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C4_306_2012
CIGRE 2012
Studies of lightning protection design for wind power generation systems in
Japan
T. SHINDO
CRIEPI
H. SHIRAISHI
NEDO
S. SEKIOKA
Shonan Institute of Technology
M. ISHII
University of Tokyo
D. NATSUNO
Toyo Sekkei Co., Ltd
Japan
SUMMARY
Recently, wind power generation systems have drastically increased in Japan. As the increase of the
wind power generation systems, outages of these systems by lightning have also increased and
establishment of effective protection methods is strongly required. New Energy and Industrial
Technology Development Organization (NEDO) in Japan has published a guideline for lightning
protection of wind power generation systems based on the field observations and model experiments.
Furthermore, many researchers in Japan have studied effective methods to reduce lightning damages
of wind power generation systems.
The results of these studies are summarized as follows.
1) Most of damage of wind turbine blades has been caused by lightning that occurs in the coastal area
of the Sea of Japan in winter season, what we call ‘winter lightning’.
2) In the case of the winter lightning, charge transfer exceeding the value of 300 C, which is the
maximum value shown in an IEC Technical Report on lightning protection of wind turbines, often
occurs.
3) Model experiments of lightning attachment shows that though the disc-type receptor is one of
effective methods to prevent the damage of wind turbine blades by lightning, discharges cannot be
always intercepted by receptors. On some conditions, lightning hits the blade and causes damage. The
breakage characteristics of wind turbine blades are also clarified from the large current experiment.
4) Surge analysis shows overvoltage characteristics inside the wind power generation systems when
lightning strikes a wind turbine and effective protection methods and grounding systems to reduce the
overvoltages are proposed.
5) A hazard map for wind power generation systems in Japan has been constructed and a cost-effective
protection scheme has been proposed based on the concept of lightning risk management.
KEYWORDS
Lightning - Surge - Wind power generation - Wind turbine - Outage - Grounding - Risk
shindo@criepi.denken.or.jp
1. Introduction
In Japan, wind power generation systems have drastically increased and the total power capacity of
wind power generation is about 2 GW at present. As the increase of the wind power generation
systems, however, outages of these systems by lightning have also increased and the establishment of
effective protection methods becomes inevitable, especially in the coastal area of the Sea of Japan,
where the extent of the lightning damages to wind power generation systems is reported to be greater
than that of other countries. New Energy and Industrial Technology Development Organization
(NEDO) in Japan has published a guideline for lightning protection of wind power generation systems
based on the field observations and model experiments [1]. Many researchers in Japan also have been
studied effective methods to reduce lightning damages of wind power generation systems. In this
report, we have summarized the studies of lightning protection of wind power generation systems in
Japan.
2. Investigation of lightning outages of wind power generation systems in Japan
The outages of wind power generation systems caused by lightning were investigated through
questionnaires sent to wind power generation developers in Japan as one of activities of NEDO [2].
From the answers obtained, we have found the outage characteristics as follows.
1) Most of damage of wind turbine blades caused by lightning occurs in the coastal area of the Sea of
Japan in winter season by what we call ‘winter lightning’. Furthermore outages more likely occur as
the size of a turbine becomes larger.
2) In summer, on the other hand, lightning damage of low voltage circuits such as electronic circuits
and minor blade damage occur all over Japan.
3. Observation of lightning to wind turbines
In Japan, lightning database based on 17-year observations by lightning location systems of electric
power utilities has been constructed [3]. Regional occurrence characteristics obtained from the
database are shown in Fig. 1.
Sea of Japan
Sea of Japan
Pacific Ocean
a) Summer (April to October)
Pacific Ocean
b) Winter (November to March)
Fig.1 Lightning flash density in Japan (Average from 2002 to 2008)
As you can see, most of lightning flashes in winter season are concentrated in the coastal area of the
Sea of Japan. Please note that the values of flash density in winter are much smaller than those in
summer. From the database, it is also clarified that the lightning current peaks are generally larger in
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winter than those in summer [3].
