Owez-R-113-Lightning-R01.Pdf

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CONTENTS Page

SUMMARY...... 4

1 General...... 5

2 Location of the OWEZ ...... 6

3 KNMI lightning data ...... 8 3.1 General Information...... 8 3.2 Extraction of relevant data...... 11

4 Determination of potential strokes on turbines ...... 14

5 Lightning data from OWEZ ...... 28

6 Comparision of the OWEZ SCADA data with the KNMI data ...... 32 6.1 Comparison of the number of strokes...... 32 6.2 Taking the position-finding of the FLITS system in closer consideration...... 32 6.3 Comparison between the FLITS and SCADA data on a daily time-scale...... 33

7 Conclusion...... 36

REFERENCES...... 37

Appendix I Discharges and strokes in the wind farm and its vicinity ...... 38

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SUMMARY

NoordzeeWind is a joint venture between utility company Nuon and oil company Shell and has been set up specifically for the development, construction and operation of the Off shore Wind farm Egmond aan Zee (OWEZ), which counts 36 wind turbines.

As part of the OWEZ project NoordzeeWind has carried out an extensive measurement and evaluation program (NSW-MEP). One of the tasks in this program is to gain knowledge on the frequency of occurrence of lightning strokes on wind turbines offshore. In this report the results of an analysis are presented that KEMA has performed on lightning strike data during the years 2006 - 2009 from the FLITS system of the Royal Netherlands Meteorological Institute (KNMI) and the SCADA system of the off shore wind farm near Egmond aan Zee. The FLITS system is based on the measurements of radio frequency pulses which are emitted by lightning.

The predicted amount of strokes in wind turbines using data from the FLITS system and the IEC 61024 method for determining the collection area of an isolated structure, are ten to hundred times lower than the amount of strokes detected by the SCADA system. Furthermore, there is a mismatch between the data of the FLITS and the SCADA system in both the position and the time of the strokes. In the same seconds or minutes the SCADA system detects strokes in wind turbines, the FLITS system hardly shows any corresponding strokes (this also applies to cloud-cloud discharges). Also on a daily time scale, there is no significant correlation between the number of strokes detected by the FLITS and the SCADA system. The reason for the mismatch between the data of the FLITS and the SCADA system is not clear.

On the basis of 1) the comparison between the FLITS data and the SCADA data for the period September 2006 – December 2009 and 2) the comparison between detected number of strokes by the FLITS system in the OWEZ area and it’s vicinity and the numbers found in literature, the conclusion can be drawn that the FLITS system seems unsuitable for detecting strokes, nor for giving an indication of lightning stroke activity, in wind turbines at the location of the Off shore Wind farm Egmond aan Zee.

Acknowledgement The Offshore wind Farm Egmond aan Zee has a subsidy of the Ministry of Economic Affairs under the CO2 Reduction Scheme of the Netherlands -5- 50863511.400-TOS/HSM 09-5341

1 GENERAL

As part of the OWEZ project NoordzeeWind will carry out an extensive measurement and evaluation program (NSW-MEP). The contents of the NSW-MEP are about generating data and knowledge. The aim of task 1.1.3 - Design conditions lightning – of the NSW-MEP program is to gain knowledge on the frequency of occurrence of lightning strokes on wind turbines offshore.

Lightning strokes hitting wind turbines or one of its components may cause failure of one or more systems and a stand still of the wind turbine. The consequences of a lightning stroke may differ to a considerable extent, from a temporarily outage of internal communication lines to a complete destruction of all rotor blades.

On all wind turbines in the OWEZ offshore park there are lightning detection systems in the tree blades. This “native” lightning system reports lightning as alarms and warnings to the SCADA system.

In this report the results of an analysis are presented that KEMA has performed on lightning strike data in the period September 2006 – December 2009 from the Royal Netherlands Meteorological Institute (KNMI) and the SCADA system of the off shore wind farm near Egmond aan Zee. -6- 50863511.400-TOS/HSM 09-5341

2 LOCATION OF THE OWEZ

The Off shore Wind farm Egmond aan Zee (OWEZ) is located 10-18 kilometres offshore from the Dutch coastal village Egmond aan Zee. Figure 1 shows the lay out of the wind farm (the boundaries of the concession area are shown as a solid line) [1]. Table 1 gives the coordinates of all individual wind turbines in the park, which are shown in figure 1 as triangles. The minimum distance between two wind turbines is 640 meters.

Figure 1 Boundary of the windpark and positions of the 36 wind turbines

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Table 1 Coordinates in WGS84 projection of the individual wind turbines of the park [1]

