Reproduction and Test Method of FRP Blade Failure for Wind ...

The 2006-02 - 04th edition 1

Reproduction and Test Method of FRP Blade
Failure for Wind Power Generators by
Lightning

Masahiro Hanai, Hiroshi Koyama, Toshiba Corporation,
Norio Kubo, Ishikawajima-Harima Heavy Industries Co. Ltd.,

Yoichi Hashimoto, JFE Engineering Corporation,
Isamu Suzuki, Daioh Construction Co. Ltd., Ueda Yoshinori, Mitsubishi Heavy Industries Co. Ltd.,

Haruo Sakamoto, Kochi University of Technology

Abstract -- In Japan, the lightning damage to the FRP Index term -- Wind power generator, Lightening
blade for wind power generators is increasing together with protection, FRP blade, Short-circuit generator, Lightning
the increase in wind power generator installation in recent impulse generator, Test method, Pressure
years. Since lightning of Japan looks severely than that of
Europe and the U.S., lightning damage is a big issue of I. INTRODUCTION
Japan. In Kochi (Japan), six grade generators which are 600
to 750 kW are installed. Moreover, some of them were According to the latest global tendency of
damaged by lightning several times. After investigating consideration of maintainable ecology like renewable
literature, we got to know what the cause of serious damage energy technology, wind power became popular. In Japan,
does not understand yet. Therefore, we are establishing a although the power in 1999 was only 2.3x104 kW, the
test method, while clarifying destructive climate by this wind power in the termination of the fiscal year in 2001
lightning, and being able to be made to perform examination reached 3x105 kW. The increasing rate of the power
of the prevention method of destruction. generation capacity in wind power is very large.
Moreover, the target of wind power generator installation
In the high voltage test, it was shown clearly that was changed by the Japanese government to 300 to 3000
lightning penetrates easily FRP that does not perform MW by 2010 in 2001. However, lightning damage is
conductive processing. Moreover, the large current increasing with the increase in installation of an wind
examination showed reappearance of the blade damaged power generator.
there, and showed the conformity of the failure mechanism
and the test method. It turned out that the breakdown The business-related department of Kochi Prefecture
situation of a real machine can be explained by the requested Kochi University of Technology in 2002 to
mechanism. If a lightning passes inside a FRP blade, the inquire by the lightning protection for a FRP blade. We
temperature of air inside a blade will rise. As a result, are going to establish protection technology, have
pressure goes up and a blade is damaged. Moreover, it consulted literature and have investigated the present
turned out that breakdown of positive polarity (winter) knowledge which can be used then since then. However,
lightning can be performed by the large current examination the acquired information was not essential about a FRP
with a short circuit generator. blade lightning damage mechanism and the direct
information about protection.
The experiment was conducted mainly in the Toshiba
Hamakawasaki High Voltage and High Power Testing A certain information about the lightning protection
Laboratory. This testing laboratory is one of the biggest for the airplane material of non-conductivity was
testing laboratories for an experiment about the high voltage aluminum coating [1] [2]. We tried to check the validity
and high power in the world. of aluminum coating from the experiment conducted on a
previous paper [3] and [4]. An experimental result to
Contact address : aluminum coating is effective in protection of lightning
Masahiro Hanai damage. However, aluminum coating will burn by
Toshiba Corporation lightning or will melt. As a result, a color changes and it
2-1 Ukishima-cho, Kawasaki-ku, Kawasaki 201-0082, Japan.
E-mail: [email protected]

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looks dirtily. For this reason, it is thought that aluminum equipment simultaneously. Therefore, the experiment
coating gives one of the methods leading new protection. examined with the equipment made to generate either the
However, it seems that aluminum coating needs to be high voltage or large current.
improved to a slight degree.
At the first step of the reappearance examination, even
Author have done various examinations, in order to if there was no conductive material behind FRP, we
investigate the damage mechanism of a FRP blade. In a checked whether a lightning would penetrate a FRP blade
paper, while explaining the mechanism of damage to a easily by the high-voltage examination. At the second
FRP blade, reference is made about that measurement step, when a lightning passed along the inside of a FRP
method. In the present experiment, we are using 1/2 scale blade by large current examination, it was checked
size model that imitated actual blade composition. whether a FRP blade would explode.

