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Cloud-to-ground lightning flash density and the number of lightning flashes hitting wind turbines in Japan
Electric Power Systems Research xxx (xxxx) xxxx
Contents lists available at ScienceDirect
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Cloud-to-ground lightning flash density and the number of lightning flashes hitting wind turbines in Japan Michihiro Matsuia,*, Koji Michishitab, Shigeru Yokoyamab a b
Franklin Japan Corporation, Japan Shizuoka University, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Cloud-to-ground (CG) lightning density Winter lightning Wind turbine Lightning risk IEC62858
The current status of the Japanese coastal region shows that winter lightning has seriously damaged wind turbines owing to its larger electrical charge than summer lightning. In Japan, 64% of serious wind turbine damage occurred in winter. Typically, lightning flash density is used to evaluate lightning risk in an area. Herein, we discuss the extent of lightning damage on wind turbines in Japan and cloud-to-ground (CG) lightning flash density based on the Japanese Lightning Detection Network, the largest lightning location system in Japan. Consequently, we found no correlation between the number of lightning flashes hitting wind turbines and summer lightning ground flash densities in the surrounding areas of a wind turbine. Approximately 95% of the lightning flashes that hit the turbine occurred in winter. The probabilities of lightning flashes hitting wind turbines were 0.12% in summer and 5.6% in winter. We developed a map showing the probability of lightning flashes hitting wind turbines in Japan. The probabilities obtained using this map were quite similar to the CG lightning flash density in winter. Therefore, we propose that lightning risk for wind turbines should not be evaluated based on the annual CG lightning flash density in Japan.
1. Introduction Lightning location systems (LLS) are operated in many countries as they effectively provide datasets to develop cloud-to-ground (CG) lightning flash density maps. LLSs are also important for risk management, assisting in enhancing a greater understanding of the extent of CG lightning flash density. However, international standard rules had not been developed regarding the lightning flash density derived from LLSs. The International Electrotechnical Commission (IEC) standardized lightning flash density based on the LLS in 2015 [1]. However, the new IEC62305 standard [2] does not integrate risk management in an international standard of lightning hitting wind turbines. However, lightning flash density NG derived from LLS data which complies with IEC62858 should be used for lightning risk management in the future. The Japanese Lightning Detection Network (JLDN) is a nationwide LLS. The JLDN observes lightning discharges that occur throughout Japan. Herein, we discussed the conformity of JLDN to IEC62858 and analyzed the relationship between the lightning risk for wind turbines in Japan and CG lightning flash density obtained from the JLDN dataset. Wind turbines have been installed in the coastal areas of the Sea of Japan to take advantage of the area’s strong winds [3]. However,
⁎
winter lightning [4,5] has seriously damaged the wind turbines in this area as electric charges of lightning discharges in winter are often larger than those in summer. According to previous study [6], 50% value of the total electric charge of downward negative lightning discharge was 5.2 C, whereas its 5% value was 24 C. A report [7] said that a lightning discharge with a charge transfer of more than 1000 C hit a wind turbine in those regions in winter. The major cause of large winter lightning charge transfer in the Sea of Japan is the long duration of continuous current compared to that in summer [8]. Moreover, many lightning discharges with large peak currents have been observed in winter. The probability of lightning discharges with peak currents greater than 100 kA in the coastal areas of the Sea of Japan in winter was 30 times more than that in the Kanto region where many lightning discharges were observed in summer [9]. When either an electric charge or the peak current of a lightning stroke was large, a possibility of serious damage on the wind turbines was increased. Therefore, clarifying the relationship between the extent of wind turbine damage caused by lightning discharges and CG lightning flash density is necessary. This would help locate appropriate places to construct wind turbines. Currently, international standards for lightning protection of wind turbines mainly focus on lightning discharges occurring in summer [10] even though the damage caused by winter lightning on wind turbines is more than
Corresponding author. E-mail address: [email protected] (M. Matsui).
https://doi.org/10.1016/j.epsr.2019.106066 Received 5 April 2019; Received in revised form 29 July 2019; Accepted 9 October 2019 0378-7796/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Michihiro Matsui, Koji Michishita and Shigeru Yokoyama, Electric Power Systems Research, https://doi.org/10.1016/j.epsr.2019.106066
Electric Power Systems Research xxx (xxxx) xxxx
M. Matsui, et al.
Fig. 1. Sensor allocation of the JLDN as of the end of November 2018.