No NB OL NB (decimal) OL (decimal) 1 52º 34’ 43.6‘’ 4º 26’ 3.2’’ 52.57878 4.43422 2 52º 34’ 59.5’’ 4º 25’ 41.2’’ 52.58319 4.42811 3 52º 35’ 15.1’’ 4º 25’ 19.5’’ 52.58753 4.42208 4 52º 35’ 31.1’’ 4º 24’ 57.4’’ 52.59197 4.41594 5 52º 35’ 47.0’’ 4º 24’ 35.4’’ 52.59639 4.40983 6 52º 36’ 2.9’’ 4º 24’ 13.3’’ 52.60081 4.40369 7 52º 36’ 19.1’’ 4º 23’ 50.9’’ 52.60531 4.39747 8 52º 36’ 35.0’’ 4º 23’ 28.8’’ 52.60972 4.39133 9 52º 36’ 50.9’’ 4º 23’ 6.7’’ 52.61414 4.38519 10 52º 37’ 7.0’’ 4º 22’ 45.0’’ 52.61861 4.37917 11 52º 37’ 22.7’’ 4º 22’ 22.5’’ 52.62297 4.37292 12 52º 37’ 38.7’’ 4º 22’ 0.4’’ 52.62742 4.36678 13 52º 34’ 54.9’’ 4º 26’ 57.1’’ 52.58192 4.44919 14 52º 35’ 10.8’’ 4º 26’ 35.1’’ 52.58633 4.44308 15 52º 35’ 26.7’’ 4º 26’ 13.0’’ 52.59075 4.43694 16 52º 35’ 42.6’’ 4º 25’ 51.0’’ 52.59517 4.43083 17 52º 36’ 8.1 ’’ 4º 25’ 15.6’’ 52.60225 4.42100 18 52º 36’ 24.0’’ 4º 24’ 53.6’’ 52.60667 4.41489 19 52º 36’ 39.9’’ 4º 24’ 31.5’’ 52.61108 4.40875 20 52º 36’ 55.9’’ 4º 24’ 9.4’’ 52.61553 4.40261 21 52º 37’ 11.8’’ 4º 23’ 47.3’’ 52.61994 4.39647 22 52º 35’ 30.4’’ 4º 27’ 17.4’’ 52.59178 4.45483 23 52º 35’ 46.3’’ 4º 26’ 55.1’’ 52.59619 4.44864 24 52º 36’ 2.4’’ 4º 26’ 33.1’’ 52.60067 4.44253 25 52º 36’ 27.0’’ 4º 25’ 59.0’’ 52.60750 4.43306 26 52º 36’ 44.9’’ 4º 25’ 34.2’’ 52.61247 4.42617 27 52º 37’ 0.8’’ 4º 25’ 12.1’’ 52.61689 4.42003 28 52º 37’ 16.7’’ 4º 24’ 50.0’’ 52.62131 4.41389 29 52º 37’ 32.7’’ 4º 24’ 27.9’’ 52.62575 4.40775 30 52º 36’ 7.2’’ 4º 27’ 35.6’’ 52.60200 4.45989 31 52º 36’ 23.3’’ 4º 27’ 13.7’’ 52.60647 4.45381 32 52º 36’ 47.6’’ 4º 26’40.0’’ 52.61322 4.44444 33 52º 37’ 5.8’’ 4º 26’ 14.8’’ 52.61828 4.43744 34 52º 37’ 21.7’’ 4º 25’ 52.7’’ 52.62269 4.43131 35 52º 37’ 37.6’’ 4º 25’ 30.7’’ 52.62711 4.42519 36 52º 37’ 53.5’’ 4º 25’ 8.6’’ 52.63153 4.41906

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3 KNMI LIGHTNING DATA

3.1 General Information

The registration of lightning by the KNMI (Royal Netherlands Meteorological Institute) is based on the measurements of radio frequency pulses which are emitted by lightning. KNMI uses the lightning detection system FLITS, which stands for Flash Localisation by Interferometry and Time of Arrival System [2]. The FLITS system uses two methods for determining the location of the discharge or stroke:

• time of arrival (TAO): this method uses the time difference between detection of radio waves in the Low Frequency (LF) area of the radio frequency (< 4 MHz) at several lightning detection stations • detection finding: this method uses the angles of the received radio waves in the Very High Frequency (VHF) area of the radio frequency (about 110 MHz) at several lightning detection stations.

Figures 3 shows the location of the lightning detection stations in the Netherlands and Belgium. Every detection station consists of a pole of 17,5 meters high and is equipped with several sensors. The data from these stations is sent to a central unit at the KNMI where the data is processed. The data covers a large area, including the Netherlands, Belgium, parts of Germany and France and a large part of the North sea.

The processed data from the FLITS system is stored in the Hierarchical Data Format (HDF). This is a general-purpose, machine-independent standard for storing scientific data in files. Table 2 gives an overview of the data columns in the KNMI lightning files.

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Table 2 The information stored in the KNMI lightning files [3]

Time offset Time difference between the reference time (which can be found in the metadata and filename of the lightning file) and the time of hit in seconds (hours : minutes : seconds) and milliseconds Longitude Place of hit in decimal degrees Latitude Place of hit in decimal degrees Event type 0=isolated point, no lightning, e.g. communication intrusions 1=starting point of a cloud-cloud discharge 2=next point in a cloud-cloud discharge 3=end point of a cloud-cloud discharge 4=ground stroke 5=return stroke Position error Error in position of the discharge in meters Rise time Rise time for a ground stroke in microseconds Decay time Decay time for a ground stroke in microseconds Current Calculated current in Ampere

Lightning data for the period 01-07-2006 till 31-12-2009 are obtained from the KNMI and analysed in this report. For further analyses the KNMI lightning data has been split in four sets:

1 for the period 01-07-2006 till 31-12-2006 (Q3 and Q4 of 2006) 2 for the whole of 2007 3 for the whole of 2008 4 for the whole of 2009.

The position error of the lightning data can be up to 7 kilometres, but at the location of the wind farm the position error is in general less than 1 kilometre (see figure 2 and 3), with a maximum of 4 kilometres (see figure 2). When analysing the KNMI lightning data one can see that in areas where the position error is more than 1 kilometre, the sensitivity of the FLITS lightning detection system decreases and less strokes are detected (see figure 4). OWEZ is located in an area where the FLITS lightning detection system should have sufficient sensitivity.

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4000 detected strikes by FLITS 3500 3000 2500 2000

1500 1000

position errorposition in meters 500

Figure 2 Ground and return strokes in the vicinity of the wind farm in the period September 2006 – September 2009 (336 in total), sorted by increasing position error

Figure 3 Locations of the FLITS detection stations. The stations are marked with a cross and circle: De Kooy, Valkenburg, Deelen, Hoogeveen, Oelegem, Mourcourt and La Gileppe. The colour scale shows the position error [4] -11- 50863511.400-TOS/HSM 09-5341

Figure 4 All cloud-to-cloud strokes (purple) and ground and return strokes (blue) in Q3-2007. The little square is the OWEZ area

3.2 Extraction of relevant data

For further analyses, all lightning events were extracted from the data files that took place within 7 kilometres of the outermost wind turbines of the OWEZ. The surface of the OWEZ including the buffer is 405 square kilometres. A buffer of 7 kilometres is chosen because this is the maximum possible location error of a stroke in the FLITS data. In practice, at the location of the wind farm the position error is less than 7 kilometres (see figure 2 and 3), but in order to be 100% sure all strokes that can theoretically occur in the wind farm are taken into account, a boundary of 7 kilometres is chosen.

Analyses of the lightning data shows that the number of days that ground- and/or return strokes occur within 7 kilometres of the outermost wind turbines of the wind farm are limited. In Q3 and Q4 of 2006 there were 12 days with ground- and/or return strokes in the wind farm and its vicinity. In 2007, 2008 and 2009 there were respectively 9, 14 and 11 of such days, see table 3.

