statistical analysis of winter lightning current and measurement of step voltage in a wind power...

7/28/2019 Statistical Analysis of Winter Lightning Current and Measurement of Step Voltage in a Wind Power Generation Site

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

Statistical Analysis of Winter Lightning Current and

Measurement of Step Voltage in a Wind Power Generation Site

Koichi Nakamura, Hitoshi Sakurano, Yoshiyuki Kubouchi, Takashi Watanabe

Chubu University, Kaela R&D Inc., Hokkei Industries Co., Unchinada Municipal Bureau

E-mail:[email protected],[email protected],[email protected],

[email protected]

Abstract The authors have carried out the measurement of

winter lightning current at a wind power generation site,

Uchinada, for eight years since 2003 through 2010. We have

presented the preliminary paper on 2006 ICLP Kanazawa and

2008 ICLP Uppsala, based on the statistical analysis of current

obtained during five years since 2003 through 2007. In this

paper the current obtained in 2010 is discussed in the first. And

more than one hundred lightning currents during eight years are

statistically analyzed, and the protection ability of lightningtower to a wind power generator is discussed. Moreover, the step

voltage in the site was measured and the potential gradient is

discussed.

Keywords-winter lightning striking wind turbine and lightning

protection tower, curr entmeasurement, lightni ng protection abili ty,

step voltage measur ement, potenti al gradient

I. INTRODUCTION (HEADING 1)The observation has been carried out at Uchinada wind

power generation site, locating near Kanazawa, the coast of

Japan Sea, which has a wind generation tower (rating output:

1500kW) and a lightning tower for lightning protection. Theirheight is 100m and 105m, respectively, and the distance

between them is 45m. The measurement was performed

almost in winter.In the measurement of lightning current, two different

types of Rogowski coil were equipped at the base of eachtower as shown in Fig. 1, and in the measurement of potential

gradient around the site, four step voltage units were

separately placed on the ground.

With the twenty six lightning current through the two towersobtained in 2010, current peak, charge, current duration time

are discussed. And more than one hundred lightning currentsduring eight years are statistically analyzed, and the protection

ability of lightning tower for wind power generator isdiscussed and compared with the theory of rolling sphere

method.

Moreover, the step voltage and the potential gradientmeasured on the site are introduced.

I. MEASUREMENTARRANGEMENTA. Lightning Crrent

Figure 1 is a photograph of wind generation tower (left)

and lightning tower (LT, right) with, and Rogowski coils (#1

and #2). Their sampling frequency is 4MHz in the first stageand 500 kHz in the later. The dynamic range and the

minimum sensitivity are 100kA, 0.5kA, and 50kA ,

0.2kA, respectively.

Figure 1 Uchinada Wind Power Generation Site.B. Potentinal Gradient

Figure 2 is a photograph of step voltage measurement unit.The grounding electrode is 1.5m long, and the distance of stepis 1m. The sampling frequency of each unit is 4MHz. Fourunits are separately placed around the site as shown in Figure 3.Location of each unit is, 6m (the closest, #1), 26.3m (#2),46.3m (#3) and 66.3m (#4) from one leg of the lightning tower.Besides a long grounding electrode with 75m is buried with thetower foundation. The total grounding resistance is around 1

.

#2 coil

#1 coil
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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Figure 2 Unit of step voltage measurement.

Figure 3 Arrangement of four step voltage measurement units

II. DATA AND ANALYSISA. Lightning Current

In 2008, 2009 and 2010, twenty, eleven and twenty six

currents have been obtained, respectively. Table 1 shows the

current peak, charge, time of current duration, and lightningstriking location in 2010. Among twenty six lightning, the

seven struck the turbine blade (TB), and the residual, nineteen,the lightning tower (LT). Some features are summarized in

the following.

(1) The positive polarity of current has short time duration inthe first step as listed in the table, No. 10-2, 10-3, 10-4,

10-6, 10-16 and 10-20. This means they have higher steep

front characteristics.(2) On the other hand, the negative current has comparatively

long duration time and the steepness is small.(3) In the current of No. 10-13, the peak value of current is -

86kA and +4kA, the corresponding charge is about -150Cand +100C, respectively. The duration time is long, in the

negative is about 60ms, and in the positive is about 65ms.

