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Electric Power Systems Research 94 (2013) 3– 9

Contents lists available at SciVerse ScienceDirect

Electric Power Systems Research

jou rn al h om epage: www.elsev ier .com/ locate /epsr

ightning protection of wind turbine blades

higeru Yokoyama ∗

entral Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japan

r t i c l e i n f o

rticle history:vailable online 13 August 2012

a b s t r a c t

The causes of lightning damages of wind turbine blades were clarified through lightning observation

eywords:ightning protection

ind power turbineinter lightning

and experiments using a high-voltage impulse generator. Upward leaders and large energy of winterlightning, which is frequent along the coast of the sea of Japan, play an important role upon lightningdamages of wind turbine blades. Lightning discharges may penetrate into the cavity of a blade withoutlightning receptors, resulting in serious damages, such as an destruction and falling of a blade. Althoughlightning receptors are totally useful for lightning protection of wind turbine blades, they are not perfect.

lade

. Introduction

The capacity of wind turbine generators has been increasing andhe most popular one is 1000–2000 kW. Lightning protection forhese large wind turbine generators is more important than thator small size. The damages of blades (Fig. 1) need much expenseecause of transportation of a large blade and replacement of it. Tolarify a generating mechanism of blade damages and to establishuitable lightning protection methods such as a proper arrange-ent of receptors are quite important for the further spread of wind

ower generation systems.Wind turbine generators have the following characteristics from

he viewpoint of lightning performance.

1) Long turbine blades.2) Wind turbine blades are composed of insulated materials such

as fiber reinforced plastics and those materials can be burnwhen struck by lightning.

3) A blade is composed of two pieces of boards made of fiber rein-forced plastics and there is a cavity between two boards stickedwith a powerful adhesive.

4) Wind turbine blades are rotating during power generation andthe lightning attachment point moves following the azimuthangle of a blade.

5) Windmills in a wind farm are separated by several hundredmeters each other.

In order to establish the better protection measures, we havenvestigated the characteristics of lightning attachment manner

∗ Tel.: +81 90 9962 6914; fax: +81 463 92 2164.E-mail addresses: rai.yokoyama@hb.tp1.jp, yokoyama@criepi.denken.or.jp

378-7796/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.epsr.2012.07.017

© 2012 Elsevier B.V. All rights reserved.

and lightning current especially for winter lightning, which occursfrequently along the coast of the sea of Japan. Also we have exper-imented the lightning attachment manners to wind turbine bladesusing a high-voltage impulse generator. Possible causes of bladedamages are estimated by above experiments.

2. Lightning outages of wind turbine

The average frequency of lightning outages is 3.9–8.0% a yearin Europe from 1991 to 1998 [1] .In Japan the average frequency oflightning outages is around 10–20% a year from 2002 to 2006 [2]. Inthe coast of the Sea of Japan that value is around 30%. Blade damagesoccupy 75% of the total damages for large wind turbine generatorsover 1000 kW. These values were got by the investigation for thewind power companies.

The damages of blades need much expense because of trans-portation of a large and heavy blade and replacement of it. It isimportant to clarify further the characteristics of winter lightningin the coast of the Sea of Japan in order to explain the reason ofincrease of lightning damages in winter seasons [3–6].

Yokoyama et al. showed the various aspects of lightning dam-ages in wind turbine blades [7].

3. Attachment manner of lightning discharges on a blade ofwind turbines and lightning current waveform

3.1. Picture of multiple strokes to a wind turbine blade taken by astill camera

Fig. 2 shows the lightning stroke to a rotating blade. The dis-charge point is drawn by the rotating blade. The rotating bladeleaves the first lightning attachment point at the back of it, becauseof the time interval of multiple strokes or long continuing current.

