IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERINGIEEJ Trans 2010; 5: 21–26Published online in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/tee.20488

Paper

Transient Grounding Characteristics of an Actual Wind Turbine GeneratorSystem at a Low-resistivity Site

Kazuo Yamamoto∗a, Member

Shunichi Yanagawa∗∗, Member

Shozo Sekioka∗∗∗, Member

Shigeru Yokoyama∗∗∗∗, Senior Member

In order to exploit high wind conditions, wind turbine generator systems are often constructed in places where few tallstructures exist; therefore, they are often struck by lightning. Much of the damage caused by lightning is from the resultingbreakdown and malfunction of the electrical, communication, and control systems inside the wind turbine generator system;these breakdowns can be attributed to a rise in electric potential both within the system and in the surroundings due to lightning.Impulse tests were conducted on a wind turbine generator system at a disposal site where the conductivity of the ground wasvery low, like that found on the surface of the sea. The rise in ground potential of the system, and around its foundation, wasmeasured. When a wind turbine generator system is constructed at a site where the grounding resistivity is very low, the potentialrise at the wave front typically becomes larger than that of the steady state because of the inductivity of the grounding system.Therefore, it is very important that the transient characteristics of the grounding system are well understood. 2010 Institute ofElectrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Keywords: lightning, wind turbine generator system, overvoltage, transient grounding characteristics, low-voltage system

Received 13 April 2009; Revised 10 June 2009

1. Introduction

In recent years, accidents associated with the use of a large num-ber of wind turbine generator systems have increased, includingthose caused by natural phenomena such as lightning and typhoons.Lightning, especially, causes extensive and serious damage [1–4].

In order to exploit high wind conditions, wind turbine generatorsystems are often constructed in hilly terrain or along the seashore.In the near future, wind turbine generators may be constructednear Japan on the sea, as are offshore wind turbine generatorsystems in Europe [5]. Few tall structures will exist in theirvicinity. Therefore, they will often be struck by lightning. Inorder to promote wind power generation, lightning protectionmethodologies for wind turbine generator systems should beestablished.

Lightning damage to wind turbine generator systems affects thesafety and reliability of these systems. Most of the breakdownsand malfunctions of the electrical and control systems insidewind turbines are caused by a rise in ground potential due tolightning [6,7]. To understand this rise in ground potential, wehave researched the transient characteristics of the groundingby experimental and analytical methods using a reduced-sizemodel of current wind turbine foundations [8–11]. Researchusing simulations of the transient and steady-state grounding

a Correspondence to: Kazuo Yamamoto.E-mail: kyamamoto@mem.iee.or.jp

∗ Department of Electrical Engineering, Kobe City College of Tech-nology, 8-3 Gakuenhigashi-machi, Nishi-ku, Kobe, Hyogo 651-2194,Japan

∗∗ Techno Center, Shoden Co. 365, Sanoucho, Inage-ku, Chiba, Chiba263-0002, Japan

∗∗∗ Faculty of Engineering, Shonan Institute of Technology, 1-1-25,Tsujidonishikaigan, Fujisawa, Kanagawa 251-8511, Japan

∗∗∗∗ Electric Power Engineering Research Lab, CRIEPI, 2-6-1 Nagasaka,Yokosuka-shi, Kanagawa 240-0196, Japan

characteristics of wind turbine foundations has already beenpresented [12–22]. However, studies using an actual wind turbinegenerator system to study transient grounding characteristics arevery few in number [23].

In this article, we present experimental studies of the impulsetests conducted on the wind turbine generator system at a disposalsite where the conductivity of the ground is very low, like thatof ocean water. The ground potential rise of the system itself,and around its foundations, was measured. When lightning strikesthe wind turbine generator system constructed at a site where thegrounding resistivity is very low, the potential rise at the wavefront typically becomes larger than that of the steady state. This isbecause of the inductivity of the grounding system. Therefore,the transient characteristics of the grounding system becomeimportant, in comparison to its steady-state characteristics.

2. Grounding of Wind Turbine Generator System

2.1. Transient characteristics Both transient andsteady-state characteristics become important for understanding thegrounding phenomena of a wind turbine generator system. How-ever, because the steady state is emphasized in the planning of thegrounding, the transient characteristics are often neglected.