Authors have so far done various examinations, in All of the high-voltage examination and large current
order to investigate the damage mechanism of a FRP examination that simulated the lightning were performed
blade. In a paper, while explaining the mechanism of in Hamakawasaki high-voltage and large current testing
damage to a FRP blade, reference is made about that laboratory of Toshiba.
measurement method. In the present experiment, we are
using 1/2 scale size model that imitated actual blade III. HIGH VOLTAGE EXPERIMENT
composition. The prepared model was prepared three
kinds. They are a FRP plate, one half of the scale size test A. High-voltage impulse test
models of a 250kW wind power generator blade, and the
250kW wind power generator full scale blade which The rated capacity 6MV impulse voltage generator is
divided into four parts. The experiment carried out the used in testing as shown in Figure 2. The size of a
high-voltage examination, which verifies whether a specimen is a FRP board with a length of 2,500mm, a
lightning penetrates a FRP blade easily, and the large width of 430mm, and a thickness of 4mm. The FRP board
current examination, which verifies whether a FRP blade fixed at the lower part, and as a test piece did not break, it
explodes with the current of a lightning. The failure of stood and installed it. The distance between a FRP board
examinations showed reproduction for the actual blade and a high-voltage electrode is 500mm. Three kinds of
failure at site by a lightning almost correctly, and the FRP materials are used for an experiment. They are two
result proved the compatibility of the measurement kinds of FRP containing glass fiber, and FRP in which the
method. copper mesh is contained with glass fiber. And aluminum
coating was performed to each FRP test specimen again.
II. THE FEATURE OF A LIGHTNING, AND ITS SIMULATION And a total of six kinds of test specimen were prepared.

The lightning has simultaneously a high voltage and As follows, the procedure of the experiment charged
large current (energy). For this reason, we need to have the impulse voltage generator to 2MV, and applied it to
test equipment that generates the high voltage and large the high voltage rod electrode. The air gap between a
current (energy) which a lightning has as shown in Figure high-voltage electrode and a test piece generated electric
1 about a method, in order to conduct the experiment that discharge in 400 to 1,000 kV, discharge current flowed
imitated the lightning. The high voltage determines the through the surface of FRP. The peak current at that time
root. Moreover, the amount of large current & electric was about 20kA. Ten electric discharge examinations
charges determines the damage that a lightning generates. were carried out to each test specimen. The days when the
However, we cannot fulfill two conditions by existing test experiment was conducted were 74% of humidity, the
temperature of 4-degree Celsius, and the atmospheric
pressure of 1,015 hPa.

High Voltage
Electrode

Discharge

FRP Plate

Figure 1 Approach to Lightning with High Voltage Figure 2 6MV Impulse Voltage Generator and Set-up
and High Power Equipment of FRP Plate for High Voltage Experiment

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B. Result Table 2 IEC Protection Level [5]

Table 1 shows the result of each experiment. All Protection Peak Specific Average rate Total current
electric discharge of the lightning flowed through the Level IEC current energy
course of aluminum coating about the sample equipped 61024-1-1 (kA) of current rise transfer
with aluminum coating. Although there were some points (kJ/Ω)
that aluminum showed dissolution partially, there was no I 200 (kA/µs) (C)
penetration of FRP. FRP damaged area of two kinds of 150 10000
FRP samples that do not carry out aluminum coating on II 100 5600 200 300
the other hand. In FRP of 45 degrees textile, electric III 2500
discharge penetrated FRP, and generated damage about 150 225
30mm in diameter through the same course with 10 times.
Also, electric discharge penetrated FRP of special textile, 100 150
also made the small hole in several places, and penetrated.
Table 3 Parametric Values in Experiments
On the other hand, although FRP containing a copper by Short Circuit Generator
mesh did not carry out penetration. But the serious burn
happened along the internal copper mesh. As mentioned Protection Peak Specific Average rate Total current
above, in FRP that does not perform conductive Level current energy
processing, it turned out that a lightning is penetrated of current rise transfer
easily and it trespasses upon an inside. I Equivalent (kA) (kJ/Ω)
II Equivalent (kA/µs) (C)
For this reason, at the following step, it was III Equivalent 47.1 11100
investigated whether a site accident could be reproduced IV (Option) 35.3 6230 0.0109 300
when large current would be sent inside a blade. 23.6 2780 0.0082 225
12.5 0.0055 150
IV. LARGE CURRENT EXAMINATION
80

1m

A. An experimental condition and a setup Figure 3 Set up of 1/2 Scale Size Model Specimens

According to the technical report of IEC-International Figure 4 Short Circuit Generator for Large
Electrotechnical Commission (TR61400-24) [5], the Current Experiment
experimental conditions were determined. Table 2 shows
the IEC values describing the lightning parameter. The small wire. The other is in the inside of the blade, and the
protection levels I is the highest total current transfer. wire is guided into the blade from the tip of the electrode.
Using the short circuit generator to conduct the large
current & charge experiment, we set the protection level For the 1/4 cut piece of the actual blade (The location
shown in Table 3. Although the peak current and average is 2/4 from the tip side), the same lightning experiment
rate of current rise between Tables 2 and 3 are different, was conducted. Figure 5 shows the set-up. The figure
the specific energy and total current transfer are almost shows that the 1/4 blade was put on the floor, and the
the same in two cases. We think that the total current electrode is upper on the blade. The electricity guidance
transfer is the most effective value. Since the lowest total to the inside of the blade is from the tip side face of the
current transfer in Table 2 is 150 C, we add the 80 C as test piece about 1m (left hand of the right figure) to the
our optional value in Table 3. artificial hole (right of the right figure)

