2012 International Conference on Lightning Protection (ICLP), Vienna, Austria

Lightning Protection of Wind Turbines against Winter Lightning in Japan

Takatoshi Shindo, Megumu Miki, Akira Asakawa Central Research Institute of Electric Power Industry (CRIEPI)

Yokosuka-shi, Kanagawa Prefecture, Japan shindo@criepi.denken.or.jp

bstractCRIEPI has been carried out observation of winter lightning to wind turbines and model experiments to understand the striking characteristics to wind turbines. The anomalous characteristics of winter lightning are clarified and lightning protection methods of wind turbine blades against winter lightning are discussed.

Keywords;lightning, lightning protection, lightning risk, wind turbine

I. INTRODUCTION Recently, wind power generation systems have drastically

increased in Japan. As the increase of the wind power generation systems, outages of these systems by winter lightning have increased. One of the reasons is the large energy of winter lightning. Furthermore upward lightning often occurs in the case of winter lightning. These anomalous features of winter lightning cause severe damage to wind turbines constructed in the coastal area of the Sea of Japan and establishment of effective protection methods is strongly required.

In order to clarify the lightning characteristics to wind turbines, we have carried out observation of winter lightning in the coastal area of the Sea of Japan. Some analytical studies also have carried out in Japan. In addition to that, we have carried out model experiments with an actual wind turbine blade to evaluate lightning protection methods for wind turbines proposed.

In this paper, studies of lightning characteristics and protection of wind turbines against winter lightning carried out in Japan are reviewed.

II. OBSERVATION OF WINTER LIGHTNING TO WIND TURBINES

A. General Characteristcis of Lightning Occurrence in Japan In Japan, lightning database based on 17-year observations

by lightning location systems of electric power utilities has been constructed [1]. Regional occurrence characteristics obtained from the database are shown in Fig. 1. From the database it was found that lightning mainly occurs in the

coastal area of the Sea of Japan in winter and lightning current characteristics are different from those of lightning in summer [1].

Figure 1. Lightning flash density in Japan.

(Average from 2002 to 2008)

a) Summer

b) Winter

Sea of Japan

Sea of Japan

Pacific Ocean

Pacific Ocean

B. Lightning observation at the Nikaho Kougen Wind Farm on the coast of the Sea of Japan [2,3] The observation of lightning flashes striking wind turbines

was conducted at the Nikaho Kougen Wind Farm in the coastal area of the Sea of Japan. Fig. 2 shows the location of the Nikaho Kougen Wind Farm in Japan and Fig. 3 shows the location of wind turbines. There are 15 wind turbines in the Wind Farm and the height of each wind turbine is about 90m. The altitude of the Wind Farm is about 500m.

We have measured lightning current waveforms and attachment features of winter lightning. The current waveforms have been measured with wide-frequency-band Rogowski coils (0.1 Hz-100kHz) [2]. The output of the Rogowski coil is sent to a PC through a fiber-optic link system. We have installed Rogowski coils at four wind turbines shown in Fig. 3. The attachment features of lightning to wind turbine blades have been observed with six still cameras and a CCTV camera.

Fig. 4 shows a typical example of still photographs of upward lightning from a wind turbine. We have obtained 86 still images of upward lightning, but only two images of downward lightning. Multiple termination strokes were also observed.

More lightning flashes struck the 1st, 4th, 8th, 12th, 14th wind turbines compared with other wind turbines. This is due to the location of wind turbines. The 1st, 12th, 14th wind turbines are located at the ends of the line of wind turbines and the 4th and 8th wind turbines are located nearer to the coast than other turbines.

Most of the flashes observed by the still cameras and CCTV camera struck at the tip of the blade. Lightning discharge channels sometimes make a form of arc along the rotation of the wind turbine blade.

We have obtained current wave form of 278 flashes. The ratio of negative, positive, bipolar flashes is 76%, 6%, 18%, respectively. Fig. 5 shows cumulative frequency distribution of the peak current, duration, and transferred charge of each type of flash.

Figure 2. Location of Nikaho Kougen Wind Farm.

a) Wind turbines in the Nikaho Kougen Wind Farm

b) Locations of wind turbines and measurement instruments

Figure 3 Wind turbines and measurement instruments in the Nikao Kougen Wind Farm

Figure 4 Photograph of upward lightning from a wind turbine.