The total amount of registered ground- and return strokes is 90 in Q3 and Q4 of 2006, 131 in 2007, 62 in 2008 and 80 in 2009 (table 3). That is 0.45, 0.32, 0.15 and 0.19 strokes/km2/year respectively. This is a considerable lower figure compared to figures found in literature for the Dutch coastal area. The KNMI gives an average of 1.3 strokes/km2/year [6] and the -12- 50863511.400-TOS/HSM 09-5341

NEN1014 publication gives about 2.5 strokes/km2/year [5]. Both in time and space large fluctuations in the number of strokes exists (see figure 5).

Table 3 Days with strokes occurring within 7 km of the outermost wind turbines of the wind farm and the total number of ground- and return strokes in Q3 and Q4 of 2006, 2007, 2008 and 2009

July - Dec. 2006 2007 2008 2009 number number number number date of strokes date of strokes date of strokes date of strokes 20060705 3 20070121 3 20080301 2 20090526 2 20060722 4 20070608 42 20080331 1 20090717 2 20060802 3 20070627 2 20080531 1 20090730 43 20060811 1 20070703 2 20080602 4 20090820 1 20060814 2 20070704 5 20080702 1 20090828 3 20060820 32 20070709 18 20080712 1 20090829 1 20060828 4 20070715 2 20080731 6 20091104 20 20060929 2 20070716 43 20080801 1 20091106 3 20061001 34 20070722 14 20080807 17 20091123 1 20061002 2 20080813 2 20091128 2 20061118 1 20080823 19 20091204 2 20061129 2 20081027 3 20081028 3 20081204 1 total 90 total 131 total 62 total 80

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Figure 5 Number of ground and return strokes per square kilometre per year according to the KNMI for the period 1995-1998 [6]

Most registered discharges are cloud tot cloud discharges. Only a small percentage of the discharges develops into a ground or return stroke. Appendix I gives an overview of all detected discharges and strokes by the FLITS system within 7 kilometres of the outermost wind turbines of the wind farm in the period September 2006 – December 2009.

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4 DETERMINATION OF POTENTIAL STROKES ON TURBINES

By using the coordinates of the 36 wind turbines (see table 1), and the coordinates of the ground- and return strokes including their position error, all strokes can be determined that could theoretically have struck a wind turbine. A GIS (Geographical Information System) was used for carrying out this analysis.

Figure 6, 7, 8 and 9 show the wind turbines (black dots), all the ground and return stokes (orange stars) and their position error (grey circles) in the vicinity of the wind farm in Q3 and Q4 of 2006, in 2007, in 2008 and in 2009. We can now select all strokes that contain a wind turbine within the range of the position error. These are all the ground and return strokes that could potentially have struck a wind turbine. In figure 10, 11, 12 and 13 all these strokes are highlighted in purple.

Figure 6 All ground and return strokes (orange stars) in Q3 and Q4 of 2006 in the vicinity of the wind farm. The grey circles around the discharge events are the position error. The black dots are the wind turbines

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Figure 7 All ground and return strokes in 2007 in the vicinity of the wind farm

Figure 8 All ground and return strokes in 2008 in the vicinity of the wind farm

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Figure 9 All ground and return strokes in 2009 in the vicinity of the wind farm

Figure 10 The ground and return strokes in Q3 and Q4 of 2006 in the vicinity of the wind farm which could potentially have struck one or more wind turbines -17- 50863511.400-TOS/HSM 09-5341

Figure 11 The ground and return strokes in 2007 in the vicinity of the wind farm which could potentially have struck one or more wind turbines

Figure 12 The ground and return strokes in 2008 in the vicinity of the wind farm which could potentially have struck one or more wind turbines -18- 50863511.400-TOS/HSM 09-5341

Figure 13 The ground and return strokes in 2009 in the vicinity of the wind farm which could potentially have struck one or more wind turbines

In Q3 and Q4 of 2006 there were 10 strokes that could potentially have struck a wind turbine in the wind farm. In 2007 this number was 15. In 2008 this number was only 3 and in 2009 this number was 8. Tables 4, 5, 6 and 7 show the details of all the potential strokes on wind turbines.

The number of purple circles in figure 10, 11, 12 and 13 do not necessarily correspond with the number of potential wind turbine strokes in table 4, 5, 6 and 7. Some strokes have the same location and position error because one discharge channel can be used two times (or more) within a fraction of a second to produce ground or return strokes.

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Table 4 All ground and return strokes in and near the wind farm that could potentially have struck a wind turbine in Q3 and Q4 of 2006

LON LAT DATE TIME SUBSEC POSERR TYPE CURRENT RISE DECAY TIME TIME potentially struck windmills 4.458 52.601 20060802 84245 0.958 560 4 -25870 800 3100 30 4.461 52.601 20060802 84245 0.9677 560 5 -15020 288 2300 30 4.461 52.601 20060802 84245 0.98 560 5 -24530 387 3600 30 4.395 52.622 20060811 14045 0.6454 600 4 -12430 487 1800 21 4.406 52.599 20060820749480.227114204-1312035030004567171819 4.397 52.609 20060820749480.370814505-127602372200678918192021 4.401 52.625 20061001 231111 0.297 590 4 -8830 5.75 25 20 21 4.393 52.619 20061001 231111 0.316 590 5 -8550 2.62 20 8 9 4.422 52.614 20061001 231213 0.713 590 4 -26140 5.88 32 35 36 4.436 52.630 20061001 231214 0.21 580 4 -14710 3.38 29 34

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Table 5 All ground and return strokes in and near the wind farm that could potentially have struck a wind turbine in 2007

LONLAT DATE TIMESUBSEC POSERR TYPE CURRENT RISE DECAY potentially struck TIME TIME windmills 4.426 52.603 20070121 43646 0.0942 750 4 8790 3.87 49 17 25 4.437 52.598 20070608 3058 0.542 570 4 -21360 612 3600 16 24 4.385 52.621 20070608 3716 0.7172 600 5 -15090 500 2200 10 4.439 52.576 20070608 3829 0.515 560 4 -25750 650 4800 1 4.444 52.585 20070608 201924 0.6067 560 4 -19810 387 3500 13 14 4.444 52.591 20070608 202059 0.2117 670 4 -33740 725 2500 14 15 23 4.38052.603200707031200260.420820504-621401.6223678910 4.422 52.621 20070709 85837 0.0006 720 4 -15310 3 17 27 28 34 35 4.369 52.625 20070716 170234 0.4672 2090 4 -34320 3.63 28 9 10 11 12 21 4.427 52.581 20070716 170715 0.57 740 4 -15170 4 30 1 2 4.357 52.626 20070716 171741 0.3553 800 4 -7710 3.5 21 12 4.424 52.580 20070716 172244 0.0797 690 4 -14550 4.25 23 2 4.399 52.643 20070716 173241 0.7816 2080 4 -52750 7.5 39 29 36 4.399 52.643 20070716 173241 0.8979 2080 5 -29790 2.88 25 29 36 4.448 52.615 20070722 63017 0.6627 730 4 -12420 4.25 40 32

