Transient Ground Impedance Measurement Using a Very Short Current Lead

Alexander Barros Lima, Jos Osvaldo S. Paulino*, Wallace C. Boaventura

Departamento de Engenharia Eltrica Universidade Federal de Minas Gerais

Belo Horizonte, Brasil josvaldo@cpdee.ufmg.br*

Maurissone Ferreira Guimares Companhia Energtica de Minas Gerais (CEMIG-D)

Belo Horizonte, Brasil mauris@cemig.com.br

AbstractThis work describes a system for the measurement of the transient impedance of ground electrodes using a portable impulse generator and a new measuring arrangement. The innovation is the use of a very short current reaction lead, which is possible due to the use of transmission line with very low propagation velocity (3% of the velocity of light). The methodology for obtaining the transient impedance from the measurements is discussed and validated. The experimental results regarding transient impedance measurements for two ground electrodes arrangements are presented. The first arrangement consists of two vertical rods and the second one is an actual 138 kV transmission line tower leg, composed by a vertical electrode in parallel with a horizontal wire. A comparison with calculated values for both cases is also presented.

Keywords lightning protection; transient ground impedance; transmission line

I. INTRODUCTION The grounding impedance of transmission line towers plays

an important role in the lightning performance of such lines. Usually, when a poor performance is observed, a survey is carried out to measure the grounding impedance of each tower. This task is cumbersome, time consuming and expensive. Recently, several efforts have been made to facilitate the measurement of grounding impedance [1,2]. This work presents some improvements in this field, proposing the use of very short current leads. The measuring arrangement is similar to that presented in [1,2], which, instead, uses long reactions leads for the potential and/or current measurements.

The new proposed methodology uses for the current reaction lead a transmission line with a very low propagation velocity, around 3% of the velocity of light in vacuum. In this study, this line will be named LIA (acronym in Portuguese for Artificial Infinite Line). Concerning the potential lead, two circuits are evaluated: i) a 20 m long, 4 mm2 isolated copper wire and ii) 6 m long LIA.

The feasibility of the proposed methodology was verified trough field tests using the developed prototype. It is a complete measuring system consisting of a portable repetitive impulse generator, four LIAs, 6 m long each, and a digital

oscilloscope to record the transient waveform. A patent registration was filed for the developed system. The first field test dealt with the measurement of transient ground impedance of two vertical rods 0.5 m long. An actual 138 kV transmission line tower leg was used in the second field test. It consist of a 30 m horizontal long electrode associated to a vertical electrode 3.5 m long (the tower foundation). For both cases, the transient ground impedance was derived and presented. A comparison with calculated values is also presented. The calculated values were obtained considering both fixed values for the soil parameters (resistivity and permittivity) and also frequency dependent.

The remaining of paper is organized as follows. Section II presents the details of the experimental setup used in the field tests and the derivation of the measured transient ground impedance. The comparison regarding the calculated results is presented in Section III and the conclusions are stated in Section IV.

II. THE EXPERIMENTAL SETUP The details of each component of the developed measuring

system and the results of the field tests are presented in the following.

A. Artificial Infinite Line (LIA) The new proposed current reaction lead, LIA, is a

transmission line made with a thin wire wrapped around a non-conductive pipe, as shown in Fig. 1. The propagation velocity can be adjusted controlling the number of turns per-unit-length. Benefiting from the reduced propagation speed, using the proposed current lead, the transient ground impedance value can be evaluated up to times around 10 s, for the case of a 20 m long LIA. For the sake of comparison, such long observation time can only be obtained using conventional leads when lengths around 1500 m are used.

Fig. 1.Transmission line with low propagation velocity (LIA) used as current or potential leads.

This work was financed by P&D 270 - Development of a System forMeasuring the Transient Ground Impedance of Transmission Line StructuresUsing impulsive waves; CEMIG- ANEEL; UFMG; CNPq (Brazilian NationalCouncil for Scientific and Technological Development) and by FAPEMIG(Minas Gerais State Research Foundation).

