1. X International Symposium on
Lightning Protection
9th-13th November, 2009 – Curitiba, Brazil
A STUDY OF TRANSIENT CHARACTERISTICS
OF AN ACTUAL WIND TURBINE GROUNDING SYSTEM
Kazuo Yamamoto1, Shunichi Yanagawa2
Koichi Yamabuki3, Shozo sekioka4, Shigeru Yokoyama5
1
Kobe City College of Technology, Japan – kyamamoto@mem.iee.or.jp
2
Shoden Company, Japan – yanagawa@sdn.co.jp
3
Wakayama National College of Technology, Japan – yamab@wakayama-nct.ac.jp
4
Shonan Institute of Technology, Japan – sekioka@elec.shonan-it.ac.jp
5
CRIEPI, Japan – yokoyama@criepi.denken.or.jp
Abstract - In order to exploit high wind conditions, wind transient characteristics of the grounding by
turbine generator systems are often constructed in places experimental and analytical methods using a reduced-
where few tall structures exist; therefore, they are often size model of current wind turbine foundations [8-11].
struck by lightning. Much of the damage caused by Research using simulations of the transient and steady-
lightning is from the resulting breakdown and malfunction
state grounding characteristics of wind turbine
of the electrical, communications, and control systems
inside the wind turbine generator system; these foundations has already been presented [12-22]. However,
breakdowns can be attributed to a rise in electric potential papers that report using an actual wind turbine generator
both within the system and in the surroundings due to system to study transient grounding characteristics are
lightning. Impulse tests were conducted on an actual wind very few in numbers [23].
turbine generator system. The rise in ground potential of
the system, and that around its foundation was measured. In this paper, we present experimental studies of the
The frequency characteristics were calculated using the impulse tests conducted on an actual wind turbine
Laplace transform to get voltage responses for all types of generator system. The ground potential rise of the system
lightning current waveforms. As a typical potential rise
itself, and around its foundations, was measured. The
response, the response to the step current which has the
peak value of 1 A was calculated. frequency characteristics were calculated using the
Laplace transform [24] to get voltage responses to all
1 INTRODUCTION types of lightning current waveforms. As a typical
potential rise response, the response to the step current
A Based on the diffusion of wind turbine generation which has the peak value of 1 A was calculated. When
systems, many accidents caused by natural disasters such lightning strikes the wind turbine generator system
as lightning and typhoons have occurred in recent years. constructed at a site where the grounding resistivity is
Especially, the damages caused by lightning become so very low, the potential rise at the wave front typically
serious [1-5]. Wind turbine generation systems are built becomes larger than that of the steady state. This is
at locations where few tall structures are found nearby so because of the inductivity of the grounding system.
as to obtain good wind conditions, and thus, they are Therefore, the transient characteristics of the grounding
often struck by lightning. To promote wind power system become important, in comparison to its steady-
generation, lightning protection methodologies for such state characteristics.
wind turbine generation systems have to be established.
2 GROUNDING OF WIND TURBINE
Lightning damage to wind turbine generator systems GENERATOR SYSTEM
affects the safety and reliability of these systems. Most of
the breakdowns and malfunctions of the electrical and 2.1 Importance of Transient Characteristics
control systems inside wind turbines are caused by a rise
in ground potential due to lightning [6, 7]. To understand Both transient and steady-state characteristics become
this rise in ground potential, we had researched the important for understanding the grounding phenomena
of a wind turbine generator system. However, because the
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2. steady state is emphasized in the planning of the
grounding, the transient characteristics are often
neglected.
When a wind turbine generator system is constructed in a 8.5 m
mountain area where resistivity is comparatively high,
the steady-state grounding resistance, in many cases,
becomes more important than the transient grounding
resistance. A potential rise caused by a lightning strike to 3m
a wind turbine generator system is more remarkable at
the wave tail than at the wave front. The potential rise at
the wave tail depends on the steady-state grounding
resistance. When a wind turbine generator system is 2m
constructed at a low resistivity site, such as a coastal area,
a significant potential rise occurs due to the inductivity of grounding mesh
the grounding system. The transient grounding resistance
foundation foot : 50 m
at the wave front, which depends on the inductivity of the
grounding system, is larger than the steady-state
resistance.