TABLE 1 DATA OF LIGHTNING CURRENT IN 2010

No. peak value

[kA]

Charge

[C]

Time of

current [ s]

(| I|0.8kA

Lightningstrikinglocation

10-1 -2.4 -0.12 83 LT

10-2 +6 and -5 +0.005

and

-0.21

3.25 and 94 LT

10-3 +3.8 /-10.8

+0.006 /-16.9

6 / 11400 LT

10-4 +10.2 and

-3.4

+0.007

and -0.14

4 and 93 LT

10-5 -6 -0.36 225 LT

10-6 +2.8 and

-2.4

+0.006

and -0.16

2 and 137 LT

10-7 -3 small - TB

10-8 -14.2 and -73 and

+3.5

(-) 8225 &

21646 and (+)

6310

TB

10-9 -2 small - LT

10-10 -2 -0.006 41 TB

10-11 +6 / -2 small - LT

10-12 -4.4 -1.3 1000 LT

10-13 -86 and+4

-148.9 and+105.2

57910 and64635

TB

10-14 -19 and

+9

-0.27 and

+0.21

23 and 127 LT

10-15 +15.8 and-12.4

+0.025and -1.59

26 and 413 LT

10-16 +18.3 and

-3.4

+0.01 and

-0.26

2 and 180 LT

10-17 +13.2 +25.0 16000 TB

10-18 +2.6 +7.0 8516 TB

10-19 +3.8 +16.0 12390 LT

10-20 +4.8 and -

12.2

+0.006

and -0.24

5 and 100 LT

10-21 -25.4 -4.6 2744 LT

10-22 +8.8 +222.0 73565 TB

10-23 -5.6 +6.9 8820 LT

10-24 -8 small - LT

10-25 -2.2 small - TB

10-26 -1.5 small - TBNote: The meaning ofand and / in the table,

and : current change is continuously from positive to negative, or

negative to positive.

/ : current change is not continuously.

75m long grounding

electrode

Ground level


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Figs. 4, 5, 6, 7 and 8 are typical waveforms through the

wind turbine blade (TB) or lightning tower (LT), respectively.

Figure 4 (No.10-16) corresponds to the positive current with

high steep front through LT. The on-set steepness in this case

is 43.2 kA/s. Figure 5 (No.10-17) shows a long term

positive current flowing through the TB during 160ms.

Figure 6 (No.10-14) shows the current through the LT with

two polarities changing from negative to positive.

Figure 4 Positive current with high steep front through LT

(10-16, steepness 43.2kA/s)

Figure 5 Positive current through TB (10-17)

Figure 6 Current with two polarities changing from negative to positive (10-

14, LT, -19 and +9kA)

The figure 7 (No.10-13) indicates the current with largecharge and long duration time. The four figures from (a) to (d)shows the current in each time domain. The charge is -148.9C innegative part and +105.2 in positive. These duration time is 57910

s and 64635 s, respectively. Figure 8 (No. 10-21) shows thenegative continuous current with two pulses.

(a) 01500 us

(b) 1500 - 9000s

(c) 9000 - 16000s


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(c) 16000 - 166000s

(d) 16000160000sFigure 7 Large current in magnitude and duration time

(10-14, LT, -19 and +9kA)

Figure 8 Negative continuous current with two pulses (No. 10-21)

B. Lightning Striking and Protction AbilityTable 2 shows the number of lightning striking counting

from the current through the lightning tower (LT) and windturbine blade (TB) in each year since 2003 through 2010. Our

optical observation indicates the total incident of lightningstriking the two towers is larger than number of current. The

protection ability of the lightning tower means the ratio of

number of lightning tower for the total number of LT and TB.

The figures of protection ability Eare indicated in Table 2.The average protection ability of LT shows 0.70.

TABLE 2 LIGHTNING STRIKING TIMES LT AND BLADE, ANDSTRIKING PROTECTION RATE OF LT

year LT TBProtection

abilityE

Note

2003 10 9 0.53

E=

LT/(LT+TB)

2004 9 6 0.60

2005 26 13 0.67

2006 13 5 0.72

2007 14 6 0.70

2008 23 1 0.96

2009 9 2 0.82

2010 17 9 0.65

total 121 51 0.70

Figure 9 shows three cumulative frequency distribution

characteristics of peak current with total one hundred seventy

two (LT +TB), one hundred one (LT) and fifty one (TB) in

Table 2, respectively. The distribution of TB looks smallerthan that of LT. Two vertical lines A and B correspond to the

value of current 10kA and 15kA, respectively.

Figure 9 Cumulative frequency distribution of peak current through

lightning tower (LT), and turbine blade (TB) and both (LT+TB)

C. Discussion on protection Ability from Rolling SphereMethod

We will discuss the protection ability of LT from the view

point of interception criterion based on the rolling spheremethod. The rolling sphere method is applied to evaluate

the interception criterion of structure, generally introducedfrom the experience of summer lightning. In summer

lightning, the discharge well starts from the cloud bottom with

downward leader. On the other hand, in winter lightning,

lightning discharge occurs well with the upward leader fromthe ground top. Assuming the rolling sphere method can be

applicable to the lightning discharge with upward leader, wewill compare the lightning protection ability and the

interception criterion.The geometrical condition is that the height of two towers is

almost the same with 100m and the distance between them is

45m. Figure 10 shows a modeling of rolling sphere space.