Table 6 All ground and return strokes in and near the wind farm that could potentially have struck a wind turbine in 2008

LONLAT DATE TIMESUBSEC POSERR TYPE CURRENT RISE DECAY potentially struck TIME TIME windmills 4.423 52.632 20080813 210111 0.1321 720 4 -24280 350 2700 35 36 4.460 52.601 20080823 52440 0.9026 660 4 -16660 363 2000 30 4.426 52.584 20081027 180400 0.1791 690 4 7850 850 4900 2 3

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Table 7 All ground and return strokes in and near the wind farm that could potentially have struck a wind turbine in 2009

LON LAT DATE TIME SUBSEC POSERR TYPE CURRENT RISE DECAY potentially struck TIME TIME windmills 4.462 52.588 20090526 24447 0.0217 650 4 -8840 2.37 23 22 4.416 52.595 20090730 13425 0.4741 580 4 -28080 8.63 41 4 5 4.448 52.617 20090730 13601 0.9116 570 5 -17880 3.5 30 32 4.436 52.625 20090730 13656 0.0104 580 4 -10870 3.12 19 34 4.423 52.63 20090730 160016 0.851 760 4 5030 8 38 35 36 4.406 52.593 20090820 161246 0.0806 710 4 4080 4.87 50 4 5 4.449 52.596 20091104 223629 0.8345 560 4 -18490 8.38 34 23 4.395 52.616 20091123 164046 0.9948 590 4 -5910 10.75 21 20 21

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On a level surface, the place where lightning strikes can be considered as random. If there is a conductive object in the vicinity of the streak, the lightning will strike in this object. The striking distance (r) in which an object will attract the lightning is a function of the current (I). Different empirical formulas exist for calculating the striking distance. We will use here the formula of Armstrong and Whitehead [7, page 226], which gives a slightly larger striking distance then other formulas found in literature, and which should therefore be considered as a worst case formula.

Armstrong and Whitehead’s formula: r=6,78*I0,8

According to this formula, and using the KNMI lightning data as input, the average striking distance of the strokes in the period July 2006 – December 2009 in the OWEZ area is 74 meter. The collection area (A) for the OWEZ wind turbines can then be calculated as a function of it’s striking distance and the height of the object, see figure 14. The collection area of a structure is defined as an area of ground surface which has the same annual frequency of direct lightning flashes as the structure. The height of the wind turbines in the OWEZ is 116 meters above Mean Sea Level [1] (tower plus blade).

The wind turbines in OWEZ are designed according to the standard IEC 61024 (Protection of structures against lightning, 1993). According to IEC 61024 the collection area is not related to the current (I) of lightning strokes, but only to the height (h) of the wind turbine: “for isolated structures the equivalent collection area is the area enclosed with a border line obtained from the intersection between the ground surface and a straight line with a 1:3 slope which passes from the upper parts of the structure (touching it there) and rotating around it.” (IEC 61024: page 21). The same method is followed in IEC 61400-24 (2002). As a consequence: A = 9πh2. For the OWEZ wind turbines this means the striking distance is 348 meters and the collection area (A) is 380459 m2.

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r < h r > h

r r-h r

h h

2 2 2hr − h r A = π·r A = π·h·(2r-h)

Figure 14 Surface (A) in which the object will attract the lightning as a function of height (h) and the striking distance(r) of the lightning stroke [7]

Using the position error in the KNMI lightning files and the collection area (A), we can now give an indication of the chance the lightning struck a wind turbine. We assume that the chance of the stroke hitting the surface is the same everywhere within the radius of the position error. The results for Q3 and Q4 of 2006, 2007, 2008 and 2009 are shown in table 8, 9, 10 and 11. The collection area according to the IEC 61024 method as well as according to Armstrong & Whitehead’s formula are given in these tables.

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Table 8 All ground and return strokes in the OWEZ area in Q3 and Q4 of 2006 and their risk of hitting a wind turbine

Armstrong & Whitehead formula POSERR CURRENT surface striking chance chance LON LAT DATE TIME SUBSEC (m) (ampère) POSERR distance surface mill is hit (%) mill is hit (%) (m2) (r) (A) IEC 61024 4.458 52.601 20060802 84245 0.958 560 -25870 985204 90 25691 2.61% 38.62% 4.461 52.601 20060802 84245 0.9677 560 -15020 985204 59 10764 1.09% 38.62% 4.461 52.601 20060802 84245 0.98 560 -24530 985204 87 23595 2.39% 38.62% 4.395 52.622 20060811 14045 0.6454 600 -12430 1130973 50 7952 0.70% 33.64% 4.406 52.599 20060820 74948 0.2271 1420 -13120 6334708 53 8669 0.96% 42.04% 4.397 52.609 20060820 74948 0.3708 1450 -12760 6605199 51 8292 1.00% 46.08% 4.401 52.625 20061001 231111 0.297 590 -8830 1093589 38 4601 0.84% 69.58% 4.393 52.619 20061001 231111 0.316 590 -8550 1093589 37 4370 0.80% 69.58% 4.422 52.614 20061001 231213 0.713 590 -26140 1093589 91 26121 4.78% 69.58% 4.436 52.630 20061001 231214 0.21 580 -14710 1056832 58 10411 0.99% 36.00%

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Table 9 All ground and return strokes in the OWEZ area in 2007 and their risk of hitting a wind turbine

