2011 International Symposium on Lightning Protection (XI SIPDA), Fortaleza, Brazil, October 3-7, 2011.
Verifications of Transient Grounding Impedance
Measurements of a Wind Turbine Generator System
Using the FDTD method
K. Yamamoto
S. Yanagawa
T. Ueda
Dept. of Electrical Engineering
Kobe City College of Technology
Kobe Hyogo, Japan
kyamamoto@mem.iee
Shoden Co.
Chiba Chiba, Japan
yanagawa@sdn.co.jp
Chubu Electric Power Co.
Nagoya Aichi, Japan
Ueda.Toshiaki@chuden.co.jp
characteristics by using the FDTD (Finite Difference Time
Domain) method [10].
Abstract— Most of the breakdowns and malfunctions of the
electrical and control systems inside or in the vicinity of a wind
turbine due to lightning are caused by a rise in ground potential.
To solve those problems, the field tests have been carried out, and
the transient characteristics of an actual wind turbine grounding
system have been obvious. This paper describes the verifications
of field tests of wind turbine grounding characteristics by using
the Finite Difference Time Domain method.
II.
Keywords-component; lightning; potential rises; grounding;
wind turbine generator system; FDTD method.
I.
GROUNDING SYSTEM
Fig. 1 shows the grounding system of the wind turbine
generator system; it consists of the foundation of the wind
turbine, grounding meshes, foundation feet and the grounding
system of a neighbor substation. A quadrangle and octagon
usually exist as shapes of the top surface of the wind turbine
foundation; it used in this measurement is an octagonal-type
foundation. The wind turbine was under construction in the
INTRODUCTION
Top view
In recent years, accidents associated with the use of a large
number of wind turbine generator systems have increased in
number. Those accidents are caused by natural phenomena
such as lightning and typhoons. Lightning especially causes
extensive and serious damages.
substation facility
1m
3m
5m
7m
9m
12 m
16.5 m
2m
In order to exploit high wind conditions, wind turbine
generator systems are often constructed on hilly terrains or
along the seashore, where few tall structures exist in their
vicinity; therefore, these structures are often struck by lightning.
In order to promote wind power generation, lightning
protection methodologies for wind turbine generator systems
should be established [1]-[3].
Damage caused to wind turbine generator systems due to
lightning affects the safety and reliability of these systems [4],
[5]. Most of the breakdowns and malfunctions of the electrical
and control systems inside and in the vicinity of a wind turbine
are caused by ground potential rise due to lightning [6], [7].
Impulse tests have been conducted at the coastal wind farm,
and the rise in ground potential around the foundations was
measured [8], [9].
4m
6m
8m
10 m
0m
4m
7m
Side view
substation facility
3.2 m
2.8 m
grounding mesh
foundation foot：10 m
Bottom view
Fields tests have been carried out on an actual wind turbine
generator system at a coastal area. The ground potential rise of
the system itself and around its foundations has been measured
to understand its characteristic [9]. This paper describes the
verifications of field tests of the wind turbine grounding
grounding mesh
Figure 1. Grounding system of the wind turbine generator system.
308
field tests, the upper parts of the wind turbine such as a tower
and blades did not exist. The underground parts of the
grounding system including the foundation had been built.
1 m. The fast front current generated by the impulse generator
was injected into the foundation through a resistance of 500 Ω
from a current lead wire. The peak value of the current was 60
A, and the wavefront was about 0.2 μs. Such current is
expressive of the lightning current with the conceivable steep
wavefront. When the frequency characteristics of the
grounding impedance are calculated, such potential responses
of the current including wide-frequency components become
important [8].
For some wind turbines, the lightning current is actively led
to the grounding mesh covering the foundation from the tower,
not to the foundation itself. However, the foundation is not
explicitly isolated from the tower; these are connected through
the anchor. When the grounding characteristics of a wind
turbine generator system are researched, a grounding system
including the foundation, the grounding meshes, the foundation
feet and the substation grounding system should be considered.
Incidentally, the grounding system was not connected to any
surrounding wind turbines.
The comparatively large resistance of 500 Ω was connected
in series with the impulse generator; the power source could
therefore be considered as a current source. The resistance also
worked as a matching impedance to reduce the current
reflections on the current lead wire. The impulse generator was
grounded by several grounding rods (length: 1.5 m; diameter:
20 mm); the steady-state grounding resistance was 8.5 Ω.
