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  :: Transient impedance of grounding rods
  :: °ü¸®ÀÚ 2007-06-13 17:17:12 , Á¶È¸ :662101  
  :: File download   [pdf : 1373 KB  Download: 8592]
TRANSIENT IMPEDANCE OF GROUNDING RODS
 
I. F. Gonas F. V. Topalis 1. A Stathopulos
National Technical University of Athens
Depanment of Electrical and Computer Enginee ring, High Voltage Laboratory
 

Abstract: The aim of this paper is the correlation of the transient impedance and its parameters ,vith the statiollary resistance of simple grounding systems. Impulse current tests of -the standard form-Si20 IJ.s were performed on several types of equilateral triangles and single driven rods of different lengths. The injected current in the grounding system and the developed potential were recorded, resulting in the determination of the time "ariation of the transient impedance. Further mathematical analysis of the experimental results led to simple linear relations between the parameter of transient impedance and the stationary resistance. The results provide useful information for the design of a grounding system a nd the measures for the protection of instal lations from lightning strokes.

Key words: Grounding system, transient impedance, stationary resistance.

 
1. Introduction
The grounding systems serve multiple purposes. Not only the) do insure a reference potential point for the electric and electronic devices but also provide a low reSIstance path for fault currents into the earth. Such fault currents Call arise either from internal sources or from ex.ternal ones e.g. by lightning strokes and industrially-generated static electricity. The
resistance of grounding systems has an essential influence on the protection of the grounded system. Grounding systems can consist of one or more vertical or horizontal ground rods, three or more vertical ground rods connected to each other and two or three¡¤dimensional grids from metal rods and foundation grounding systems.

The behaviour of the grounding system under lightning determines the degree of protection provided. This makes obvious the purpose of analysis procedures predicting the trans ient response of grounding systems. If an equivalent circuit approach is adopted these procedure can be implemented in a simulation model [1-7].

The specific value of impulse impedance which is of main interest is tlle one corresponding to the beginning of the steep ascent for the wave-froot. The resu lts reveal its value to be quite higher than the stationary value of its ground
resistance and reduces to this latter value [3, 5].

The work presented in this paper refers to the prob lem of transient analysis of practical grounding systems cons isting of grounding rods under impulse lightning currents.
 
2. Fundamentals
The driven rod is one of the simplest and most economical form of electrodes. The stationary resislance R of a driven rod is given by the following fonnula [7]:
When the electrode voltage changes with time, there will be a conductive current in add ition to a capacitive current. The equivalent circuit of a driven rod under impulse current is shovm in Fig. 1. The resistance R of the rod is given by the Eq. (1) and the inductance L of such a rod is equal to [7, 8]:


where:
Z1 , is the maximum value of the ratio of impulse voltage to impulse current. Z2 is the ratio of tho maximum value of voltage to the respecti ve value of current when voltage reaches its maximum, Z3 is the ratio of maximum value of voltage to the maximum value of current and Z4 is the ratio of voltage when current reaches its maximum to the maximum value of cu rrent.

It is obvious:
ZI >Z2 > Z3 >Z4>R             (9)
A lightning discha rge affects the resistance of a grounding system in two ways. The current is up to 100 kA or more and has a much higher frequency spectrum than the stationary case.
The transient impedance becomes greater as:
¡Ü the inducti vity of the wire and of the connection becomes grenter
¡Ü the high value of current can dry the ground,
¡Ü the high frequency spectrum shortens the electrical length of long grounding wires
¡Ü the skin effect rises the resistance and the inductivity of wires due to the value of the frequency.
The transient impedance becomes smaller as the electrical field strength on the surfuce of grounding system can reach values where predischarges in the ground start: these discharges can lead to ground ionisation that destroy layers with high resistance [4].
 