2. Japanese lightning detection network
that caused by lightning in summer. A study [7] revealed that although the number of lightning flashes in summer is overwhelmingly more than that in winter, the number of wind turbine accidents in winter is more than that in summer in the coastal areas of the Sea of Japan. Hence, the annual CG lightning flash density is dominated by the number of lightning flashes in summer. Therefore, estimating the probability of lightning damage using the annual CG lightning flash density is difficult to select appropriate sites for installing wind turbines. This study aims to estimate the density of lightning flashes hitting the wind turbines in the coastal areas of the Sea of Japan. We calculate the density of lightning flashes hitting the wind turbines (DLFHW) using the following Eq. (1). DLFHW= Ngs · Hrates + Ngw · Hratew [flash · km−2 ·Year−1]
The JLDN is the largest lightning detection network in Japan. Since 1998, the JLDN has been owned and operated by the Franklin Japan Corporation (FJC). Before the JLDN was built, the FJC operated a small lightning detection network with six sensors in the central area called “Kanto” in Japan. However, the FJC expanded lightning observation network in central and western areas of Japan in 1998. Moreover, sensors were added to the network in the northern area of Japan in 1999 and in the southwestern islands called “Ryukyu” in 2002. The sensor network to observe lightning discharges in the entire area of Japan was named the “Japanese Lightning Detection Network (JLDN).” By the end of November 2018, the JLDN included 31 sensors. Fig. 1 shows the sensor allocation of the JLDN. The location accuracy of the JLDN has been improved year by year. The average location accuracy for negative downward lightning in summer was reported to be approximately 440 m in southern Kyushu [12]. However, the lightning discharges in winter developed from wind turbines in the coastal areas of the Sea of Japan were approximately 580 m [13]. The propagation delay correction (PDC) was applied to the JLDN in December 2015. As a result of the recalculation of the PDC, the average location error reported by [12] was improved from 440 to 310 m. That of lightning strokes hitting the wind turbine at the Nikaho windfarm in the coastal area of the Sea of Japan was improved from 790 to 120 m [14]. The minimum peak current of lightning strokes hitting the wind turbine, detected by JLDN, was −3.3 kA [15]. Matsui and Michishita [16] investigated the median location accuracy of the JLDN for the wind
(1)
where Ngs is the CG lightning flash density in summer (April to October); Ngw is the CG lightning flash density in winter (November to March); Hrates is the ratio of the number of CG lightning flash hitting the wind turbines to the number of lightning flashes detected by the JLDN within a radius of 10 km; and Hratew is the same measured in winter. The number of lightning strokes located by the JLDN is considered the number of lightning in its regular operation. However, we converted the number of strokes to the number of flashes based on the standard of “stroke to flash grouping” defined in the IEC62858. This paper is an extended version of the paper presented at the ICLP2018 [11].
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Fig. 2. Conformity areas of the JLDN to IEC62858.
3. Characteristics of winter lightning in Japan and accidents stemming from lightning hitting wind turbines
turbines in Nikaho, which was calculated to be approximately260 m using real-time calculation. We examined the validity of the CG lightning flash density observed by the JLDN to discuss the relationship between CG lightning flash density and lightning damage on the wind turbines. According to IEC62858 Ed.1.0 [1], at least 10 years is the minimum observation period, using data not older than 5 years. The relation between ground flash density (NG), observation years (Tobs) and the size of grid cell (Acell) should satisfy Eq. (2). NG × Tobs × ACell ≥ 80
Fig. 3 shows the location of wind turbines investigated by the New Energy and Industrial Technology Development Organization [17] from 2009 to 2013 [7]. A previous study [7] reported that lightning flashes hitting the wind turbines were observed by Rogowski coils alone or by Rogowski coils and video cameras. Fig. 4 shows the number of damages caused by lightning hitting wind turbines. The serious damage shown in Fig. 4 was the accident of the suspension of operation for three months or more. In Fig. 4, the maximum number of damages caused by lightning was in December. The extent of damage afflicted on wind turbines in winter was larger than that in summer. 64% of this serious damage occurred between November and March. Winter lightning is a major cause of the increase in damage afflicted on wind turbines. Characteristics of winter lightning are discussed in the next section. Because winter lightning has lower height at the bottom of a thundercloud than summer lightning, the initial high structures of electric field on the ground, such as wind turbines, are stronger in winter than in summer. Therefore, lightning discharge occurs and develops from high structures in winter. Consequently, the discharge channel branches out upward [18,19]. From lightning observation at the chimney of the Fukui thermal power plant of height 200 m [20], lightning discharges hit 174 times in winter for 7 years; in contrast, lightning discharges have hit only once in summer for the last 7 years. Fig. 5 shows spatial distributions of lightning discharges in the vicinity of wind turbines in the coastal area of the Sea of Japan. Fig. 5 (a) and
(2)
where NG is the ground flash density (flash × km−2 × year−1); Tobs is the observation period (year); and Acell is the area of each single cell (km2). We analyzed the CG lightning flash datasets observed by the JLDN from January 1, 2003 to December 31, 2017. The observation period (Tobs) was 15 years. The cell size (Acell) was 5 × 5 km. NG was the number of CG lightning flashes divided by the observation period and cell size. Therefore, the areas conformed to Eq. (2) as defined in IEC6258 when NG was larger than 0.213 flash/ (km2 × year). Gray areas shown in Fig. 2 indicate the areas that satisfy Eq. (2). Fig. 2 shows that NG obtained from lightning discharges observed by the JLDN conforms to IEC62858 in many areas of Japan except the eastern Hokkaido region. Thus, evaluating the density of lightning flash hitting wind turbines in the coastal areas of the Sea of Japan is reasonable.
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Fig. 3. Locations of the wind turbines investigated by NEDO.
lightning current. A great deal of winter lightning can exceed 300 C, the maximum value of lightning protection level defined by the IEC [2]. Therefore, the number of accidents caused by winter lightning is larger than that caused by summer lightning. Table 1 compares the peak current and electric charge between downward negative lightning and lightning hitting the wind turbines in winter. We show the parameters for downward negative lightning reported by Berger et al. [6]. In Table 1, we compare peak current and electric charge between downward negative lightning and lightning hitting the wind turbines in coastal areas in winter. Both positive and negative charges of lightning hitting the wind turbines were measured by the Rogowski coils equipped with the wind turbines and peak currents estimated by JLDN. Table 1 shows 50% value and 5% value of charges of both positive lightning discharges hit wind turbines and negative ones. These were overwhelmingly larger than those of downward negative lightning. Table 1 also shows that the 5% value of peak currents of positive lightning that hit the wind turbines is larger than that of downward negative lightning. This indicates that lightning discharges that hit the wind turbines in winter have larger energy and peak current than typical downward negative lightning discharges. Winter lightning frequently occurs in the coastal areas of the Sea of Japan, in which the incidence of winter lightning is approximately half of the annual thunderstorm days. Winter lightning occurs within 35 km of the coastline of the Sea of Japan. We defined the period of November to March as winter and the period of April to October as summer.
Fig. 4. Monthly variation of accidents caused by lightning hitting wind turbines.
