Measurement of impulse grounding impedance in frequency domain

Yang Lin Jianmin Li Wu Guangning Sichuan Electrical power Test and Research Institute School of Electrical Eng.SWJTU School of Electrical Eng.SWJTU Chengdu, China

Chengdu, China gnwu@swjtu.cn yangcome@163.com

Abstract— Impulse grounding impedance cannot

be described as an absolute index, because it

depends on the characteristic of the impulse

current waveform, soil structure type and

grounding system configuration, etc. This paper

presents a test system to measure the impulse

grounding impedance directly in time domain,

moreover, calculated and defined in frequency

domain. A field test for 110kV tower footing has

been performed to measure the impulse ground

impedance, and compared it with the ground

impedance measured by fall-of-potential method.

Key word- grounding impedance, impulse current,

transient characteristic

I. INTRODUCTION

Grounding device plays an important role in power system protection, it is often necessary to evaluate the performance of grounding system. The behavior of grounding systems at nominal frequency and DC current is well understood from the previous studies[1-3].Impulse characteristic, is more complicated as it involves frequency -dependence. The impulse grounding impedance cannot be described as an absolute index, because it depends on the waveform of the impulse current, soil structure type and grounding system configuration, etc[4-6]. Field measurement of grounding impulse impedance is an effective method to investigate the performance of grounding system when it dissipating lightning current. Grounding impulse impedance is calculated as Z=V/I after measured the grounding system dissipated impulse current and it's transient potential rise[7], but this definition of grounding impedance concealed the

impulse characteristics of grounding system which is verified by Silverio Visacro[8]. It is ideality that employ real amplitude voltage and current impulses to perform grounding impedance measurement which include nonlinear characteristics in impedance[3], but such measurement system need a mobile large capability impulse generator which could hardly be prepared in testing site. to solve this problem, This paper devises a test system to measure the impulse grounding impedance as follows: Firstly, records the impulse current and transient potential rise of ground system when it excited by impulse current in time-domain; And then, transform it from time-domain to frequency-domain through fast fourier transform(FFT); Last, calculate impulse ground impedance in frequency-domain. thereby impulse ground impedance can be analyzed more appropriately. A field test for 110kV tower footing has been performed to measure the impulse ground impedance, and compared it with the ground impedance measured by fall-of-potential method.

II. GROUNDING IMPULSE IMPEDANCE

DEFINITION

Generally, the grounding system impulse impedance is defined as[1,10]:

/i p pZ V I (1)

Or /p ip ipZ V I (2)

Where Vp, Vip is the peak of the applied voltage and the magnitude of applied impulse voltage at the instant of the peak of the impulse current,

2010 International Conference on Intelligent System Design and Engineering Application

978-0-7695-4212-6/10 $26.00 © 2010 IEEE

DOI 10.1109/ISDEA.2010.144

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2010 International Conference on Intelligent System Design and Engineering Application

978-0-7695-4212-6/10 $26.00 © 2010 IEEE

DOI 10.1109/ISDEA.2010.144

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respectively; Ip is the peak of the impulse current dissipating through the grounding system. It has also been defined as[11,12]:

( ) ( ) / ( )Z t V t I t (3)

Or /avg avg avgZ V I (4)

where V(t) and I(t) are the instantaneous value of applied impulse voltage and current; Vavg

and Iavg are the average value of applied impulse voltage and current. It was verified that these definition of grounding impulse impedance can not express the impulse characteristics of grounding system which appears to be a capacitive impedance and inductance impedance for high-resistivity soil and fast rising applied voltage and current or large scale of grounding system. We propose a new definition of impulse impedance in frequency domain as Z(f)=V(f)/I(f). It is deduced as follow: first of all, measure the ground impulse current and transient potential rise excited by impulse current in time-domain; and then, Transform the impulse voltage and current from time-domain(V(t),I(t)) to frequency-domain (V(f),I(f)); after all, it is easy to calculate the grounding impedance when the grounding system is excited by impulse current. The frequency-domain impedance of a grounding system is defined as:

( )( )( )

U kZ kI k

k=0,1,...n (5)

Where 2 f is the fundamental angular

frequency.

III. FIELD TEST

A. Test arrangement

The principle of fall-of-potential method is also applicable to TGR of grounding system excited by impulse current. The test arrangement is illustrated in fig.1.

Figure 1. Arrangement of impulse impedance test In Fig.1, ICG(Impulse current Generator) is charged voltage doubling rectifing circuit with AC source. So the discharging impulse voltage can be up to 2kV, and impulse current can be up to 500A by add charging capacitor. Oscilloscope records the transient potential difference between grounding electrode and reference electrode and the potential drop of R respectively. It should be noticed that there is no soil ionization during the test because of low amplitude of applied impulse voltage and current[9]. So the differential probe of oscilloscope can measure the voltage directly because of its range form -2600V to 2600V.