When a wind turbine generator system is constructed in amountain area where resistivity is comparatively high, the steady-state grounding resistance, in many cases, becomes more importantthan the transient grounding resistance. A potential rise causedby a lightning strike to a wind turbine generator system ismore remarkable at the wave tail than at the wave front. Thepotential rise at the wave tail depends on the steady-state groundingresistance. When a wind turbine generator system is constructed ata low-resistivity site, such as a coastal area, a significant potentialrise occurs due to the inductivity of the grounding system. Thetransient grounding resistance at the wave front, which depends on

2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

K. YAMAMOTO ET AL.

0

0.2

0.4

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1

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0 50 100 150 200

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rent

[A

]

Time [ns]

(a) Current injected into the reduced-size foundation model

−5

0

5

10

15

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25

0 50 100 150 200

Vol

tage

[V

]

Time [ns]

(b) Potential rise of the reduced-size foundation model

Fig. 1. Transient characteristic of the 1/20 foundation model.(a) Current injected into the reduced-size foundation model. (b).

Potential rise of the reduced-size foundation model

the inductivity of the grounding system, is larger than the steady-state resistance.

Some studies have used the reduced-size model to investigatethe grounding characteristics for a wind turbine generator system,taking into account transient phenomena [8–11]. For example,Fig. 1 shows the results of reduced-size experiments [11]. In thoseexperiments, a 1/20 reduced-size foundation model was used, andcurrent with a steep wave front was injected. Figure 1(a) and (b)show, the injected current and the potential rise waveforms ofthe foundation, respectively. An octagonal reduced-size foundationmodel, as shown in Fig. 2, was utilized in our experiments. Theconducting meshes were embedded in the concrete foundationto model a reinforced concrete foundation. The experimentalsetup of the reduced-size experiment is illustrated in Fig. 3. Themeasurement of the potential rise reached a maximum level at thewave front because of the inductive component of the foundationmodel. The steeper the wave front of the injected lightning currentis, the larger the maximum potential rise becomes. The lower theresistivity of the ground is, the more remarkable the steep wavefront of the potential rise is.

In this manner, we should consider both the transient andsteady-state grounding characteristics for an optimum design ofthe grounding system.

It should be noted that the characteristic of the ground in caseof the reduced-size experiments may have some differences fromthat of actual ground. Therefore, the above-mentioned featuresof the experimental results need to be confirmed in actual scaleexperiments.

2.2. Grounding of an offshore wind turbine Thesoil around the actual wind turbine generator system on thedisposal site had electrical characteristics similar to seawater,because the soil on the disposal site contained a lot of seawater.The target wind turbine generator system had four long foundationfeet, like those of offshore wind turbines, to increase the bearingcapacity of the soil. The grounding characteristics of the foundationconstructed on the disposal site exhibited inductivity in the wayexplained in the previous section. Construction of offshore wind

775 mm 321 mm

150 mm

400 mm

100 mm

70 mm

50 mm

Fig. 2. Dimensions of the foundation model

VI

Co-axial cable

5 kΩVoltage measuring wire

0.5 m

30 m

Current lead wire

Pulse generator

0.5 m30 m

Foundation model

Fig. 3. Experimental setup of the reduced-size experiment

turbine generator systems is prohibited in Japan because of fisheryrights, destruction of the environment, and so on. However, thereare wind turbine generator systems on the seawalls. Depending onthe governmental energy policy, offshore wind turbine generatorsystems may be constructed in the future [5]. Therefore, thegrounding characteristics of the wind turbine foundation ondisposal sites should be researched to estimate the groundingcharacteristics of low-resistivity sites.

3. Measurements

3.1. Foundations Figure 4 shows in detail the founda-tion of the actual wind turbine generator system that was used inour measurements. The shape was rectangular and parallel-piped,and 8.5 m × 8.5 m × 2 m in size. The foundation was reinforcedconcrete; the intervals between reinforcing were about 30 cm.The tower was connected to the foundation at ground level. Thedepth of the foundation was 2 m, and the length of the foun-dation feet was 50 m, to enhance the bearing capacity of soil.Grounding mesh existed underneath the foundation; its size wasabout 8.5 m × 8.5 m. The stratiform resistivity around the windturbine generator system is shown in Fig. 5. The Wenner methodwas utilized to measure the resistivity. The steady-state groundingresistance of the grounding system of the wind turbine generatorsystem was 0.062 .