(b) show the abovementioned spatial distributions in summer and winter, respectively. Fig. 5 (a) indicates that lightning did not concentrate on the vicinity of the wind turbines in summer. However, Fig. 5 (b) shows that lightning discharges concentrated in the vicinity of the wind turbines in winter because they initiated and developed upward from high structures. Winter lightning is also called “single-discharge lightning,” comprising few lightning discharges compared with summer lightning, making it difficult to predict its lightning approach. The duration of the current of winter lightning is quite longer than that of summer. Sometimes, it exceeds hundreds of milliseconds [19]. The amount of charge transfer is expressed by the integral value of
4. CG lightning flash density and the number of lightning flashes hitting wind turbines in Japan 4.1. CG lightning flash density Fig. 6 (a) shows the annual CG lightning flash density, and Fig. 6 (b) 4
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Fig. 5. Spatial distribution of lightning discharges occurred in the vicinity of wind turbines. Table 1 Comparison between negative downward lightning flashes measured by Berger (1975) and lightning flashes hitting wind turbines in Japan.
Charge [C] Peak Current [kA]
Lightning Type
95%
50%
5%
Downward negative [6] Positive lightning hit WT [7] Negative lightning hit WT [7] Downward negative [6] Positive lightning hit WT [7] Negative lightning hit WT [7]
1.1 2.0 1.6 14 9 4
5.2 57.9 61.1 30 35 10
24 286 294.7 80 138.5 35
shows the CG lightning flash density in winter. The colors of density values are different in Fig. 6 (a) and (b). CG lightning flash densities shown in Fig. 6 were obtained from datasets observed by JLDN between 2003 and 2017. The grid cell size of that density was 10 × 10 km. Fig. 6 (b) shows that the CG lightning flash density in the coastal area of the Sea of Japan is overwhelmingly higher than other areas in winter. However, Fig. 6 (a) showed that lightning flash densities in those areas
Fig. 7. Monthly variation of the number of lightning flashes occurred within 10 km from wind turbines and the number of lightning flashes hitting them.
Fig. 6. Lightning ground flash density in Japan. The observation period was between 2003 and 2017. The grid cell sizes were 10 × 10 km. 5
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45 14143
38876 788
Summer
Winter
Summer
Winter
(b) Number of lightning flashes hitting wind turbines
(a) Number of lightning flashes detected by the JLDN within 10 km from the wind turbines
Fig. 8. Number of lightning flashes occurred in surrounding areas of wind turbines and number of lightning flashes hitting them in the coastal areas of the Sea of Japan.
summer is overwhelmingly larger than that in winter, more lightning flashes hit the wind turbines in winter than summer. Fig. 8 (a) shows the number of lightning flashes detected by JLDN within 10 km of the wind turbine in summer and winter. Fig. 8 (b) shows the number of lightning flashes hitting wind turbines in summer and winter. Fig. 8 showed that 95% of lightning flashes hitting the wind turbines were observed in winter, whereas only 27% of lightning flashes were detected by the JLDN within 10 km in winter. We investigated the relationship between the number of lightning flashes hitting the wind turbines and the number of accidents caused by lightning. Regardless of the scale of failure or damaged component, when a component of a wind turbine (e.g. communication devices, electric control system, generator, step-up gear and wind turbine blade) was broken due to lightning discharge, we counted it as one accident. The correlation coefficient between them was 0.86. This showed that accident due to lightning increased on the wind turbines when the number of lightning flashes hitting them increased. Fig. 8 indicated that the number of CG lightning flashes hitting wind turbines and the number of lightning flashes detected by the JLDN within 10 km was 0.12% and 5.6% in summer and winter, respectively. This shows that the probability of winter lightning hitting a wind turbine is approximately 47 times more than that of summer lightning hitting the wind turbines. This indicates that the probability of lightning hitting wind turbines is independent of the annual CG lightning flash density. Fig. 9 shows the relationships between the number of lightning flashes hitting the wind turbines and CG lightning flash density in winter and summer. We calculated their correlation coefficients, revealing that the coefficient for winter was 0.58 and that for summer was 0.17. This showed that no correlation exists between lightning flash density in summer and the number of accidents caused by lightning hitting the wind turbines.
Fig. 9. Relationships between number of lightning flashes hitting wind turbines and CG lightning flash density in winter (Nov. to Mar.) and summer (Apr. to Oct.). CC is the correlation coefficient.
were nearly similar to other areas during the year. Approximately 64% of accidents on all wind turbines owing to lightning in Japan occurred in coastal areas of the Sea of Japan in winter [5]. 4.2. Seasonal variation of the number of lightning flashes hitting wind turbines
4.3. Density of lightning flashes hitting the wind turbines
The lightning risk on wind turbines is a probability of damaging turbines by lightning. The damage depends on the characteristics of lightning discharges such as their peak current, steepness of current waveforms, and electric charges. We compared the number of lightning flashes that occurred in the surrounding area of the wind turbines and the number of lightning directly hitting the turbines. Fig. 7 shows the monthly variation of the CG lightning flash detected by JLDN within 10 km from the wind turbines shown in Fig. 3 and the number of lightning flashes hitting these wind turbines as detected by the Rogowski coils and/or the video cameras. Although Fig. 7 shows that the number of lightning discharges within 10 km from the wind turbines in
We showed the rates of the number of CG lightning flashes hitting the wind turbines and the number of lightning flashes detected by the JLDN within a 10 km range. Ngs was 0.0012 and Ngw was 0.056 within this range. Fig. 10 demonstrates the DLFHW calculated using Equation (1) per 10 × 10 km grid cell in Japan. High probability areas for turbines being hit by lightning are the coastal areas of the Sea of Japan. Figs. 8 and 9 show the corresponding results, indicating that the annual CG lightning flash density is more dominated by the CG lightning flash density in summer than that in winter. Moreover, a correlation is recorded between that in winter and the number of accidents at wind turbines. Therefore, these are dissimilar to annual CG lightning flash 6
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Fig. 10. Probability of lightning flashes hitting wind turbines in Japan per grid cell of 10 × 10 km.
