field measurement of the grounding impedance of a wind farm in venezuela

7/28/2019 Field Measurement of the Grounding Impedance of a Wind Farm in Venezuela

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2012 International Conference on Lightning Protection (ICLP), Vienna, Austria

Field Measurement of the Grounding Impedance of a

Wind Farm in Venezuela

Miguel Martnez Lozano

Dpt. Conversion and Power Delivery

Universidad Simn Bolvar

Caracas - Venezuela

[email protected]

Luis G. Daz Pulgar

Dpt. Conversion and Power Delivery

Universidad Simn Bolvar

Caracas - Venezuela

[email protected]

AbstractOne of the main parameters involved in lightning

studies in wind mill parks, is the grounding system. For this

reason, it is necessary to know not only the grounding resistance

at low frequency (measured with traditional equipments), butalso the transient behavior especially in the frequency domain. In

Venezuela, recently was installed a wind mills park with 100 MW

of capacity, in a region with high lightning activity. In this

context, a variety of measurements of the grounding systems

were carried out during this year, including: soil resistivity,

traditional resistance measurement and impedance measurement

using a portable lightning generator at 3 kV. The main results

are showed and analyzed in the context of knowing more detailed

the behavior of grounding systems in this special kind of

applications.

Keywords: Grounding, Transient Response, Lightning, Wind

Mills

I. INTRODUCTION

In the past ten years the world has taken special interest in

the welfare of the environment, especially in the electric

power generation area. That is why the installation of

alternative energy system generators, powered by non-

combustible fuel has increased.

Latin-America has not escaped from this reality and

recently has started to project and install wind farms, such as

the one in Paraguan, Venezuela. This facility has 76 units of

1.2MW aprox. For a total of 100MW installed power.

Nevertheless, tropical countries as Venezuela, has an

important lightning activity (Paraguan has approximately

60TD/year). This is the reason for paying special attention tothe behavior of the generation units with heights above 60m

located in unoccupied zones, where they are by far the most

elevated points from the ground (figure 1). The park location

is in the west coast of Venezuela.

This is why it is a real necessity to develop transients and

outage rate studies [1]. So the protection and design of certain

elements of the park can be selected, tuned and adapted

according to the most reliable amount of parameters.

One of the most relevant subjects concerning the modeling

of the lightning protection system is the grounding system [1],

and its influence on the behavior of the transients in the wind

turbine [2].

Fig. 1. The windmills are the highest objects around.

The grounding system must be studied in a wide range of

frequencies from a few Hz to MHz [2, 3, 4]. To make these

measurements in the field, one portable impulse generator wasdeveloped [5]. With the impulse injection, the transient

voltage and current waveforms are obtained for a variety of

temporal windows and then processed in the frequency

domain using FFT. The information in time and frequency

domains is analyzed looking the best design of the grounding

system. In this article these results and analysis are showed in

two stages of construction: in first term considering only the

foundation as the grounding electrode, and finally when the

additional grounding arrangement was installed. The

information includes the soil resistivity and low frequency

resistance values.

II. GROUNDING SYSTEM OF A WIND FARMThe grounding system of a wind turbine has three

fundamental parts:

A. The Foundation (Figure 2).The foundation is a very important part of the grounding

system, because of its size and quantity of metal in contact

with soil through the concrete [6]. The foundation has

permanent contact with tower. Then the lightning current flow

occurs through this component to the soil. The typical

dimension of this component is appreciable and it provides


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generally a low resistance value. By other hand, the fact that

the metal (reinforcement steel) is in contact to soil through

concrete, have two important advantages: the permanent

content of humidity (making an stable value of the resistance)

and the high permittivity of the cement in comparison with the

surrounding soil. All these facts indicate that the foundationcontributes to the best transient performance of the grounding

system and its effect must be taken into account in any studyfor this kind of phenomena [6].

Figure 2. Typical foundation of a windmill.

The size of the foundation is shown in Figure 3.

13 m

5

1,6

1,5

2

Figure 3. Foundation size.

2.6 m

13m

Figure 4. Grounding system mesh.

B. The grounding electrodesThe additional conductors disposed in the soil forms the

grounding electrodes. Generally, the designers call to this

array, the grounding mesh because of its geometrical

configuration. However, more important that the control of

touch and step potentials would be the array design to

minimize the transient impedance.

The final design depends of the soil resistivity and other

parameters imposed by the factory of the windmill or the finalclient (electric facility).

In the figure 4 the array implemented in the wind farm isshown. The conductor used was the 4/0 AWG Cu.

C. The possibility of additional electrodes for connectingwith the rest of the units of the park and main substation.

(With different strategies).

The interconnection between the wind generators to form a

giant equipotential mesh in the park is a common practice in

this kind of application, but the cost associated with it, is very

high. The strategy to the interconnection is a very important

aspect because between the units, exist many services shared(by example: power cables, control cables, fiber optic, etc).

The typical implementation of this aspect is shown in figure 5.

Figure 5. Scheme for interconnection between Windmills.