Observations of the winter lightning to isolated towers have been carried out by many researchers in
Japan and it has been reported that the winter lightning often transfer much larger amount of charges
than those of usual summer lightning [4-7]. The continuous or very low frequency components of
lightning currents are dominant for the large charge transfer and it is not an easy task to measure them
accurately at actual wind turbines. Recently observations of the winter lightning to wind turbines at
Nikaho Wind Park have been conducted using a special Rogowski coil of which frequency response is
as low as 0.1 Hz [8]. In addition to current observations, optical observations of lightning striking
characteristics to wind turbines have been made with high speed camera systems and still cameras and
they show that upward lightning from wind turbines often occur in the case of winter lightning [9].
Three types of winter lightning currents have been usually observed; i. e. positive, negative and bipolar.
Fig. 2 shows cumulative distributions of observed current peaks and transferred charges of lightning to
wind turbines at Nikaho Wind Park for these types of currents [9].
a) Current peak
b) Transferred charge
Fig. 2 Cumulative distribution of current peaks and transferred charge observed at Nikaho Wind Park
Table 1 summarizes current observation results of the winter lightning at several sites on the coast of
the Sea of Japan. It is clear that charge transfer exceeding the value of 300 C, which is the value
shown in an IEC Technical Report on lightning protection of wind turbines [10], often occurs in the
case of the winter lightning.
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Table 1 Occurrence of lightning with large amount of charges in winter in Japan
Observation site
Year
Number of
Percentage of lightning
Maximum
samples
with a charge of more
transferred
than 300 C
charge
Goishigamine
2004-2006
Total 110
4%
430C
Wind turbine:
H=60m
Kashiwazaki &
1978-1986
Total 97
7%
>1000C
Fukui
Positive 32
12%
Tower K: H=80m
Negative 65
3%
Tower F: H=200m
Nikaho
2005-2008
Positive 16
6%
687 C
Wind turbine:
Negative 147
0%
(Bipolar)
H=90m
Bipolar 42
12%
4. Model experiments of lightning to wind turbine blades and protection measures
4.1 Model experiments of lightning striking characteristics to wind turbine blade
For protection of wind turbine blades, the most promising one is an external receptor- conductor
system and several receptor systems have been proposed. In order to understand lightning striking
characteristics to wind turbine blades and to verify the effects of these protection schemes, model
experiments have been carried out in Japan [11-13]. In these experiments, a tip part of 3m of an actual
wind turbine blade is used and the model blade was set on various conditions.
a) Vertical arrangement
b) Horizontal arrangement
Fig. 3 Model experiments of lightning to a wind turbine blade
Fig.3 a) and b) show examples of experimental arrangement. Several types of receptors have been
investigated, that is, disc-type receptors of different numbers, edge conductors, a conducting cap that
covers at the top of the model blade and so on. The effect of pollution on the blade surface has been
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also investigated.
The conclusions obtained from these model experiments are as follows.
1) Even in the case of non-conductive blades, surface discharge occurs, especially when the blade
surface is polluted. The surface discharge sometimes causes penetrative destruction on the blade.
2) The disc-type receptor is one of effective methods to prevent the damage of wind turbine blades
from lightning, especially in the case of negative lightning. However, in some conditions, discharges
are not directly captured by receptors but hit the surface or edge of the blade and surface discharges
and/or penetrating discharges occur and may cause damage of the blade.
3) The blade covered with a conducting cap at the top of it shows relatively high protection efficiency.
4) It is clarified that the direction that a lightning leader approaches to is an important factor for the
interception efficiency of the receptor system. Furthermore, discharge may develop from a down
conductor inside the blade and penetrating discharge occurs. These factors should be considered for
the design of lightning protection systems of wind turbine blades.