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TRANSIENT GROUNDING CHARACTERISTICS OF AN ACTUAL WIND TURBINE

3 m

2 m

8.5 m

Grounding mesh

Foundation foot : 50 m

Fig. 4. Foundation of the actual wind turbine generator system

d1 = 3.0 [m]r1 = 15 [Ωm]

r2 = 1 [Ωm] ∞

Fig. 5. Resistivity around the wind turbine generator systemestimated by Wenner method

3.2. Experimental conditions Figure 6 shows theexperimental setup. The current was led to the foundation from theimpulse generator by using insulated copper wire (length: 150 m;cross section: 5.5 mm2) as the current lead wire. The height of thecurrent lead wire was about 1 m. The fast-front current generatedby the impulse generator was injected into the foundation througha resistance of 500 from a current lead wire, as shown in Fig. 6.The peak value of the current was 60 A and the wave front wasabout 0.4 µs. A comparatively large resistance of 500 was con-nected in series with the impulse generator; it can therefore beconsidered a current source.

The injected current was measured at the end of the currentlead wire near the foundation by using a current prove as shownin Fig. 6. The potential rise of the foundation was measured asthe voltage difference between the top of the foundation and thevoltage measurement wire. The height of the wire was 1 m, and itwas grounded at the remote end. The potential rise around the windturbine generation system was measured as the voltage betweenthe conductive rods placed at the measurement points, as shownin Fig. 7, and the voltage measurement wire. As shown in Fig. 6,the current lead wire and the voltage measurement wire wereorthogonalized to decrease their mutual electromagnetic induction.The surge impedance of the voltage measuring wire was about500 ; therefore, the 400 resistance was connected betweenthe remote end of the voltage measuring wire and grounding rod,which had about 120 grounding resistance. This was how thenoise induced on the voltage measuring wire was discharged tothe ground readily.

V

I

Ground

Foundation

Current lead wire

I.G.

Voltage measuring wire

150 m

70 m

Fig. 6. Experimental setup

3.3. Experimental data The current into the founda-tion and potential rise at the foundation were recorded to studythe transient and steady-state characteristics of the foundation. Thepotential rises around the wind turbine generator system were mea-sured at intervals of 1 m (0–10 m from the edge of the foundation,as shown in Fig. 7) around the foundation, and from 2 to 4 m (over10 m from the edge of the foundation, as shown in Fig. 7). Thepotential rise was measured at 21 locations. An additional rodwas buried about 0.1 m deep, at each measured point, to measurepotential rise.

3.4. Measuring instruments The impulse generatorhad a capacitance of 1.5 µF, and was discharged by using a gapswitch. The charging voltage was 30 kV for these measurements.

A TDS3054C oscilloscope (Tektronix) was used to measure thevoltage and current waveforms; its bandwidth was DC-500 MHz.

A P6139A passive probe (Tektronix) was used for voltagemeasurements; its bandwidth was DC-500 MHz and its inputcapacitance was up to 8 pF. A PEARSON 150 was used asthe current probe; its bandwidth and usable rise time were inthe range of 40 kHz–20 MHz and over 20 ns, respectively. Themeasurements performed using these instruments were accurate,with a rise time of several hundred nanoseconds.

3.5. Measured results The measurement results areshown in Fig. 8. Figure 8(a) and (b) show the injected current I

and the potential rise V at the top of the foundation, respectively.The injected current showed a ramp wave, and its peak andrise time were approximately 60 A and 0.4 µs, respectively.The voltage was inductive at the wave front. The ratio of themaximum voltage at the wave front to the current at the sametime was approximately 13 V/A. This value was greater than thesteady-state grounding resistance. The voltage waveform oscillatedafter the wave front. The medium value of the voltage graduallydecreased to the value of the steady-state grounding resistance.It is believed that the oscillations were caused by the inductanceand capacitance of the grounding system. It is possible that thesteady-state grounding resistance of 0.062 was the convergencevalue.