References
densities shown in Fig. 8 (a); however, these are similar to that of winter. Therefore, we propose that the annual lightning flash density should not be used to evaluate lightning risk for wind turbines in Japan. However, the DLFHW showed in Fig. 10 disregards an increase in the number of lightning flashes after a high structure installation such as a wind turbine [21]. Therefore, the DLFHW turbine will exceed that shown in Fig.10 after wind turbine installation.
[1] IEC, “Lightning density based on lightning location system (LLS),” IEC62858 Ed.1.0, International Standard, (2015). [2] IEC, “Protection against lightning – part 2: risk management,” IEC62305-2 Ed.3, 81/570/CDV(Committee Draft for Vote), September (2017). [3] NEDO, “Wind power generation introduction guidebook,” Ed. 9, (2008), pp. 85–88 (in Japanese). [4] S. Yokoyama, S. Sekioka, Lightning protection of wind turbine generation system, IEEJ Trans. PE. 129 (No. 5) (2009) 572–575 (in Japanese). [5] S. Yokoyama, Lightning damages of wind turbine blades and protection methods of them, IEEJ Trans. PE. 124 (No. 2) (2004) 177–180 (In Japanese). [6] K. Berger, R.B. Anderson, H. Kroninger, Parameter of lightning flashes, Electra 80 (1975) 223–237. [7] NEDO: “Research and Development of Next-Generation Wind Power Generation Technology for Technology Corresponding to Natural Environment etc. for Measures of lightning protection,” Management No. 20150000000080, 2015 (in Japanese). [8] K. Miyake, I. Kishizima, T. Suzuki, H. Mitani, M. Tkashima, Characteristics of winter lightning current in coastal area of the sea of Japan, IEEJ Trans., PE. 110 (11) (1990) 944–951 (In Japanese). [9] M. Ishii, F. Fujii, M. Saito, M. Matsui, Evaluation of risk of lightning hits on tall structures in Japan, IEEJ Trans. PE. 129 (No. 5) (2009) 621–627 (In Japanese). [10] T. Sato, The investigation committee of lightning protection for wind power generation facilities considering lightning characteristics, IEEJ Trans. PE 133 (No. 3) (2013) 23 (in Japanese). [11] M. Matsui, K. MIchishita, S. Yokoyama, Lightning ground flash density and evaluation of lightning risk for wind turbines in coastal areas of the sea of Japan, 34th International Conference on Lightning Protection, Rzeszow, Poland, 2018. [12] M. Matsui, K. Michishita, S. Kurihara, Accuracies of location and estimated peak current for negative downward return-strokes to wind turbine observed by JLDN, IEEJ Trans. PE 135 (No. 10) (2015) 644–645 (in Japanese). [13] M. Ishii, F. Fujii, M. saito, D. Natsuno, A. Sugita, Detection of lightning return strokes hitting wind turbines in winter by JLDN, IEEJ Trans. Power Energy 133 (No 12) (2013) 1009–1010 (in Japanese). [14] M. Matsui, K. Michishita, N. Honjo, Discussion on improving error of located
5. Conclusion Despite the fact that only 27% of the total number of CG lightning flashes occurred within 10 km of the wind turbines in winter, 95% of CG lightning flashes hitting the wind turbines occurred in winter. The correlation coefficient between the number of lightning flashes within a 10 km radius from the wind turbines and the number of accidents caused by lightning discharges was −0.27. Therefore, no correlation exists between these factors. The correlation coefficient between the number of lightning flashes hitting the wind turbines and the number of accidents caused by lightning was 0.86. The density of lightning flashes hitting wind turbines to the number of lightning flashes detected by the JLDN within 10 km from them were 0.12% and 5.6% in summer and winter, respectively. The probability map of lightning flashes hitting the wind turbines in Japan presented herein indicate a high probability that CG lightning flashes hitting wind turbines were concentrated in the coastal areas of the Sea of Japan, similar to areas of high CG lightning flash density in winter. Therefore, we proposed that annual lightning flash density should not be used to evaluate lightning risk for wind turbines in Japan. 7
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[18] D. Wang, N. Takagi, Characteristics of winter lightning rhar occurred on a windmill and its lightning protection tower in Japan, IEEJ Trans. PE. 132 (No. 6) (2012) 568–572. [19] M. Miki, Observation of current and leader development characterustics of winter lightning, IEEJ Trans. PE. 127 (No. 12) (2007) 1247–1252. [20] IEIEJ, Lightning protection for electrical and electrronics equipment, Oham Publishing Co., Ltd., 2011, pp. 28–31. [21] M. Saito, M. Ishii, A. Onishi, F. Fujii, M. Matsui, D. Natsuno, Frequency of upward lightning hits to wind turbines in winter, IEEJ Trans. P.E. 131 (No. 12) (2011) 979–985 (in Japanese).
position and estimated peak current of winter lightning to a wind turbine by JLDN, 10th Asia-Pacific International Conference on Lightning (APL 2017), Krabi, Thailand, 2017. [15] M. Matsui, K. Michishita, N. Honjo, Accuracy of location and estimated peak current of winter lightning detected by JLDN after adapting propagation delay correction, Annual Conference of Power & Energy Society IEE of Japan, Tokushima, Japan, 2018. [16] M. Matsui, K. Michishita, Transition of Location Accuracy of the JLDN”, High Voltage Engineering, IEE of Japan, Ishigaki-jima, Japan, 2019. [17] NEDO: http://www.nedo.go.jp/english/index.html.
