2014 International Conference on Lightning Protection [ICLP), Shanghai, China

Measurement and Modeling of Lightning Current at Wind Turbine in Summer

Koji Michishita Dept. of Electrical and Electronic Engineering

Shizuoka University Hamamatsu, Japan

Satoshi Kurihara/ Koji Takano Research Laboratory

Kyushu Electric Power Co. Fukuoka, Japan

Abstract— The authors have carried out measurement of the current waveforms associated with lightning strokes to wind turbines at south Kyushu. In this paper measured waveforms on summer in 2013 are reported and it is shown that the return strokes with steep rate of rise and/or with high energy are observed in summer. This result is of practical importance for the insulation design of the wind turbine at the area of high lightning activity on summer.

Keywords- Lightning; return stroke; lightning flash; lightning discharge

I. INTRODUCTION [HEADING 1]

For the effective insulation design of the power distribution line, it is necessary to estimate the lightning current parameters at the area of interest because the current parameters are intrinsically dependent on the season and region.

It is straightforward to measure the lightning current by using the Rogowski coil or current shunt. The lightning return stroke current is usually measured at the top of high buildings, which are susceptible to frequent lightning strikes. Current waveforms are observed in many places around the world [1]; and, the obtained parameters of return lightning stroke current waveforms are reflected in the design of lightning protection for power lines and facilities. However, even tall buildings, which are hit by lighting relatively frequently, are struck only a few dozen times a year. Therefore, measurement for a long period is required to accumulate statistically enough amount of data.

In the case that lightning parameters are collected effectively, it is useful to estimate the current parameters from the electric and/or the magnetic field waveforms by assuming a return-stoke model defining the temporal and spatial distribution of the return stroke current along the channel.

The authors measure electric and magnetic field waveforms at four points in south Kyushu since 2001 to accumulate the return stroke current parameters effectively. We have estimated the lightning current waveforms by assuming the DU [Diendorfer-Uman] model [2] as the return-stroke model [3], and have shown that the statistical distribution of the estimated current parameters agrees with that obtained by the direct current measurement [4].

The authors have also measured the lightning current at a small tower of 40m in height in south Kyushu since 2005. Up to now no data was obtained because the tower height is so small that the lightning seldom hits the tower and at the same time the tower locates in the area of high lightning activity in summer where strokes do not concentrate to such a low tower compared with the lightning activity in winter. Since 2007, the authors also have measured the lightning current at the bottom of a wind turbine of 60 m in height from the ground to a nacelle. It is usual that the lightning hits the top of the blade and in this case the top of the blade is about 100 m at maximum. In 2009, the authors obtained the current waveform in the event of the negative-polarity lightning hit to the wind turbine initiated by upward propagating leader [5, 6]. The pulse current is observed with the current peak of -11kA, a front duration of 20 µs and the stroke duration of 80 µs. The pulse is followed by the long continuing current and the stroke charge was higher than 150 C. In [5, 6], it is shown that the measured current waveform is well reproduced from the measured E-field waveforms by using DU model, and the charge is also well estimated from the E-field waveforms measured by a slow antenna by assuming a point charge model with the accuracy of 4%.

In 2013 two wind turbines are newly instrumented. In this paper, the measurements of lightning current at south Kyushu are reported together with the E-field waveforms measured at multiple points. The current waveforms are also reproduced in the same way as in [4], and it is shown that the estimated current waveforms fairly agree with the measurements.

II. MEASUREMENT OF RETURN STROKE CURRENT AND E-FIELD

A. Location of Measuring Site

Figure 1 shows observation sites at southern Kyushu, famous for summer lightning. Electric field waveforms associated with lightning discharge have been observed at four sites and at two of them magnetic field has been observed too. Direct measurement of lightning current has been carried out at three sites for comparison with the estimated return-stroke current parameters.

Shibushi

Kokubu

Miyazaki

Miyakonojo

：Electric-Magnetic

：Electiric

：Current

Muregaoka

Figure 1. Location of the observation sites.

B. Current Measurement

A Rogowski coil for the measurement of the lightning current was installed at the bottom of a wind tribune generator at Muregaoka. Six turbines are instrumented by a Rogowski coil. The signals of the lightning current waveforms observed by the Rogowski coil were transmitted via optical links and digitized every 100 ns or 400 ns with the resolution of 12 bits. The total recording time was about 400 ms or 2 s with pre-trigger of 25 % or 5 %. The trigger time is also stamped on the recorded current waveforms with the precision of 0.2 µs or 2 µs. The current was measured in the frequency range from 0.1 Hz to 1 MHz. The calibrations are made in site by applying the known current.