As mentioned above, the grounding characteristics of the systemshowed strong inductiveness at the wave front because the steady-state grounding resistance was as low as 0.062 . In the caseof offshore wind turbine generator systems, similar groundingcharacteristics should be observed. Transient phenomena obviouslybecome more important than steady-state phenomena for lightningprotection design.

The potential around a wind turbine generation system increaseswhen it is struck by lightning. To investigate the potential rise, thefast-front current was injected into the grounding system, as shownin Figs 6 and 7. The injected current was very similar to the resultsshown in Fig. 8(a); the peak value was 60 A, and the wave frontwas about 0.4 µs.

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K. YAMAMOTO ET AL.

Foundation

0 m1 m 3 m 5 m 7 m

2 m 4 m 6 m 8 m 10 m 14 m 18 m

16 m

0 m

1 m 3 m 5 m 7 m 9 m

2 m 4 m 6 m 8 m 10 m

12 m

14 m 18 m

16 m

Top view

Side view

9 m 12 m

Fig. 7. Measuring point of the potential rise around the foundation

0

20

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60

80

0 2 3 4 5

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rent

[A

]

Time [µs]

(a) Injected current into the foundation

−100

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100

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400

500

0 1 2 4

Vol

tage

[V

]

Time [µs]

(b) Potential rise of the foundation

1

3 5

Fig. 8. Transient characteristic of the actual grounding systemof the wind turbine generator system at the disposal site.(a) Injected current into the foundation. (b) Potential rise of the

foundation

Figure 9(a) shows the measured potential rise around thewind turbine generator system. Figure 9(b) shows the relationshipbetween the maximum potential rise and the distance.

The wave shape, shown in Fig. 9(a), was almost analogousto the potential rise shown in Fig. 8(b). If the skin effect ofthe ground is not considered, and the grounding impedance isassumed to be a pure resistance, the maximum potential rise isinversely proportional to the distance from the foundations. Afew differences exist in comparison with the inversely propotionalwaveform because the measurement results in Fig. 9(a) show surgebehavior.

−50

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tage

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0 m

40 m

Fig. 9. Potential rise around the wind turbine generator system.(a) Waveforms of the potential rise. (b) Peak values of the

potential rise

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TRANSIENT GROUNDING CHARACTERISTICS OF AN ACTUAL WIND TURBINE

3.6. Comparisons of the reduced and actual scaleexperiments It has been confirmed that the features on theactual scale measurements agree well with those on the reducedscale measurements. However, there are some difficulties of highfrequency measurements on the reduced scale such as the reductionof errors caused by using a high-frequency voltage probe for thesurge phenomena. It is also doubtful for the actual ground to bemodeled on the reduced-size measurements accurately, because itis not easy that the particle size of the actual ground is not scaleddown. For these reasons, the measurements on the actual scale aremore desirable than those on the reduced scale.

4. Overvoltage due to Potential Rise

4.1. Overvoltage at power, communication, and con-trol equipments The maximum potential rise of 460 V isshown in Fig. 8(b), when the peak value of the injected current was60 A and the wave front was about 0.4 µs. This result means thatan overvoltage of about 7.7 V/A may occur between an incomingconductor, or incoming conductors such as power, communica-tions, and control lines, and certain equipments in the tower. Ofcause, the overvoltage depends on the installation of the surge pro-tective devices (SPDs). If lightning with an average peak currentof 24 kA [24] were to strike the wind turbine generation system,the overvoltage for power, communication, and control equipmentin the tower would be assumed to be approximately 184 kV. Ifthe wave front of lightning current were steeper, the overvoltagewould become larger.

4.2. Outermost insulation layer overvoltage Whenincoming conductors such as cables used for power transmission,communication, or control are buried, the measured voltage inFig. 9(b) represents the potential of the soil around the cables.The voltage at the outermost insulating layer was the differencebetween the measured voltage in Fig. 9(b) and the potential of themetal sheath of the cable.