As shown in Fig. 1, the foundation is octagonal type with
the width of 16.5 m. The foundation was constructed with
reinforced concrete; the intervals between reinforcing were
about 30 cm. The tower bottom was connected to the
foundation and the anchor at ground level. The depth of the
anchor and the foundation was 2.8 m and 3.2 m respectively.
The length, diameter and number of the foundation feet were
10 m, 1 m and 17 respectively to enhance the bearing capacity
of soil. The foundation feet were made of steel. The cross
section of the wires for the grounding meshes was 60 mm2; it is
connected to the anchor and the foundation feet. The details of
the grounding mesh covering the foundation are shown in Fig.
1.
The injected current was measured at the end of the current
lead wire near the foundation with a current probe, as shown in
Fig. 3. The potential rise of the foundation was measured as the
voltage difference between the top of the foundation and the
voltage measurement wire (length: 68 m; cross-section: 2 mm2).
The ground terminal of the passive probe was connected to the
top of the foundation so that the case of the oscilloscope can be
same potential at the measuring point. The height of the
voltage measurement wire was 1 m, and it was grounded at the
remote end through a matching impedance of 300 Ω. The surge
impedance of the voltage measuring wire was about 400 Ω;
therefore, the 300 Ω resistance was connected between the
remote end of the voltage measuring wire and a grounding rod
(length: 0.5 m, diameter: 20 mm, grounding resistance: 81 Ω).
This was how the noise induced on the voltage measuring wire
was discharged to the ground readily. The potential rises
around the wind turbine generator system were measured as the
voltage between the voltage measurement wire and the
conductive rods at intervals of 1 m around the foundation, and
2 – 4 m (over 10 m from the edge of the foundation, as shown
in Fig. 1) for further distance. The potential rises were
measured at 21 locations. The bottom of the conductive rod
(length: 0.3 m; diameter: 10 mm) was buried about a 0.1 m
depth at each measured point to measure the potential rises, the
The stratiform ground resistivity at the site of the wind
turbine generator system is shown in Fig. 2. The test place is a
low resistivity site, and is a few hundred meter away from the
coast. The Wenner method was utilized to measure the
resistivity. The steady-state grounding resistance of the
grounding system of the wind turbine generator system was
0.21 Ω.
III.
EXPERIMENTAL CONDITIONS
Fig. 3 shows the experimental setup. The current was led to
the foundation from the impulse generator with insulated
copper wire (length: 90 m; cross section: 5.5 mm2) as the
current lead wire. The height of the current lead wire was about
IV cable
500 
I
V
50 m
V
anchor
I
I.G.
current lead wire
90 m
18 m
substation facility
voltage measuring wire
300 
Figure 2. Stratiform ground resistivity at the site of field tests.
Figure 3. Experimental setup.
309
ground terminal of the passive probe was connected to
conductive rod so that the case of the oscilloscope can be same
potential at the measuring point. As shown in Fig. 3, the
current lead wire and voltage measurement wire were not
orthogonalized around the wind turbine. Mutual
electromagnetic induction between the current lead and voltage
measuring wires may have existed.
voltage [V]
80
The details of the measuring instruments are shown in Ref.
[9]. Those instruments are enough to measure the potential
responses of the injected current with a rise time of several
hundred nanoseconds.
The dimensions of the analytical space were 96 m × 204 m
× 92 m, and it was divided into cube cells with a side length
of 0.5 m. The absorbing boundary condition was 2nd order Liao.
The ground level was 53 m from the bottom of the analytical
space; the resistivity of the ground was the same as that shown
in Fig. 2. Thin-wire models to model the current lead wire,
voltage measuring wire, and grounding mesh were used [11],
[12]. The foundation was an octagonal-type; the foundation
foot was modeled as a rectangular parallelepiped of 1 m × 1 m
× 10 m. The current source parallel with the resistance of 500
Ω was connected between the foundation and current lead wire.
COMPARISONS OF MEASUREMENTS AND CALCULATIONS
A. Injected current into the wind turbine foundation and its
potential rise
Fig. 5(a) shows the calculated and measured injected
currents I. The injected current showed a ramp wave which
includes wide frequency component, and its peak value and
rise time were approximately 60 A and 0.2 μs, respectively.