The test piece of the 1/2 size model is 1/2 width and
1/2 thickness of those for the 250 kW power generator.
However, the length was determined to be 1 m, although
the length of actual blade is 12.6 m. Figure 3 shows the
set-up of the 1/2 scale size model blade. The electricity to
the electrode shown at the upper side of the figure comes
from the short circuit generator shown in Figure 4. The
guidance of the electric was selected in two ways. One is
on the surface of the blade, being guided by a copper

Table 1 High Voltage Experiment Results

45 degree With aluminum Without aluminum
GPRP coating coating

Special weave Passed the aluminum Penetrated on the back
GPRP coating portion side of FRP

GFRP with Passed the aluminum Penetrated on the back
copper mesh coating portion side of FRP

Passed the aluminum Penetrated into the copper
coating portion mesh portion in FRP

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1m

Figure 5 Set-up of Full Scale 1/4 Cut Model Figure 6 Results for 1/2 Scale Size Model Specimens
Specimens (Surface Current of 281C)
10ms
B. Result
Terminal Voltage
1) The experimental result by one half of size
specimens Current

A result is summarized to Table 4. The first three Arc Voltage
experiments are the ones for the surface current. Table 4 1kV
also shows the number of the tests at the left hand. No. 1-
3 are sides A-1 and A-2. Figures 6 shows the result of No. Figure 7 Current and Arc Voltage Waveform of Large
3, that is one of blade burned. Although No.1 and 2 in Current Test (Inside Current of 146C)
Table 4 is not shown in figures, bu the result is almost
same, and the blade was burned. Therefore, if the current
flows on the surface of the blade, the surface is burned,
and no explosion. There remains smoke and soot. It looks
that the function of the FRP blade is capable to be used
again.

However, when the lightning current once flows into
the blade, the blade explodes. Figure 7 shows the results
of Actual current and Arc voltage waveform for the No. 4
in Table 4 test, which is the one of the inside current case
using TP-1. The TP-1 was used after the tests No.1-3
were finished. The result was explosion of the blade as
shown in figure 8. As well as the case of No. 4, No. 5
using TP-2 exploded.

In the tests of 1-3, the lightning current flew on the
surface of the blade. The state of smoke and soot did not
depend on the total current transferred.

2) The experimental result by enough size specimens
The result of the full scale 1/4 cut model specimens is
shown in Figure 9. Size blade with a sufficient portion of
2/4 from the chip side also exploded. Since the power of
the burst was frightful, the blade was fixed with the rope,

Table 4 Conditions and Results of Large Current Test

No. Test Copper Planned Actual condition Result &
Piece Wire Level Peak Total Figures
Location Current Current
(kA) (C) Surface Burned
1 Surface IV 11.9 70.3 Surface Burned
III Surface Burned
2 1/2 Side A-1 I 21.4 129 Figure 8 Results for 1/2 Scale Size Model Specimens
3 (Inside Current of 66.4C)
Model Surface 45.6 281
TP-1 Side A-2 but the blade was damaged at the place where this rope
was rolled. Immediately after conducting a lightning
4 Inside IV 11.1 66.4 Exploded experiment, as we needed to use the fire extinguisher,
smoke and fire arose.
5 1/2 Inside III 22.9 146 Exploded
TP-2 I 42.2 264 Exploded

6 FULL Inside
-1(2/4)

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be occurred also at the total charges of 75C. The

following things can be considered as this cause.

First, the heat by current flowing occurs. For this

4m reason, air inside a blade expands and high pressure is

Figure 9 Results for Full Scale 1/4 Cut Model occurred. As a result, a blade explodes, without the ability
Specimens
to bear high internal pressure.
V. DISCUTTION
As shown in Figure 8, it turned out that the arc in an
A. The simulation of a positive polarity (winter) lightning
with a short circuit air is shared in the voltage of 1000 V/m at 23kA current