2.2 Grounding of an Offshore Wind Turbine
The soil around the actual wind turbine generator system Fig. 1 - Foundation of the actual wind turbine generator system
on the disposal site where the measurements has been
performed has electrical characteristics similar to
seawater, because the soil on the disposal site contained a
lot of seawater. The target wind turbine generator system
had four long foundation feet, like those of offshore wind ρ1 = 15 [Ωm] d1 = 3.0 [m]
turbines, to increase the bearing capacity of the soil. The
grounding characteristics of the foundation constructed
on the disposal site exhibited inductivity in the way
explained in the previous section. Construction of
offshore wind turbine generator systems is prohibited in
Japan because of fishery rights, destruction of the ρ2 = 1 [Ωm] ∞
environment, and so on. However, there are wind turbine
generator systems on the coastal area. Depending on the Fig. 2 - Resistivity around the wind turbine generator system
governmental energy policy, offshore wind turbine estimated by Wenner method
generator systems may be constructed in the future [5].
Therefore, the grounding characteristics of the wind Grounding mesh existed underneath the foundation; its
turbine foundation on disposal sites should be researched size was about 8.5 m × 8.5 m. The stratiform resistivity
to estimate the grounding characteristics of low around the wind turbine generator system is shown in Fig.
resistivity sites. 2. The Wenner method was utilized to measure the
resistivity. The steady-state grounding resistance of the
3 MEASURMENTS grounding system of the wind turbine generator system
was 0.062 Ω.
3.1 Foundations
3.2 Experimental Conditions
Fig. 1 shows in detail the foundation of the actual wind
t ur bi n e gen er a t or syst em t h a t wa s used in our Fig. 3 shows the experimental set up. The current was
measurements. The shape was rectangular and parallel- led to the foundation from the impulse generator by using
piped, and 8.5 m × 8.5 m × 2 m in size. The foundation insulated copper wire (length: 100 m; cross section: 5.5
wa s r ei n for ced con cr et e; t h e i ntervals between mm2) as the current lead wire. The height of the current
reinforcing were about 30 cm. The tower was connected lead wire was about 1 m. The fast front current generated
to the foundation at ground level. The depth of the by the impulse generator was injected into the foundation
foundation was 2 m, and the length of the foundation feet through a resistance of 500 Ω from a current lead wire,
was 50 m, to enhance the bearing capacity of soil. as shown in Fig. 3. The peak value of the current was 60
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3. was connected between the remote end of the voltage
measuring wire and grounding rod, which had about 120
Ω grounding resistance. That was how the noise induced
on the voltage measuring wire was discharged to the
current lead wire ground readily.
150 m
ground I.G. 3.3 Experimental Data
I Voltage measuring wire
The current into the foundation and potential rise at the
foundation 70 m foundation was recorded to study the transient and
V steady-state characteristics of the foundation. The
potential rises around the wind turbine generator system
were measured at intervals of 1 m (0 to 10 m from the
edge of the foundation, as shown in Fig. 4) around the
foundation, and from 2 m to 4 m (over 10 m from the
Fig. 3 - Experimental set up
edge of the foundation, as shown in Fig. 4). The potential
Top view
rise was measured at 21 locations. An additional rod was
buried about 0.1 m deep, at each measured point, to
measure potential rise.
1m 3m 5m 7m 9m 12 m 16 m
0m
2 m 4 m 6 m 8 m 10 m 14 m 18 m 3.4 Measuring Instruments
foundation The impulse generator had a capacitance of 1.5 μF, and
Side view was discharged by using a gap switch. The charging
1m 3m 5m 7m 9m 12 m 16 m voltage was 30 kV for these measurements.
0m
2 m 4 m 6 m 8 m 10 m 14 m 18 m
A TDS3054C oscilloscope (Tektronix) was used to
measure the voltage and current waveforms; its
bandwidth was DC–500 MHz. A P6139A passive probe
(Tektronix) was used for voltage measurements; its
bandwidth was DC–500 MHz and its input capacitance
was up to 8 pF. A PEARSON 150 was used as the
current probe; its bandwidth and usable rise time were in
Fig. 4 - Measuring point of the potential rise around the the range of 40 kHz to 20 MHz and over 20 ns
foundation respectively. The measurements performed using these
instruments were accurate, with a rise time of several
A and the wave front was about 0.4 μs. A comparatively hundred nanoseconds.
large resistance of 500 Ω was connected in series with
the impulse generator; it can therefore be considered a 3.5 Measured Results
current source.