Compared with the measurements shown in Fig. 5(a), the
calculated injected current agreed well with the measured
current. The potential rise V at the top of the foundation as
shown in Fig. 5(b) showed inductive characteristics. The
transient grounding resistance is greater than that of steadystate. The voltage waveform oscillated at the wavefront. The
39 m
voltage measuring wire
2m
7m
V
44 m
20
0
2
4
6
8
10
8
10
time [s]
ANALYTICAL CONDITIONS
204 m
Simulation
(a) Injected current waveform
The measurements were reproduced by using
electromagnetic field analysis through the FDTD method. The
analytical setup is shown in Fig. 4.
V.
Experiment
40
0
voltage [V]
IV.
60
100
80
60
40
20
0
-20
-40
Experiment
Simulation
0
2
4
6
time [s]
(b) Potential rise waveform
Figure 5. Calculated and measured results of the injected current and the
potential rise.
potential rise at the wave tail converges to that correspond to
the steady-state grounding resistance of 0.21 Ω. At the wave
front, there were a few differences. However, the peak and
wave tail value of the calculated potential rise agreed well with
the measured values. The field tests were carried out soon after
the construction of the wind turbine grounding system, the soil
around the grounding system was unstable. That may be why
the differences between calculated and measured results
appeared.
The grounding characteristics of the system showed strong
inductiveness at the wavefront; the steady-state grounding
resistance was as low as 0.21 Ω. The steady-state grounding
resistance is usually lower than 10 Ω [3]. In such grounding
system of low steady-state grounding resistance, similar
inductive characteristics should be observed [8]. Transient
phenomena obviously become more important than steadystate phenomena for lightning protection design in these types
of wind turbine grounding systems.
voltage source
current read wire
B. Potential rises around the wind turbine foundation
The potential around a wind turbine generator system
increases when it is struck by lightning. To investigate the
potential rises, the fast-front current was injected into the
grounding system, as shown in Fig. 3. The injected current was
much the same as the results shown in Fig. 5(a), where the
peak value was 60 A and the wavefront was about 0.2 μs.
96 m
I
V
foundation foot
10 m
substation facility
Soil
Fig. 6 shows comparisons of the potential rise around the
foundation at several points. As seen in the potential rise of the
foundation, inductivity was seen in the potential rises around
the foundation. The wave shape shown in Fig. 6(a) was almost
analogous to the potential rise of the wind turbine grounding
system shown in Fig. 5(b).
Figure 4. Analytical space for the FDTD simulations.
310
Experiment
voltage [V]
voltage [V]
100
80
60
40
20
0
-20
-40
Simulation
0
2
4
6
8
10
100
80
60
40
20
0
-20
-40
Experiment
Simulation
0
2
4
time [s]
Experiment
Simulation
0
2
4
6
8
100
80
60
40
20
0
-20
-40
voltage [V]
voltage [V]
Simulation
4
6
8
voltage [V]
Simulation
4
6
8
10
time [s]
voltage [V]
(d) Result at 6 m from the foundation
100
80
60
40
20
0
-20
-40
Simulation
2
4
6
6
8
10
8
100
80
60
40
20
0
-20
-40
Experiment
Simulation
0
2
4
6
8
10
If the skin effect of the ground is not considered, the shape
of the grounding system is assumed to be a semi-sphere and the
grounding impedance is assumed to be a pure resistance, the
potential rises around the foundation shown in Fig. 7 are found
to be inversely proportional to the distance from the
foundations. Big differences existed in comparison with the
inversely proportional waveform in Fig. 7(a) because the
measured and calculated results in Fig. 7(a) shows transient
behavior.
Experiment
0
4
Fig. 7 shows the comparisons of the calculated and
measured values of the grounding potential rise around the
foundation. The calculated results agreed well with the
measured results, as shown in Fig. 7. However, the values at
several points had small differences. The straiform ground
resistivity measured by Wenner method as shown in Fig. 2 may
not agree with the actual stratiform ground resistivity
completely. It is an approximate result, and might influence the
above mentioned differences.