C. Field Measurement

The hemispheric-shaped electrostatic antenna is used for the observation of electric field and the crossed loop antenna is used to measure the magnetic fields. Signals of the electric and magnetic fields measured in the frequency range from 1.6 kHz to 4.3 MHz are digitized every 40 ns with the resolution of 10 or 12 bits [3]. The calibrations are made by applying the known electric field [7]. The recording time after the trigger was either 4 ms, 120ms or 240ms. Signals of the electric fields obtained by a slow antenna in the frequency range from 8.4Hz to 100 kHz were digitized every 1 μs with the resolution of 12 bits for 1 s with the pre-trigger of 25 %. At each observation site, the trigger time obtained by the GPS clock with a precision of 0.2 μs is stamped on the field records.

III. RESULTS

A. Measured current at 10:35:50 on August 4 in 2013

Figure 2 shows the current waveform measured at 10:35:50 on August 4 in 2013. In the figure positive current is defined as the positive current propagating upward along the lightning channel or the negative current propagating downward. No initial continuous current is observed in this event and, as will be shown later, the field change associated with stepped leader is observed preceding the field change associated with the pulse current at about 0 ms in Fig. 2, therefore, it is inferred that the lightning flash is initiated by downward propagating leaders. Figure 2 shows that the lightning flash to No. 3 wind turbine at Muregaoka was consisted of two strokes with the time interval of 55 ms, which is longer than the 50 % value of 33 ms observed by Berger et al. [4].

Tables 1 shows the current waveform parameters of first strokes in Fig. 2 along with the observed statistical values by Berger et al and Figure 3 shows the first return-stroke current waveform in the expanded time scale.

The current peak of first return stroke was as high as 77 kA, almost equal to the 5 % value in [4]. The front duration of the first stroke current waveform was 30 µs, long as that of first strokes (5% value in the Berger’s dataset was 18 µs), with the stroke duration being 60 µs. Long front duration is shown to be due to the dominant contribution to the total current following the DU model. However, long front duration might be influenced by the upward leaders in response to the downward

leaders. Such influence of the upward leaders cannot be known without high-speed video observations.

The maximum steepness of the first stroke current waveform was 25 kA/µs, higher than the Berger’s 50 % value of 12 kA/µs, which is pointed out to be biased toward lower values [8]. Because the current waveform in Fig. 2 was obtained by the recording system with the sampling interval of 400 ns, the observed steepness might be the underestimate of the actual value.

The impulse charge, defined as the time integral of the current from the onset of the return stroke until the time when the absolute value of the current decreases below 1 kA, was 7.2 C. The stroke charge, defined as the time integral of the current from the onset of the return stroke until the time when the absolute value of the current becomes zero, was also 7.2 C due to absence of the continuing current following the return stroke.

Table 2 shows the current waveform parameters of the second stroke in Fig. 2, along with the observed statistical values by Berger et al.

The peak of the subsequent strokes was 40kA, higher than the 5 % value of 30 kA in the Berger’s dataset with the front duration of 2 µs and the stroke duration of 31µs. The measured front duration is underestimated due to the long sampling interval of 400 ns with the maximum error of 40 %.

The observed maximum steepness was 36 kA/µs, higher than that of the preceding first stroke of 25 kA/µs and around the Berger’s 50 % value of 40 kA/µs. However, measured steepness is probably underestimated because of the long sampling interval of 400 ns.

-80

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-40 -20 0 20 40 60 80 100

I [k

A]

Time [ms]

Figure 2. Measured current at 10:35:50 on August 4 in 2013.

TABLE I. WAVEFORM PARAMETERS OF FIRST STROKE IN FIG. 2.

Fig. 2 Berger 50% 5%

Current [kA] 77 30 80 Front duration [µs] 30 5.5 18 Stroke duration [µs] 60 75 200 Steepness [kA/µs] 25 12 32 Impulse charge [C] 7.2 4.5 20 Stroke charge [C] 7.2 5.2 24

TABLE II. WAVEFORM PARAMETERS OF SECOND STROKE IN FIG. 2.

Fig. 2 Berger 50% 5%

Current [kA] 40 12 30 Front duration [µs] 2.0 1.1 4.5 Stroke duration [µs] 31 32 140 Steepness [kA/µs] 36 40 120 Impulse charge [C] 4.1 0.95 4.0 Stroke charge [C] 4.1 1.4 11

The impulse charge, estimated by the above-mentioned definition, was 4.1 C and is almost equal to the 5 % value of the Berger’s dataset. The stroke charge of 4.1 C is equal to the impulse charge due to absence of the continuing current.

