owez r 113 20090803 lightning - noordzeewind · analysis and evaluation on collected lightning...

KEMA Nederland B.V. Utrechtseweg 310, 6812 AR Arnhem P.O. Box 9035, 6800 ET Arnhem The Netherlands T +31 26 3 56 91 11 F +31 26 3 89 24 77 [email protected] www.kema.com Registered Arnhem 09080262

50863511.400-TOS/ECC 09-5341 OWEZ_R_113_20090803

NSW-MEP - design conditions lightning

Analysis and evaluation on collected lightning stri ke data

Arnhem, August 3rd 2009 Author: Luc Verhees

By order of

author : L. Verhees 09-08-03 reviewed : R. de Graaff 09-08-

B 31 pages annexes WSc approved : M.P. de Jong 09-08-

© KEMA Nederland B.V., Arnhem, the Netherlands. All rights reserved. It is prohibited to change any and all versions of this document in any manner whatsoever, including but not limited to dividing it into parts. In case of a conflict between the electronic version (e.g. PDF file) and the original paper version provided by KEMA, the latter will prevail. KEMA Nederland B.V. and/or its associated companies disclaim liability for any direct, indirect, consequential or incidental damages that may result from the use of the information or data, or from the inability to use the information or data contained in this document. The contents of this report may only be transmitted to third parties in its entirety and provided with the copyright notice, prohibition to change, electronic versions’ validity notice and disclaimer.

-3- OWEZ_R_113_20090803 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 ...................................................................................22

6 Comparision of the OWEZ SCADA data with the KNMI data ...............................26 6.1 Comparison of the number of strokes...................................................................26 6.2 Taking the position-finding of the FLITS system in closer consideration..............26 6.3 Comparison between the FLITS and SCADA data on a daily time-scale.............28

7 Conclusion............................................................................................................30

REFERENCES.......................................................................................................................31

Appendix I Discharges and strokes in the wind farm and its vicinity ................................32

-4- OWEZ_R_113_20090803 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 will carry 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 is presented that KEMA has performed on lightning strike data during

the years 2006 - 2008 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 61400-24 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. Therefore the FLITS system is 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.

-5- OWEZ_R_113_20090803 1 GENERAL

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).

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 is presented that KEMA has performed on lightning

strike data during 2006, 2007 and 2008 from the Royal Netherlands Meteorological Institute

(KNMI) and the SCADA system of the off shore wind farm near Egmond aan Zee.

The Off Shore wind Farm Egmond aan Zee has a subsidy of the Ministry of Economic Affairs

under the CO2 Reduction Scheme of the Netherlands.

-6- OWEZ_R_113_20090803 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 wind turbines

-7- OWEZ_R_113_20090803 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

-8- OWEZ_R_113_20090803 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 the

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 out of a pole of 17,5 meters high and is equipped

with several sensors. The data from these stations is send 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.

-9- OWEZ_R_113_20090803 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 31-12-2008 are obtained from the KNMI and

analysed in this report. For the year 2009, KEMA continues to receive the lightning data.

For further analyses the KNMI lightning data has been split in three 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.

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.

-10- OWEZ_R_113_20090803

0

500

1000

1500

2000

2500

3000

3500

4000

posi

tion

erro

r in

met

ers

detected strikes by FLITS

Figure 2 Ground and return strokes in the vicinity of the wind farm in Q3Q4-2006, 2007 and

2008 (283 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- OWEZ_R_113_20090803

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 and 2008 there were respectively 9 and 14 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 and 62 in 2008 (table 3). That is 0,45, 0,32 and 0,15 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 NEN1014 publication

-12- OWEZ_R_113_20090803 gives about 2,5 strokes/km2/year [5]. Both in time and space (see figure 5) large fluctuations

in the number of strokes exists.

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 and 2008

datenumber

of strikes datenumber

of strikes datenumber

of strikes

20060705 3 20070121 3 20080301 220060722 4 20070608 42 20080331 120060802 3 20070627 2 20080531 120060811 1 20070703 2 20080602 420060814 2 20070704 5 20080702 120060820 32 20070709 18 20080712 120060828 4 20070715 2 20080731 620060929 2 20070716 43 20080801 120061001 34 20070722 14 20080807 1720061002 2 20080813 220061118 1 20080823 1920061129 2 20081027 3

20081028 320081204 1

total 90 total 131 total 62

July - Dec. 2006 20082007

Figure 5 Number of ground and return strokes per square kilometre per year according to

the KNMI for the period 1995-1998 [6]

-13- OWEZ_R_113_20090803 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 within 7 kilometres of the outermost wind turbines of the

wind farm in Q3 and Q4 of 2006, in 2007 and in 2008.