4.2 Model experiments of breakage of wind turbine blades by large current
When lightning penetrates into a wind turbine blade and arc discharge occurs, breakage of the blade
sometimes occurs. In order to understand the phenomena, model experiments have been carried out
with a short circuit generator [14, 15]. An example of experimental arrangement is shown in Fig. 4.
Copper wire for arc ignition
(The experiments were carried out
with different arc positions)
Current
Receptor
Arc
Down
conductor
2.2m
3m
Pressure sensor
a) Blade model
Blade
Plate
b) Experimental arrangement (horizontal position)
Fig.4 An example of experimental arrangement of large current tests [15]
From these experiments, it is concluded that
1) When lightning strikes a receptor, the receptor is partially melted and the surface near the receptor
is burned. However, the damage of the blade is not severe even if arc discharge develops on the blade
surface.
2) When arc discharge occurs inside the blade, breakage of the blade may occur by the pressure rise in
the blade by the arc discharge. The magnitude of the damage mainly depends on the total charge
injected into the arc discharge.
3) If the injected charge is same, the damage of the blade is larger as the current peak increased and
water exists in the blade.
5. Surge analysis for wind power generation systems
Not only the physical damage of wind turbine blades by lightning, surge voltages generated at low
voltage circuits such as control and communication systems are important for lightning protection
design. The magnitude of the surge voltages depends on the configuration of internal circuits and surge
protection methods of a wind power generation system and its grounding systems. In order to establish
effective protection schemes, numerical analysis with the EMTP (Electro-magnetic Transients
Program) and the FDTD (Finite-Difference Time-Domain) method, and scale model experiments have
been carried out [16-21]. The FDTD method is useful for high frequency phenomenon. Considering
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winter lightning sometimes has large energy as mentioned in section 3, The EMTP is still useful tool.
Based on the results, following conclusions are obtained for effective protection methods and
grounding systems to reduce the overvoltages.
1) When lightning strikes a wind turbine, overvoltages are generated between lines coming from
outside of a wind power generation tower and frames of devices inside the tower. The overvoltages
can be reduced by connecting the sheath of the lines to the tower grounding system.
2) If there is a loop circuit, the induced overvoltage should be taken into consideration.
3) Generally speaking, ring earth electrodes are effective to reduce touch and step voltages generated
by lightning.
4) In a wind farm where several wind turbines are connected in series, lightning surges are likely to
propagate towards the end of a distribution line in the wind farm and there is a possibility that failure
occurs at a wind turbine that is not struck by lightning.
5) When lightning strikes a wind turbine, lightning currents may flow from wind turbines to
distribution lines and the currents often cause serious damages in surge protective devises in the case
of the winter lightning with large energy.
6. Lightning risk management for wind power generation systems
Considering the cost performance for the lightning protection design, the concept of risk management
is effective [22]. Risk is defined as a product of loss and its occurrence frequency and risk
management considers both the risk and cost of various protection measures and finds the most costeffective way to protect wind power generation systems. In the case of wind power generation systems,
damage of wind turbines is the most severe from the viewpoint of the cost and period for repair [23].
From the studies shown above, it was found that the important parameters of lightning to cause
damage are frequency of lightning occurrence and the energy of lightning flashes. We have made a
lightning hazard map in Japan considering these parameters. An example of the hazard map is shown
in Fig. 5. Based on the hazard map and the concept of lightning risk management, effectiveness of
various lightning protection measures has been evaluated [1, 2]. In the high-risk area, either placing a
receptor on the tip of a blade or covering the tip of a blade with metal is recommended, and the current
capacity of down conductors should be taken into consideration with care. In the case of the winter
lightning, concentration of lightning hits to tall structures is observed [24] and these effects should be
taken into consideration for detailed risk assessment. An independent lightning tower is also
mentioned as a possible lightning protection measure against winter lightning in the guide [1].
Installation of it on the upwind side of the plant is recommended considering the prevailing wind
direction in winter.