Figure 4 shows the E-field waveforms measured at Miyakonojyo, Kokubu and Miyazaki. The authors estimated the current waveform at the bottom of the lightning channel by the method in [3]. In [3], the current is assumed to be consisted of two components, namely the discharge current and the corona current following the DU model and the DU model is adopted as the return stroke model since the model seems theoretically reasonable. The current is estimated by the agreement between the measured and the calculated E-field waveforms. The estimated current waveform shown in Fig.4 agrees with the measurements up to 70 µs after the onset of the return stroke. The field waveforms associated with the first return stroke in Fig. 3 were obtained at plural points but in other strokes the field waveform is obtained at one or no point. In such cases method in [3] can’t be applied, therefore, current can’t be modeled either.

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I [k

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Measurement

Calculation

Breakdown current

Corona current

Figure 3. Measured current of first stroke at 10:35:50 on August 4 in 2013.

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/m]

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Measurement at MiyakonojyoMeasurement at KokubuMeasurement at MiyazakiCalculation at MiyakonojyoCalculation at MiyazakiCalculation at Kokubu

Figure 4. Measured and calculated E-field waveforms.

B. Measured current at 10:38:46 on August 4 in 2013

Figure 5 shows the current waveform measured at 10:38:46 on August 4 in 2013 at the No. 3 wind turbine. This lightning flash occurred following the flash in Fig. 5. No initial continuous current is observed in this event as in Fig. 2, therefore, it is inferred that the lightning flash is initiated by downward propagating leaders. The lightning flash was consisted of two strokes with the time interval of 72 ms, which is longer than the 50 % value of 33 ms observed by Berger et al. and the interval of the strokes in Fig. 2.

Table 3 shows the current waveform parameters of first strokes in Fig. 4 along with the observed statistical values by Berger et al.

The current peak of first return stroke was 40 kA, higher than the 50 % value of 30 kA in [4]. The front duration of the first stroke current waveform was 23 µs, being long as that of first strokes [5% value in the Berger’s dataset was 18 µs], with the stroke duration of 86 µs.

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I [k

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Figure 5. Measured current at 10:38:46 on August 4 in 2013.

The maximum steepness of the first stroke current waveform was 18 kA/µs, higher than the Berger’s 50 % value of 12 kA/µs. As mentioned before, the observed steepness might be the underestimate of the actual value.

The impulse charge was 9.4 C and the stroke charge was 10 C. Both charges are higher than those shown in Table 1, although the current was high for the stroke in Table 1. This is because of the long stroke duration for the stroke in Fig. 4 and also of the continuing current with the duration of 4.9ms.

Table 4 shows the current waveform parameters of the second stroke in Fig. 4, along with the observed statistical values by Berger et al.

The peak of the subsequent strokes was 23kA, higher than the 50 % value of 12 kA in the Berger’s dataset with the front duration of 30 µs and the stroke duration of 70µs.

The observed maximum steepness was 7.4 kA/µs, lower than the Berger’s 95 % value of 12 kA/µs. However, measured steepness is probably underestimated because of the long sampling interval of 400 ns.

The impulse charge was 7.3 C and the stroke charge was 8.1 C. Both charges are higher than those shown in Table 2,

although the current was low for the stroke in Table 4. This is because of the long stroke duration for the stroke in Fig. 4 and also of the continuing current with the duration of 5.5ms.

In the insulation design of the wind turbines, the peak, the steepness and the charge transfer of the return stroke current waveforms are the important parameters. The peak and the steepness are related to the sparkover caused by the overvoltage and the charge transfer is related to the damage of the SPD (Surge Protective Device). The current waveforms shown in this paper had high current peak, high steepness and also large charge transfer. These results are of practical importance for the insulation design of the wind turbine in the area of high lightning activity in summer.

The current waveforms in this paper are obtained at the low instrumented structure of 100 m in height including the blade. Therefore, the lightning discharge is little influenced by the existence of the structure, although the rotating blade might influence the attachment process.

TABLE III. WAVEFORM PARAMETERS OF FIRST STROKE IN FIG. 4.

Fig. 5 Berger 50% 5%

Current [kA] 40 30 80 Front duration [µs] 23 5.5 18 Stroke duration [µs] 86 75 200 Steepness [kA/µs] 18 12 32 Impulse charge [C] 9.4 4.5 20 Stroke charge [C] 10 5.2 24

TABLE IV. WAVEFORM PARAMETERS OF SECOND STROKE IN FIG. 4.