If the metal sheath is grounded at the remote end, its potential isalmost equivalent to that of the soil at the remote end, as shown inFig. 10(a). The potential of the soil around the incoming conductornear the foundation was larger than that at the remote end. Thevoltage at the outermost insulating layer near the foundation alsoexceeded that at the remote end. The voltage per ampere at theoutermost insulating layer near the foundation was 5.2 V/A, asshown in Fig. 9(b). If lightning with an average peak current of24 kA [5] were to strike the wind turbine generation system, theovervoltage at the outermost insulating layer would be assumed tobe approximately 125 kV.

When a metal sheath was grounded at the near end, its potentialwas almost equivalent to that of the soil near the foundations, asshown in Fig. 10(b). The voltage at the outermost insulating layerat the remote end also exceeded that near the foundation.

It should be noted that the above-mentioned overvoltages arein most severe case, because the potential of the metal sheath isusually affected by that of the ground around the cable transiently.The steeper the wave front of lightning current was, the larger theovervoltage became. Overvoltage at the outermost insulating layercould produce dielectric breakdown or partial discharge betweenthe metal sheath and the soil around it.

5. Conclusions

This article has presented the results of experimental studiesthat investigated the grounding characteristics of a wind turbinegeneration system at a low-resistivity site, and the voltage risearound it. The grounding characteristics of the grounding systemshowed strong inductivity at the wave front. The overvoltages

Voltage at theoutermostinsulating layer

Distance0

(a) Sheath grounded at the remote end

Distance

Potential of the sheath

Potential of the ground

Voltage at theoutermostinsulating layer

0

(b) Sheath grounded at the near end

Fig. 10. Overvoltage at the outermost insulation layer.(a) Sheath grounded at the remote end. (b) Sheath grounded

at the near end

for the power, communication, and control equipment placed inthe tower and at the outermost insulating layer were considered.Potential rise in the foundations and the surrounding groundsoil could cause an overvoltage of the power, communications,and control equipment placed in the tower and at the outermostinsulating layer of any incoming cable, which could result in thebreakdown or deterioration of the insulation.

The installation features of the wind turbine generator systemthat were employed in this paper were very similar to those used atsea. The long foundation feet were much like those of an offshorewind turbine generator system. The results given in this articlewill be very useful as basic data for lightning protection of windturbine generator systems at low-resistivity sites, including thoseof offshore wind turbine generator systems.

References

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(2) IEC. Wind Turbine Generator Systems—part 24: Lightning Protection .TR 61400-24, 2002.

(3) NEDO. Wind Turbine Failures and Troubles Investigating CommitteeAnnual Report . 2006; (in Japanese).

(4) NEDO. Wind Turbine Failures and Troubles Investigating CommitteeAnnual Report . 2007; (in Japanese).

(5) NEDO. NEDO report NP-9801 , 1999.(6) Yamamoto K, Noda T, Yokoyama S, Ametani A. An experimental

study of lightning overvoltages in wind turbine generation systemsusing a reduced-size model. Electrical Engineering in Japan 2007;158(4):22–30.

(7) Yamamoto K, Noda T, Yokoyama S, Ametani A. Experimental andanalytical studies of lightning overvoltages in wind turbine generatorsystems. Electric Power Systems Research 2009; 79(3):436–442,ISSN:0378–7796.

(8) Yamamoto K, Noda T, Yokoyama S, Ametani A. Groundingcharacteristics of a wind turbine generation system and voltagerise around it. International Conference on Grounding and Earthing(Ground2006), Maceio, Brazil, 2006; 415–419.

(9) Yamamoto K, Senoo T, Ametani A, Noda T, Yokoyama S. Groundingcharacteristics of the foundations of a wind turbine generation system.Annual Meeting of the Institute of Electrical Engineers of Japan ,7–094, 2007; (in Japanese).

(10) Yamamoto K, Senoo T, Fukuoka A, Ametani A. Effects of groundingconductors around the foundation of a wind turbine. Annual Meetingof the Institute of Electrical Engineers of Japan , 7–091, 2008; (inJapanese).

(11) Yamamoto K Ueda T. A study of the grounding effect due to theconnection of two foundations of wind turbine generator systems.Annual Meeting of the Institute of Electrical Engineers of Japan ,7–112, 2009; (in Japanese).

