Field tests: Performances of practical earthing systems under lightning impulses

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<ul><li><p>th</p><p>gor,</p><p>l tets arlectdy ata frnituinvpesconr w</p><p>ce of enalyse] and er modare noiques (difcusignicd to th</p><p>for the validation and to provide the full picture of the earthingsystems under eld conditions. Moreover, the eld site tests havenon-uniform soil structures with both lateral and vertical varia-tions which are difcult to be presented in laboratory and compu-tational methods. Thus, this gives difculties when relatinglaboratory and computational with experimental at real eld sites.Due to these reasons, this paper presents the experimental set and</p><p>of 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 [19] is not clearly seen in thestudy [16] which could be due to limitations in its voltage level,</p><p> Corresponding author. Tel.: +60 383125387; fax: +60 383183029.</p><p>Electrical Power and Energy Systems 45 (2013) 223228</p><p>Contents lists available at</p><p>n</p><p>.e lE-mail address: normiza.nor@mmu.edu.my (N. Mohamad Nor).powerful computational tools for earthing systems [13,14], andmake the computational techniques more popular. The laboratoryscaled models [15] 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 suitable</p><p>measurements 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 purpose1. Introduction</p><p>Investigations on the performanhigh impulse currents have been athe literature using laboratory [159] and also analytical and computthe approaches using the eld testsas compared to other types of techntational methods) due to its cost andcent decades, there have beencomputer technology, which have le0142-0615/$ - see front matter Crown Copyright 2http://dx.doi.org/10.1016/j.ijepes.2012.08.077arthing systems underd and can be found ineld measurements [6els [1014]. However,t yet widely publishedlaboratory and compu-lty. Furthermore, in re-ant developments ine development of very</p><p>test results of impulse tests on rod bed electrodes at real eld site.Practical earthing systems can be of various electrode congura-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 eld sites and using typical congurations of practicalearthing systems can therefore represent the real scenario of earth-ing systems characteristics under lightning response.</p><p>In this present study, there are 3 types of congurations used: 2,3 and 4 rod electrodes to provide variations in the earth resistancevalues. For the same site and earthing systems, the calculation andEarth resistance valuesField tests: Performances of practical ear</p><p>N. Mohamad Nor , S. Abdullah, R. Rajab, K. RamarFaculty of Engineering, Multimedia University Malaysia, Jalan Multimedia, 63100 Selan</p><p>a r t i c l e i n f o</p><p>Article history:Received 27 April 2012Received in revised form 7 August 2012Accepted 29 August 2012Available online 13 October 2012</p><p>Keywords:Earthing systemsSoil ionisationNon-linearVoltage and current shapesTime delays</p><p>a b s t r a c t</p><p>In this paper, experimentaunder high impulse currengurations, 2, 3 and 4 rod educers adopted for this sturemote earth and some dagurations under low magreal earthing systems wereits voltage and current shafound that under impulsewas found to be non-linea</p><p>Electrical Power a</p><p>journal homepage: www012 Published by Elsevier Ltd. All ring systems under lightning impulses</p><p>Malaysia</p><p>st set and test results on the performance of practical earthing systemse presented where the earthing systems consist of 3 earthing systems con-rodes. Calibration tests using linear test load in order to validate the trans-re also discussed in this paper. This paper also presents the selection of theom previously published work of the same site and earthing systems con-de currents and at power frequency. Soil ionisation characteristics on theestigated. The performance of earthing systems are investigated based on, earth pre- and post-ionisation resistance values and time delays. It wasditions, earthing systems behave differently where the earth resistancehich are due to thermal and ionisation processes.</p><p>Crown Copyright 2012 Published by Elsevier Ltd. All rights reserved.</p><p>SciVerse ScienceDirect</p><p>d Energy Systems</p><p>sevier .com/locate / i jepesights reserved.</p></li><li><p>of which the impulse generator which could generate only up to4 kV was used.</p><p>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.</p><p>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 congurations 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.</p><p>2. Experimental arrangement</p><p>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.</p><p>2.2. Earthing systems</p><p>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 andwith 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 Curves</p><p>Compressed DC-Charging Marx G</p><p>ren</p><p>lec</p><p>224 N. Mohamad Nor et al. / Electrical Power and Energy Systems 45 (2013) 223228Air Cylinder Unit</p><p>DSO</p><p>High Voltage Probe</p><p>Cur</p><p>E2.1. Test set-up</p><p>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,</p><p>AC Diesel GeneratorFig. 1. Impulseand 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-tionwhen 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 ismoremoist and of clay terra type. Due to this soil type,it is rather expected to have lower soil resistivity on the secondlayer.</p><p>2.3. Remote earth</p><p>ANSI/IEEE Std 812000 [17] discusses and outlines manyimportant aspects of earthing systems including measurement</p><p>enerator</p><p>t Transformer</p><p>trode under Test</p><p>Remote Earthtest circuit.</p></li><li><p>er anN. Mohamad Nor et al. / Electrical Powand 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 signicantly 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 congurations are in arange of 73122X. Since there is a nearby 4-oor 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 ismeasured using the Fall-ofPotential(FOP) method, as expected the remote earth was found to be lower</p><p>Fig. 2. Rod bed electrodes for (i) 2-rods, (ii)</p><p>Table 1Earth resistivity and resistance values under steady state.</p><p>Rod-bedscongurations</p><p>Steady stateearth resistancevalues (X) bycalculations[15]</p><p>Steady state earthresistance values(X) bymeasurements in2008 [15]</p><p>New steady stateearth resistancevalues (X) bymeasurements in2010</p><p>2 rods 126.34 122.23 130.533 rods 94.58 95.27 105.574 rods 76.2 72.67 80.33</p><p>Table 2Soil resistivity prole.</p><p>Upper resistivity (Om) 780Lower resistivity (Om) 195Thickness, h (m) 1.3than that of earthing systems under test (2, 3 and 4 rod electrodes),which is 25.2X.</p><p>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 6.23X has been designed and investigatedin Ref. [18].</p><p>2.4. Calibration impulse tests</p><p>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 transformers</p><p>3-rods and (iii) 4 rods congurations.</p><p>d Energy Systems 45 (2013) 223228 225adopted in this work were rst 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.</p><p>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 to760X (see Fig. 4). The results proved that the voltage and currenttransducers used provide an accurate and reliable measurementfor high magnitude fast impulse tests.</p><p>3. Investigations of earthing systems under impulse conditions</p><p>Impulse tests were conducted on three sets of earthing systems;2, 3 and 4 rod electrodes with different current magnitudes in</p></li><li><p>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, inde...</p></li></ul>