8
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Cloud-to-ground lightning flash density and the number of lightning flashes hitting wind turbines in Japan Michihiro Matsuia,*, Koji Michishitab, Shigeru Yokoyamab a b
Franklin Japan Corporation, Japan Shizuoka University, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Cloud-to-ground (CG) lightning density Winter lightning Wind turbine Lightning risk IEC62858
The current status of the Japanese coastal region shows that winter lightning has seriously damaged wind turbines owing to its larger electrical charge than summer lightning. In Japan, 64% of serious wind turbine damage occurred in winter. Typically, lightning flash density is used to evaluate lightning risk in an area. Herein, we discuss the extent of lightning damage on wind turbines in Japan and cloud-to-ground (CG) lightning flash density based on the Japanese Lightning Detection Network, the largest lightning location system in Japan. Consequently, we found no correlation between the number of lightning flashes hitting wind turbines and summer lightning ground flash densities in the surrounding areas of a wind turbine. Approximately 95% of the lightning flashes that hit the turbine occurred in winter. The probabilities of lightning flashes hitting wind turbines were 0.12% in summer and 5.6% in winter. We developed a map showing the probability of lightning flashes hitting wind turbines in Japan. The probabilities obtained using this map were quite similar to the CG lightning flash density in winter. Therefore, we propose that lightning risk for wind turbines should not be evaluated based on the annual CG lightning flash density in Japan.
1. Introduction Lightning location systems (LLS) are operated in many countries as they effectively provide datasets to develop cloud-to-ground (CG) lightning flash density maps. LLSs are also important for risk management, assisting in enhancing a greater understanding of the extent of CG lightning flash density. However, international standard rules had not been developed regarding the lightning flash density derived from LLSs. The International Electrotechnical Commission (IEC) standardized lightning flash density based on the LLS in 2015 [1]. However, the new IEC62305 standard [2] does not integrate risk management in an international standard of lightning hitting wind turbines. However, lightning flash density NG derived from LLS data which complies with IEC62858 should be used for lightning risk management in the future. The Japanese Lightning Detection Network (JLDN) is a nationwide LLS. The JLDN observes lightning discharges that occur throughout Japan. Herein, we discussed the conformity of JLDN to IEC62858 and analyzed the relationship between the lightning risk for wind turbines in Japan and CG lightning flash density obtained from the JLDN dataset. Wind turbines have been installed in the coastal areas of the Sea of Japan to take advantage of the area’s strong winds [3]. However,
⁎
winter lightning [4,5] has seriously damaged the wind turbines in this area as electric charges of lightning discharges in winter are often larger than those in summer. According to previous study [6], 50% value of the total electric charge of downward negative lightning discharge was 5.2 C, whereas its 5% value was 24 C. A report [7] said that a lightning discharge with a charge transfer of more than 1000 C hit a wind turbine in those regions in winter. The major cause of large winter lightning charge transfer in the Sea of Japan is the long duration of continuous current compared to that in summer [8]. Moreover, many lightning discharges with large peak currents have been observed in winter. The probability of lightning discharges with peak currents greater than 100 kA in the coastal areas of the Sea of Japan in winter was 30 times more than that in the Kanto region where many lightning discharges were observed in summer [9]. When either an electric charge or the peak current of a lightning stroke was large, a possibility of serious damage on the wind turbines was increased. Therefore, clarifying the relationship between the extent of wind turbine damage caused by lightning discharges and CG lightning flash density is necessary. This would help locate appropriate places to construct wind turbines. Currently, international standards for lightning protection of wind turbines mainly focus on lightning discharges occurring in summer [10] even though the damage caused by winter lightning on wind turbines is more than
Corresponding author. E-mail address: [email protected] (M. Matsui).
https://doi.org/10.1016/j.epsr.2019.106066 Received 5 April 2019; Received in revised form 29 July 2019; Accepted 9 October 2019 0378-7796/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Michihiro Matsui, Koji Michishita and Shigeru Yokoyama, Electric Power Systems Research, https://doi.org/10.1016/j.epsr.2019.106066
Electric Power Systems Research xxx (xxxx) xxxx
M. Matsui, et al.
Fig. 1. Sensor allocation of the JLDN as of the end of November 2018.
2. Japanese lightning detection network
that caused by lightning in summer. A study [7] revealed that although the number of lightning flashes in summer is overwhelmingly more than that in winter, the number of wind turbine accidents in winter is more than that in summer in the coastal areas of the Sea of Japan. Hence, the annual CG lightning flash density is dominated by the number of lightning flashes in summer. Therefore, estimating the probability of lightning damage using the annual CG lightning flash density is difficult to select appropriate sites for installing wind turbines. This study aims to estimate the density of lightning flashes hitting the wind turbines in the coastal areas of the Sea of Japan. We calculate the density of lightning flashes hitting the wind turbines (DLFHW) using the following Eq. (1). DLFHW= Ngs · Hrates + Ngw · Hratew [flash · km−2 ·Year−1]
The JLDN is the largest lightning detection network in Japan. Since 1998, the JLDN has been owned and operated by the Franklin Japan Corporation (FJC). Before the JLDN was built, the FJC operated a small lightning detection network with six sensors in the central area called “Kanto” in Japan. However, the FJC expanded lightning observation network in central and western areas of Japan in 1998. Moreover, sensors were added to the network in the northern area of Japan in 1999 and in the southwestern islands called “Ryukyu” in 2002. The sensor network to observe lightning discharges in the entire area of Japan was named the “Japanese Lightning Detection Network (JLDN).” By the end of November 2018, the JLDN included 31 sensors. Fig. 1 shows the sensor allocation of the JLDN. The location accuracy of the JLDN has been improved year by year. The average location accuracy for negative downward lightning in summer was reported to be approximately 440 m in southern Kyushu [12]. However, the lightning discharges in winter developed from wind turbines in the coastal areas of the Sea of Japan were approximately 580 m [13]. The propagation delay correction (PDC) was applied to the JLDN in December 2015. As a result of the recalculation of the PDC, the average location error reported by [12] was improved from 440 to 310 m. That of lightning strokes hitting the wind turbine at the Nikaho windfarm in the coastal area of the Sea of Japan was improved from 790 to 120 m [14]. The minimum peak current of lightning strokes hitting the wind turbine, detected by JLDN, was −3.3 kA [15]. Matsui and Michishita [16] investigated the median location accuracy of the JLDN for the wind
(1)
where Ngs is the CG lightning flash density in summer (April to October); Ngw is the CG lightning flash density in winter (November to March); Hrates is the ratio of the number of CG lightning flash hitting the wind turbines to the number of lightning flashes detected by the JLDN within a radius of 10 km; and Hratew is the same measured in winter. The number of lightning strokes located by the JLDN is considered the number of lightning in its regular operation. However, we converted the number of strokes to the number of flashes based on the standard of “stroke to flash grouping” defined in the IEC62858. This paper is an extended version of the paper presented at the ICLP2018 [11].
