65Earthing Measurements for Power Line Towers

EARTHING MEASUREMENTS FOR POWER LINE TOWERS

Stanisław Wojtas / Gdańsk University of Technology

1. INTRODUCTION

Earthing is an important and necessary element of any energy system. Properly designed and constructed earthing guarantees safety for both people and devices located in places where a flow of dangerous short circuit or surge current caused by a lightning discharge can occur. Therefore, the earthing resistance should be made as low as possible, and its value should meet the guidelines contained in the specified standards and regula-tions.

During its construction and later operation, the earthing should undergo periodic inspection, mainly through measurements of its resistance. Control tests of the resistance carried out using the traditional method are often very time consuming, especially for earthing of power line towers. For example, a 100 km section of 110 kV line may consist of more than 300 towers; the earthing of each should be tested at least every four [2] or five [5] years. It is therefore important that the tests can be carried out without disconnecting the lines. The measurement time for one tower should be as short as possible, and the instruments should be light and easy to transport.

The purpose of this paper is to describe the procedures for measuring and assessing the earthing for pow-er transmission line poles equipped with lightning conductors. The subject of analysis is primarily the influence of the span length and front time of the used measurement impulses on the results of earthing impedance. The presented results of theoretical calculations and computer simulations have been supplemented with measure-ments on real objects.

2. CLASSIC METHODS FOR ASSESSMENT OF LINE TOWER EARTHING

Static resistance of earthing of line towers is usually determined using meters operating at low frequency and implementing various types of technical methods. In the case of high voltage transmission line towers, their earthings are connected in parallel by lightning conductors, as shown in fig. 1 Therefore, there are two main methods of measurement: disconnection of artificial earth electrode from the tower and using a meter equipped with a current clamp.

2.1. Disconnecting the earth electrode from tower structureWhen using a low frequency excitation, the control terminals should be disconnected for the time of

measurement, and thus the connection between the artificial earth electrode and the tower structure should be removed. Such a procedure is quite cumbersome and requires removing four connections – one at each leg of

Abstract

The elements of artificial earth electrode of the line tower and its foundation participate in the discharge of short circuit or lightning current to the ground. Both ear-thing elements should be taken into consideration during the control measurements of resistance or impedance of such earthings. In addition, the measuring procedu-re must take into account the fact that the earthings of transmission poles are connected in parallel by lightning conductors. The article discusses the issue of measuring and assessing the features of power line pole earthing using slow- and fast-changing waveform. The measure-ments of earthing resistance of the poles using meters

with frequencies similar to those in the network are cumbersome and laborious. The influence of earthings of adjacent poles can be reduced by using wave impedance of lightning conductors at high frequency or impulse wa-veforms. As a result of comparing the two methods based on fast-changing waveforms, it turns out that impulse meters are much more resistant to interferences caused by electromagnetic fields of the lines. The study analysed the effect of impulse front time and the line span length on errors made during these meassurements using impul-se meters.

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the tower (fig. 1b). In addition, the resistance value obtained in this way is caused solely by the artificial earth electrode, whereas the natural foundation earth electrode does not affect the measurement result. It should also be noted that such measurements should be made when the line has been turned-off.

Fig. 1. The connection of measured earth electrode, including the bypass effect of adjacent towers – a) and foundations of the tower with ring earth – b)

In real earthing systems, the foundation earth electrode can significantly affect the resultant value of earthing resistance and determine the final assessment of the measurement result. The measurement results for the tower in the ground with a resistivity of about 200 Ωm shown in fig. 2. show that such a situation may take place. When measuring the resistance of an artificial earth electrode separated from the tower, a result Rs equal to 18 Ω was obtained, which is too high a value in relation to the standard requirements [1]. The resist-ance value of the analysed tower foundation is 12 Ω, and the parallel connection of both earthing elements gives the value of 7,7, which means that the requirements of the aforementioned standards are met.