-14- OWEZ_R_113_20090803 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 and 8 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 and in 2008. 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 9, 10 and 11 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

-15- OWEZ_R_113_20090803

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

-16- OWEZ_R_113_20090803

Figure 9 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

Figure 10 The ground and return strokes in 2007 in the vicinity of the wind farm which could

potentially have struck one or more wind turbines

-17- OWEZ_R_113_20090803

Figure 11 The ground and return strokes in 2008 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 and in 2008 this number was only 3. Tables 4,

5 and 6 show the details of all the potential strokes on wind turbines.

The number of purple circles in figure 9, 10 and 11 do not necessarily correspond with the

number of potential wind turbine strokes in table 4, 5 and 6. 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.

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

4.458 52.601 20060802 84245 0.958 560 4 -25870 800 3100 304.461 52.601 20060802 84245 0.9677 560 5 -15020 288 2300 304.461 52.601 20060802 84245 0.98 560 5 -24530 387 3600 304.395 52.622 20060811 14045 0.6454 600 4 -12430 487 1800 214.406 52.599 20060820 74948 0.2271 1420 4 -13120 350 3000 4 5 6 7 17 18 194.397 52.609 20060820 74948 0.3708 1450 5 -12760 237 2200 6 7 8 9 18 19 20 214.401 52.625 20061001 231111 0.297 590 4 -8830 5.75 25 20 214.393 52.619 20061001 231111 0.316 590 5 -8550 2.62 20 8 94.422 52.614 20061001 231213 0.713 590 4 -26140 5.88 32 35 364.436 52.630 20061001 231214 0.21 580 4 -14710 3.38 29 34

LON LAT DATE TIME SUBSEC POSERR TYPE CURRENT

potentially struck windmills

RISETIME

DECAYTIME

-18- OWEZ_R_113_20090803 Table 5 All ground and return strokes in and near the wind farm that could potentially have

struck a wind turbine in 2007

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

struck a wind turbine in 2008

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 2008 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 10. The collection

4.423 52.632 20080813 210111 0.1321 720 4 -24280 350 2700 35 364.460 52.601 20080823 52440 0.9026 660 4 -16660 363 2000 304.426 52.584 20081027 180400 0.1791 690 4 7850 850 4900 2 3

SUBSEC POSERRLATLON DATE TIME TYPE CURRENT potentially struck windmills

RISETIME

DECAYTIME

4.426 52.603 20070121 43646 0.0942 750 4 8790 3.87 49 17 254.437 52.598 20070608 3058 0.542 570 4 -21360 612 3600 16 244.385 52.621 20070608 3716 0.7172 600 5 -15090 500 2200 104.439 52.576 20070608 3829 0.515 560 4 -25750 650 4800 14.444 52.585 20070608 201924 0.6067 560 4 -19810 387 3500 13 144.444 52.591 20070608 202059 0.2117 670 4 -33740 725 2500 14 15 234.380 52.603 20070703 120026 0.4208 2050 4 -62140 1.62 23 6 7 8 9 104.422 52.621 20070709 85837 0.0006 720 4 -15310 3 17 27 28 34 354.369 52.625 20070716 170234 0.4672 2090 4 -34320 3.63 28 9 10 11 12 214.427 52.581 20070716 170715 0.57 740 4 -15170 4 30 1 24.357 52.626 20070716 171741 0.3553 800 4 -7710 3.5 21 124.424 52.580 20070716 172244 0.0797 690 4 -14550 4.25 23 24.399 52.643 20070716 173241 0.7816 2080 4 -52750 7.5 39 29 364.399 52.643 20070716 173241 0.8979 2080 5 -29790 2.88 25 29 364.448 52.615 20070722 63017 0.6627 730 4 -12420 4.25 40 32

SUBSEC POSERRLATLON DATE TIME TYPE CURRENT potentially struck windmills

RISETIME

DECAYTIME

-19- OWEZ_R_113_20090803 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.

r < h r > h

h

A = π·r2 A = π·h·(2r-h)

r

r r-h

h

r22 hhr −

Figure 10 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 and 2008 are shown in table 7, 8 and

-20- OWEZ_R_113_20090803 9. The collection area according to the IEC 61024 method as well as according to Armstrong

& Whitehead’s formula are given in these tables.

Table 7 All ground and return strokes in the OWEZ area in Q3 and Q4 of 2006 and their

chance of hitting a wind turbine Armstrong & Whitehead formula

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%

CURRENT (ampère)

striking distance

(r)

chancemill is hit (%)

IEC 61024LON LAT DATE TIME surface

(A)

surfacePOSERR

(m2)

chancemill is hit (%)SUBSEC

POSERR(m)

Table 8 All ground and return strokes in the OWEZ area in 2007 and their chance of

hitting a wind turbine Armstrong & Whitehead formula

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%

LON LAT DATE TIMEchance

mill is hit (%)IEC 61024

SUBSECPOSERR

(m)CURRENT (ampère)

striking distance

(r)

chancemill is hit (%)surface

(A)

surfacePOSERR

(m2)

Table 9 All ground and return strokes in the OWEZ area in 2008 and their chance of

hitting a wind turbine Armstrong & Whitehead formula

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%

LON LAT DATE TIMEchance

mill is hit (%)IEC 61024

SUBSECPOSERR

(m)CURRENT (ampère)

striking distance

(r)

chancemill is hit (%)surface

(A)

surfacePOSERR

(m2)

-21- OWEZ_R_113_20090803 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 and 6) 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 and 0.04 for the year 2008. 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 and 1.25 for the year

2008.