4 S. Yokoyama / Electric Power Systems Research 94 (2013) 3– 9

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Fig. 1. Blade dam

.2. Lightning with unusually large amount of electric charges

As for winter lightning in Japan, lightning currents with unusu-lly long time duration are frequently observed. The amount oflectric charge is expressed by the time integration of an instantightning current value. For winter lightning, there are many light-ing flashes, the electric charge of which exceeds 300 C. This valuemounts to 10 or more times as much as the value of usual summerightning. Due to this phenomenon, it is easy to ignite a windmilllade or to break a blade when the arc of a lightning discharge

nvades into an inside of a blade cavernous part.Moreover, there are damages of surge arresters inserted for

ightning protection of electrical and an electronic circuit. Uedat al. report statistical value of the amount of electric charges forwo winter seasons at four winter lightning regions along the coastf the Sea of Japan [8]. This report shows that lightning with the big

mount of electric charges over 300 C has occurred 8 times. 4% ofotal lightning flashes exceeds the value of protection level I shownn IEC TC62305-01or IEC 61400-24 [9]. Hongo et al. reported that

Fig. 2. Lightning discharge to rotating blade.

due to lightning.

winter lightning with the charge of 910 coulombs was observed inthe tower for mobile phones located in the coast of the Sea of Japan[10] (Fig. 3).

4. Experimental results of lightning attachment mannersusing high voltage impulse generator

4.1. Experimental equipments

Lightning attachment manner to wind turbine blades was inves-tigated by using a 12MV high voltage impulse generator at Shiobaratesting yard of Central Research Institute of Electric Power Indus-try (Fig. 4). The voltage waveforms are lightning impulse (L.I.) andswitching impulse (S.I.) as shown in Fig. 5 [11,12].

Three-meter long blade-sample was cut from an actual twelve-meter long wind turbine blade made of GFRP (glass fiber reinforcedplastics). The blade-sample was fixed vertically on a wooden foun-dation, and a gap length between an electrode and the tip of theblade-sample was set to four meters.

4.2. Experiments on non-conductive blade

In order to investigate an influence of pollution on a blade, anartificial pollution test was performed. The polluted blade-sampleswere prepared by spraying the water mixed with salt and powdered

Fig. 3. Long continuing current.

S. Yokoyama / Electric Power Systems Research 94 (2013) 3– 9 5

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Table 1Experimental results of non-conductive blade-sample.

Discharge manner Number of times

Non-polluted Electrode-to-ground 28Creeping 8

Fig. 4. 12 MV high-voltage impulse generator at Shiobara testing yard.

lay. The degree of pollution was set at the equivalent salt depositensity (ESDD) value of 0.1 mg/cm2, which corresponds to high pol-

uted condition near the sea coast. The 50% flashover voltages (FOV)f polluted and non-polluted blade-samples were measured forositive switching impulse voltages by the up-and-down method.s a reference, the 50%FOV of three-meter long metallic pipe waslso measured.

Table 1 shows the experimental results and Fig. 6 shows theypical discharge manner of the non-conductive blade-samples.ince the blade is made of dielectric materials, electrode-to-ground

ig. 5. Waveforms of lightning impulse and switching impulse. (a) Lightningmpulse and (b) switching impulse.

Polluted Creeping 14

discharge occurred frequently as shown in Fig. 6(a). Creeping dis-charges along the edge of the lower half of a blade-sample were alsoobserved several times as shown in Fig. 6(b). On the other hand, thedischarge manner was quite different under the polluted condition.Electrode-to-ground discharge did not occur any more, and creep-ing discharges along the whole surface of a blade-sample alwaysoccurred as shown in Fig. 6(c).

Some discharges penetrated the polluted blade-sample into thecavity of a blade as shown in Fig. 7.

4.3. Experiments on blade-sample with receptors

The effect of a small metal receptor embedded on the faceof a blade tip was investigated. A copper receptor of 25 mm indiameter was fitted on the face of a three-meter long blade-sample. The receptor was installed 250 mm away from the tipand 130 mm away from the both edges. A down-conductor fromthe receptor was connected to the ground running through thecavity of a blade-sample. Since wind turbine blades revolvearound a nacelle in practice, experiments were performed forfour typical arrangements of the blade-sample such as vertical,oblique 45◦, horizontal 1 (with a leading edge facing the elec-trode) and horizontal 2 (with a trailing edge facing the electrode)(Fig. 8).

