part E). The current leads the voltage couldbe associated with the capacitive effects of the earthing systems.This shows that the earthing systems under high impulse condi-tions can be represented as capacitive and resistive circuit, asshown in the literature [1,10–12].
From previous laboratory and field work [1–9], when the earth-ing systems study were injected with high impulse currents, twoobservations were observed on its current traces, where most stud-ies [3–9] found with one current peak, and some publications [1,2]found with two current peaks. On the other hand, for the experi-mental work at field sites recorded so far, only Ref. [18] foundthe current with two current peaks, whereas the rest of the fieldwork [6–9] found with one current peak only. However, the trendof the resistances with current magnitudes is not discussed in
Table 3Measured Rdc and Rimpulse of test electrodes.
Rod-beds configurations 2-rods 3-rods 4-rods
RDC (O) 130.53 105.57 80.33Rimpulse,average, R1 (O) 101.85 83.07 74.26Rimpulse,average, R2 (O) 71.83 55.6 42.4Difference between RDC and R1 (%) 22 21 7.56Difference between RDC and R2 (%) 45 47.33 47.13
8
N. Mohamad Nor et al. / Electrical Power and Energy Systems 45 (2013) 223–228 227
previous paper [19]. In this present study, when impulse tests wereconducted on the earthing systems at field site, two current peakswere observed, where the voltage shapes decrease smoothly. Thetwo current peaks could be associated with the thermal and ioni-sation processes as suggested in the literature [1,2]. Since thereare two current peaks observed in this study, hence there aretwo resistances derived, pre- and post-ionisation resistances,which will be discussed in the next part.
At tail times, it was observed that the voltage decays moreslowly than current. This could be due to high impedance of earth-ing systems on the tail, hence slowing down the voltage decay.
0
2
4
6
0 5 10 15 20 25 30V at corresponding t (kV)
Tim
e (µ
s)
2rods_t13rods_t14rods_t12rods_t23rods_t24rods_t2
Fig. 7. Time to current peak vs. the peak current magnitudes.
3.2. Resistance of earthing systems
As mentioned in Section 2, Part D, the resistance is measured asthe ratio of V at Ipeak to the Ipeak throughout this study. Since twocurrent peaks were observed, the resistance values were measuredusing the two peak currents and their corresponding instantaneousvoltages, as defined in previously published work [1,2], where thefirst peak current is used to define for the pre-ionisation resistanceR1 and the second current peak is used to define for R2, post-ioni-sation resistance. It has been well discussed in the literature [1,2]that the R1 is associated with thermal process and R2 is mainlycaused by the ionisation processes.
Fig. 6 shows the resistance values obtained for different voltagelevels. As expected, both R1 and R2 for 4 rod electrodes are the low-est, since it has the most number of rods. Resistance values, R1
were found to decrease with increasing current magnitudes whichcould be due to a non-linear behaviour of earthing systems underhigh impulse conditions. Similar pattern of reduction of earthresistance was also observed in the literature [1–12]. In Refs.[1,2] particularly, the reduction of earth resistance with increasingcurrents are explained as due to thermal processes, where due toI2R heating in soil, the temperature of the soil increases, which willincrease the conductivity and reduce the resistivity of the soil,hence reduce the earth resistance value. However, R2 are almostconstant, independent of current magnitudes for all rod bed config-urations. This could be due to a fully developed ionisation region insoil when at R2, thus earthing systems becomes very conductive,and has similar characteristics to a linear load.
For easier analysis, Table 3 presents the comparison of earthresistance values obtained at low and high voltage conditions forR1 and R2. The difference in R1 is highest in 2-rods configurations,followed by 3-rods and the least earth resistance reduction is for4-rods. This finding is similar to what was found in literature [1–9] that the reduction of earth resistance depends on its RDC, wherethe higher the RDC, the higher the decrease in earth resistance val-ues under impulse conditions. Also, as can be seen, the earth resis-tance values are found to decrease under high impulse conditions,
0
20
40
60
80
100
120
0 100 200 300 400
Ipeak1(A)
R ( Ω
)
2-rods_R1
3-rods_R14-rods_R1
2-rods_R2
3-rods_R2
4-rods_R2
Fig. 6. Resistances vs. current magnitudes of 2, 3 and 4 rod electrodes.
and the decrease is the highest at R2 when the soil is considerablyfully ionised at second current peak. It is also observed that the dif-ference in R2 for all earthing systems is close, around 45–48%.
3.3. Time delay
In many studies [1,2,19,20], time delays are estimated to de-scribe the thermal and ionisation processes occurring in soil underhigh impulse current conditions. Time delays are generally definedas the time to peak currents. Since two peak currents were ob-served in this present study, two time delays were defined, t1 isthe time corresponded to the first peak current, and t2 is the timecorresponded for the second peak current. It was observed inpreviously published work [1,2] and rather expected that the timedelays are the lowest/fastest for high conducitivity soil and athigher currents. However, these present tests revealed that thetime-to-peak currents (time delays) t1 are the lowest for the high-est steady-state earth resistance (2-rod bed), which are followedby 3-rod and 4-rod beds configurations (see Fig. 7). Also t1 and t2
are independent of voltage/current magnitudes for all configura-tions. Similar trend was observed when both t1 and t2 are plottedagainst its corresponding voltages. Fig. 6 shows the relationshipbetween the time-to-current peak, t1 and t2 and the amplitude ofthe voltages at its corresponding time, V at t1 and V at t2 for threeearthing systems consisting of rod electrodes (2, 3 and 4 rods). Thispattern of time delays is different than that is described in the lit-erature [1,2,19,20], where in these references, the time delays werefound to decrease with increasing currents/voltages. The differencein the pattern of time delays could be due to uncontrolled effects ofthermal and ionisation processes, non-uniform water settling pro-cess, drying and heating in wet sand, and of course, the statisticalnature of air breakdown inside the soil.
4. Conclusions
Investigations on the performances of real/imitative earthingsystems under lightning impulses are conducted. Before the im-pulse tests on the earthing systems, calibration tests were first
228 N. Mohamad Nor et al. / Electrical Power and Energy Systems 45 (2013) 223–228
conducted on linear and non-linear test loads in order to see theeffectiveness of the transducers, and the results are discussedand presented in this paper. It was found that the transducersare suitable for the impulse test in this study.
When impulse tests were conducted on the earthing systemsconsisted of 2, 3 and 4 rod electrodes, it was found that the earthresistances of the poor earthing systems (2 rod electrodes) werefound to be the highest. The non-linearity effect or the reductionof earth resistance values as the currents increased was also ob-served in this study. For the current responses, it was found thatthe relation between the time to peak current (time delay) andthe current peak magnitudes is different that those seen in litera-ture, which could be due to a few factors. A lot more studies arehoped to be achieved by the authors in future by comparing the re-sults of various configurations and resistivity of earthing systems, inorder to better understand the characteristics of earthing systemsunder high impulse conditions, particularly for the real field site.
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