Annals of the „Constantin Brancusi” University of Targu Jiu, Engineering Series, No. 3/2018

13

GROUNDING RESISTANCE MEASURMENTS AND CONTROL FOR WIND

GENERATORS

Georgi Tsonev Velev, Technical University of Gabrovo, Gabrovo, BULGARIA

Krasimir Marinov Ivanov, Technical University of Gabrovo, Gabrovo, BULGARIA

Petar Kolev Petrov, Technical University of Gabrovo, Gabrovo, BULGARIA

Adriana Foanene, University „Constantin Brȃncuşi” in Targu Jiu, ROMANIA

ABSTRACT: The paper inhere describes the measuring procedure and results for grounding

resistance audit and control in regard with wind generators having EMC problems, working at the

locality of “Buzludzha” in the “Stara Planina” mountain, owned by “VETROKOM” LTD.

KEY WORDS: wind generator, grounding (earth) resistance, grounding (earthing) system,

electromagnetic compatibility (EMC).

INTRODUCTION Until 2011 “Vetrokom” LTD, has installed

in the locality of “Buzludzha” in the “Stara

Planina” mountain 29 wind generators with

total installed capacity of 72,5 MW at an

altitude of 1490 m. The single aggregates are

with rated capacity of 2,5 MW each, having

their own step-up transformers in the gondola

with secondary rated voltage of 20 kV. Wind

aggregates have locally their own grounding

installations, around their foundations and are

connected electrically in groups of 4(or 5)

units by loop cable power line 20kV with

main step-up substation 20/110 kV. The

interconnection between the wind power

station and the transmission power grid has

been established by 9 km cable power line at

110 kV.

In 2017 “Vetrokom” LTD turned to the

Technical University of Gabrovo with a

request for support about EMC problems with

5 of their generators. Most of the problems

were related to frequent cases of damaged

power-electronic blocks, situated in the wind

turbine gondolas, after intensive lightning

activity. The first approach in the study was to

check grounding efficiency, having in mind

that the problem exists only with 5 of the

wind generators.

CHOOSING THE CORRECT

METHOD TO MEASURE

GROUNDING RESISTANCE OF

WIND TURBINES. The problematic wind turbines, intended

for the measurement procedure were with

numbers №725, №789, №797, №731 and

№736.

FL 725R1

R2

R3

R4

Power Cable 20 kV

Figure 1. Reinforced concrete foundation of

wind generators

Annals of the „Constantin Brancusi” University of Targu Jiu, Engineering Series, No. 3/2018

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Each wind generator has four grounding

electrodes, connected in parallel (Fig. 1) with

earth resistances 1 2 3 4, , and R R R R .

Having in mind the large size of the wind

turbine steel-concrete foundation (a square

with side edge of 40 meters) and the lack of

any engineering plans of the wind turbines

grounding installation, the grounding

installation size was assumed to be treated as

an large-scale [2].

For large-scale grounding installations, the

generally used in practice “Fall of potentials

method” is not applicable, because it requires

extremely long cable leads covering the

required extended distance from the ground

electrode center to the current probe(200-

500m)[1,3-6,8].

Long distances to current probe results in

limiting the current injected through the earth,

and respectively to week voltage signal on the

potential probe. All that, combined with the

extremely high soil resistivity on site (around

2800Ω/m) leads to insufficient instrument

sensitivity.

In order to obtain accurate measurement

results the “Slope” method was chosen.

“SLOPE” METHOD BASICS

“Slope” method has been shown to give

reliable results, even when the soil is

nonhomogeneous.

It has been designed to eliminate the need

for impractically long leads by the ability to

interpolate the correct distance along the

combined resistance curve, i.e. the curve of

the current probe’s resistance superimposed

upon that of the tested grid, without sufficient

spacing to produce the characteristic “flat

portion” between [5, 6].

CD

0,2 CD 0,4 CD 0,6 CD

Potential electrocde

Current electrode

Grounding grid

PD

Figure 2. Set-up for the “Slope” method,

where PD - distance from grounding grid to

potential probe; CD - distance from grounding

grid to current probe;

Procedure consists of the following steps:

1. Earth tester is connected to the grounding

grid on convenient place;

2. Current probe is inserted at distance CD

from the grounding grid (normally 2 to 3

times the maximum dimension of the

system);

3. Potential probe are located consequently at

distances equal to 20% of CD , 40% of CD

and 60% of CD and measurements are

performed for every one of the cases. The

obtained resistances are RE1, RE2 and RE3

respectively.

4. The coefficient µ is calculated. It represents

the change of slope in the

resistance/distance curve.