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 Armstong & 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.

-22- OWEZ_R_113_20090803 5 LIGHTNING DATA FROM OWEZ

On all wind turbines in the OWEZ offshore park there are lightning detection systems in the

tree 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 2008, 194 strokes in wind turbines have

been detected, see table 10. From these 194 strokes, 47 had a current of over 10 kAmpère.

Table 10 Number of strokes in wind turbines per day detected by the SCADA system

all >10kA all >10kA all >10kA29/09/2006 1 01/01/2007 3 1 11/03/2008 1 102/10/2006 3 21/01/2007 18 8 21/03/2008 7 104/10/2006 1 23/01/2007 3 2 24/03/2008 110/10/2006 1 24/01/2007 1 25/03/2008 117/10/2006 2 1 07/02/2007 5 1 01/08/2008 1 129/11/2006 8 5 19/03/2007 2 1 07/08/2008 130/11/2006 2 08/06/2007 8 2 13/08/2008 3 104/12/2006 4 2 27/06/2007 1 23/08/2008 2 1

09/07/2007 13 2 03/10/2008 29 116/07/2007 3 1 27/10/2008 3 122/07/2007 8 5 28/10/2008 23 411/11/2007 1 21/11/2008 101/12/2007 3 22/11/2008 7 407/12/2007 1 23/11/2008 210/12/2007 1 02/12/2008 215/12/2007 1 05/12/2008 15 1

09/12/2008 1total 22 8 total 72 23 total 100 16

20082007sept-dec 2006number

of strikesnumber

of strikesnumber

of strikesdate date date

Sometimes several strokes were detected by the SCADA system within a few seconds. The

most extreme example of this phenomenon was on October 3rd 2008, when 27 out of the 36

wind turbines were hit within a time-span of 5 seconds. Find below a list with dates when

several strokes were detected within a few seconds:

• 29-11-2006 7 strokes within 6 seconds




-23- OWEZ_R_113_20090803 • 22-7-2007 5 strokes within 4 seconds



• 28-10-2008 5 strokes within 4 seconds and later that day 6 strokes within 5 seconds

• 4-12-2008 13 strokes within 6 seconds.

It’s possible that a forked lightning strikes at several places at the same time, but these

places then are close to each other. The wind turbines at OWEZ are however placed 700

meters apart. It seems impossible that one discharge causes simultaneous strokes several

kilometres apart. The current of these simultaneous strokes is rarely higher than 10

kAmpère. A possible explanation for this phenomenon is the following: when a heavy

thundercloud is located above the wind farm, the top of the wind turbines will have an

opposite load compared to the cloud. When a lightning stroke causes the cloud to discharge

(both cloud-to-cloud and ground strokes causes the cloud to discharge), the load in the wind

turbine will flow away in a very short time span into the earth. It could be possible this causes

a current larger than the threshold value of the SCADA system (6 kAmpère).

Whereas ‘normal’ lightning strokes can have currents as low as 6 kAmpère, there is no

reason to assume all individual strokes (not coinciding with another stokes in the same

second) with a current below 10 kAmpère are caused by this phenomenon, also because

heavy thunderstorms will have dimensions larger than the area around one individual wind

turbine.

Figure 11 and 12 show the stroke current distribution and the number of strokes per wind

turbine. The maximum current registered in over the period September 2006 – December

2008 is 75 kAmpère.

-24- OWEZ_R_113_20090803

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

6-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90

Current [kA]

No

of S

trik

es

200820072006 sept-dec

Total Hits: 194Max. Current: 75 kA

Figure 11 Stroke current distribution at OWEZ in the period September 2006 – December

2008

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

wind turbine

Str

ikes

Figure 12 Number of strokes per wind turbine in the period September 2006 – December

2008

-25- OWEZ_R_113_20090803 From the 194 strokes detected in the period September 2006 – December 2008 by the

SCADA system (see table 10), 9 had a very powerful current of over 50 kAmpère and where

given the status ‘alarm’. The other 185 strokes where given the status ‘warning’. Table 11

shows the 9 strokes which had a current of over 50 kAmpère.