B. Interference of test

The test circuit employs shield wire as conducting wire, and the shield layer should be grounded at single terminal for preventing the noise signal which induced by electromagnetic coupling between the connecting wire. Note that the shield layer can not be double end grounded, because it will create a loop for impulse current. It is shown form Fig.2 that TGR measured in case that adopt insulated conductor as connecting line the have more fluctuation than the shield wire is.

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Figure 2. Influence of TGR caused by connecting wire

Electromagnetic interference caused by the corona can be eliminated by low pass digital filtering. In Fig3, the impulse current is filtered by low pass(<10MHz).

0.00000 0.00002 0.00004 0.00006 0.00008 0.00010

0

5

10

15

20

25

30

35

t (s) Figure 3. Digital low pass filtering of impulse current

IV. TEST RESULT

A 110kV transmission line tower footing which located at SiChuan power test and research institute was tested to verify the method presented by this paper. Fig.4 illustrated the impulse voltage and current of grounding electrode measured during the test.

Figure 4. Impulse current and voltage of 110kV tower

footing After converted the impulse current and voltage from time domain to frequency domain, it is easy to calculate the frequency response of grounding electrode. As it shown in fig 5, the impulse impedance is stable at low frequency, and maximizing at 0.7MHz. The regularity of impulse impedance in high frequency can be obtained by perform more filed test for different grounding systems in deferent condition.

Figure 5. Frequency domain impedance of 110kV tower

footing The AC grounding impedance of the same tower footing was measured by fall of potential method, the test result is shown in table 1. TABLE 1. COMPARISON OF IMPULSE IMPEDANCE AND AC

IMPEDANCE

method Grounding impedance/

Fall of potential 3.6 Impulse impedance/50Hz 3.5

It is seem form the table 1 that the AC impedance is close to impulse impedance at 50Hz, because there is no soil ionization occurred during the field test for impulse impedance. Although frequency response have advantage over the time response to express the performance of grounding systems, time response is also effective and more intuitive to understand the transient performance of grounding systems. The dissipating time that be defined as the time of grounding system retain in high potential is a rational and effective index to estimate the performance of grounding system.

V. CONCLUSION

In this paper, impulse grounding impedance of 110kV tower footing have been gained by directly measuring the impulse current and voltage when it excited by ICG. It is shown that the impulse grounding impedance at low frequency is close to AC power grounding impedance. For future work, laboratory test for soil ionization will be done and investigate out the

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correlation between field and laboratory tests.

REFERENCES

[1] Guangrun Xie. Grounding technology of electrical power system Water Conservancy and Electric Power publishing company 1996

[2] Rong zeng, Jinliang He. Novel Measurement System for Grounding Impedance of Substation IEEE TRANSACTIONS ON POWER DELIVERY,VOL.21,NO.2,APRIL 2006(reference) [3] IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System, IEEE Std. 81-1983. [4] Leonid Grcev. Impulse Efficiency of Ground Electrodes, IEEE TRANSACTIONS ON POWER DELIVERY, VOL.24, NO.1, JANUARY 2009:441-451 [5] M.Ramamoorty, M.M.Babu Narayanan, S.Parameswaran, D.Mukhedkar, Transient Performance of Grounding Grids, IEEE Transactions on Power Delivery[J], Vol.4, No.4, October 1989:2053-2059 [6] W.Xiong, F.P.Dawalibi. Transient Performance of Substation Grounding Systems Subjected to Lightning and Similar Surge Currents, IEEE Transactions on Power Delivery[J],Vol.9, No.3,July 1994:1412-1420 [7] Hai Wu, Cheng Gao, Xiao Liu. etc. TGR analysis of a kind of grounding electrode,2008 IEEE:pp:1185-1188 [8] Silverio Visacro. A Comprehensive Approach to the Grounding Response to Lightning Currents, IEEE Transactions on power delivery[J], vol.22,No.1,January 2007, pp:381-386 [9]A. M. Mousa. The soil ionization gradient associated with discharge of high currents into concentrated electrodes. IEEE Trans. Power Del, vol. 9, no. 3, Jul. 1994, pp: 1669–1677 [10] Jinliang He, Yanqing Gao, Rong Zeng et. Effective Length of Counterpoise Wire Under Lightning Current, IEEE Transactions on power delivery[J], vol.20,No.2,April 2005,

pp:1585-1591 [11] Y.Chen, P. Chowdhuri. Correlation between laboratory and field tests on the impulse impedance of rod-type ground electrodes.IEE Proceedings[J] online no. 20030246, pp:420-426 [12] P. Chowdhuri. Impulse impedance tests on laboratory model ground electrodes. IEE. Proceedings[J] online no.20030500, pp:427-433

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