This time, the large current examination that from this experimental result. As a result, the calorific
determines the damage of a lightning was performed
using the waveform of the commercial frequency current value (W) per 1 meter by an arc is following.
of 50Hz with the short circuit generator. On the other
hand, as shown in Tables 2 and 3, in the examination W=∫(V*I) dt
using the current of the lightning parameter decided by
IEC TR-61400-24, and this commercial frequency, = V*∫ (I) dt
current increasing rates differ greatly.
=V*Q (1)
However, the lightning strike current of the positive
polarity (winter) lightning actually surveyed in Japan Sea where W is the calorific value (J), V is the arc voltage (V),
has a very small current increasing rate to Figure 10 [6]. It
turns out that especially the first peak current has rise time I is the arc current (A) and Q is the total amounts of
with a frequency of about 50Hz. For this reason, even if
simulating current using a short circuit generator, it is electric charges (C).
considered that the current of a positive polarity (winter)
lightning with such a low current increasing rate is When arc voltages are assumed to be 1000 V/m and
satisfactory.
the total amount 75C of electric charges, the quantity of
15 10ms: 1/2 cycle of 50Hz
heat of 90 kJ/m will occur inside a blade. We can
10
calculate the temperature rise of the air in the 1/2 Scale

Size Model Specimen using the specific heat and volume

of air. At total current is 146C, the temperature rise of the

air will become about 14000 degrees Celsius and air will

expand, if it calculates by making it a base, and it

contributes to the pressure increase of a blade altogether,

the pressure inside a blade will rise to about 50 atm.

Since it seems that it has still sufficiently high pressure

even if air by this expansion escapes through the hole of a

blade etc., it can explain that a blade explodes. Since an

actual blade is 4 times the volume of this 1/2 scale size

model, the almost same pressure increase occurs in 300C

whose amount of electric charges is 4 times of the 75C,

and it can explain that a blade explodes similarly. As

mentioned above, FRP blade failure of the wind power

generator by a lightning showed clearly that it is

reproducible with the high-voltage examination by

lightning impulse voltage generating equipment, and the

large current examination, which uses a short circuit

Current (kA) generator.

5 VI. CONCLUSION

0 In order to solve the mechanism in which a lightning
does damage to the blade of a wind power generator and
0 10 20 30 40 50 to work on the measure, the high-voltage examination and
Time (ms) the large current examination were carried out. As a result,
the following things became clear.
Figure 10 Observed Waveform of Positive Polarity 1. By high-voltage examination, a lightning penetrates the
Lightning Current [6] board made from FRP of an insulator easily.
2. At the blade model of 1/2 scale, in the large current
B. The mechanism of blade failure examination which uses a short circuit generator, when
From the result of a high-voltage examination, when the course of the lightning went into the inside of a blade,
the explosion was occurred at 66C. This value is below
the blade has conductivity, a lightning does not invade the the total amount 80C of electric charge transfers, which is
inside of a blade. On the other hand, when the blade is a 50% value of a positive polarity (winter) lightning.
using FRP with high insulation performance, a lightning 3. When the course of a lightning was the blade surface, it
penetrates FRP easily and can invade the inside of a blade. became damage only to a little burn of the surface also in
From the result of a large current test examination, if a the total amount 281C of electric charge transfers
lightning invades the inside of a blade, an explosion will

The 2006-02 - 04th edition 6

equivalent to level I.
4. The survey current wave type of the positive polarity
(winter) lightning resembles well the half-wave of the
alternating voltage, which uses a short circuit generator.
5. The damage situation of the blade model in the large
current examination performed with alternating voltage
resembles damage to the blade of a real machine. And it
turned out that verification of the lightning strike damage
decided by IEC TR 61400-24 could be performed in the
large current examination, which used the short circuit
generator.
6. FRP blade failure of the wind power generator by a
lightning showed clearly that it is reproducible with the
high-voltage examination by lightning impulse voltage
generating equipment, and the large current examination
with a short circuit generator.

VII. RECOGNITION

The authors greatly acknowledge the support from the
Kochi prefecture government

VIII. REFERENCE

[1] R.H. Golde, Lightning Volume 2, Lightning Protection, Academic
Press,1977

[2] F.A. Fisher, J.A. Plumer, and et al, Lightning Protection of Aircraft,
Lightning Technology Inc., 1999

[3] H. Sakamoto, et al, “Lightning Protection of FRP Blades for Wind
Power Generators”, World Renewable Energy Conference Denver,
Colorado, Sept. 2004

[4] H. Sakamoto and M. Hanai, “Reproduction and Mechanism of
FRP Blades Failure for Wind Power Generators”, Journal, JSME,
to be submitted 2005.

[5] Wind turbine generator systems-Part24: Lightning Protection, IEC
TR 61400-24

[6] K. Yamashita, Y. Mizutani, D. Tanaka, M.Yoda and I.
Miyachi, ”Positive and Negative Current Wveform of Rocket-
Triggered Lightning”, in Proc. 1999 Annual Meeting Record of
IEE of Japan , No.1639, pp.7-30