2013 International Symposium on Lightning Protection (XII SIPDA), Belo Horizonte, Brazil, October 7-11, 2013

978-1-4799-1344-2/13/$31.00 '2013 IEEE 177

Fig. 8. Measuring arrangement #1.

The several components of the developed measuring system used for the first site experiments are shown in Fig. 9. The impulse generator, batteries, digital oscilloscope, vertical rod and the two LIAs used as current and potential leads can be easily identified.

Fig. 9. Measuring site #1 located at the campus of the Federal University of Minas Gerais.

Fig. 10 shows the resulting measured current and voltage obtained using the LIAs, 12 m long, for both, current reaction lead and potential lead. The measured transient ground impedance, computed according to (1), is shown in Fig. 11. The time of arrival (3.0 s) of the reflected current wave, at the LIA open end, is marked in Fig. 10 with a vertical dotted line.

The transient impedance is defined as:

( )( )

( )

v tZ t

i t= , (1)

where, Z(t) is the transient impedance, t is the time, v(t) is the voltage in the ground electrode and i(t) is the current, obtained through the voltage and current probes depicted in Fig. 9.

Fig. 10. Measured voltage and current. Grounding grids consisting of one and two rods, in parallel.

Fig. 11. Measured transient impedance for one and two vertical rods, in parallel.

The grounding resistance (RDC) of the rods were measured using a conventional megger and the obtained values are 430 and 289 , considering one and two rods, respectively. The measured value for the apparent soil resistivity was 335 m. Regarding the proposed methodology, the transient impedances values, measured at 3 s, were 350 and 210 for one and two rods, respectively. As one can note, the measured transient impedance value at 3 s is around 75% of the resistance values. The differences between the measured and calculated values will be discussed in Section III.

D. Transient Impedance Measurement Site #2. The ground electrode arrangement in site #2 is shown in

Fig. 12. It is a leg of a 138 kV transmission line tower (vertical electrode 3.5 m long inside a concrete cylindrical structure with a diameter of 1 m) connected to a buried 30 m long horizontal electrode. Fig. 13 shows a picture of the tower foundation. The obtained value for the soil apparent resistivity was 1600 m, which considers the fact that the measurement was done in rainy day and the soil was saturated with water.

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Fig. 12. Ground electrode arrangement for site #2: horizontal wire 30 m long and a 3.5 m vertical wire, inside a concrete cylindrical structure.

Fig. 13. Concrete foundation of a 138 kV transmission line tower.

For this site, a different measurement arrangement was used. It is shown in Fig. 14 and a picture of the measuring site is presented in Fig. 15.

This measuring arrangement is similar to that of site #1. A LIA was used as a current lead but, differently, an insulated copper wire, 20 m long, was used as a potential lead.

Fig. 14. Measurement arrangement for site #2.

Fig. 15. Measuring site #2 located in the City of Jaboticatubas, where a 138 kV transmission line is being built. The measurements were done in a muddy rainy day.

The measured current and voltage are shown in Fig. 16, while Fig. 17 shows the resulting transient ground impedance. The time of arrival (1.5 s) of the reflected current wave due to the open end of the current LIA is highlighted with a dotted line. The grounding resistance measured using a megger is RDC = 65 and the transient impedance value measured at 1.5 s is Z = 32 . The transient impedance value, at 1.5 s, is around 50% of the measured resistance value. This difference is discussed in Section III.

Fig. 16. Measured voltage and current. Grounding grid consisting of one tower foot and a horizontal wire, 30 m long.

Fig. 17. Measured transient impedance for a 138 kV tower foot in parallel with a horizontal wire, 30 m long.

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Fig. 21. Calculated transient impedance for a 138 kV tower foot, 3.5 m long, in parallel with a horizontal wire, 30 m long. The soil parameters are frequency invariant.

Concerning the time behavior of the calculated impedance, the initial part exhibits the transient impedance value and the late time gives the low frequency resistance value of the ground electrode. As shown in Fig. 21, the late time impedance value is Z = 63 , which agrees quite well with the measured value using a megger, RDC = 65 . The difference is less than 3%.