2
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Fig. 2. Conformity areas of the JLDN to IEC62858.
3. Characteristics of winter lightning in Japan and accidents stemming from lightning hitting wind turbines
turbines in Nikaho, which was calculated to be approximately260 m using real-time calculation. We examined the validity of the CG lightning flash density observed by the JLDN to discuss the relationship between CG lightning flash density and lightning damage on the wind turbines. According to IEC62858 Ed.1.0 [1], at least 10 years is the minimum observation period, using data not older than 5 years. The relation between ground flash density (NG), observation years (Tobs) and the size of grid cell (Acell) should satisfy Eq. (2). NG × Tobs × ACell ≥ 80
Fig. 3 shows the location of wind turbines investigated by the New Energy and Industrial Technology Development Organization [17] from 2009 to 2013 [7]. A previous study [7] reported that lightning flashes hitting the wind turbines were observed by Rogowski coils alone or by Rogowski coils and video cameras. Fig. 4 shows the number of damages caused by lightning hitting wind turbines. The serious damage shown in Fig. 4 was the accident of the suspension of operation for three months or more. In Fig. 4, the maximum number of damages caused by lightning was in December. The extent of damage afflicted on wind turbines in winter was larger than that in summer. 64% of this serious damage occurred between November and March. Winter lightning is a major cause of the increase in damage afflicted on wind turbines. Characteristics of winter lightning are discussed in the next section. Because winter lightning has lower height at the bottom of a thundercloud than summer lightning, the initial high structures of electric field on the ground, such as wind turbines, are stronger in winter than in summer. Therefore, lightning discharge occurs and develops from high structures in winter. Consequently, the discharge channel branches out upward [18,19]. From lightning observation at the chimney of the Fukui thermal power plant of height 200 m [20], lightning discharges hit 174 times in winter for 7 years; in contrast, lightning discharges have hit only once in summer for the last 7 years. Fig. 5 shows spatial distributions of lightning discharges in the vicinity of wind turbines in the coastal area of the Sea of Japan. Fig. 5 (a) and
(2)
where NG is the ground flash density (flash × km−2 × year−1); Tobs is the observation period (year); and Acell is the area of each single cell (km2). We analyzed the CG lightning flash datasets observed by the JLDN from January 1, 2003 to December 31, 2017. The observation period (Tobs) was 15 years. The cell size (Acell) was 5 × 5 km. NG was the number of CG lightning flashes divided by the observation period and cell size. Therefore, the areas conformed to Eq. (2) as defined in IEC6258 when NG was larger than 0.213 flash/ (km2 × year). Gray areas shown in Fig. 2 indicate the areas that satisfy Eq. (2). Fig. 2 shows that NG obtained from lightning discharges observed by the JLDN conforms to IEC62858 in many areas of Japan except the eastern Hokkaido region. Thus, evaluating the density of lightning flash hitting wind turbines in the coastal areas of the Sea of Japan is reasonable.
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Fig. 3. Locations of the wind turbines investigated by NEDO.
lightning current. A great deal of winter lightning can exceed 300 C, the maximum value of lightning protection level defined by the IEC [2]. Therefore, the number of accidents caused by winter lightning is larger than that caused by summer lightning. Table 1 compares the peak current and electric charge between downward negative lightning and lightning hitting the wind turbines in winter. We show the parameters for downward negative lightning reported by Berger et al. [6]. In Table 1, we compare peak current and electric charge between downward negative lightning and lightning hitting the wind turbines in coastal areas in winter. Both positive and negative charges of lightning hitting the wind turbines were measured by the Rogowski coils equipped with the wind turbines and peak currents estimated by JLDN. Table 1 shows 50% value and 5% value of charges of both positive lightning discharges hit wind turbines and negative ones. These were overwhelmingly larger than those of downward negative lightning. Table 1 also shows that the 5% value of peak currents of positive lightning that hit the wind turbines is larger than that of downward negative lightning. This indicates that lightning discharges that hit the wind turbines in winter have larger energy and peak current than typical downward negative lightning discharges. Winter lightning frequently occurs in the coastal areas of the Sea of Japan, in which the incidence of winter lightning is approximately half of the annual thunderstorm days. Winter lightning occurs within 35 km of the coastline of the Sea of Japan. We defined the period of November to March as winter and the period of April to October as summer.
Fig. 4. Monthly variation of accidents caused by lightning hitting wind turbines.