3. Experimental apparatus and test techniques
The layouts of grounding system (Fig. 3) werle tested experimental ly under impul se lightning current of waveshapc 8/20 ¥ìs The maximum value of the current vas varying up to 3 kA The first grounding layout was a single driven rod and the second one was an equilateral triangle with three vertical rods. Cooper rods with diameter 20 mm were used, The measured value
of the earth res istivity was found to be equal to 30 .§Ù . m. The wavetbnns of the impulse cunent and of the potential of grounding system were recorded directly by a data acquisition system controlled by a personal computer, with
measuring bandwidth of 20 MHz.
 
4. Test results
The measurements values of peak volrage. peak current and impedance often different grounding layouts are presented in Table 1,
In these figures, the test results for the grou nding system of a driven rod wi th diameter 20mm and length 75cm arc presented. The waveforms of the injected current is Show in Fig. 4 The measured potential with reference to the ideal earth is shovm in Fig 5. The transient impedance of the grounding system under this stress is the one of Fig. 6.

 
5. Conclusions
The perfonned measurements, show that the transienr impedance reaches its maximum value very fast (fraction of microsecond) and consecutively is reduced (0 rhe value of the stationary resistance. the one corresponding to
the beginning of the steep ascent for the wave~ front. The results reveal the value of the transient impedance to b ~ quite higher than the stationary resistance. The determined analytical relations between the parameters of the transient impedance and the stationar) resistance anow the limitation or even elimination of time and money consuming experiments. It will also facilitate the optimisation of any planned grounding system. The computer aided
optimisation of grounding systems is very useful, since the improvement of them after their installation is a difficult task and sometimes not possible,
 
References.
[1] SufIis, S.A., Gonos, l.F :. Topahs, F.Y. and Stathoplilos LA.: Transient behaviour of a horizontal grounding rod under impulse current", Recent Advances in Circuits and Systems, Word Scientific Publishing Company, Singapore, 1998, pp. 61 ~ 64,

[2]Suflis, S.A., Gonos, I. F., Topaiis, F. Y. and Stathopulos I. A.: "Transient behaviour of a horizontal grounding rod under impulse current", 2nd International Conference on Circuits, Systems and Computers (lMACSCSC"
98), October 1998, Piraeus, Greece, pp. 289¡¤292

[3] Gonos, l.F., Antoniou, M.K., Topalis, F.V. and Stathopulos I. A. : "Behaviour of a grounding system under impulse lighming current", 6lh International Conference and Exllibition on Optimisation of Electrical and
Electronic Equipment (OPTIM 98), May 1998, Brasov, Romania, pp. 171 ~ 174.

[4] Bogensperger, H.J., Frei, J. and Pack, S.: Resistance of grounding systems, stationary and transient behaviour", 9th lntematiom S}mposium on High Voltage Engineering, August! 995, Graz, Austria, pp. 6715-1-4.

[5] Verma, R. and Mukhedkar D .. "Impulse impedance of buried ground wire", iEEE Trans. on Power ApparatUs and Systems, [980, PAS¡¤99 (5) pp. 2003¡¤2007.

[6]Meliopoulos, P.A. and Moharam, G.M. "Transient Analysis of Grounding S}stems", iEEE Trans. on Power Appdratus and ,Iystems, [983, PAS¡¤[02 (2) pp .389¡¤397.

[7] Kalifa, M.; "High Vorlage Engineeri ng, Theory and Praclice", Dekker, USA, 1990, pp.3 31¡¤356.

[8] Gupta, R.B., and Thapar, B, "Impulse Impedance of Grounding Grids", IEEE Tran s. on Power Apparatu~ and Systems, 1980. PAS,99 (6) pp. 2357¡¤2362.

Address of Authors
National Teclmica! University of Athens
Dept. of Electrical and Computer Engineering
42, Patission Str., GR~1068 2 Athens, Greece
Tel.: + 30 ~1- 7723539, 7723627, 7723582 .
Fax.: +30¡¤ 1¡¤ 7723628. 7723504
Email .: igonos@soft lab:ece.ntua.gr
topalis@Softlab.ece.ntua.gr
stathop@:power. ece.nlua.gr
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