The measurement results are shown in Fig. 5. Figs. 5 (a)
The injected current was measured at the end of the and (b) show, respectively, the injected current I and the
current lead wire near the foundation by using a current potential rise V at the top of the foundation. The injected
prove as shown in Fig. 3. The potential rise of the current showed a ramp wave, and its peak and rise time
foundation was measured as the voltage difference were, respectively, approximately 60 A and 0.4 μs. The
between the top of the foundations and the voltage voltage was inductive at the wave front. The ratio of the
measurement wire. The height of the wire was 1 m, and maximum voltage at the wave front to the current at the
it was grounded at the remote end. The potential rise same time was approximately 13 V/A. This value was
around the wind turbine generation system was measured greater than the steady-state grounding resistance. The
as the voltage between the conductive rods placed at the voltage waveform oscillated after the wave front. The
measurement points, as shown in Fig. 4, and the voltage medium value of the voltage gradually decreased to the
measurement wire. As shown in Fig. 3, the current lead value of the steady-state grounding resistance. It is
wire and the voltage measurement wire were believed that the oscillations were caused by the
orthogonalized to decrease their mutual electromagnetic inductance and capacitance of the grounding system. It is
induction. The surge impedance of the voltage measuring possible that the steady-state grounding resistance of
wire was about 500 Ω; therefore, the 400 Ω resistance 0.062 Ω was the convergence value.
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4. As mentioned above, the grounding characteristics of the
system showed strong inductiveness at the wave front
because the steady state grounding resistance was as low
as 0 0 6 2 In the case of offshore wind tu bine
. Ω. r
generator systems, similar grounding characteristics
should be observed. Transient phenomena obviously
become more important than steady-state phenomena for
lightning protection design.
The potential around a wind turbine generation system
increases when it is struck by lightning. To investigate
the potential rise, the fast-front current was injected into
the grounding system, as shown in Figs. 3 and 4. The
injected current was very similar to the results shown in
Fig. 5 (a), the peak value was 60 A and the wave front
was about 0.4 μs.
Fig. 6 (a) shows the measured potential rise around the
wind turbine generator system. Fig. 6 (b) shows the
relationship between the maximum potential rise and the
distance.
The wave shape, shown in Fig. 6 (a), was almost
analogous to the potential rise shown in Fig. 5 (b). If the
skin effect of the ground is not considered, and the
80
60
current [A]
40
20
0
0 1 2 3 4 5
time [µs]
(a) Injected current into the foundation
500
400
voltage [V]
300
200
100
0
-100
0 1 2 3 4 5
time [µs]
(b) Potential rise of the foundation
Fig. 5 - Transient characteristic of the grounding system of the
actual wind turbine generator system

5. responses, the time response to lightning of several wave 4 CONCLUSIONS
shapes can be calculated.
This paper has presented the results of experimental
It should be noted that above mentioned measured results studies that investigated the grounding characteristics of
include the influence of the surge propagations on the a actual wind turbine generation system, and the voltage
tower and blades, the induced voltage on the voltage rise around it. The grounding characteristics of the
measuring wire from the current lead wire and so on. If grounding system showed strong inductivity at the wave
we want to obtain the independent grounding front. The frequency and step responses of the grounding
characteristic of the foundation, the model of the wind system have been presented to get voltage responses to all
turbine with the grounding system should be established types of lightning current waveforms.
in the numerical electromagnetic field analyses such as
FDTD (Finite-Difference Time-Domain) method, and the The installation features of the wind turbine generator
independent model of the grounding system should be system that were employed in this paper were very
calculated. similar to those used at sea. The long foundation feet
were much like those of an offshore wind turbine
generator system. The results given in this paper will be
30 very useful as basic data for lightning protection of wind
25 turbine generator systems at low resistivity sites,
impedance [Ω]
including those of offshore wind turbine generator
20
systems.
15
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