7. Conclusions
We have surveyed lightning damage of wind power generation systems in actual fields in Japan and
found that lightning that occurs in the coastal area of the Sea of Japan in winter season, what we call
winter lightning, has anomalous characteristics and severely damages wind power generation systems.
We have also studied outage phenomena of wind power generation systems and countermeasures from
various aspects such as lightning observation, model experiments and numerical analysis. Based on
these results, a lightning hazard map for wind power generation systems in Japan has been made and
the most cost-effective scheme for lightning protection of wind power generation systems has been
established. However, there still are many points to be clarified in establishing lightning protection of
wind turbines in Japan, especially the lightning phenomena, testing methods and quantitative
evaluation of lightning protection measures. For that purpose, a project by NEDO to observe lightning
current waveform has been carried out.
It has been reported that similar lightning phenomena occur not only in Japan but also in some other
areas in the world. Reflection of the research results shown above to international standards on
lightning protection design for wind power generation systems will be of benefit to international
community.
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a) Summer
b) Winter
0-249 flashes/year
0-49
flashes/year
250-499 flashes/year
50-99
flashes/year
500-749 flashes /year
100-149 flashes/year
750<
150<
flashes/year
Number of wind turbines
flashes/year
Damage of wind turbines
: very severe
: 1-10
: 11-30
: 30<
: relatively severe
: light
: very light
Fig. 5 An example of lightning hazard map. The colors in the map show the number of lightning
flashes of which currents are more than 50 kA in a mesh size of 20 km by 20 km. The size of each
circle indicates the number of wind turbines. The color of each circle indicates the reported damage
level of wind turbines. For more detailed classification of the damage levels, please see Ref. 1.
Bibliography
[1]
[2]
[3]
[4]
[5]
NEDO, “Guideline for wind power generation in Japan – Lightning protection” (2008). (in
Japanese).
D. Natsuno, S. Yokoyama, T. Shindo, M. Ishii, H. Shiraishi, “Guideline for lightning protection
of wind turbines in Japan” (30th International Conference on Lightning Protection (ICLP), No.
SSA-1259, Cagliari, 2010).
T. Shindo, H. Motoyama, A. Sakai, N. Honma, J. Takami, M. Shimizu, K. Tamura, K. Shinjo, F.
Ishikawa, K. Miyazaki, M. Ikuta, D. Takahashi, “Lightning occurrence data observed wit
lightning location systems of electric power companies in Japan:1992-2008” (30th International
Conference on Lightning Protection (ICLP), No. 2A-1032, Cagliari, 2010).
K. Miyake, T. Suzuki, M. Takashima, M. Takuma, T. Tada, “Winter lightning on Japan Sea
coast -lightning striking frequency to tall structures-” (IEEE Transactions on Power Delivery,
Vol.5, No.3, 1990, pages 1370-1376).
K. Miyake, T. Suzuki, K. Shinjou, “Characteristics of winter lightning current on Japan Sea
Coast” ( IEEE Transactions on Power Delivery, Vol.7, No.3, 1992, pages 1450-1457).
6
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
A. Asakawa, K. Miyake, S. Yokoyama, T. Shindo, T. Yokota, T. Sakai, “Two types of lightning
discharges to a high stack on the coast of the sea of Japan in winter” (IEEE Transactions on
Power Delivery, Vol.12, No.3, 1997, pages 1222-1231).
M. Miki, V. A. Rakov, T. Shindo, G. Diendorfer, M. Mair, F. Heidler, W. Zieschank, M. A.
Uman, R. Thottappillil, D. Wang, “Initial stage in lightning initiated form tall objects and in
rocket-triggered lightning” (J. Geophys. Res., Vol.110, D02109, 2005).
A. Asakawa, T. Shindo, S. Yokoyama, H. Hyodo, “Direct lightning hits on wind turbines in
winter season: Lightning observation results for wind turbines at Nikaho wind park in winter”
(IEEJ Trans. Vol. 5, No. 1, 2010, pages 14-20).