4.3.1. Vertical arrangementTable 2 shows the experimental results. In many cases, dis-

charges progressed along the surface of a blade-sample tip andattached to the receptor, and sometimes discharges passed throughthe air and attached to the receptor directly. In Table 2, these dif-ferent discharge manners were treated as the same phenomenon

Fig. 6. Discharge manner to the non-conductive blade-sample. (a) Electrode toground discharge, (b) surface discharge (non-polluted condition) and (c) surfacedischarge (polluted condition).

6 S. Yokoyama / Electric Power Systems Research 94 (2013) 3– 9

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Fig. 9. Discharge manner to a vertically arranged blade with a receptor. (a) Front

ig. 7. Penetration of discharge into cavounous part of a blade. (a) Discharge man-er, (b) front side and (c) back side.

nd described by a word “Receptor”. When the discharge struck the

dge of the blade tip on the way to a receptor, the tip portion wasamaged as shown in Fig. 9. The damage ratios are 50% for negative.I. and 10% for positive S.I.

ig. 8. Experimental setup for blade-sample with a receptor. (a) Vertical, (b) oblique5◦ , (c) horizontal 1 and (d) horizontal 2.

view, (b) right view and (c) damaged area.

4.3.2. Oblique (45◦) arrangementExperiments were performed 10 times each for four wave-

forms, such as positive lightning impulse (L.I.), negative L.I., positiveswitching impulse (S.I.) and negative S.I., and no damage wascaused. For most of the tests the discharge propagated along thesurface of a blade-sample and attached to a receptor.

4.3.3. Horizontal arrangement 1 (trailing edge facing ahigh-voltage electrode)

Table 3 shows the experimental results. In the case of nega-tive discharges, a receptor successfully intercepted the dischargemany times. However in the case of positive discharge, the dis-charge penetrated through the trailing edge around the middle partof the blade-sample frequently. The new destruction of the blade-sample and striking the damaged area again were treated as 1 timedestruction in Table 3. Fig. 10 shows the discharge manner and thedamaged area. The damage ratios are 100% for positive L.I., 0% fornegative L.I., 86% for positive S.I. and 6% for negative S.I.

Table 2Experimental results on vertically arranged blades with a receptor.

Waveform Discharge manner Number of times

L.I.+ Receptor 6

− Receptor 3Damaged the blade tip 3

S.I.

+ Receptor 9Damaged the blade tip 1

– Receptor 10

L.I.: lightning impulse; S.I.: switching impulse.

Table 3Experimental results of the blade in horizontal 1 position with a receptor.

Waveform Discharge manner Number of times

L.I.+ Penetration 3− Receptor 20

S.I.

+Penetration 12Creeping 2

–Receptor 16Penetration 1

S. Yokoyama / Electric Power Systems Research 94 (2013) 3– 9 7

Fig. 10. Discharge manner to horizontal arranged blade-sample. (a) Front view, (b)re

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ight view and (c) damaged area. (a) Struck the conducting-cap, (b) struck the bladedge and (c) damaged area.

.3.4. Horizontal arrangement 2 (leading edge facing a higholtage electrode)

Table 4 shows the experimental results. Similar to the resultsf horizontal arrangement 1, positive discharge damaged thelade-sample many times and negative discharge attached to theeceptor. These results are attributed to the fact that when a nega-ive impulse voltage is applied to the electrode, a positive upwardeader from a grounded object tends to progress easily. Since thehick leading edge faced the electrode, the frequency of the destruc-ion was lower compared to the horizontal arrangement 1. Theamage ratios are 100% for positive L.I., 20% for negative L.I., 28%or positive S.I. and 9% for negative S.I.

.4. Experiments on blade-sample covered with conducting-cap

As a protection method other than a round receptor system, metallic-cap with a down-conductor can be employed at thelade tip. The top part of 260 mm length of a blade-sample wasompletely covered with a copper tape, and was connected tohe down-conductor. The blade-sample was arranged obliquely45◦).