The Sea of Japan

The Pacific Ocean

Figure 5. Cumulative distributions of lighting parameters which struck

wind turbines in winter

The median values of the peak current, duration and charge transfer in the bipolar flash are larger than those of the positive and negative flashes. Especially, bipolar flashes transfer larger charges than other two types do and percentage of flashes with a charge transfer of more than 300 C is 12 % for bipolar flashes. The value of 300 C is the protection level I in the IEC standard [4].

In 2008, We have observed a bipolar flash with a charge transfer of 687 C. Fig. 6 shows the current waveform of the flash. This is the largest charge transfer in our observation.

Table 1 shows observed transferred charge of winter lightning at other sites on the coast of the Sea of Japan. It is clearly shown that the charge transfer of more than 300 C often occurs in the case of winter lightning.

Figure 6 Current waveform of bipolar flash with a charge transfer of 687 C.

TABLE 1. Observation results of transferred charge in winter lightning

Presently, in a 5-year-project of NEDO (New Energy and Industrial Technology Development Organization), direct lightning current observation has been carried out at 25 wind turbines in Japan [5]. Fig. 7 shows cumulative distribution of transferred charge of 284 data for three seasons. The obtained results show same tendency described above.

Figure 7 Distribution of transferred charge at 16 wind turbine sites [5].

Observation site Year Number of samples

Percentage of lightning

with a charge of more than

300 C

Maximum transferred

charge

Goishigamine Wind turbine: H=60m

2004-2006

Total 110 4% 430C

Kashiwazaki & Fukui Tower K: H=80m Tower F: H=200m

1978-1986

Total 97 Positive 32 Negative

65

7% 12% 3%

>1000C

Nikaho Wind turbine: H=90m

2005-2008

Positive 16 Negative

147 Bipolar 42

6% 0% 12%

687 C (Bipolar)

III. MODEL EXPERIMENTS 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 clarify the effects of receptors for lightning protection, some model experiments has been carried out [6-10], however, the related important parameters such as arrangement of the receptors, polarity of an applied voltage, surface contamination have been not studied systematically. We have conducted model discharge experiments of several meters using the tip part of an actual 1MW class wind turbine blade and investigated the effects of the important parameters [11, 12].

Applied voltage waveform is a switching impulse (250/1200 micro sec). The model blades used in the experiments are, a) a model blade without receptor, b) model blade with one disk type receptor, c) model blade with three disk type receptors and a tip conductor (Fig.8), and d) model blade of which top part is covered with metal. In the test, effects of several parameters such as a) polarity of the applied voltage, b) arrangement of the model blade (vertical and horizontal as shown in Fig.9) and c) salt pollution on the blade surface were investigated.

Figure 8 Model blade with a tip conductor.

a) Vertical position b) Horizontal position

Figure 9 Typical experimental configurations.

Figure 10 Photograph of model experiments

An example of the experiments is shown in Fig. 10

Main results obtained from the experiments are as follows.

1) Even without receptors, discharges to the model blade occur. Receptors are therefore one of useful method to protect blades from lightning.

2) In the case of vertical position, whether there is pollution or not, the receptors attract more than 90% and 100% of discharges for positive and negative polarity, respectively.

Fig. 11 shows distribution of discharge points on the blade when positive voltage is applied. Without receptor, they are scattered all over the blade. With a receptor, on the other hand, they concentrate on the top part of the blade. This characteristic is explained from the electric field distribution as shown in Fig. 12. As shown in the figure, lines of electric field of force are not distorted with the blade only, but they concentrate with the earth potential by the receptor and the grounding conductor running through the blade.

Figure 11 Discharge points on the blade. Polarity of the applied voltage is

positive. Scales in the figure is shown in cm.

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340

-50 -30 -10 10 30 50 70

2

0

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-50 -30 -10 10 30 50 70

twice

Tip conductor

receptor

a) With a receptor b) Without a receptor

Fig. 12 Calculated potential distribution and lines of electric field force.

3) In the case of horizontal position, most of discharge first contacts the edge of the model blade, then it reaches a receptor via surface discharge even if the top part of the blade is covered with a metal as shown in Fig. 13. This phenomenon is more likely to occur if the applied voltage is negative and there is pollution on the blade surface. This result suggests that even with receptors, surface discharges on actual wind turbine blade occur if lightning closes to the blade from horizontal direction.