Armstrong & Whitehead formula POSERR CURRENT surface striking chance chance LON LAT DATE TIME SUBSEC (m) (ampère) POSERR distance surface mill is hit (%) mill is hit (%) (m2) (r) (A) IEC 61024 4.426 52.603 20070121 43646 0.0942 750 8790 1767146 38 4568 0.52% 43.06% 4.437 52.598 20070608 3058 0.542 570 -21360 1020704 78 18909 3.71% 74.55% 4.385 52.621 20070608 3716 0.7172 600 -15090 1130973 59 10844 0.96% 33.64% 4.439 52.576 20070608 3829 0.515 560 -25750 985204 90 25500 2.59% 38.62% 4.444 52.585 20070608 201924 0.6067 560 -19810 985204 73 16762 3.40% 77.23% 4.444 52.591 20070608 202059 0.2117 670 -33740 1410261 112 39294 8.36% 80.93% 4.380 52.603 20070703 120026 0.4208 2050 -62140 13202545 182 90591 3.43% 14.41% 4.422 52.621 20070709 85837 0.0006 720 -15310 1628602 59 11098 2.73% 93.44% 4.369 52.625 20070716 170234 0.4672 2090 -34320 13722792 113 40381 1.47% 13.86% 4.427 52.581 20070716 170715 0.57 740 -15170 1720336 59 10936 1.27% 44.23% 4.357 52.626 20070716 171741 0.3553 800 -7710 2010620 34 3703 0.18% 18.92% 4.424 52.580 20070716 172244 0.0797 690 -14550 1495712 57 10230 0.68% 25.44% 4.399 52.643 20070716 173241 0.7816 2080 -52750 13591788 160 74271 1.09% 5.60% 4.399 52.643 20070716 173241 0.8979 2080 -29790 13591788 101 32197 0.47% 5.60% 4.448 52.615 20070722 63017 0.6627 730 -12420 1674155 50 7941 0.47% 22.73%

Table 10 All ground and return strokes in the OWEZ area in 2008 and their risk of hitting a wind turbine

Armstrong & Whitehead formula POSERR CURRENT surface striking chance chance LON LAT DATE TIME SUBSEC (m) (ampère) POSERR distance surface mill is hit (%) mill is hit (%) (m2) (r) (A) IEC 61024 4.423 52.632 20080813 210111 0.1321 720 -24280 1628602 86 23211 2.85% 46.72% 4.460 52.601 20080823 52440 0.9026 660 -16660 1368478 64 12705 0.93% 27.80% 4.426 52.584 20081027 180400 0.1791 690 7850 1495712 35 3811 0.51% 50.87% -26- 50863511.400-TOS/HSM 09-5341

Table 11 All ground and return strokes in the OWEZ area in 2009 and their risk of hitting a wind turbine

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The chance that a stroke hits a wind turbine is calculated by multiplying the surface (A) with the number of potentially hit wind turbines (see table 4, 5, 6 and 7) and divided by the surface area of the position error. By adding up all the chances in the table, one obtains the number of lightning strokes in wind turbines according to the theory of probability. Using Armstrong and Whitehead’s formula, this number is 0.16 strokes for Q3 and Q4 in 2006, 0.31 strokes for the year 2007, 0.04 strokes for the year 2008 and 0.10 strokes for 2009. Using the IEC 61024 approach, the amount of strokes is 4.82 for Q3 and Q4 in 2006, 5.92 strokes for the year 2007, 1.25 strokes for the year 2008 and 3.72 strokes for the year 2009.

The IEC 61024 method for determining the collection area gives much greater values compared to the formula’s found in literature, which besides the height of the structure, also include the current of a lightning stroke. The number of potential strokes determined by the IEC 61024 method is still much smaller than the registered number of strokes by the SCADA system, as can be read in chapter 5. This means the IEC 61024 method for determining the collection area seems to be closer to reality than Armstrong & Whitehead’s or other comparative formula’s. Therefore, from this point on forwards, only the number of potential strokes according to the IEC 61024 method will be used.

The position error is a factor which introduces a large uncertainty in the analyses carried out in this chapter. The meaning of the position error is unclear. The manufacturer does not provide information on this. According to the KNMI (oral communication) the position error of the lightning stroke could well be larger than the position error as given in the FLITS files.

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5 LIGHTNING DATA FROM OWEZ

On all wind turbines in the OWEZ offshore park there are lightning detection systems in the three blades. The lightning system reports the maximum detected current peak of the stroke to the SCADA system.

The registration of lightning strokes in wind turbines by the SCADA system in the OWEZ started in September 2006. Up to December 31st 2009, 290 strokes in wind turbines have been detected, see table 12. From these 290 strokes, 79 had a current of over 10 kAmpère.

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Table 12 Number of strokes in wind turbines per day detected by the SCADA system

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Figure 15 and 16 show the stroke current distribution and the number of strokes per wind turbine. As can be seen in table 12 and figure 15, most lightning strokes (73%) have a current between 6 kAmpère (the threshold value for the SCADA system) and 10 kAmpère.

From the 290 strokes detected in the period September 2006 – December 2009 by the SCADA system, 13 had a current of over 50 kAmpère and were given the status ‘alarm’. The other 277 strokes were given the status ‘warning’. Table 13 shows the 13 strokes which had a current of over 50 kAmpère. The maximum current registered in the period September 2006 – December 2009 is 75 kAmpère.

220 210 200 190 Total Hits: 290 180 Max. Current: 75 kA 170 160 2009 150 2008 140 130 2007 120 2006 Q3 Q4 110 100 90 No of Strikes 80 70 60 50 40 30 20 10 0 6-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 Current [kA]

Figure 15 Stroke current distribution at OWEZ in the period September 2006 – December 2009

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14 13 12 11 10 9 8 7 6 nr. of strokesnr. of 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627282930313233343536 wind turbine

Figure 16 Number of strokes per wind turbine in the period September 2006 – December 2009

Table 13 Strokes with a current > 50 kAmpère (September 2006 - December 2009) date time (UTC) turbine current (kAmpère) 21-1-2007 04:44:49 32 72 21-1-2007 05:35:40 20 63 21-1-2007 05:41:03 7 75 7-2-2007 08:46:19 24 52 19-3-2007 21:02:17 14 61 8-6-2007 00:40:23 36 64 8-6-2007 20:28:47 35 64 1-8-2008 00:48:34 15 60 28-10-2008 02:00:14 8 59 23-3-2009 20:14:00 27 52 25-5-2009 20:45:00 36 55 26-11-2009 11:55:16 33 61 28-11-2009 03:44:00 36 51

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6 COMPARISION OF THE OWEZ SCADA DATA WITH THE KNMI DATA

6.1 Comparison of the number of strokes

The number of strokes per square kilometre detected by the FLITS system is much lower than the number found in literature: 0.15 – 0.45 strokes/km2/year for the area within 7 kilometres of the outermost wind turbines of the wind farm (see chapter 3) versus 1.3 – 2.5 strokes/km2/year found in literature [5][6]. The numbers from the SCADA system are 2.2 – 5.0 strokes/km2/year in the area where the wind turbines are placed (about 20 km2). This figure is however not corrected for the limited number of strokes that can occur between the wind turbines. Therefore it can be concluded that the number of strokes per square kilometre detected by the SCADA system are about two times higher than found in literature.