III. MEASUREMENTS

The field measurements that were carried out in this work,

are:

The soil resistivity using the Wenner method (IEEEStd 81.1) [7]

The static resistance using fall of potential (IEEE Std81.1) [7]

The surge transient response using a portable 3kVimpulse generator [5].

For comparison purposes, each measurement was developed

in the following situation:

a) For a Windmill with only the foundation (without thegrounding mesh constructed).

b) For a Windmill with foundation and ground mesh,but without interconection with others generators.

The measurement campaign was developed in March and

April, 2012. And the strategy (distances and topology of the

instruments in the field) is shown in figure 6. The climatologic

conditions in the place are the typical in a tropical region

(figure 7) and with the generators located at no more than 2

Tower

Concrete with reinforcement

steel


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km of the sea (at 10 m height over sea level). Ta=35C and

relative humidity around 70%. The days were cloudy with

sporadic rains at night.

For the impulse test, the equipment array for the

measurement is shown in figure 8.

180 m

80

Resistivity profiles (RP)

RP2

RP1

RP3

100 m

Fall of potential profile

Passageway

Generator i Generator j

Figure 6. Field measurement distances.

Figure 7. Photograph of the environment.

(a)

(b)

Generator i GR under test Generator j

GR1 GR2# 12 AWG Cu

180 m# 12 AWG Cu

180 m

(c)Figure 8. Impulse test arrangement. (a) General description of impulse

generator. (b) Measurement technique. (c) Disposition of grounds for

reference (GR1 y GR2).

The equipment used for the measurement, was: forresistivity and resistance the equipment was a tellurometer

Fluke 1625 (figure 7). For the impulse test, the generator was

fabricated at the high voltage laboratory and it has a capacity

of 3kV in voltage charge. For the waveforms (current and

voltage) capture, the voltage probe was a Nicolet 4kV x100, a

Rogowski coil 5kA, 0.1 V/A and one oscilloscope Tektronix

TDS 3032 300 MHz (figure 9). The waveforms were digitized

by one Ethernet connection with a PC for the post-processing.

Figure 9. Impulse generator connections.

IV. RESULTS

The results are presented for each windmill tested. The first(AG1) was the windmill with only the foundation constructed,

and the second (AG2) with foundations and grounding meshinterconnected.

A. Soil Resistivity:Several measurements were carried out (three profiles) for

each AG (1 and 2) and the processed soil models (two layer

model) are presented in table I. The Wenner method was used

with separations of: 1, 3, 6, 9 and 12 m.

TABLE I.SOIL RESISTIVITY MEASUREMENTS RESULTS.

Two Layer Soil Model AG1 AG2

1 (-m) 56.48 93.12 (-m) 13.11 5.4

h(m) 3.62 2.63Correlation error in data (%) 0.89 16.7


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Figure 10. "Caliche" stone extracted in the location.

The soil resistivity measurements show that the second

layer has a very low value (in order of 10 -m). This value

indicates the presence of water with high content of salts. The

soil is rocky with presence of sea fossils (crustaceans) and it is

identified as Caliche. It is a very rich carbonated rock and avery good water absorbent (hydrophilic). In figure 10 is shown

a piece of the rock extracted during the construction of

foundation.

B. Ground Resistance:The measurement of the ground resistance for each case

was carried out using the fall of potential method [7] with a

current probe separation of 100 m. Twenty measurements of

potential were taken to obtain the graphs shown in figures 11

and 12. For these graphs the values of the low frequency

resistances are obtained for AG1 and AG2.

Figure 11. Fall of potential for AG1.

Figure 12. Fall of potential for AG2.

In Table II, are resumed the values of the low frequency

ground resistances (Rg) for AG1 and AG2.

TABLE II Ground Resistances values.

AG1 AG2

Rg () 0.29 0.15

C. Ground Impedance:All the previous measurements were made to obtain the data

for analysis and comparison, but the importance of the study is

precisely try to obtain the impedance of the grounding system(GS) through the injection of an impulse current from a

portable generator. With the response of the GS, two types of

analyzes can be performed. The first is in the time domain,

trying to characterize the relationship between instant values

of the voltage and current waveforms. The second is to

transform the waveforms from the time domain to thefrequency domain using FFT. Then, the relation between

voltage and current is the impedance in the frequency domain.The procedure followed for the obtaining and processing of

the signals is resumed in the figure 13.

Figure 13. Procedure followed.

For the time domain analysis, the expressions used are:

maxmax /IVZo = (1)

)(/ maxmax2 VIVZo = (2)

maxmax3 /)( IIVZo = (3)

These relationships let us to know the coherence of the data,

the impulse impedance (1) and the behavior between low andhigh frequencies, knowing the difference obtained by

comparison between (2) and (3) with (1). For example, in a

grounding system where the resistive behavior domains, the

three expressions tend to the same value because the

waveforms of voltage and current have a time coincidence at

its maximum. In other hand, the discordance between thevalues indicates a soft or stronger inductive or capacitive

behavior, depending if (2) is higher than (3) or vice versa.

For the frequency domain, the analysis is located in the


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modulus of the impedance, Z(w), as is indicated in (4).