3 2

2 1

E E

E E

R R

R R

(1)

5. From appendix 1, the respective value of

P

C

D

D is taken in regard with µ.

7. Since the distance to the current probe CD

is already known, a new value for

PD (distance to the potential probe) is

calculated.

PP C

C

DD D

D

(2)

Potential probe is relocated to its new

“Actual” position.

6. The actual grounding resistance is

measured for the new set-up, placing the

potential probe at its new distance PD .

7. Measurement procedure is recommended to

be repeated for other current probe distances

(larger values for CD ). Measurements are

assumed with sufficient accuracy when for

the different current probe distances the

results for earth resistance are stable.

MEASURMENT RESULTS FOR

WIND GENERATORS EARTH

RESISTANCE

The procedure described in the previous

chapter is used to calculate earth resistance

Annals of the „Constantin Brancusi” University of Targu Jiu, Engineering Series, No. 3/2018

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for all of the wind generators that have been

under test. In details the intermediate

calculations are displayed only for Generator

№731 (table 1).

Table 1

Cu

rren

t p

rob

e d

ista

nce

DC

R1

( D

P =

0,2

.DC

)

R2

( D

P =

0,4

.DC

)

R3

(

DP

=

0,6

.DC

)

µ P

C

D

D

DP

_R

EA

L

RGR

m - - m 80 6,2 10,2 15,8 1,4 0,43 34,4 11,7

All results are summarized in Table 2, where

the real measured value for earth resistance of

each wind generator GRR is corrected by the

season coefficient , which takes an account

of the season in which the measurement is

performed (wet/dry) and the dimensions and

geometry of grounding electrodes used. For

each wind generator a comparison is made

between the measured earth resistance and the

norm regulated value by Bulgarian legislation

[3, 7].

Table 2

Win

d g

ener

ato

r №

:

RG

R

S

easo

n c

oef

fici

ent

RC

OR

=

φ

. R

GR

NORMR

Gro

un

din

g E

ffic

ien

cy

- - FL-725 28,9 1,3 37,5 30 NO

FL-789 21,3 1,3 27,6 30 YES

FL-797 - 1,3 - 30 NO

FL-731 11,7 1,3 15,2 30 YES

FL-736 5,35 1,3 6,96 30 YES

Protective earth resistance of wind turbine

№ 725 in wind farm “Vetrokom” does not

comply with standards [3] and [7]. The

single earth electrode 1R does not have an

electrical connection with the grounding

contour. The connecting bus-bar is probably

interrupted at the point of interconnection

with the grounding system, inside the

reinforced concrete foundation;

Protective earth resistance of wind turbine

№ 797 does not comply with standards [3]

and [7]. Single earth electrodes

1 2 3 4, , ,R R R R does not have an electrical

connection with the grounding contour. The

measured resistances for each of the earth

electrodes is in the order of kΩ -s. Wind

generator FL 797 is actually grounded only

via the lightning bus-bar and the grounded

armor of the power cable, through which the

machine is partially grounded to the earth

contours of the neighboring wind turbines in

series in the group.

ADDITIONAL STUDIES AND

RECOMMENDATIONS

For the period of audit and control, also

measurements in regard with soil resistivity

were made for the locality adjacent to wind

turbine FL 725 using Wener’s method.

In the surface soil layer with a depth of up

to 5 meters soil resistivity varies in the range

1500 – 5000Ω.m , which means that surface

soil layer is very inhomogeneous. For deep

soil layers (depth 10 m and more) soil

resistivity decreases and falls below

1000Ω.m . Measurements results were

processed in specialized software, and an

average soil resistivity of 2800 Ω.m was

obtained. Average soil resistivity was used for

further engineering calculations.

If an additional grounding system is

designed and built, which consists of

horizontal galvanized flat grounding bars with

standard dimensions of 40/4 mm, buried in

trenches at a depth of 1.5 m, according to the

diagram below (Fig. 3), grounding resistance

will comply with the prescriptions in

standards [3] and [7] about resistance to

ground in regard with the wind generators

with EMC problems ( 29,5 GRR ).

If the horizontal grounding bus-bar is

covered with a layer of loamy soil, earth

resistance will be further reduced to a value

of 23 GRR . Using a layer of loamy soil

will also enhance electrical contact of the

grounding bus-bar with surrounding soil.

