Measurement Experience ofLightning Currents to Wind
Turbines
YOSHINORI UEDA* 1

SHINJI ARINAGA* 2

MITSUYOSHI FUKUDA* 3NOBUKI IWAI* 3

TAKATOSHI MATSUSHITA* 3

KOSUKE INOUE* 1

Wind power generation is growing rapidly and is acting a main role for preventing global warming. Yet the number of wind turbine failures due to severe weather conditions in Japan has been increasing. The foremost cause forthis increase in the failure rate of wind turbines has been lightning. The coastal area of the Japan Sea in the
Hokuriku region are prone to "Winter Lightning," a form of high-energy lightning which is rare in most other partsof the world. Winter lightning damages many wind turbines in Japan every year. Mitsubishi Heavy Industries, Ltd.
(MHI) has observed and measured "Winter lightning" which strike actual wind turbines in Japan since 2002. By
doing so, the company hopes to establish a technology for rapidly releasing the lightning energy to the ground whenever lightning strikes. In this paper, the authors analyze lightning current measurements recorded by wind turbines
struck by lightning in various regions of Japan. Observed data reveals that IEC61400-24 protection level 1 is not
sufficient for “Winter lightning.” These results confirm the necessity of Japanese national research project “Guideline for Wind Turbines in Japan.”Fig. 1the most frequent is lightning strikes4 (Fig.
1). The winter lightning in the coastal area of the Japan Sea in the
Hokuriku region causes especially great damage, mainly
Fig. 2).
2 Winter light‑
to the wind turbine blades 2, 5, 6 (Fig.
ning has much larger energy than ordinary lightning.
The temperature differential between the warm ocean current in the Japan Sea and cold air mass in the atmospherecauses vast lightning clouds to develop and to discharge
the accumulated electricity at once. To design wind turbine blades capable of withstanding the large energy
discharged, the properties of the winter lightning must
be grasped in quantitative terms. To attain such a grasp,
MHI has observed and measured “Winter lightning” which
strike actual wind turbines in Japan since 2002. The company has also participated in the NEDO research
projects described above.

1. Introduction
As of March 2007, there were 1,314 wind turbines, witha total capacity of 1,491 MW, installed in Japan.1 As morewind turbines are introduced, the number of wind turbine failures increase in parallel. This has led to concernthat the current international design standard for windturbines (IEC 61400), a standard established mainly by
Europeans, has insufficient provisions for meteorologicalphenomena typical of Japan such as typhoons, mountainwinds, and winter lightning. This concern has compelled
the New Energy and Industrial Technology Development
Organization (NEDO) to research measures to deal withwinter lightning2 and to begin drawing up "Guideline forWind Turbines in Japan." 3Among the causes of wind turbine failures in Japan,

22

Typhoon 1%

Others 2%

Ratio of damage(cases/100 units; Per year)

20

Unclear 17%
Beingexamined 14%

Lightning strike
36%

Maintenance
4%18: Winter lightning areas: Other areas

16
14
12
10
8
6
4
2

Freezing 1%

0Construction 7%

Total damage Blade damage, Control equipment,
etc.
etc.

Fig. 1 Causes of failures of domestic wind turbines (4)Fig. 2 Frequency of damage to wind turbines caused bylightning 5

Design 4%

Manufacturing 14%*1 Power Systems Headquarters*2 Nagasaki Research & Development Center, Technical Headquarters*3 Nagasaki Shipyard & Machinery Works