The radius R of rolling sphere gives 45m and 60m atlightning protection level IV and III, respectively. Then the

interception criterion Eiof lightning tower are given as 0.84

(IV) and 0.91 (

) as noted in Table 3, and the correspondingminimum peak values of current are 15.7 kA and 10.1 kA,

respectively.Considering the cumulative distribution frequency

characteristics in Figure 9, the following discussion can bemade.

(1) Total number of current larger than 15kA is 47.Number of current through LT larger than 15kA is 37.

Lightning protection ability of LT becomes 37/47 = 0.79

This condition corresponds to the lightning protection

level IV.

A B


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(2) Total number of current larger than 10kA is 69.Number of current through LT larger than 10kA is 56.

Lightning protection ability of LT becomes 56/69 = 0.81

This condition corresponds to the lightning protection

level III.

Comparing the interception criterion in Table 3 and

protection ability of the LT actually obtained, the protectionability is somewhat small. Such difference may be caused by

the difference conditions with the propagating mechanism of

leader and the turbine blade to be protected is rotating, not

stable.

Figure 10 Application of rolling sphere method to the lightning tower

TABLE 3 COMPARISONS OF INTERCEPTION CRITERION AND

LIGHTNING PROTECTION ABILITY

Lightning

protectionlevel

Radius of

rolling sphere

R [m]I

Min. peak value

of current

I[kA]

Interception

criterion

Ei

60 15.7 0.84

45 10.1 0.91

(1) Total number of current larger than 15kA: 47Number of current through LT larger than 15kA: 37

Lightning protection ability of LT: 37/47 = 0.79

(2) Total number of current larger than 10kA: 69Number of current through LT larger than 10kA: 56

Lightning protection ability of LT: 56/69 = 0.81

D. Step Voltage and Potentil GradientFour units of step voltage measurement were separately

placed around the site as shown in Figure 3.

Figure 11 indicates waveforms of lightning current and four

step voltage (2010/01/01 14:48) measured. The figure (a) is

lightning current through the LT, and (b) is four step voltage.The current change shows somehow complex, first negative,

second positive and final negative. The positive peak is+16kA and negative one is -26kA.

The four step voltage were triggered independently; those

waveforms are displayed mutually shifting within several tens

microsecond. Comparing the current and step voltage, they are

alike in waveform. The figures of four step voltage are also

well alike in waveform.Fig. 12 shows the characteristics of step voltage vs. distance

from the tower foot, i.e. potential gradient. Two arecorresponding to positive and negative peak of the current. It

is interesting that the step voltage in negative case is smaller

than in the positive in spite of large current. The reason of

such difference is not clear.

R =45m R =60m

(a) Lightning current through the lightning tower

(b) Step voltage of four unitFigure 11 current and step voltage measured


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Figure 12 Potential gradient

IV CONCLUSIONS

(1) For twenty six current waveforms obtained in 2010, theirstatistical distribution of current magnitude, charge andspecific energy are obtained, and compared to the past

data. It cannot be found large difference from the dataobtained in the ordinal year.

(2) Using one hundred twenty one lightning current throughthe lightning tower, fifty one through the turbine blade,

and their total one hundred seventy lightning current, theircumulative frequency distribution was obtained, and the

lightning protection ability of lightning tower wasevaluated. The statistics indicates the protection ability of

the LT was similar to the interception criterion obtainedby rolling sphere method.

(3) Potential gradient was obtained by step voltagemeasurement. It is interesting that the different

magnitude of step voltage was measured in a currentwaveform.

ACKNOWLEDGMENT

The authors wish to thank Mr. Kenji Asai, Ryo Tsubouchi,

and Yokota for their cooperation.

REFERENCES

[1] K. Nakamura, H. Sakurano, Lightning Damage and Protection

Technique for Wind Farm in Winter Japan , Proc. of International Conf.

on Grounding and Farthing, Brazil, 2004

[2] H. Sakurano, M. Hashimoto, K. Nakamura, Observation of Winter

Lightning Striking A Wind Power Generation Tower and A Lightning

Tower, Proc. of 28th ICLP, XI-8 28th International Conference on

Lightning Protection (ICLP), Kanazawa, Japan, 2006

[3] K. Nakamura, H. Sakurano, A. Nakanishi, Observation of Winter

Lightning Striking a Wind Power Generation Tower/a Lightning Tower

and its Statistical Analysis, Proc. of 29th International Conf. on

Lightning Protection (ICLP), Uppsala, Sweden, 2008.[4] K.Nakamura, H. Sakurano, S. Yasui, A. Nakanishi, Lightning Current

Measurement on a Wind Power Generator Tower / a Lightning Tower

and those Grounding Electrodes, Proc. of 6th ALPF, E-03, Yokohama,

Japan, 2009.

[5] Dehn + Sohne, Lightning Protection Guide, Book, 2004.

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