For most of the experiments, discharges struck the conducting-ap directly as shown in Fig. 11(a) and lightning current wasransmitted to the ground through a down-conductor safely. How-ver, in the case of positive switching impulse voltages, the

ischarge attached to the blade edge just below the conducting-ap as shown in Fig. 11(b). Fig. 11(c) shows the damage caused byhe attachment of the discharge. After the discharge, damaged areaas burned with flame during several seconds.

able 4xperimental results of the blade in horizontal 2 position with a receptor.

Waveform Discharge manner Number of times

L.I.+ Penetration 4

– Receptor 4Penetration 1

S.I.

+ Creeping 8Penetration 4Receptor 2

– Receptor 10Creeping 1

Fig. 11. Discharge manner to the blade-sample with a conducting-cap.

Although the damage was caused rarely, this type of blade hasrelatively better protection performance than the blade with apoint-receptor.

5. Possible mechanism of breakage and combustion ofblades

5.1. Blades made of only insulated materials

5.1.1. Under non contaminated conditionIt is expected that lightning attachment probability to blades

without metallic parts becomes less than that to blades with metal-lic receptors. However, when a blade is once struck by lightning,breakage or combustion occurs easily by the following mechanism.

- Discharge penetrates into the blade cavity and the inner pres-sure increases rapidly by heating of air or evaporation of moisture(Fig. 7).

- Current flowing into the pasting portion of FRP makes the crackin an edge part.

- Ignition in a tip part or an edge part occurs.

5.1.2. Under contaminated conditionUnder the condition of contamination even blades made of only

insulated materials have the similar attachment characteristic asthose with metal parts. The probability of lightning attachmentto blades contaminated with sea salt becomes larger than thosewithout contamination.

5.2. Blades with lightning receptors and a down conductor

The experiment of long gap electric discharges shows that thedischarge possibly attaches to the insulated part of a blade withreceptors, when a positive switching impulse voltage is applied toa rod electrode, which is hung over the blade model. As the upwardleader from receptors cannot extend sufficiently, the downwardleader from the rod electrode is likely to attach to the insulatedsurface. On the other hand, an upward leader from receptors canextend, when a negative switching impulse voltage is applied to arod electrode.

In the case that a blade side becomes negative electrode siderelatively to a rod electrode, a possibility of the discharge attach-ment on the insulated surface of a blade becomes large because anupward leader does not extend enough.

8 S. Yokoyama / Electric Power Syste

Fig. 12. Winter lightning (upward leader).

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. Lightning protection of wind turbine blades

.1. Capture of lightning by an isolated lightning tower

This protection method is to construct an isolated tower, whichs a little apart from a windmill and blocks lightning dischargerom it. This method is expected to be fairly effective under theondition that wind direction is constant during thunderstorms.he lightning tower should be constructed on the windward sidef a windmill. If the wind direction is widely diverse, it is neces-ary to construct two or more towers. It is not practical from theconomical viewpoint.

.2. Effect of a lightning rod on a nacelle as countermeasuregainst blade breakage

In order to make lightning strokes attach to lightning rod on nacelle sufficiently, a long lightning rod would be necessary. Butrom the viewpoint of risk management, even if the length of a light-ing rod on a nacelle is shorter than that of a blade, some possibleamages can be protected by following reasons.

1) The sparkover voltage to a blade made of insulating mate-rials and a metal blade were checked experimentally usingmodel blades. According to the result, we can expect thatsome lightning discharges can be caught by a lightning rodon a nacelle even if the top of a blade made of insulatingmaterials is higher than the top of a lightning rod. How-ever, if the surface of a blade is contaminated, this effectdecreases.

2) When a blade, which occupies the highest position of threeblades, is perpendicular to the ground level, the height of thetop of the blade above the ground is the sum of the bladelength itself and the height of a nacelle. When the highestblade angle is 60◦ from a perpendicular position, it is the sumof a half of the length of a blade and the height of a nacelle.So totally the effect of a lightning rod on a nacelle is not sosmall.