foundationsearthing ring

control terminals

2.2. The use of a meter equipped with a current clampA special type of technical method is implemented using a current clamp meter. When using such a me-

ter, the test terminals are not disconnected, and the current generated in the meter flows to the ground in the system of connected earthings and is divided into two parts. One of them passes through the tested conductor and earth electrode, while the other (IS) through the rest of the earthing system. The above case is illustrated in fig. 3. The measurement result is determined on the basis of the part of the current that flows through the tested earthing. The voltage drop is determined in relation to the auxiliary probe placed in the zone of reference potential.

a) b)

Fig. 2. The results of static resistance for earthing of 110 kV line tower with the lightning conductor disconnected from its structure: R – resistance of the parallel connection of the foundation and artificial earth electrode, Rs – artificial earth electrode resistance, Rf – tower foundation resistance

Stanisław Wojtas / Gdańsk University of Technology

67

Measurements of tower earthings using this method are possible only when the meter is equipped with current clamps with a very large diameter to cover a single leg of transmission line tower. In order to determine the tower earthing resistance, four separate partial measurements should be made, one for each leg of the tower. The final result is determined by calculation as a parallel connection of the measured partial resistances.

Electricity generated in the meter enters the tower structure at the point of galvanic connection (P). From there, the current spreads in all directions through conductive structure of the tower. Part of the current flowsupward as IS and flows to the other towers in the system through lightning protection wire. The rest of the cur-rent flows into the tested earthing and then to the ground through the four legs of the tower. Hence, the currentflowing into the ground is the sum of currents from I1 to I4 in individual legs of the tower. Voltage U designated in relation to the zone of reference potential should have the same value for the measurements of each leg of the tower. Therefore, the differences in the results of those measurements can be caused only by the differ-ences in currents discharged to the ground by each leg of the tower. So, it can be stated that the total earthing resistance of the tower is a result of the parallel connection of partial resistances obtained for each leg:

(1)

However, it should be noted that the above dependence is correct only if all the partial measurements have the same point P connecting the meter to the tower [10].

1

4321

1111

RRRR

R

3. IMPULSE METHOD

3.1. Measuring principleThis method allows measuring the earthing of transmission line towers using a suitable measuring instru-

ment without disconnecting the earthing from the tower construction. In most cases, the span length in lines exceeds 150 m, and the wave impedance Zfp in the conductor – ground system is equal to about 500 Ω [9]. Dur-ing the measurements the tested earthing with impedance Zx is bypassed with wave impedances Zfp of lightning conductors running to the two adjacent towers and wave impedances Zfs of the towers, as shown in fig. 4. Theimpedance value for earthing of each tower is marked as Zu and Zx.

3I 2I

1I

4I

sI

Pv Pi

MRCPP

Fig. 3. Determination of static resistance of a high voltage transmission tower using current clamps, where: MR – resistance meter, CP – current clamps, P – galvanic connection point, PV, P – auxilliary voltage and current probes

Earthing Measurements for Power Line Towers

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Fig. 4. Pole earthing with adjacent poles and selected values of wave impedances for each element of the system

In such a system the impedance value at the terminal of measured earthing Z can be calculated according to the following formula:

(2)

Fig. 5. shows the effect of bypassing the adjacent towers during the earthing measurement Zx as a func-tion of this earthing. The relative error of the measured value Zm as a result of bypassing is determined based on the formula (2) as (Zx – Zm)/Zx. Calculations were made using the following assumed values of impedances: Zfp = 500 Ω, Zfs = 100 Ω [9] and Zu = 10 Ω. The presented chart shows that for the most frequently used value Zx, that does not exceed 20 Ω, the relevant error made during the impulse measurement when the earthings of adjacent towers are connected is maintained at 5%.

The presented procedure for measuring earthing of power lines with lightning conductors without dis-connecting the earthing wires from the tower structure allows this type of inspection and measurement work without turning off the line. In addition, impulse measurements without disconnection of control terminals are affected by the tower foundation, which also participates in the discharge of actual lightning currents, and whose resistance is often comparable to the resistance of an additional artificial earth electrode; therefore, itshould not be overlooked in assessing the earthing effectiveness.