In 2007, 2008 and 2009, respectively 72, 100 and 96 strokes have been detected by the SCADA system. This contrasts sharply with the predicted 5.92 strokes in 2007, 1.25 strokes in 2008 and 3.72 strokes in 2009 using the data from the FLITS system from the KNMI (see chapter 4).

According to NoordzeeWind there is no reason to cast doubt on the recordings of the SCADA system. The reason for the mismatch between the FLITS data and the SCADA data being the SCADA system detecting too many strokes is therefore not investigated.

6.2 Taking the position-finding of the FLITS system in closer consideration

Possible explanations for the huge difference between the number of detected strokes in wind turbines by the SCADA system and the number of predicted strokes by the FLITS system (72 versus 5.92 in 2007, 100 versus 1.25 in 2008 and 96 versus 3.72 in 2009) are: a Wind turbines have a much greater attraction on ground and return strokes as is suggested by literature (IEC 61024, Armstrong and Whitehead’s formula and other formula’s [7]) b The position error in the FLITS data is larger in reality (but below 7 kilometres) c Not only ground and return strokes but also cloud-to-cloud discharges will strike wind turbines when they are inside the range of it’s position error.

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In order to investigate the three possible reasons mentioned above, it’s being assumed that the time recordings of both the SCADA and the FLITS system are correct. The SCADA system registers lightning strokes with a precision of 1 second and the FLITS system with a precision of 1 millisecond.

We determine if in the same second as the SCADA system has detected a lightning stroke, the FLITS system also detected a stroke within 7 kilometres of the outermost wind turbines of the wind farm. This is the case for only 1 out of 290 detected strokes by the SCADA system. This was at January 21st 2007, when the SCADA system detected a stroke in wind turbine 5 and the FLITS system detected a stroke between wind turbine 17 and 25. Now we use a time span of a minute instead of a second, so it is determined whether the two systems detect a stroke in the same minute. This is the case for only 3 out of the 290 strokes. If we also take into consideration cloud-to-cloud discharges (type 1 – 3, table 2) the score is 50 out of the 290 strokes. The average distance between the SCADA system location and the FLITS system location of these 50 strokes is however large, namely 9.4 kilometres, which is larger than the maximum position error. The above mentioned reasons a), b) and c) as possible reasons for the mismatch between the number of strokes detected by the SCADA and the FLITS system should on the basis of the this information be rejected.

From the 290 strokes detected in the period September 2006 – December 2009 by the SCADA system, 13 had a current of over 50 kAmpère and were given the status ‘alarm’ (see table 13). In the same minute as the SCADA system detected these 13 ‘alarm’ strokes, the FLITS system never detected a ground or return stroke within 7 kilometres of the outermost wind turbines of the wind farm. By 4 out of these 13 ‘alarm’ strokes, cloud-to-cloud strokes were detected by the FLITS system in this area in the same minute. A clear illustration of the mismatch between the SCADA and the FLITS system is the lightning stroke detected by the SCADA system on March 19th 2007. This was an ‘alarm’ stroke with current of 61 kAmpère. On March 19th 2007 there was no lightning activity at all in the vicinity of the wind farm according to the FLITS system, not even a single cloud-to-cloud discharge.

6.3 Comparison between the FLITS and SCADA data on a daily time-scale

Although in general on days of high lightning activity there are strokes detected by both the SCADA system and the FLITS system, the correlation between the two is very low. See figure 17. Here the number of detected strokes by the SCADA system is plotted on the x-axis and the number of strokes (ground- and return strokes) within 7 kilometres of the outermost wind turbines of the wind farm according to the FLITS system (see appendix I) is plotted on -34- 50863511.400-TOS/HSM 09-5341

the y-axis. Every dot represents one day with lightning activity. The period taken into account is again September 2006 – December 2009.

A comparable low correlation is found between strokes detected by the SCADA system and cloud-to-cloud discharges detected by the FLITS system (type 0 – 3, see table 2), as can be seen in figure 18. The blue points in figure 18 are isolated point discharges, these are not lightning discharges but events such as communication intrusions. The red squares are the starting points of cloud to cloud discharges and the green triangles the end of cloud to cloud discharges. Intermediate discharges during cloud to cloud discharges are also registered by the FLITS system and are represented in graph 18 as yellow dots. They are indicated as “next point cloud-cloud discharge”. There is hardly any correlation between amount of type 0 – 3 discharges from the FLITS system and the amount of detected strokes by the SCADA system. The highest correlation is R2 = 0.30 between the number of strokes detected by the SCADA system(x) and ‘next point of cloud-cloud‘ discharges (y): 64.23e0.2112x. The ‘next point of cloud-cloud’ discharges are apparently the best - but still insufficient - indicator for lightning stroke activity in the OWEZ area. Figure 18 doesn’t show the cases having zero strokes detected by the SCADA system, but one or more cloud-to-cloud discharges detected by the FLITS system. The number of days for which this applies are numerous.

50 45 40 35 30 25 20 FLITS data FLITS R2 = 0.1 15 10 5 0 number of strikes in and around OWEZ by OWEZ and around in strikes of number 0 2 4 6 8 1012141618 number of detected strikes by SCADA system

Figure 17 Number of strokes detected by the SCADA system and by the FLITS system in and around OWEZ on a daily time scale

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10000

y = 64.23e0.2112x 2 R = 0.3036 isolated point 1000 discharge start cloud-cloud discharge

next point cloud-cloud discharge

100 end cloud-cloud discharge

10 number of cloud-cloudnumber discharges in and around OWEZ by FLITSin OWEZ by aroundand data

1 01234567891011121314151617 number of detected strikes by SCADA system

Figure 18 Number of strokes detected by the SCADA system and number of cloud-to-cloud discharges by the FLITS system in and around OWEZ on a daily time scale -36- 50863511.400-TOS/HSM 09-5341

7 CONCLUSION

The number of strokes per square kilometre detected by the FLITS system in the OWEZ area and it’s vicinity is much lower (five to ten times) than the numbers found in literature. The amount of strokes per square kilometre detected by the SCADA system in the OWEZ area are about two times higher than the numbers stated in literature. The predicted amount of strokes in wind turbines using data from the FLITS system and the IEC 61024 method for determining the collection area of an isolated structure, are ten to hundred times lower than the amount of strokes detected by the SCADA system.