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)1

ment to measure lightning strikes.8
MHI started its lightning observations in the 600 kW
wind turbine set up at the peak of Goishigamine, at theroot of the Noto Peninsula in Ishikawa Prefecture, in
2002. The Rogowski coil was set in two places, i.e., themain shaft and tower base, to distinguish between thelightning strikes on the turbine blades and those on thelightning conductor on the nacelle (Fig. 4). MHI also inFig. 6
stalled a lightning strike observation camera (Fig.
6) incooperation with the Lightning Center of Hokuriku Electric Power Co., in order to establish a system to visuallyobserve areas struck by lightning.
2.2 Lightning observation implementation sites
Based on the observation experience in Goishigamine,
MHI started to increase the number of measurementsites in 2005. Over the past two winters, 2005-2006 and2006-2007, the company has collected lightning-observation data using 37 units installed at ten measurement
Fig. 7 and Table 1 ).
sites in various regions of the country (Fig.
able
The sites are geographically arranged in a configurationthat will hopefully determine the boundaries of areas
threatened by winter lightning dangers, with help fromsupplementary data collected at four surrounding sites
operated under the NEDO studies on winter lightning
To share information, the NEDO study also relies on
measurement data from MHI's lightning observation in
Goishigamine, Ishikawa Prefecture. In addition, Electric Power Development Co., Ltd. is measuring lightning
strikes in Nikaho, Akita Prefecture, independently fromthe NEDO research2 (Table 1).

2. Measurement of lightning current
2.1 Measurement method
With any attempt to measure a large lightning current directly, the measurement equipment is threatenedby the risk of burnout. As an alternative, the lightning
current can be measured indirectly based on a non-conFig. 33) with a large bore
tact method.7 A Rogowski coil (Fig.
diameter is looped around the tower base of a wind turbine (and the main shaft at the Goishigamine site), forFig. 4use as a current sensor (Fig.
4). When lightning strikes
the turbine and the current passes inside the loop, amagnetic field is generated and an induced current flows
through the Rogowski coil. The magnitude of the original lightning current (charge amount; coulomb) can be
determined by calculating the rate of change in the magnetic field from the induced current measured, and thenintegrating the rate of change. The composition of themeasurement device is shown in Fig. 5
5. The Central Research Institute of the Electric Power Industry (CRIEPI)and Hokuriku Electric Power Co. originally developed thismeasurement technique while researching ways to protect transmission line towers from lightning strikes. Later,
Toko Electric Co. applied this measurement technique tothe tower base of a wind turbine and used actual equip-

External
magnetic
field

Antenna
Current

GPS

Magnetic field

CPU

MemoryI
Fig. 3 Structure of Rogowski coil

A/D
M

Serial I/F

BatteryPower
SupplyI/F
Main shaft (For detectionof lightning strikes on
blades; Installed only in
Goishigamine)

Rogowski coilTA

System bus

Conductor

Mobilepbone

Control
AC100V

Fig. 5 Composition of lightning current measurement device

Tower base(Common to
37 units)

Fig. 6 Camera observation device forlightning strikes

Fig. 4 Lightning current measurement device

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)2

: Measurement sites for MHI
A to D: Measurement sites for NEDOZ:
Measurement sites for Electric Power Development Co., Ltd.
Soyamisaki WF

10

B Tomamae WF

Hamatonbetsu

15
20

10

Z Nikaho WF

5
5

15

15

10

20 5

15

10

Nadachi

20

15 10
20
25
25
30

10

Kamaishi WF
10

30 30

30 35
10
25
10
35
Muroran
25
15
35
30
15
C Uchinada WF
30
30
20
30
15
3030 35
35
Tottori D Taikoyama WF 35
20
30
25
35
15
35
30
Yurihama
25
35
15 15
35 30
20
25
30
25
15
30
25
20
15
Seto WH
Yokohama
30
2020
30
20 20
25
20
30
25 20 2025
20
25
: 35 days or more/year
2025 30
30
30
25
25
35
30
: 25 days or more/year
30
25
25
20
30
30
: 15 days or more/year
20
35
35 25
: Up to 15 days/year
30

A Goishigamine

30

Japan Meteorological Agency, 10-year report on lightning storms, 1968

Fig. 7 Lightning observation implementation sites

MHI

Table 1 List of measurement sites and the number of lightning strikes
Colored portions are winter lightning areas.