Fig. 5 Berger 50% 5%

Current [kA] 23 12 30 Front duration [µs] 30 1.1 4.5 Stroke duration [µs] 70 32 140 Steepness [kA/µs] 7.4 40 120 Impulse charge [C] 7.3 0.95 4.0 Stroke charge [C] 8.1 1.4 11

C. Measured current at 7:27:16 on August 20 in 2014

Figure 6 shows the current waveform measured at 7:27:16 on August 20 in 2014 at the No. 3 wind turbine. No initial continuous current is observed in this event as in Figs. 2 and 5, therefore, it is inferred that the lightning flash is initiated by downward propagating leaders. The lightning flash was consisted of two strokes with the time interval of 37 ms, which is longer than the 50 % value of 33 ms observed by Berger et al. and the interval of the strokes in Fig. 2.

The front duration of the second stroke of 0.8 µs, not influenced by the noise floor as can be seen from Fig.6, is very short compared with other cases.

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I [k

A]

Time [ms]

Muregaoka wind turbine No. 314 08/20 07:27:16.226769

Figure 6. Measured current at 7:27:16 on August 20 in 2014.

TABLE V. WAVEFORM PARAMETERS OF FIRST STROKE IN FIG. 6

Fig. 6 Berger 50% 5%

Current [kA] 38 30 80 Front duration [µs] 4.8 5.5 18 Stroke duration [µs] 48 75 200 Steepness [kA/µs] 28 12 32 Impulse charge [C] 3.5 4.5 20 Stroke charge [C] 3.9 5.2 24

TABLE VI. WAVEFORM PARAMETERS OF SECOND STROKE IN FIG. 6.

Fig. 6 Berger 50% 5%

Current [kA] 15 12 30 Front duration [µs] 0.8 1.1 4.5 Stroke duration [µs] 8.2 32 140 Steepness [kA/µs] 28.7 40 120 Impulse charge [C] 2.0 0.95 4.0 Stroke charge [C] 2.9 1.4 11

D. Discussion on measurement

Current parameters shown in this paper is within the scope of the IEC61400-24 [9], although some of the measured charges on the coast of the Sea of Japan [10] are beyond the expected range of 300 C. Our measurement supports the validity of the standard in the summer lightning area.

IV. CONCLUSION

In this paper the authors show the measurement of lightning current at south Kyushu in 2013. The three sets of the current waveforms of negative downward two-stroke lightning flash were obtained. The following insights are obtained in this paper.

(1) The E-field waveforms obtained at multiple points at south Kyushu are shown, and it is shown that the current waveforms estimated by the method in [3] from the measured E-field waveforms agree with measurements up to 70 µs after the onset of the return stroke.

(2) Current parameters shown in this paper is within the scope of the IEC61400-24 [9]. Our measurement supports the validity of the standard in the summer lightning area.

ACKNOWLEDGMENT

This research is financially supported by the Power Academy. The authors would like to show the sincere thanks to the support.

REFERENCES

[1] M. A. Uman , ”The Lightning Discharge”, Academic Press,1987

[2] G. Diendorfer and M. A. Uman, “An improved return stroke model with specified channel-base current”, Journal of Geophysical Research, 95, 13621-13644, 1990

[3] T. Harada and K. Michishita, “Estimation of Lightning Current Parameters Based on Measurement of Electric Field Waveforms”, International conference on lightning protection, I-33, Kanazawa,2006

[4] K. Berger, R. B. Anderson, and H. Kroeninger : "Parameters of lightning flashes", ELECTRA, No. 41, pp. 23-37, 1975.

[5] Hidehiro Nakata, Fumihiro Kinoshita, Koji Michishita,” Simultaneous measurement of return-stroke current and E-field waveforms at southern Kyushu in Japan”, 29th International conference on lightning protection, 76, Vienna [2012.9.3]

[6] Satoshi Kurihara, Hidehiro Nakata, Yousuke hashimoto, Koji Michishita, ”A verification of estimation accuracy of lightning current waveform and charge transfer from measured E-field waveform”, IEEJ Transaction on Power & Energy, Vol 135 No. 2, 2015 (in Japanese)

[7] K. Michishita, M. Ishii, J. Hojo, “Measurement of horizontal electric fields associated with distant cloud-to-ground strokes”, Journal of Geophysical Research, 101, No. D2, pp.3861-3867, (1996, 2)

[8] V. A. Rakov and M. A. Uman, “Lightning : Physics and Effects”, Cambridge University Press, 2003.

[9] T. S. Sorensen et al., ”The update of IEC31400-24 Lightning Protection of Wind Turbines”, Proc. of 29th ICLP, Uppsala, Sweden, 10-13, 2008

[10] K. Michishita, M. Furukawa, N. Honjo and S. Yokoyama, “Measurement of Lightning Current at Wind Turbine near Coast of Sea of Japan in Winter”, Proc. of 33rd ICLP, Estoril, portugal (2016)