From the information in chapter 4, 5 and 6 it can be concluded that there is a mismatch between the data of the FLITS and the SCADA system in both the location and time of the strokes. In the same seconds or minutes the SCADA system detects strokes in wind turbines, the FLITS system hardly shows any corresponding strokes (this also applies to cloud-cloud discharges). Also on a daily time scale, there is no significant correlation between the number of strokes detected by the FLITS and the SCADA system (see paragraph 6.3).

On the basis of 1) the comparison between detected number of strokes by the FLITS system in the OWEZ area and it’s vicinity and the numbers found in literature and 2) the comparison between the FLITS data and the SCADA data for the period September 2006 – December 2009, the conclusion can be drawn that the FLITS system seems unsuitable for detecting strokes, nor for giving an indication of lightning stroke activity, in wind turbines at the location of the Off shore Wind farm Egmond aan Zee. The reason for the mismatch between the data of the FLITS and the SCADA system is not clear. A further research project, outside the OWEZ monitoring program, might shed light on this.

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REFERENCES

[1] Near Shore Windpark Wbr/Wm vergunningaanvraag NSW, 2003.

[2] Processing, validatie, en analyse van bliksemdata uit het SAFIR/FLITS systeem, Saskia Noteboom.

[3] Output format description of the FLITS HDF file converter program hdf2dis, version 2.1_20041028, KNMI.

[4] Informatie over het bliksemdetectie systeem, Hans Beekhuis KNMI.

[5] DOWEC CONCEPT STUDY TASK 7, Standards and criteria for offshore wind turbines, ECN-CX--00-039, H.B. Hendriks, P.P. Soullié, 2000.

[6] KNMI, Onweerswaarnemingen in Nederland, H.R.A. Wessels, 1999. Published in Zenit, 26, 1999, pages 260 – 264.

[7] Insulation Coordination for power systems, Andrew Hileman, 1999.

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APPENDIX I DISCHARGES AND STROKES IN THE WIND FARM AND ITS VICINITY

Detected discharges and strokes by the FLITS system within 7 kilometres of the outermost wind turbines of the wind farm in Q3 and Q4 of 2006.

JUL AUG SEP OKT NOV DEC datum : 20060705 datum : 20060801 datum : 20060914 datum : 20061001 datum : 20061118 datum : 20061204 type 0 6 type 0 4 type 0 4 type 0 48 type 1 4 type 1 1 type 1 15 type 1 9 type 1 6 type 1 166 type 2 336 type 2 125 type 2 254 type 2 100 type 2 68 type 2 2002 type 3 5 type 3 1 type 3 13 type 3 7 type 3 10 type 3 171 type 4 1 datum : 20061206 type 4 3datum : 20060802 datum : 20060929 type 4 26datum : 20061129 type 1 1 datum : 20060722 type 0 1 type 0 12 type 5 8 type 0 1 type 2 28 type 0 26 type 1 3 type 1 18datum : 20061002 type 1 6 type 3 1 type 1 47 type 2 22 type 2 444 type 0 5 type 2 72 datum : 20061207 type 2 245 type 3 2 type 3 20 type 1 7 type 3 5 type 0 2 type 3 48 type 4 1 type 4 2 type 2 124 type 4 2 type 1 5 type 4 4 type 5 2datum : 20060930 type 3 5 type 2 267 datum : 20060725 datum : 20060811 type 0 2 type 4 2 type 3 4 type 1 1 type 1 9 datum : 20061006 type 3 1 type 2 121 type 1 2 type 3 8 type 2 28 type 4 1 type 3 2 datum : 20060814 datum : 20061023 type 0 6 type 0 1 type 1 16 type 2 561 type 3 15 type 4 2 datum : 20060817 type 0 1 datum : 20060820 type 0 13 type 0=isolated point, no lightning, e.g. communication intrusions type 1 46 type 1=start cloud-cloud discharge type 2 984 type 2=next point in cloud-cloud discharge type 3 44 type 3=end cloud-cloud discharge type 4 24 type 4=ground stroke type 5 8 type 5=return stroke datum : 20060825 type 1 5 type 2 43 type 3 8 datum : 20060828 type 0 1 type 1 5 type 2 163 type 3 4 type 4 4 datum : 20060829 type 0 1 type 1 6 type 2 93 type 3 6 -39- 50863511.400-TOS/HSM 09-5341

Appendix I page 2

Detected discharges and strokes by the FLITS system within 7 kilometres of the outermost wind turbines of the wind farm in 2007.

JAN FEB MAA APR MEI JUN JUL AUG SEP OKT NOV DEC datum : 20070101 datum : 20070207 datum : 20070507 datum : 20070607 datum : 20070703 datum : 20070822 datum : 20070917 datum : 20071201 type 14type 11 type 11type 12type 21type 01type 11 type 12 type 2 24 type 2 106 type 2 2 type 2 6 type 3 1 type 1 1 type 3 1 type 2 112 type 3 5 type 3 1 type 3 1 type 3 1 type 4 2 type 2 22datum : 20070918 type 3 2 datum : 20070110 datum : 20070228 datum : 20070608 datum : 20070704 type 2 36 datum : 20071207 type 0 1 type 1 1 type 0 636 type 0 3 type 3 1 type 1 1 type 1 4 type 2 96 type 1 1186 type 1 19datum : 20070925 type 2 7 type 2 80 type 3 1 type 2 5167 type 2 286 type 1 1 type 3 1 type 3 4 type 3 1199 type 3 18 type 3 5 datum : 20070111 type 4 33 type 4 4 type 1 1 type 5 9 type 5 1 type 2 27 datum : 20070620 datum : 20070709 type 3 1 type 0 2 type 0 31 datum : 20070118 type 113type 195 type 0 1 type 2 157 type 2 1658 type 2 116 type 3 12 type 3 90 type 3 2 datum : 20070627 type 4 14 datum : 20070121 type 0 3 type 5 4 type 0 6 type 1 15 datum : 20070715 type 1 26 type 2 363 type 0 33 type 2 760 type 3 13 type 1 48 type 3 25 type 4 1 type 2 216 type 4 3 type 5 1 type 3 47 type 4 2 datum : 20070716 type 0 443 type 1 592 type 2 1754 type 3 610 type 4 27 type 5 16 datum : 20070720 type 0 3 type 1 4 type 2 59 type 3 3 datum : 20070722 type 0 3 type 1 19 type 2 548 type 3 23 type 0=isolated point, no lightning, e.g. communication intrusions type 4 10 type 1=start cloud-cloud discharge type 5 4 type 2=next point in cloud-cloud discharge datum : 20070723 type 3=end cloud-cloud discharge type 0 1 type 4=ground stroke datum : 20070730 type 5=return stroke type 0 1