Experiment
2
2
(h) Result at 30 m from the foundation
Figure 6. Potential rises around the foundation.
time [s]
0
0
time [s]
10
(c) Result at 4 m from the foundation
100
80
60
40
20
0
-20
-40
Simulation
(g) Result at 20 m from the foundation
Experiment
2
Experiment
10
(b) Result at 2 m from the foundation
0
10
time [s]
time [s]
100
80
60
40
20
0
-20
-40
8
(f) Result at 10 m from the foundation
voltage [V]
voltage [V]
(a) Result at the edge of the foundation
(The distance from the foundation is 0 m)
100
80
60
40
20
0
-20
-40
6
time [s]
On another front, the value of the steady-state grounding
potential rises around the foundation in Fig. 7(b) is almost
10
time [s]
(e) Result at 8 m from the foundation
311
80
204 m
70
current read wire
Experiment
Simulation
500 Ω
voltage [V]
60
I
151 m
50
40
40 m
voltage measuring wire
30
2m
7m
20
44 m
80 m
V
foundation foot
10 m
substation facility
10
0
96 m
current source
Soil
0
5
10
15
20
25
30
35
40
distance [m]
(a) Relation between the peak values of transient potential rises and the
distance from the foundation
Figure 8. Analytical space for the FDTD simulations.
1.2
10
voltage [V]
8
current [A]
Experiment
Simulation
6
1
0.8
0.6
0.4
0.2
0
4
0
5
10
15
20
15
20
time [s]
(a) Injected current waveform
2
0
0
5
10
15
20
25
30 35
voltage [V]
1.5
40
distance [m]
(b) Relation between the steady-state values of potential rises and the distance
from the foundation
Figure 7. Potential rises around the foundation.
0.5
0
inversely proportional to the distance from the foundation
except for the vicinity of the foundation. The potential rises in
the vicinity of the foundation are affected by that of the
substation facility.
VI.
1
0
5
10
time [s]
(b) Potential rise waveform
Figure 9. Calculated results in case of the complete wind turbine
generator system.
POTENTIAL RISE ON THE COMPLETE WIND TURBINE
lightning-channel is about 500 Ω; therefore, the 500 Ω
resistance was connected in parallel with the current source.
The total impedance is almost equal to 1 k which is the surge
impedance of the lightning-channel [13].
In this chapter, the potential rise on the complete wind
turbine was simulated using the FDTD method. The analytical
setup is shown in Fig. 8.
The dimensions of the analytical space were 96 m × 204 m
× 204 m. The length of each blade modeled as a thin wire was
40 m. The nacelle is modeled as a rectangular parallelepiped
conductor with the dimensions of 10 m × 5 m × 5 m. A
lightning-channel is also modeled as a thin wire.
The injected current is shown in Fig. 9(a). The wave front
is about 5.5 s [14]. The potential rise of the foundation is
shown in Fig. 9(b). In this way, the potential rise responses for
all kinds of lightning currents can be simulated using the
FDTD method, once the measurements are verified using the
FDTD method.
The current source parallel with the resistance of 500 Ω
was connected between the top of the blade and the lightningchannel model. The surge impedance of the thin wire model for
312
To solve the mechanism of generation of the potential rise
in the multifarious ground system, impulse tests and FDTD
simulations at many kinds of wind turbine sites should
continue in cooperation with owners of wind turbines.
VII. CONCLUSIONS
This paper has presented the results of experimental and
analytical studies investigating the grounding characteristics of
an actual wind turbine generator system and the potential rises
around it.
REFERENCES
[1]
The calculated injected current agreed well with the
measured current.
There were a few differences on the potential rise of the
wind turbine grounding system. However, the peak and wave
tail value of the calculated potential rise agreed well with the
measured values.
[2]
[3]
The grounding characteristics of the system showed strong
inductiveness at the wavefront; the steady-state grounding
resistance was as low as 0.21 Ω. The steady-state grounding
resistance is usually lower than 10 Ω. In such low resistivity
sites, similar inductive characteristics should be observed.
[4]
[5]
[6]
As seen in the potential rise of the foundation, inductivity
was seen in the potential rises around the foundation. The wave
shape of the potential rises around the foundation was almost
analogous to that of the wind turbine grounding system.
[7]
The calculated results of the potential rises around
foundation agreed well with the measured results. However,
the values at several points had small differences. The
straiform ground resistivity measured by Wenner method may
not agree with the actual stratiform ground resistivity
completely.
[8]
[9]
The peak values of transient potential rises were not in
inverse proportion to the distance from the foundation because
the results showed surge behavior.
[10]
The value of the steady-state grounding potential rises was
almost in inverse proportion to the distance from the
foundation except for the vicinity of the foundation. The
potential rises in the vicinity of the foundation are affected by
that of the substation facility.
[11]
[12]
The potential rise calculation with the complete wind
turbine model had been done for the lightning current with a
typical rise time of 5.5 s. In this way, after verifying the
measurements using the FDTD method, potential responses for
all kinds of lightning currents can be calculated.
[13]
[14]
313
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