Regarding the transient impedance, the calculated values were initially compared with the measured ones in Fig. 22. The calculation was done considering the soil resistivity equal to 1600 m and the relative permittivity equal to 80. At 1.5 s, the difference between the calculated and the measured value is around 20%.

The comparison between measured and calculated values is finally done considering the soil parameters frequency dependent, using the same approach as for the cases in site #1. The result is presented in Fig. 23.

Fig. 22. Measured and calculated curves comparison. The calculated transient impedance for a 138 kV tower foot was done considering the soil parameters frequency invariant.

Fig. 23. Measured and calculated curves comparison. The calculated transient impedance for a 138 kV tower foot was done considering the soil parameters frequency dependent.

As shown in Fig. 23, the measured values match very well with calculated values for times higher than 0.5 s. For lower times, the oscillations in the measured curve are due to the voltage wave reflections on the potential lead, since for this case, an insulated copper wire, 20 m long, was used. These oscillations do not appear in case of the vertical rods, because in that situation the potential lead was a LIA and up to 1.5 s, there are not reflected waves at the measuring point.

IV. CONCLUSIONS The proposed transient impedance measurement

methodology is simple and the use of very short leads presents great advantages, especially in terms of time saving. As shown, even in a muddy rainy day, the measurements could be easily accomplished.

The overall performance of the proposed methodology was evaluated through measurements for two cases: a simple grounding arrangement, consisting of two short vertical rods, and for a complex arrangement, consisting of an actual tower foot of a 138 kV transmission line, which is composed by a vertical electrode, 3.5 m long, inside a concrete structure, in parallel with a 30 m long horizontal wire. For both cases, the obtained measurements results were consistent. Additionally, the measured values were compared to calculated ones and matched very well.

The proposed low propagation velocity lines (LIAs) are low cost, low weight and very simple to built, presenting an interesting alternative to the conventional long leads. It is important to mention that the LIAs can be used either as a current or potential lead. The portable repetitive impulse generator built is safe, very simple to operate and also low cost.

Concluding, this new methodology can be very efficient, especially in urban environment and in areas with dense vegetation where the use of long leads is very cumbersome. The authors expect to present some results further evaluating the methodology in near future.

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of low frequency resistance and lightning impulse impedance on transmission towers, in Proceedings of the X International Symposium on Lightning Protection, 9 a 13 de novembro de 2009, Curitiba, Brasil.

[2] Nayel, M.; Zhao, Jie; Ametani, Akihiro; He, Jin-Liang L.; Cai, Zongyuan; Wang, Qi. A study of zero reference effects on electromagnetic transient measurements of a grounding electrode. IEEE International Symposium on Electromagnetic Compatibility, EMC 2006.

[3] Alexander Barros Lima, Jos Osvaldo Saldanha Paulino, Wallace do Couto Boaventura, Maurissone Ferreira Guimares. A simplified method for calculating the tower grounding impedance by means of PSPICE. ICLP 2012 28th International Conference on Lightning Protection, 2-7 September, Vienna, Austria.

[4] James H. Scott. Electrical and magnetic properties of rock and soil. United States Department of the Interior Geological Survey. Open-File Report 83-915. Prepared in cooperation with the U.S. Air Force, 1983.

[5] Alpio R., Visacro, S.; Frequency Dependence of Soil Parameters: Effect on the Lightning Response of Grounding Electrodes. IEEE Transactions on Electromagnetic Compatibility, volume 55, issue 1,pages 132-139, 2013.

[6] Jos Osvaldo Saldanha Paulino, Wallace do Couto Boaventura, Alexander Barros Lima, Maurissone Ferreira Guimares. Transient voltage response of ground electrodes in the time-domain. ICLP 2012 28th International Conference on Lightning Protection, 2-7 September, Vienna, Austria.

[7] Fagan, Eugene J. ; Lee, Ralph H.; The Use of Concrete-Enclosed Reinforcing Rods as Grounding Electrodes. IEEE Transactions on Industry and General Applications. Volume IGA-6, Issue 4, pp. 337-348, 1970.

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