Measurement site

Presenceorabsenceof winter
lightning

Terrain

Rated
outputNumber of Number of
Numbercameralightning
Presence or
Number of lightning strikesof windobservation
current
absence ofturbines
devicesmeasurement
receptorin the site
installed 2002-03 2003-04 2004-05 2005-06 2006-07
devices

1 Goishigamine,
Ishikawa Prefecture

Presence

Mountainpeak

600 kW(Absence )Presence1

1

2 Soyamisaki WF,
Hokkaido

Absence

Hill on the
1 000 kW
cape

Presence

57

3 Hamatonbetsu,
Hokkaido

Absence Sea coast 1 000 kW

Presence

Absence Sea coast 1 000 kW
4 Muroran, Hokkaido
5 Kamaishi WF, Iwate
Absence Mountains 1 000 kW
Prefecture
6 Nadachi, Niigata
Presence Sea coast 600 kW
Prefecture
7 Yokohama,
Absence Sea coast 2 400 kW
Kanagawa Prefecture
Sea coast
8 Tottori, Tottori
1 000 kW
Presence
hill
Prefecture
9 Yurihama, Tottori
Prefecture
10 Seto WH, Ehime
Prefecture

11

-

16

119

23

10

-

-

-

3

-

1

1

-

-

-

0

-

Absence

1

1

-

-

-

6

-

Presence

43

7

-

-

-

8

2

Absence

1

1

-

-

-

1

2

Presence

1

1

-

-

-

5

-

Presence

3

3

-

-

-

34

11

Sea coast 600 kW Presence
hill
Ridge ofPresence /Absencethe cape 1 000 kW
absence

1

1

-

-

-

12

5

11

11

-

-

-

2

-

120

37

11

0

16

190

43

Presence

NEDO, etc.

Total
A Goishigamine,
Ishikawa Prefecture
B Tomamae WF,
Hokkaido
C Uchinada WF,
Ishikawa Prefecture

1

1

2

The same as 1
Sea coast
1 000 kW
hill

Presence

20

1

-

-

0

4

-

Presence Sea coast 1 500 kW

Presence

1

1

-

-

14

28

-

D Taikoyama WF,
Kyoto Prefecture

Presence Mountains

750 kW

Presence

6

1

-

-

-

-

-

Z Nikaho WF,
Akita Prefecture

Presence Sea coast 1 650 kW
hill

Presence

15

2

2

-

10

12

-

-

162

42

4

11

10

42

222

43

Grand total

Absence

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)

3

Number of lightning strikes

3.1 Results of lightning strike measurement
Table l shows the results of observations of lightning
currents for the last 5 winters. Most observations arelast 2 winters, during the period from September 2005
through April 2007. According to MHI's observations,
233 lightning strikes hit wind turbines at nine sites. Outof those strikes, 142 (61%) hit Goishigamine. The period
from 2005 to 2006 was a lightning bonanza. On several
occasions, the Goishigamine turbines were barraged bylightning strikes in quick succession over short periods.
In December 2005, for example, the turbines were hit by
13 strikes on the 8th, 18 strikes on the 16th, and 10
strikes on the 21st.
When the NEDO data is included, 222 lightning strikes
were measured at 12 sites with 39 wind turbines from 2005
to 2006. As this was the richest sampling of data, we selected this season for our analysis of the frequency oflightning strikes. As it turned out, the strike frequency was
about 5.7 times per wind turbine per season (as opposed toabout 2.7 times per wind turbine per season if excluding
Goishigamine, the site subjected to the 'bonanza year' for
Table 2lightning) (T2). The frequency of lightning strikes is
at least tenfold higher in winter lightning areas than inother areas. We should point out, however, that this should
only be considered a rough standard for areas where lightning strikes are frequent. When setting up the
Table 2 Frequency of lightning strikes on wind turbines (2005 to 2006)
Number of
Number ofmeasurement lightning
devices
strikes

Site

Frequency

70

140

60

120

50

100

40

80

: Summer season: Winter season: Total: Summer season: Winter season

30
20

60
40
20

10

Accumulated frequency (%)

measurement apparatus for lightning stoke measurement,
there was a bias for wind turbines with a higher risk of
attracting lightning (based on past records of lightning
strikes, terrain, and region). In addition, this frequency is
one digit higher than the rate of damage due to lightning
strikes calculated by NEDO (0.2 times per unit per year;
see Fig. 2). It seems that in some cases, lightning inflicts
no damage to a wind turbine when it strikes.
3.2 Results of lightning current measurement
Figure 8 shows the results of observations of the lightning current for two winters during the period from
September 2005 through April 2007. According to ouranalysis of frequency distribution of lightning currents
by magnitude shown in the figure, larger currents tended
to be less frequent. Observation values exceeded protection level I (300 coulombs: Refer to Table 33) of theinternational design standard IEC 61400-24 9 ten strikes
at four sites: Goishigamine, Tottori, Yurihama andUchinada (NEDO) (all in winter lightning areas)Table 4 ). On December 22, 2005, large lightning strikes
able(T
were measured in multiple sites. The weather map10 on
Fig. 9this day showed a strong winter pattern (Fig.
9).