(12) Jenkins N, Vaudin A. Earthing of wind farms. Wind Engineering1994; 18(1):37–43.

(13) Lorentzou M, Cotton I, Hatziargyriou N, Jenkins N. Electromagneticanalysis of wind turbine grounding systems. Proceedings of theEuropean Wind Energy Conference (EWEC97), 1997.

(14) Hatziargyriou N, Lorentzou M, Cotton I, Jenkins N. Windfarmearthing. Proceedings of IEE Colloquium on “Lightning protectionof Wind Turbines , IEE Publication No. 97/303, 1997.

(15) Hatziargyriou N, Lorentzou M, Cotton I, Jenkins N. Transferredovervoltages by windfarm grounding systems. Proceedings ofInternational Conference on High Quality Power (ICHQP98), 1998;342–347.

(16) Cotton I. Windfarm earthing. Proceedings of 11th InternationalSymposium on High Voltage Engineering (ISH 99), vol. 2, London,UK, 1999; 288–291.

(17) Cotton I Jenkins N. Windfarm earthing. Proceedings of EuropeanWind Energy Conference (EWEC99), 1999; 725–728.

(18) Lorentzou M, Hatziargyriou N, Papadias B. Analysis of windturbine grounding systems. Proceedings of 10th MediterraneanElectrotechnical Conference (MELECON 2000), 2000; 936–939.

(19) Prousalidis JM, Philippakou MP, Hatziargyriou N, Papadias B. Theeffects of ionization in wind turbine grounding modeling. Proceedingsof 10th Mediterranean Electrotechnical Conference (MELECON2000), 2000; 940–943.

(20) Kontargyri VT, Gonos IF, Stathopulos IA. Frequency response ofgrounding system of wind turbine generators. Proceedings of the 24thInternational Symposium on High Voltage Engineering (ISH2005),2005.

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Kazuo Yamamoto (Member) was born in Osaka, Japan, in1974. He received his Bachelor’s, Master’s,and Doctoral degrees, all in engineering, fromDoshisha University, Kyoto, Japan, in 1997,2000, and 2007 respectively. He had beenwith Nara National College of Technologyfrom 2000 to 2006. He had been an assistantprofessor at Kobe City College of Technology

from 2006 to 2007. He is currently an associate professor. From1998 to 1999, he was a Visiting Researcher at the ManitobaHVDC Research Centre, Winnipeg, Manitoba, Canada. From2008, he has been a research fellow of the InfrastructureResearch Center at Doshisha University. His research interestincludes lightning protection. He is a member of IEEE andCIGRE.

Shunichi Yanagawa (Member) was born in Kanagawa, Japan,in 1961. He received his Bachelor’s degreesin engineering from Tokai University, Kana-gawa, Japan, in 1985, and joined Shoden Cor-poration, where he is currently an ExecutiveResearch Scientist. He had been the Directorof the Techno Center in Shoden Corporationfrom 2004 to 2008. He is currently a man-

ager of the technology development department. His researchinterest includes the research and product developments abouta lightning protection.

Shozo Sekioka (Member) was born on December 30, 1963.He received the B.Sc. and Dr Eng. degrees inelectrical engineering from Doshisha Univer-sity, Kyoto, Japan, in 1986 and 1997, respec-tively. He joined Kansai Tech Corporation in1987. He had been an associate professor atShonan Institute of Technology from 2005 to2007. He is currently a professor. He has been

engaged in the lightning surge analysis in electric power sys-tems. He is a member of IEEE and IET.

Shigeru Yokoyama (Senior Member) was born in Sendai,Japan, on March 5, 1947. He received theB.Sc. and Ph.D. Degrees from the Universityof Tokyo, Japan, in 1969 and 1986, respec-tively. He joined Central Research Instituteof Electric Power Industry (CRIEPI), Japan,in 1969. Since then he has been engaged inthe research of lightning protection on power

system, buildings, and wind turbines. He is a Research advisorat CRIEPI presently. He was Professor at Kyushu Universitysince 2001 through 2007. He was a vice president of the IEEJapan in 2001. He is a fellow of IEEE and a chairman of IECTC 81(Lightning Protection) Japanese committee.

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