(b) show the abovementioned spatial distributions in summer and winter, respectively. Fig. 5 (a) indicates that lightning did not concentrate on the vicinity of the wind turbines in summer. However, Fig. 5 (b) shows that lightning discharges concentrated in the vicinity of the wind turbines in winter because they initiated and developed upward from high structures. Winter lightning is also called “single-discharge lightning,” comprising few lightning discharges compared with summer lightning, making it difficult to predict its lightning approach. The duration of the current of winter lightning is quite longer than that of summer. Sometimes, it exceeds hundreds of milliseconds [19]. The amount of charge transfer is expressed by the integral value of
4. CG lightning flash density and the number of lightning flashes hitting wind turbines in Japan 4.1. CG lightning flash density Fig. 6 (a) shows the annual CG lightning flash density, and Fig. 6 (b) 4
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Fig. 5. Spatial distribution of lightning discharges occurred in the vicinity of wind turbines. Table 1 Comparison between negative downward lightning flashes measured by Berger (1975) and lightning flashes hitting wind turbines in Japan.
Charge [C] Peak Current [kA]
Lightning Type
95%
50%
5%
Downward negative [6] Positive lightning hit WT [7] Negative lightning hit WT [7] Downward negative [6] Positive lightning hit WT [7] Negative lightning hit WT [7]
1.1 2.0 1.6 14 9 4
5.2 57.9 61.1 30 35 10
24 286 294.7 80 138.5 35
shows the CG lightning flash density in winter. The colors of density values are different in Fig. 6 (a) and (b). CG lightning flash densities shown in Fig. 6 were obtained from datasets observed by JLDN between 2003 and 2017. The grid cell size of that density was 10 × 10 km. Fig. 6 (b) shows that the CG lightning flash density in the coastal area of the Sea of Japan is overwhelmingly higher than other areas in winter. However, Fig. 6 (a) showed that lightning flash densities in those areas
Fig. 7. Monthly variation of the number of lightning flashes occurred within 10 km from wind turbines and the number of lightning flashes hitting them.
Fig. 6. Lightning ground flash density in Japan. The observation period was between 2003 and 2017. The grid cell sizes were 10 × 10 km. 5
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45 14143
38876 788
Summer
Winter
Summer
Winter
(b) Number of lightning flashes hitting wind turbines
(a) Number of lightning flashes detected by the JLDN within 10 km from the wind turbines
Fig. 8. Number of lightning flashes occurred in surrounding areas of wind turbines and number of lightning flashes hitting them in the coastal areas of the Sea of Japan.
summer is overwhelmingly larger than that in winter, more lightning flashes hit the wind turbines in winter than summer. Fig. 8 (a) shows the number of lightning flashes detected by JLDN within 10 km of the wind turbine in summer and winter. Fig. 8 (b) shows the number of lightning flashes hitting wind turbines in summer and winter. Fig. 8 showed that 95% of lightning flashes hitting the wind turbines were observed in winter, whereas only 27% of lightning flashes were detected by the JLDN within 10 km in winter. We investigated the relationship between the number of lightning flashes hitting the wind turbines and the number of accidents caused by lightning. Regardless of the scale of failure or damaged component, when a component of a wind turbine (e.g. communication devices, electric control system, generator, step-up gear and wind turbine blade) was broken due to lightning discharge, we counted it as one accident. The correlation coefficient between them was 0.86. This showed that accident due to lightning increased on the wind turbines when the number of lightning flashes hitting them increased. Fig. 8 indicated that the number of CG lightning flashes hitting wind turbines and the number of lightning flashes detected by the JLDN within 10 km was 0.12% and 5.6% in summer and winter, respectively. This shows that the probability of winter lightning hitting a wind turbine is approximately 47 times more than that of summer lightning hitting the wind turbines. This indicates that the probability of lightning hitting wind turbines is independent of the annual CG lightning flash density. Fig. 9 shows the relationships between the number of lightning flashes hitting the wind turbines and CG lightning flash density in winter and summer. We calculated their correlation coefficients, revealing that the coefficient for winter was 0.58 and that for summer was 0.17. This showed that no correlation exists between lightning flash density in summer and the number of accidents caused by lightning hitting the wind turbines.
Fig. 9. Relationships between number of lightning flashes hitting wind turbines and CG lightning flash density in winter (Nov. to Mar.) and summer (Apr. to Oct.). CC is the correlation coefficient.
were nearly similar to other areas during the year. Approximately 64% of accidents on all wind turbines owing to lightning in Japan occurred in coastal areas of the Sea of Japan in winter [5]. 4.2. Seasonal variation of the number of lightning flashes hitting wind turbines
4.3. Density of lightning flashes hitting the wind turbines
The lightning risk on wind turbines is a probability of damaging turbines by lightning. The damage depends on the characteristics of lightning discharges such as their peak current, steepness of current waveforms, and electric charges. We compared the number of lightning flashes that occurred in the surrounding area of the wind turbines and the number of lightning directly hitting the turbines. Fig. 7 shows the monthly variation of the CG lightning flash detected by JLDN within 10 km from the wind turbines shown in Fig. 3 and the number of lightning flashes hitting these wind turbines as detected by the Rogowski coils and/or the video cameras. Although Fig. 7 shows that the number of lightning discharges within 10 km from the wind turbines in
We showed the rates of the number of CG lightning flashes hitting the wind turbines and the number of lightning flashes detected by the JLDN within a 10 km range. Ngs was 0.0012 and Ngw was 0.056 within this range. Fig. 10 demonstrates the DLFHW calculated using Equation (1) per 10 × 10 km grid cell in Japan. High probability areas for turbines being hit by lightning are the coastal areas of the Sea of Japan. Figs. 8 and 9 show the corresponding results, indicating that the annual CG lightning flash density is more dominated by the CG lightning flash density in summer than that in winter. Moreover, a correlation is recorded between that in winter and the number of accidents at wind turbines. Therefore, these are dissimilar to annual CG lightning flash 6
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Fig. 10. Probability of lightning flashes hitting wind turbines in Japan per grid cell of 10 × 10 km.