M. Miki, T. Miki, A. Wada, A. Asakawa, Y. Asuka, N. Honjo, “Observation of lightning flashes
to wind turbines” (30th International Conference on Lightning Protection (ICLP), No. 1A-1149,
Cagliari, 2010).
IEC TR61400-24, “Wind turbine generator systems part-24: Lightning protection” (2002).
T. Naka, N. J. Vasa, S. Yokoyama, A. Wada, A. Asakawa, H. Honda, K. Tsutsumi, S. Arinaga,
“Study on lightning protection methods for wind turbine blade” (IEEJ Transactions on PE,
Vol.125, No. 10, 2005, pages 993-999).
T. Shindo, A. Asakawa, M. Miki, “A study of lightning striking characteristics to wind turbines”
(29th International Conference on Lightning Protection (ICLP), No.9c-4, Uppsala, 2008).
T. Shindo, A. Asakawa, M. Miki, “Lightning Striking Characteristics to Wind Turbine Blades Experimental study of effects of the receptor configuration and other parameters-” (IEEJ
Transactions on PE, Vol.129, No.2, 2009, pages 331-339). (in Japanese)
M. Hanai, H. Ikeda, M. Nakadate, H. Sakamoto, “Large current lightning mimic test for FRP
blades of wind turbine generators” (IEEJ Transactions on PE, Vol.127, No.3, 2007, pages 531536). (in Japanese)
Y. Goda, S. Tanaka, T. Ohtaka, “Arc tests of wind turbine blades simulating high energy
lightning strikes” (29th International Conference on Lightning Protection (ICLP), No.9c-5,
Uppsala, 2008).
Y. Yasuda, T. Hara, T. Funabashi, “Analysis on lightning surge propagation in wind farm”
(IEEJ Transactions on PE, Vol.125, No.7, 2005, pages 709-716). (in Japanese)
K. Yamamoto, T. Noda, T. Yokoyama, S. Ametani, “An experimental study of lightning
overvoltages in wind turbine generator systems using a reduced-size model” (Electrical
Engineering in Japan, Vol.158, No.4, 2007, pages 22-30).
Y. Yasuda, N. Uno, H. Kobayashi, T. Funabashi, “Surge analysis on wind farm when winter
lightning strikes” (IEEE Transactions on Energy Conversion, Vol.23, No.1, 2008, pages 257262).
M. Nagao, N. Nagaoka, Y. Baba, A. Ametani, “FDTD analysis of the current distribution within
the grounding system for a wind turbine generation tower struck by lightning” (IEEJ
Transactions on PE, Vol.128, No.11, 2008, pages 1393-1399). (in Japanese)
K. Yamamoto, S. Yanagawa, S. Sekioka, S. Yokoyama, “ Transient grounding characteristics of
an actual wind turbine generator system at a low resistivity site” (IEEJ Transactions Vol.5, 2010,
pages 21-26).
H. Okamoto, S. Sekioka, Y. Ebinuma, K. Yamamoto, Y. Yoh, T. Funabashi, S. Yokoyama,
“Energy absorption of distribution line arresters due to lightning back flow current and ground
potential rise for Lightning hit to wind turbine generator system” (IEEJ Transactions on PE, Vol.
129, No.5, 2009, pages 668-674). (in Japanese).
T. Shindo, T. Suda, “A study of lightning risk” (IEEJ Transactions on Electrical and Electronic
Engineering, Vol.3, No.5, 2008, pages 583-589).
T. Shindo, T. Suda, “Lightning risk on wind turbine generator systems” (IEEJ Transactions
on PE, Vol.129, No.10, 2009, pages 1219-1224).
M. Ishii, M. Saito, F. Fujii, M. Matsui, D. Natsuno, “Frequency of upward lightning from tall
structures in winter in Japan” (7th Asian-Pacific International Conference on Lightning (APL),
TH-AM-A2-4 No.304, Chengdu, 2011).
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