Annals of the „Constantin Brancusi” University of Targu Jiu, Engineering Series, No. 3/2018

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If horizontal bus-bar width is

increased 2 times, earth resistance will

become 9,8 GRR

tower

foundation

Grounding bus-bars

Grounding bus-bars

50 m 50 m

50

m5

0 m

Two layers of loamy soil

Soil from the site

Grounding electrode

Section of the earth conductor trench

1,5

m0

,25

m

0,2

5 m

0,5 m

Figure 3. Additional grounding system in

order to reduce grounding resistance

According to Fig. 3 the following procedure

must be followed:

1. Trenches with a depth of 1.5 m and a

width of 0.5 m are pre-excavated;

2. A layer of clay soil at the bottom of the

trench with a thickness of 25 cm is placed

and trampled;

3. Zinc-coated horizontal bus-bar is laid

down on it and a second layer of clay soil

with a thickness of 25 cm is placed,

which is also tramped.

4. The rest of the trench is filled with the

dredged soil and compacted.

The application of deep vertical

earthing electrodes by pre-drilling holes in the

soil will be difficult because of the stony

nature of the soil layer. In this case it will be

necessary to use additional low resistivity

materials to improve the soil resistance, which

in liquid form have to be poured into the

drilled hole after the vertical earth electrode is

placed in it in order to seal and improve the

contact of the electrode with surrounding soil.

Construction of vertical earth electrodes of

this kind would require large investments and

the results are not guaranteed;

Realization of additional grounding

systems will require more thorough

engineering calculations and design, as well

as additional field measurements of soil

resistivity at various locations at the region of

the “Vetrocom” wind farm.

CONCLUSIONS Wind turbines are connected by loop

power cable 20 kV in groups of 4 (5) power

unit. The metal screen of the power cable

connects electrically the metal towers of the

machines in the group to each other and to the

substation grounding system. Separately, the

power cable is protected from lightning by a

steel reinforcement along its length, which is

attached to the reinforced concrete structure

of the foundation of each machine at an

unknown point. These facts explain why the

most distant machines in the group have the

highest earth resistance, which is exceeding

the requirements. In fact, the nearby machines

to the substation and those along the power

cable line between two neighboring

generators are additionally partially grounded

by the grounding systems of neighboring

machines and the substation earthing system.

The last wind generators in the group due to

their high distance from the power substation

are only partially grounded only to the

previous machine;

In some wind turbines it was found

that bus-bars connecting the tower of the unit

with the local lightning protection grounding

electrodes are interrupted somewhere in the

machine steel-concrete foundation (for FL

797 the four connecting bus-bars do not make

a connection with the grounding system, thus

the machine is grounded only through the

earth systems of neighboring wind turbines

trough the lightning protection conductor of

the cable and its grounded screen. For FL 797

one of the connecting bus-bars has been

disconnected in the foundation). We assume

that the same problem may exist for other

wind generators also;

Since soil resistivity in the region as a

result of the rocky soil type is very high (with

average of 2800 Ω.m ), the local lightning

protection grounding resistance for some of

the aggregates do not comply with the 30

limit, as stated in Bulgarian standards [3, 7],

which seem to be the most liberal, compared

to other standards. For example in IEC

Annals of the „Constantin Brancusi” University of Targu Jiu, Engineering Series, No. 3/2018

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61400-24 [4], earth resistance limit for

lightning protection of wind turbines is 10 ;

A reason for the high grounding

resistance measurement results in some of the

wind generators can also be found in the

imprecise dimensioning and technical

construction of wind generators’ grounding

systems and their elements and the reinforced

concrete foundation, since the grounding

system is built simultaneously with the

reinforced concrete foundation.

REFERENCES 1. IEEE Std 81-1983, “IEEE Guide for

Measuring Earth Resistivity, Ground

Impedance, and Earth Surface Potentials of a

ground system”, American National Standards

Institute, September 1984;

2. IEEE Std 80-2000, “IEEE Guide for Safety in

AC Substation Grounding”, IEEE-SA

Standards Board, January 2000;

3. Ordinance No 3 from 9 June 2004 for the

design of electrical systems and power

lines(Bulgarian standard);

4. IEC 61400-24:2010, Wind turbines - Part 24:

Lightning protection;

5. Megger, Getting Down to Earth - A practical

guide to earth resistance testing, 2010

6. Whitham D. Reeve, Principles and Practice of

Earth Electrode Measurements, Reeve

Engineers, 2008

7. Ordinance No 16-116 from 8 February 2008

for the Technical Operation of the Electrical

Energy Equipment (Bulgarian standard).

8. L.M. Cîrţînă, C. Militaru, C. Rădulescu, Study

of compensation errors due to temperatures, as

elements that are component of the chains of

dimensions formed at assembly,7th Youth

Symposium on Experimental Solid, Jurnal

Mechanics, 2008/5/14,Wojcieszyce, Poland

.

APPENDICES

Appendix A1. Dependence of the ratio P

C

D

D

on the coefficient µ