3. Observation results

445 617

Winter lightning areas 5 (4)7 (6)194 (75)

27.7 (12.5)

Other areas

7 (7)32 (32)28 (28)0.9 (0.9)

Total

12 (11)39 (38)222 (103)

5.7 (2.7)

0
0

50

100

150

200 250300 350 400 525 739

Charge amount (C)Fig. 8 Distribution of lightning currents observed

Note: Values in ( ) are values when Goishigamine is excluded.

Table 3 Protection level standard in IEC 61400-24
IEC's protection level
I

Charge amount (C)II
III & IV

225

300
150

Table 4 Observation data exceeding the IEC protection level I
Charge amountMeasurement site

1

739 C

Tottori

December 6, 2005

2

617 C

Tottori

December 22, 2005

3

525 C

Tottori

December 22, 2005

4

445 C

Uchinada (NEDO)

January 19, 2005

5

415 C

Goishigamine

December 16, 2005

At 9:00 on the 22nd day

6

386 C

Goishigamine

December 16, 2005

7

383 C

Goishigamine

December 8, 2005

8

356 C

Tottori

December 22, 2005

9

337 C

Yurihama

January 7, 2007

10

325 C

Uchinada (NEDO)

December 22, 2005

Fig. 9 Weather map (December 22, 2005)
On the 22nd day (Thursday), a strong winter pattern prevailed again.
The temperature at an altitude of about 5 200 m over Yonago City,
Tottori Prefecture was -40oC or below. Akita City had 56 cm snowfall andKagoshima City had 11 cm. Both cities broke their deepest snow records
for December for the first time in 88 years. Hachijojima-machi, Tokyo,
had a maximum instantaneous wind speed of 38.2m/s (northwestward).

Date of measurement

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)4

3.3 Concerning the receptor
The wind turbine in Goishigamine had non-receptor
blades (nonconducting FRP solid) when it was first built.
Later, in May 2005, the non-receptor blades were replaced with receptor blades (blades with a lightning
conductor run through).
When the turbine operated with non-receptor
blades, lightning struck the blades and the lightning
conductor on the nacelle at a nearly fifty-fifty ratio.
After the receptor blades were adopted, lightning
struck the blade receptor with 100% probabilityTabl
e 5 and Fig. 12
(T
able
12).

A cold air mass moved over even the western part ofJapan. The temperature difference from the warm ocean
current in the Japan Sea expanded, and lightning clouds
developed over wide areas, bringing about snowfall andlightning strikes. The biggest lightning charge, a 739coulomb strike, struck Tottori on December 6, 2005. Ason the 22nd later in the month, the weather map on thisFig. 10day (Fig.
10) showed a cold air mass over even the western part of Japan. Winter lightning apparently resultsfrom "a large temperature differential in the terrestrial
airspace between the warm ocean current of the Japan
Sea and the cold air mass up in the sky."
Figure 1
1 shows the probability lightning strikes
11
based on magnitude, systematically arranged according
to the IEC's protection level. The probability of a lightning strike at IEC protection level I or more is about
4%, and that of a strike with a magnitude of double level
1 or greater is 1% or more. When the high lightningstrike frequency shown in Table 2 is also considered, we
can see that winter lightning in the Hokuriku region isa weather phenomenon specific to Japan and beyond theassumptions of IEC.