References
densities shown in Fig. 8 (a); however, these are similar to that of winter. Therefore, we propose that the annual lightning flash density should not be used to evaluate lightning risk for wind turbines in Japan. However, the DLFHW showed in Fig. 10 disregards an increase in the number of lightning flashes after a high structure installation such as a wind turbine [21]. Therefore, the DLFHW turbine will exceed that shown in Fig.10 after wind turbine installation.
[1] IEC, “Lightning density based on lightning location system (LLS),” IEC62858 Ed.1.0, International Standard, (2015). [2] IEC, “Protection against lightning – part 2: risk management,” IEC62305-2 Ed.3, 81/570/CDV(Committee Draft for Vote), September (2017). [3] NEDO, “Wind power generation introduction guidebook,” Ed. 9, (2008), pp. 85–88 (in Japanese). [4] S. Yokoyama, S. Sekioka, Lightning protection of wind turbine generation system, IEEJ Trans. PE. 129 (No. 5) (2009) 572–575 (in Japanese). [5] S. Yokoyama, Lightning damages of wind turbine blades and protection methods of them, IEEJ Trans. PE. 124 (No. 2) (2004) 177–180 (In Japanese). [6] K. Berger, R.B. Anderson, H. Kroninger, Parameter of lightning flashes, Electra 80 (1975) 223–237. [7] NEDO: “Research and Development of Next-Generation Wind Power Generation Technology for Technology Corresponding to Natural Environment etc. for Measures of lightning protection,” Management No. 20150000000080, 2015 (in Japanese). [8] K. Miyake, I. Kishizima, T. Suzuki, H. Mitani, M. Tkashima, Characteristics of winter lightning current in coastal area of the sea of Japan, IEEJ Trans., PE. 110 (11) (1990) 944–951 (In Japanese). [9] M. Ishii, F. Fujii, M. Saito, M. Matsui, Evaluation of risk of lightning hits on tall structures in Japan, IEEJ Trans. PE. 129 (No. 5) (2009) 621–627 (In Japanese). [10] T. Sato, The investigation committee of lightning protection for wind power generation facilities considering lightning characteristics, IEEJ Trans. PE 133 (No. 3) (2013) 23 (in Japanese). [11] M. Matsui, K. MIchishita, S. Yokoyama, Lightning ground flash density and evaluation of lightning risk for wind turbines in coastal areas of the sea of Japan, 34th International Conference on Lightning Protection, Rzeszow, Poland, 2018. [12] M. Matsui, K. Michishita, S. Kurihara, Accuracies of location and estimated peak current for negative downward return-strokes to wind turbine observed by JLDN, IEEJ Trans. PE 135 (No. 10) (2015) 644–645 (in Japanese). [13] M. Ishii, F. Fujii, M. saito, D. Natsuno, A. Sugita, Detection of lightning return strokes hitting wind turbines in winter by JLDN, IEEJ Trans. Power Energy 133 (No 12) (2013) 1009–1010 (in Japanese). [14] M. Matsui, K. Michishita, N. Honjo, Discussion on improving error of located
5. Conclusion Despite the fact that only 27% of the total number of CG lightning flashes occurred within 10 km of the wind turbines in winter, 95% of CG lightning flashes hitting the wind turbines occurred in winter. The correlation coefficient between the number of lightning flashes within a 10 km radius from the wind turbines and the number of accidents caused by lightning discharges was −0.27. Therefore, no correlation exists between these factors. The correlation coefficient between the number of lightning flashes hitting the wind turbines and the number of accidents caused by lightning was 0.86. The density of lightning flashes hitting wind turbines to the number of lightning flashes detected by the JLDN within 10 km from them were 0.12% and 5.6% in summer and winter, respectively. The probability map of lightning flashes hitting the wind turbines in Japan presented herein indicate a high probability that CG lightning flashes hitting wind turbines were concentrated in the coastal areas of the Sea of Japan, similar to areas of high CG lightning flash density in winter. Therefore, we proposed that annual lightning flash density should not be used to evaluate lightning risk for wind turbines in Japan. 7
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[18] D. Wang, N. Takagi, Characteristics of winter lightning rhar occurred on a windmill and its lightning protection tower in Japan, IEEJ Trans. PE. 132 (No. 6) (2012) 568–572. [19] M. Miki, Observation of current and leader development characterustics of winter lightning, IEEJ Trans. PE. 127 (No. 12) (2007) 1247–1252. [20] IEIEJ, Lightning protection for electrical and electrronics equipment, Oham Publishing Co., Ltd., 2011, pp. 28–31. [21] M. Saito, M. Ishii, A. Onishi, F. Fujii, M. Matsui, D. Natsuno, Frequency of upward lightning hits to wind turbines in winter, IEEJ Trans. P.E. 131 (No. 12) (2011) 979–985 (in Japanese).
position and estimated peak current of winter lightning to a wind turbine by JLDN, 10th Asia-Pacific International Conference on Lightning (APL 2017), Krabi, Thailand, 2017. [15] M. Matsui, K. Michishita, N. Honjo, Accuracy of location and estimated peak current of winter lightning detected by JLDN after adapting propagation delay correction, Annual Conference of Power & Energy Society IEE of Japan, Tokushima, Japan, 2018. [16] M. Matsui, K. Michishita, Transition of Location Accuracy of the JLDN”, High Voltage Engineering, IEE of Japan, Ishigaki-jima, Japan, 2019. [17] NEDO: http://www.nedo.go.jp/english/index.html.
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