Zfp

ring earth

control terminals

Zx

Zfs

Zu

Zfs

Zfp

Zu

Zfs

foundations

Zfp Zfp

Zfs ZfsZfs

Zu ZuZx

1

2

)(5,0](5,0[

ufsfpx

xufsfpfsm ZZZxZ

xZZZZxZZ

Fig. 5. Measurement relative error as a function of the measured value Zx based on the expression (2)

Zx [Ω]

Erro

r[%]

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3.2. Comparison of high-frequency and impulse metersBasic recommendations for the measurement of earthing in towers are contained in Annex N to the stand-

ard PN-E 05115: 2001, designated as information annex [3]. According to the above standard “Various methods can be used for measuring earthing resistance and impedance. Selection of the right method depends on the size of earthing system and level of disturbance.” It is recommended to measure earthing resistance with an earthing tester with a frequency of measuring voltage not exceeding 150 Hz using a current and voltage probe. In the case of connection to the system of lightning conductors of the power line, all earthings of the line towers have an influence on the obtained result. For such large systems the discussed standard allows any measuringmethod that is useful under the circumstances. The given examples use a high-frequency earthing tester to avoid turning off the line and disconnecting earthings from tower structures. Test frequency should be high, so that impedance of lightning conductors to adjacent towers is high enough to avoid this route of the measured current flow. In this case, a meter that generates impulses with a proper front time can be used instead of ahigh-frequency meter. Fig. 6 shows the results of comparative measurements of the impedance of horizontal earthing with a length of 70 m, made using the impulse and high frequency methods. Impulses with a front time of 4 µs were used in the measurements. The results obtained using both methods are comparable and show an increase in earthing impedance in relation to the resistance obtained using the static method [6].

In Poland the resistance of line tower earthing is commonly measured using the impulse method, with-out disconnecting the control terminals [8, 11]. Amplitude of the measuring current impulse is about 1 A. In the case of high frequency testers, the measuring current is at the level of miliampers, which can make such measurements unresistant to interference from stray currents and currents induced by electromagnetic fieldsof the lines.

The results of measuring a 400 kV line tower obtained using both methods confirm the above problem.The tower was placed in the ground of low resistivity and the value of 2.5 Ω obtained using the impulse method is justified. The curve Z = f (f) obtained using a high frequency tester presented in fig. 7 shows a clear influenceof external interferences (field, earthing currents), which overstate the impedance results. Values similar to theimpulse results were obtained for very low frequencies – about 150 Hz. Such a frequency range is to be carried out using the static method and then the earthing resistance refers to the parallel connection of all towers, so it should reach the value of 1Ω. No influence of adjacent earthing should be present for a frequency of severalkilohertz – the meter shows the earthing impedance value of 20 for such a frequency range, which is definitelytoo high a result. The observed differences were caused by external interferences – their source in the working high voltage line. Due to a significantly higher test current amplitude, the impulse meters are much more resist-ant to such interferences.

010203040

50607080

100 1 k 10 k 100 k 1000 kf [Hz]

Z [

]

41k

4s

Fig. 6. The measurement results for horizontal earth electrode impedance with a length of 70 m as a function of frequency, made using a high frequency tester; the measuring point obtained using the impulse method for a front time of 4 µs [8] is marked on the curve

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4. TEST RESULTSThe subject of the tests was the influence of the impulse front time and span length on the obtained values

of the impedance of earthing for power transmission line towers equipped with lightning conductors. The tests were carried out using both computer simulations and measurements on real towers earthings.

4.1. Computer simulationsCalculations based on computer simulations were made using Matlab software with the Simulink pack-

age. The tower earth electrode consists of parallel artificial earthing ring and foundation earth electrode. The earthing ring is modelled using the elements R, L and C, which were determined according to the methodology developed by R. Verm [12]. The foundation is modelled by the resistance Rf calculated based on the dimensions of the foundation footing. The whole replacement model of earth electrode is presented in fig. 8.

?