225-300:
IEC level I 5%

300-600:
Level I or
above 3%600-: Twiceor more 1%

150-225:
IEC level II 3%

0-150: IEC level III or
below 88%

Unit: coulomb
Fig. 11 Distribution of lightning current probability

500

Blade (C)400

At 9:00 on the 6th day
Fig. 10 Weather map (December 6, 2005)
On the 6th day (Tuesday), heavy snow fell mainlyin the western part of Japan.
Cold air continued to flow mainly into the westernpart of Japan. Snowfall over the 24-hour period
up to 9:00 was 71 cm in Gujo City, Gifu Prefecture;
40 cm in Maniwa City, Okayama Prefecture; and38 cm in Shobara City, Hiroshima Prefecture. In
Gujo City, Gifu Prefecture, the accumulation ofsnow reached 119 cm.

300
200
100

0

100

200

300

400

500

Tower (C)Fig. 12 Comparison between the current value in theblade and that in the tower baseTable 5 Change in the frequency of lightning strikes due to the presence or absence of receptor

PeriodPresence or absenceof receptor

Lightning current
measurement
Blade

Tower

Number of lightning strikes

CameraObservation

Blade Nacelle Unclear

Total

Percentage oflightning
strikes on blades

2002-03
2004-05

Absence
Absence

Presence

Presence

Presence
Presence

6
6

5
4

0
6

11
16

55%
60%

2005-06
2006-07

Presence
Presence

Presence
Presence

Presence
Presence

Presence
Presence

113
22

0
0

6
1

119
23

100%
100%

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)5

Blade
58%

Blade
100%

Nacelle lightning
conductor 42%

Fig. 14 After a receptor was adopted

Fig. 13 No-receptor period

Fig. 13This was confirmed by camera observation (Fig.
14 It also appears that the overall frequency
and Fig. 14).
of lightning strikes on the wind turbines increased by
1.5 to 10 times after the receptor blades were replaced.
But given that the seasonal variation was as large as
1.5 times to 5 times, we cannot affirm that the blade
replacement had a decisive effect.

References
1. NEDO, Wind Turbine Generators Installation Record in Japan (Japanese), March 2007
http://www.nedo.go.jp/enetai/other/fuuryoku/
2. NEDO, “Study for Winter Lightning to Windturbines Annual
Report 2006” (Japanese)
3, NEDO, Solicitation of "Settlement of ‚vind Power Guideline
for Japan" (Japanese), June 28, 2005.
http://www.nedo.go.jp/informations/koubo/170628_1/
170628_1.html
4, NEDO, "Wind Turbine Failures and Troubles Investigating
Committee Annual Report 2005"
5, NEDO & Touyou-Sekkei, "Lightning Protection for WTGS in
Japan (Japanese)", 7th Wind Energy General Seminar at AIT,
June 22-23, 2007
6, NEF, "Wind Turbine Technical Lecture 2004 (Japanese)"
7, Y. Ueda et al, "Measurement of Lightning Currents Fell to
the Actual Wind Turbines", RE2006 at Chiba, Oct.10, 2006
8, T. Otsuka et al, "Direct Measurement of Lightning Current
Through a Wind-Turbine-Generator-Structure", IEEJ Papers
B, Vol.124, No.12, 2004
9, IEC TR 61400-24, "Wind turbine generator systems-Part 24:
Lightning protection", 2002
10,Weather map (No.47, December 2005) at the web site of Japan Meteorological Agency

4. Conclusion
In the two winters during the period from September
2005 through April 2007, MHI observed and measured
lightning strikes on 37 wind turbines set up in 10 sites
in Japan. In total, 233 lightning strikes were recorded.
The measurement results show that the IEC 61400-24
protection level setting is not sufficient for winter lightning.
(1) 4% of lightning strikes in winter exceeded IEC protection level I (300 coulombs).
(2) 1% of lightning strikes in winter exceeded IEC protection level I by more than twofold.
(3) The largest measured value was 739 coulombs, on
December 6, 2005.
Every region on earth has its own meteorological phenomena and climate. Winter lightning is typicalexamples of a meteorological regionality. Similar meteorological phenomena specific to Japan will probably
be found in typhoons and "mountain winds" as well. In
view of this, we recommend that NEDO create a more
appropriate system by establishing "Guideline for Wind
Turbines in Japan."

Yoshinori Ueda

Shinji Arinaga

Mitsuyoshi Fukuda

Nobuki Iwai

Takatoshi Matsushita

Kosuke Inoue

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)6