)(/)()( wIwVwZ = (4)

To know the behavior in a wide range of frequencies is

important to obtain the time response in a variety of time

windows. This implies, to repeat the impulse injection in atleast four time windows (100 ns/div, 1s/div, 10s/div and

100s/div). The performance ofZ(w) as the transfer function

of a complex grounding circuit, let us to obtain an important

information about its transient behavior. In the figure 14, thisinformation is explained graphically.

Z(w)

w

Inductive

Resistive

Capacitive

FFigure 14. Different types of frequency behavior.

In figures 15 and 16, some photos of the experiments

carried out in AG1 and AG2, are showed.

Figure 15. Measurement of ground impedance in AG1. Windmill without

ground mesh (only foundation).

Figure 16. Measurement of ground impedance in AG2. Windmill withgrounding mesh connected to foundation.

In the figures 17 and 18, the time waveforms of voltage and

current are shown for AG1 and AG2, respectively. And thetime analysis, using expressions (1) to (3) can be observed in

tables III and IV.

Figure 17. Voltage and current in AG1 in time domain.

Figure 18. Voltage and current in AG2 in time domain.

TABLE III.THE EXPRESSIONS OF Z, FOR TIME DOMAIN IN AG1.

Relationship ()

Zo 0.28

Zo1 0.30

Zo2 0.26

Rg 0.29

TABLE IV.EXPRESSIONS OF Z, FOR TIME DOMAIN IN AG2.

Relationship ()

Zo 0.167Zo1 0.33Zo2 0.12Rg 0.15

Clearly, the time responses of AG1 and AG2 are totally

different. In both cases, the coherence between the value of Rg

and Zo is good, it means that Zo tends to Rg. In the case ofAG1, the values Zo1 and Zo2, are very similar to Zo,

indicating the predominance of the resistance in the complex

circuit. However, in the case of AG2, Zo1 is higher than Zo

and Zo2 is lower than Zo; typically, this relations indicate an

inductive performance, but not strong, because the differences

are not important.

In the figures 19 and 20, the waveforms of voltage and

current, are shown in the frequency domain for AG1 and AG2,

respectively. And the Z(w) as the relation between these

function, are in figures 21 and 22.


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Figure 19. Representation of voltage and current in frequency domain for

AG1.

Figure 20. Representation of voltage and current in frequency domain for

AG2.

Figure 21. Modulus ofZ(w) for AG1.

Figure 22. Modulus ofZ(w) for AG2.

In the comparison of figures 21 and 22, can be seen some

similarities with the observations made in the tables 3 and 4.

In this sense, the performance of AG1 is more resistive: the

impedance dont show significant variations from low

frequency to 10 kHz and then an small inductive componentappear, but it isnt strong (the variation is less than one order

of magnitude). However, in the case of AG2 the inductivebehavior begins from kHz. In both cases the impedances are

very low, in order of 1 in AG1 at 1 MHz and less than 5

in AG2.

V. CONCLUSIONS

The grounding model is an essential part for the correctsimulation of the transient events in a wind park.

The influence of the foundations indicates that its

consideration in the simulation, measurement, etc, is very

important. This fact is special because the correct

representation of the grounding system is the interaction of all

the components involved. And the foundation in most ofapplications is always present.

The evaluation performed in this work, allows the designer

to have better information and more complete and real models

of the components like the grounding system.

Both, the information in time domain and frequency

domain, are important and complementary. The three differentrelationships shown in this work let us to obtain a quick view

of the transient response only by a simple comparison of the

instant values of voltage and current.

REFERENCES

[1] UNE-EN 61400-24. 2011. Wind Turbines. Part 24: Protection against

Lightning.[2] S. Yanagawa, D. Natsuno, K. Yamamoto. A measurement of transient

grounding characteristics of a wind turbine generator system and itsconsiderations. 7th Asia-Pacific International Conference on Lightning,

November 1-4, 2011, Chengdu, China.[3] K. Yamamoto, S. Yanagawa, K. Yamabuki, S. Sekioka, S. Yokoyama.

Analytical surveys of transient and frequency-dependent grounding

characteristics of a wind turbine generator system on the basis of fieldtests. IEEE Transactions on Power Delivery, Vol. 25, No 4, October

2010.

[4] E. Badran, M. Rizk, M. Abdel-Rahman. Analysis and suppression ofbackflow lightning surges in onshore wind farms. Journal of lightning

research, 2011, 3, 1-9.

[5] Ramrez J., Martinez M., RODRIGUEZ J., O. Zurga, E. Jaspe.Comportamiento Transitorio de electrodos sencillos de puesta a

tierra. VI Encuentro Regional Latinoamericano del CIGRE, Paraguay.

Junio1999. Volumen I. Pginas 124-129.[6] Yamamoto K, et al. Verifications of transient grounding impedance

measurements of a wind turbine generator system using the FDTD

method. 2011 International Symposium on lightning protection (XISIPDA), Fortaleza, Brazil, October. Pp.255-260.

[7] IEEE Std 81-1983, IEEE Guide for Measuring Earth Resistivity, Ground

Impedance, and Earth Surface Potentials of a Ground System (Part I).

field measurement of the grounding impedance of a wind farm in venezuela

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