Fig. 7. The results of impedance measu-rements for earthing of a 400 kV line tower as a function of frequency; the dotted line indicates the value of 2.5 Ω obtained using the impulse method with an impulse ftront time of 4 μs

The parameters of a square earthing ring with a side of 12 m and foundation of 0.9 m3 were designated for the assumed ground resistivity – 200 Ωm. The impedance of such a modelled earth electrode was determined at the impulse current with an amplitude of 1 A and front times equal to 0.5; 1.0; 4.0 and 8.0 µs and at alternating current with network frequency. The simulation results are shown in fig. 9. With the increase of front time, the impedance value of earthing decreases, but at the time of 4 µs its value reaches the state close to the fixed state obtained for network frequency, which is primarily due to the presence of resistive elements.

Fig. 8. Replacement model of earthing ring (R, L, C) with foundations (Rf )

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In the next stage of calculations in Matlab, the analysed tower earthing was bypassed by two correspond-ing earthings, connected by a lightning conductor with the configuration shown in fig. 4. Wave impedance in the lightning conductor – earth system was modelled as a long line using fixed parameters. The assumed span length was equal to 200, 300 and 400 m. Wave impedance of towers was omitted, since the used impulse front lengths cause multiple wave reflections at the ends of towers, which reduces their influence on the resultant waveforms in the analysed connection system. The simulation measurement results for impulse impedance be-tween terminals 1 and 2 in fig. 4 as a function of the impulse front time for the assumed span lengths are shown in fig. 10. The curve marked with a description “without lightning conductors” corresponds to the results shown in fig. 10 and shows how the impedance of the modelled earthing decreases with the increasing front time of the measuring impulse. Subsequent curves show the influence of parallel connection of earthings of adjacent towers on the obtained results, and their deviation from the initial curve (without lightning conductors) is a measure of error made when measuring without isolating the lightning conductors on top of the tower. Errors caused by bypassing earthings increase with the decrease in span length and increase in front of the measuring impulse, as can be seen in fig. 10.

The current Polish practice for measuring power line pole earthings uses impulse front times of 1 and 4 µs as the values provided in the standard PN 04060:1992 [4]. In the case of impulses with the front time of 1 µs the decrease of the measuring impedance value for earthing caused by the bypassed influence of adjacent earthings is at the lowest level and does not exceed 2-3%. However, it should be noted that the same value of impulse impedance at such a short time of front significantly exceeds the earthing resistance measured in static conditions, which usually is a reference point in assessing earthings. The impulse coefficient for the tower earth-ing is defined as the ratio of the impulse impedance to the static resistance; in the case of impulse with a front time of 1 µs it can achieve high values, usually in the range of 1.2-2.5. Higher values refer to earthings in towers located in grounds of high resistivity where it is necessary to use extended artificial earth electrodes [7, 10]. In such cases, the impedance of tower earthing measured with the impulse with a front time of 1 µs can too often exceed the normative values reported for static conditions.

Fig. 9. Results of simulation calculations for the impedance of tower earthing at the impulse current with an amplitude of 1 A, given front times and the frequency of 50 Hzstatic

.

.

. . .

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Measurements of tower earthings using impulses of front time equal to 4 µs may include errors at short spans due to bypassing with adjacent earthings. This error does not exceed 10%, even in extreme cases. Impulse coefficient of tower earthings measured at impulse of 4 µs is not very high and usually does not exceed the value of 1.5. Compared to the static resistance, the higher value of measured impedance is partially offset by an error introduced by the bypassing with adjacent earthings, so the results obtained in impulse measurements without isolating earthings from the lightning conductors may be related to the requirements for earthing static resist-ance with a good approximation.