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Appendix I page 3

Detected discharges and strokes by the FLITS system within 7 kilometres of the outermost wind turbines of the wind farm in 2008.

JAN FEB MAA APR MEI JUN JUL AUG SEP OKT NOV DEC 20080201 20080321 20080408 20080531 20080601 20080702 20080801 20080904 20081003 20081122 20081201 type 0 1 type 0 3 type 1 1 type 0 37 type 1 1 type 0 4 type 0 17 type 1 1 type 1 15 type 0 1 type 0 1 20080301 type 1 2 type 2 6 type 1 72 type 3 1 type 1 11 type 1 22 type 2 5 type 2 600 type 1 2 20081204 type 0 6 type 2 39 type 3 1 type 2 51220080602 type 2 232 type 2 62 type 3 1 type 3 16 type 2 180 type 1 6 type 1 18 type 3 220080423 type 3 65 type 0 23 type 3 10 type 3 2420080911 20081017 type 3 2 type 2 192 type 2 26120080324 type 0 2 type 4 1 type 1 45 type 4 1 type 4 1 type 2 3 type 0 120081123 type 3 6 type 3 20 type 0 1 type 2 481 20080708 20080807 20081027 type 0 1 type 4 1 type 4 220080331 type 3 42 type 0 1 type 0 31 type 1 5 type 0 1 type 4 4 type 1 1 type 1 53 type 2 165 type 1 120080605 type 2 66 type 2 373 type 3 4 type 2 18 type 2 3 type 3 1 type 3 53 type 4 3 type 4 1 20080712 type 4 8 20081028 type 0 1 type 5 9 type 0 6 type 1 320080808 type 1 13 type 2 38 type 1 1 type 2 302 type 3 3 type 3 2 type 3 14 type 4 120080812 type 4 3 20080719 type 2 29 type 1 1 20080813 type 2 7 type 0 3 type 3 1 type 1 17 20080726 type 2 246 type 0 1 type 3 13 type 1 3 type 4 2 type 3 3 20080814 20080728 type 0 1 type 0 8 20080823 type 1 8 type 0 5 type 2 33 type 1 19 type 3 6 type 2 281 type 0=isolated point, no lightning, e.g. communication intrusions 20080731 type 3 18 type 1=start cloud-cloud discharge type 1 2 type 4 16 type 2=next point in cloud-cloud discharge type 2 9 type 5 3 type 3=end cloud-cloud discharge type 3 3 type 4=ground stroke type 4 5 type 5=return stroke type 5 1

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Appendix I page 4

Detected discharges and strokes by the FLITS system within 7 kilometres of the outermost wind turbines of the wind farm in 2009.

JAN FEB MAA APR MEI JUN JUL AUG SEP OKT NOV DEC 20080323 20080410 20090514 20090616 20090717 20090819 20090903 20091011 20091102 20091203

type 1 2 type 0 1 type 2 14 type 2 16 type 0 2 type 0 1 type 0 1 type 0 2 type 0 1 type 0 1 type 2 67 20080415 type 3 1 type 3 1 type 1 1120090820 20090924 type 1 5 type 1 4 type 1 5 type 3 1 type 1 3 20090516 20090618 type 2 57 type 0 71 type 0 1 type 2 85 type 2 65 type 2 21 type 2 15 type 1 1 type 0 9 type 3 10 type 1 16620090925 type 3 6 type 3 4 type 3 4 type 3 5 type 2 2 type 1 1 type 4 2 type 2 775 type 0 1 datum : 20091103 20091204 20080427 type 3 1 type 3 120090721 type 3 163 type 0 1 type 0 1 type 0 3 type 1 1 20090517 type 0 1 type 4 1datum : type 2 1 type 1 2 type 2 5 type 0 1 type 2 420090828 type 0 1 20091104 type 2 79 type 1 4 type 3 1 type 0 9 type 0 39 type 3 2 type 2 3020090724 type 1 24 type 1 101 type 4 1 type 3 5 type 1 1 type 2 197 type 2 3087 type 5 1 20090525 type 2 3 type 3 26 type 3 98 20091211 type 0 71 type 3 2 type 4 1 type 4 14 type 0 1 type 1 7420090730 type 5 2 type 5 6 20091220 type 2 238 type 0 2720090829 20091105 type 1 1 type 3 70 type 1 124 type 1 5 type 1 1 type 2 3 20090526 type 2 1958 type 2 155 type 2 78 type 3 1 type 0 56 type 3 126 type 3 5 type 3 2 20091221 type 1 68 type 4 29 type 4 120091106 type 1 1 type 2 364 type 5 1420090831 type 0 1 type 2 43 type 3 69 type 0 2 type 1 9 type 3 1 type 4 2 type 3 1 type 2 578 type 3 10 type 4 3 20091107 type 0 1 20091122 type 0 1 20091123 type 0 5 type 1 8 type 2 49 type 3 8 type 4 1 20091126 type 0 3 type 1 15 type 2 324 type 3 15 20091127 type 0 3 type 1 5 type 2 185 type 3 5 type 0=isolated point, no lightning, e.g. communication intrusions 20091128 type 1=start cloud-cloud discharge type 0 2 type 2=next point in cloud-cloud discharge type 1 13 type 3=end cloud-cloud discharge type 2 183 type 4=ground stroke type 3 13 type 5=return stroke type 4 2