4) In order to clarify the characteristics for upward lightning from wind turbine blades, experiments which simulate upward lightning have been made. At first, the model blade was hung vertically as shown in Fig. 14. In the case of positive polarity, discharge starting from a receptor directly develops to the ground if there is no pollution. With pollution, however, surface discharge occurs. In the case of negative polarity, surface discharge occurs with or without pollution. Without pollution, puncture discharges sometimes occur.

When the model blade was hung horizontally as shown in Fig. 15, puncture discharges occurred for the negative voltage application even with a tip conductor. Puncture discharges of model wind turbine blades were also observed by other researchers [9, 13]. The structure of a down conductor is important to avoid the puncture discharge. However, puncture discharge does not always cause the destruction of the blade and it depends on the amount of the charge and the peak value of the current [14].

Figure 13 Experiment of a model blade of which top part is covered with metal

(Horizontal position).

Figure 14 Model experiments simulating upward lightning from a blade

(Vertical position).

Figure 15 Model experiments simulating upward lightning from a blade (Horizontal position).

In conclusion, the receptor is an effective measure, especially, with a tip conductor or if the tip of the blade is covered by metal. However, the attractive characteristics depend on the direction that lightning approaches from and care should be taken.

Recently, it is found that large wind turbines have effect to collect winter lightning [15]. Analytical studies of lightning striking characteristics to high structures have been also carried out [16]. For the establishment of lightning protection design of wind turbine systems, further investigations taking lightning leader development into consideration and field observation of lightning striking characteristics to wind turbines are indispensable.

Discharges that cause a puncture discharge through a blade have occurred from a down conductor inside the blade. Though it is not clarified yet that this phenomenon occurs in the actual wind turbine blade, it is one of the points that should be taken care for the design of receptors.

Model blade

Earthed plane

IV. LIGHTNING RISK MANAGEMENT For rational lightning protection design, the concept of

lightning risk management is effective. CRIEPI has already proposed a basic concept of lightning risk management [17,18], which consists of lightning hazard evaluation (LHE), lightning risk assessment (LRA), and lightning risk management (LRM) as shown in Fig. 16.

The lightning hazard evaluation, the first process, is to evaluate the severity of lightning. The severity of lightning differs from region to region, of course, but it is not sufficient to consider the frequency of cloud-to-ground lightning of the area. Other factors such as the peak values of lightning currents and the energy of lightning strokes should be also taken into consideration.

The lightning risk assessment, the second phase, is a process to assess the lightning risk from the loss of damage caused by lightning and their occurrence frequency under a given lightning hazard.

The lightning risk management, the third phase, is a process to determine the best policy taking the lightning risk, the loss of the damage and cost of protection schemes into consideration. If the cost of protection is very large, it is necessary to transfer the risk by insurance and so on.

Based on the concept of the lightning risk management shown above, we have developed a Lightning Risk Assessment Program called LIRAP(Lightning Risk Assessment Program) that has two functions of lightning hazard evaluation and lightning risk assessment [19]. Fig. 17 shows a basic structure of the program.

The application of the concept to lightning protection for wind turbines has been carried out and a lightning hazard map for wind turbines has been constructed in Japan as shown in Fig. 18 [20].

Figure 16 Process of lightning risk management

Figure 17 Basic structure of the lightning risk assessment program (LIRAP).

Figure 18. Lightning hazard map of Japan.

V. CONCLUSIONS Lightning that occurs in the coastal area of the Sea of Japan

in winter season, what we call winter lightning, has several anomalous characteristics and severely damages wind power generation systems.

We have studied lightning striking characteristics and possible countermeasures from various aspects such as lightning observation, model experiments and numerical analysis. Based on a concept of lightning risk management, a lightning hazard map for wind power generation systems in Japan has been established.

In Japan, lightning observation and research on damage of wind turbines caused by lightning have been carried out as one of NEDO projects. In CIGRE, a WG has also been working on lightning protection of wind turbine blades. With these activities and results shown in this paper, a reasonable lightning protection design will be established.

High risk area Normal area

Lightning HazardEvaluation

Lightning RiskAssessment

Lightning RiskManagement

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