4.2. Measurements of the actual line tower earthingThe simulation calculations for the bypassing influence of the adjacent towers on the measurement results

described in the previous section were verified with tests conducted on the actual power line. The tests were conducted on seven towers belonging to two lines with a voltage of 110 k V, and the test program included the measurement of impulse impedance with the impulse front times of 1 and 4 µs and static resistance. Measure-ments were made with closed control terminals connecting the artificial earth electrode to the tower structure in two connection configurations: with no lightning conductors and with conductors mounted at the top of the tested tower. Average values of the obtained impedances and resistances are shown in fig.11. Errors resulting from bypassing the measured earthings with the earthings of adjacent towers are shown in the bottom part of the figure. The biggest error, exceeding 40%, was observed in static measurements; this confirms that the static method can not be used to measure the tower earthing without disconnecting the control terminals or isolating the lightning conductors from the tower structure. Much smaller errors occurred during measurements using the impulse method: at impulse front time of 1 μs the average error was 3.5%, and 4.3% at the front time of 4 μs. Lowering of the obtained earthing values due to bypassing with earthings of adjacent towers does not exceed the error values obtained from computer simulations and shown in fig. 5 and 10.

Fig. 10. Influence of the cur-rent front time of measuring impulse on the impedance of the tower bypassed with ear-things of two adjacent towers and lightning conductors of various span lengths.

Fig. 11. Influence of lightning conductors on the impedance of actual earthings measured at given impulse front times and on their static resistance

impu

lse

resi

stan

ce [Ω

]

impulse front time [µs]

without lightning conductors

400 metres

300 metres200 metres

without lightning conductor

with lightning conductor

static

.

.

.

. .

. .

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5. CONCLUSIONS

The discharge of the line tower current to the ground is done by an artificial earth electrode and the foun-dations of that tower. Therefore, assessment of earthing resistance for the power line tower should be made when both earthing elements are parallel. Measurement using low frequency meters with disconnection of control terminals from the tower structure does not meet the aforementioned condition. Moreover, it requires that the line is turned-off during measurement. Although there are methods of measurement at low frequency using current clamp meters, which allow testing the complete tower earthing without disconnecting the control terminals, they are quite cumbersome as they require analysis of the current flowing into the ground througheach of the four legs of the tower and due to the relatively low accuracy of such measurements.

The use of fast-changing waveforms (impulse or high frequency meters) allows the measurement of earthing without disconnecting the control terminals from the tower structure, because the earthings of adja-cent towers are connected in parallel to the tested earthing through lightning conductors, whose impedance increases to the value of wave impedance in the conductor – ground system in the case of fast-changing wave-forms. In practice, impulse meters are used for the earthing measurements in the case of high-voltage line towers, due to a very high susceptibility to interference of the meters operating at high frequency. Measuring current of high-frequency meters is at the level of milliamperes and their work is interfered by voltages induced in the measuring circuits by electromagnetic field under the line, as well as by stray currents. Impulse metersoperate at currents at the ampere level, which makes them much more resistant to this type of interference.

Parallel connection of earthings in individual line towers slightly lowers the impedance value measured using the impulse method. The difference between the actual and measured value of the earthing impedance increases with increasing impulse front time, and decreases with increasing length of the line spans. The pro-posed 4 µs impulse front time is a compromise between the required accuracy of measurement and the ob-tained impedance values referred to the earthing resistance specified in standardization rules. Even under themost adverse conditions, the theoretical error made in applying the proposed impulse method of measurement does not exceed 10%, which is acceptable in earthing tests.

The calculations and computer simulations were confirmed by the results of measurements carried outon the actual earthings of 110 kV towers. These tests show that the error caused by the bypassing influence ofearthings of other towers under real conditions is smaller than the one resulting from theoretical calculations and does not exceed 5%.

Earthing Measurements for Power Line Towers

BIBLIOGRAPHYBIBLIOGRAPHY

1. PN-EN 62305-1:2008 – Protection against lightning, Part 1, General principles.2. BS EN 62305-3:2009 – Protection against lightning, Part 3, Physical damage to structures and life hazard.3. PN-E 05115:2002 – Power installations exceeding 1 kV a.c.4. PN-E 04060:1992 – High voltage test technique. General principles and test requirements.5. Construction Law, 1994, consolidated text: Journal of Laws of 2006, No. 156, item 1118.6. Wojtas S., Ocena uziemień odgromowych metodami: udarową i wysokoczęstotliwościową, Pomiary, Automatyka,

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