.3. Receptors and a down conductor

A protective measure using receptors and a down conduc-or can reduce the blade damages considerably, but receptors

ms Research 94 (2013) 3– 9

for lightning protection was not perfect. Experimental resultsshow that the vertically arranged blade was damaged at thetip, and in the case of horizontally arranged blade, dischargespenetrated the edge of the middle part of blades. Experimentsshowed that obliquely arranged blade was not damaged, butsince the discharge progressed on the surface of the blade tothe receptor, it possibly causes damages. The blade covered withconducting-cap at the top of the blade has relatively high protectionefficiency.

7. Summary

The analysis of characteristics of winter lightning and theexperimental results clarified the following matters for thedesign of better lightning protection measures for wind turbineblades;

(1) Characteristics of winter lightning relevant to outages of blades• Upward leader—concentration of lightning strokes to wind-

mill (Fig. 12)• Lightning discharges of positive polarity—increase of unex-

pected discharge attachment to the insulated part of a blade• Large energy of a lightning current—explosion or combustion

of a blade, melting of receptors(2) Results of long discharge experiments

Regarding non-conductive blades, creeping dischargesoccurred more frequently under the polluted condition, andsometimes penetrative discharges into the cavity of a bladeoccurred.

Lightning protection method using receptors was not perfect.Vertically arranged blade-samples were damaged at the tip, and inthe case of horizontally arranged blade-samples, discharge pene-trated the edge of the middle part of blade-samples. Blade-samplescovered with conducting-cap at the top of a blade showed highprotection efficiency.

It is necessary to develop better protection measures for windturbine blades. As an example, protection performance of variousblade materials, such as carbon reinforced plastics, should beexamined.

References

[1] Wind turbine generator systems—part 24: lightning protection, IEC TR 61400-24, 2002, 7.

[2] M. Ideno, K. Seki, Study on improvement of performance of wind powergeneration system and lightning damage, in: Proc. of the 28th Interna-tional Conference on Lightning Protection, Kanazawa, September, 2006, pp.1585–1589.

[3] K. Michimoto, Meteorological aspects of winter thunderstorms alongthe Hokuriku Coast of Japan, in: Proc. of the 28th International Con-ference on Lightning Protection, Kanazawa, September, 2006, pp.9–13.

[4] M. Miki, Observation of current and leader development characteristics ofwinter lightning, in: Proc. of the 28th International Conference on LightningProtection, Kanazawa, September, 2006, pp. 14–19.

[5] M. Ishii, Electromagnetic fields associated with lightning discharges in win-ter, in: Proc. of the 28th International Conference on Lightning Protection,Kanazawa, September, 2006, pp. 20–25.

[6] H. Sugimoto, Lightning protection against winter lightning, in: Proc. of the 28thInternational Conference on Lightning Protection, Kanazawa, September, 2006,pp. 26–32.

[7] S. Yokoyama, Y. Yasuda, M. Minowa, T. Sato, Investigation on causes oflightning damages in wind turbine blades, in: The Seventh International Work-shop on High Voltage Engineering (IWHV2010), ED-10-92(SP-10-59,HV-1054),Kitakyushu, Japan, 2010.

[8] Y. Ueda, S. Arinaga, K. Inoue, T. Matsushita, Research for Wind TurbineBlade Protection against Strong Winter Lightning, Wind Power Shanhai,2007.

[9] IEC 61400-24, Wind turbines—part 24: lightning protection,2010-6.

Syste

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International Conference on Lightning Protection, Kanazawa, September, 2006,

S. Yokoyama / Electric Power

10] Y. Hongo, M. Nagano, M. Chihara, S. Yokoyama, Observation of lightning current

waveforms at Ogami Mountainous Area Niigata Prefecture, in: The 2008 AnnualMeeting Record I.E.E. Japan.

11] N.J. Vasa, T. Naka, S. Yokoyama, A. Wada, A. Asakawa, S. Arinaga, Experi-mental study on lightning attachment manner considering various types oflightning protection measures on wind turbine blades, in: Proc. of the 28th

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pp. 1483–1487.12] S. Yokoyama, A. Wada, A. Asakawa, T. Shindo, Study on Lightning Outage Mech-

anism of Wind Turbine Blades and Evaluation of Lightning Protection Methodsfor Them, CRIEPI Report no. H06018.