Table 11 Strokes with a current > 50 kAmpère (September 2006 - December 2008)

date time (UTC) turbine current (kAmpère)

21-1-2007 05:44:49 32 72

21-1-2007 06:35:40 20 63

21-1-2007 06:41:03 7 75

7-2-2007 09:46:19 24 52

19-3-2007 22:02:17 14 61

8-6-2007 01:40:23 36 64

8-6-2007 21:28:47 35 64

1-8-2008 01:48:34 15 60

28-10-2008 03:00:14 8 59

-26- OWEZ_R_113_20090803 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

somewhat higher as found in literature, being 3.4 – 5.1 strokes/km2/year in the area where

the wind turbines are placed (about 20 km2). When not taking into account the several

strokes detected by the SCADA system which take place within a few seconds - they count

only as one stroke – (see also chapter 5), the number of strokes per square kilometre lies

between 2 to 3 strokes/km2/year, which is in accordance with the numbers stated in

literature. However, this number could be higher again because there can also be strokes in

between the wind turbines.

In 2007 and 2008, respectively 72 and 100 strokes have been detected by the SCADA

system (respectively 59 and 60 strokes when not taking into account subsequent strokes that

follow in the same second). This contrasts sharply with the predicted 5.92 strokes in 2007

and 1.25 strokes in 2008 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 should therefore be rejected.

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 and 100 versus 1.25 in 2008) 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)

-27- OWEZ_R_113_20090803 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.

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. Because the precision of the SCADA system is one second,

several strokes that occur during 1 second only count as one in this analyses; the in total 194

registered strokes during the period September 2006 – December 2008 occurred in 138

different seconds.

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 138 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 4 out of the 138 strokes. If we also take

into consideration cloud-to-cloud discharges (type 1 – 3, table 2) the score is 42 out of the

138 strokes. The average distance between the SCADA system location and the FLITS

system location of these 42 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 194 strokes detected in the period September 2006 – December 2008 by the

SCADA system (see table 10), 9 had a very powerful current of over 50 kAmpère and where

given the status ‘alarm’ (see table 11). In the same minute as the SCADA system detected

these 9 ‘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 9 ‘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.

-28- OWEZ_R_113_20090803 6.3 Comparison between the FLITS and SCADA data on a da ily 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 13. 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

the y-axis. Every dot represents one day. The period taken into account is again September

2006 – December 2008.

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 14. There is hardly any correlation (R2 < 0.10) between amount of type 0 – 3

discharges from the FLITS system and the amount of detected strokes by the SCADA

system. For both figure 13 and figure 14 applies that if more than one stroke is registered by

the SCADA system within one second, it only counts as one stroke. Figure 14 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.

R2 = 0,0379

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12 14 16

number of detected strikes by SCADA system

num

ber o

f stri

kes

in a

nd a

roun

d O

WE

Z b

y F

LIT

S d

ata

Figure 13 Number of strokes detected by the SCADA system and by the FLITS system in

and around OWEZ

-29- OWEZ_R_113_20090803

1

10

100

1000

10000

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

number of detected strikes by SCADA system

num

ber

of c

loud

-clo

ud d

isch

arge

s in

and

aro

und

OW

EZ

by

FLI

TS

dat

a

isolated point discharge

start cloud-cloud discharge

next point cloud-cloud discharge

end cloud-cloud discharge

Figure 14 Number of strokes detected by the SCADA system and number of cloud-to-cloud

discharges by the FLITS system in and around OWEZ

-30- OWEZ_R_113_20090803 7 CONCLUSION

From the information in chapter 4, 5 and 6 it can be concluded that there is no significant

match between the SCADA data and the data from the FLITS system from the KNMI.

The amount of strokes in wind turbines predicted using data from the FLITS system are ten

to hundred times lower than the amount of strokes detected by the SCADA system. The

number of strokes per square kilometre detected by the FLITS system in the OWEZ area and

it’s vicinity is also much lower than the number found in literature. The amount of strokes per

square kilometre detected by the SCADA system are more or less in accordance with the

numbers stated in literature.

There is a mismatch between the data of the FLITS and the SCADA system in both the

position and the time (see paragraph 6.2) 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).

The reason for the mismatch between de data of the FLITS and the SCADA system is not

clear. A possible explanation could be that the position error of the FLITS data is well over

the stated maximum of 7 kilometres. In that case, strokes which coordinates were more than

7 kilometres outside the outermost wind turbines, in reality could have been located in the

OWEZ area, but they were not included in the analysis in this report. One can think of

numerous other possible reasons for the mismatch in both time and location of the strokes

detected by the FLITS and SCADA systems. Fact is that the FLITS system is not suitable 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.

-31- OWEZ_R_113_20090803 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.

-32- OWEZ_R_113_20090803 APPENDIX I DISCHARGES AND STROKES IN THE WIND FARM AND

ITS VICINITY

Detected discharges and strokes within 7 kilometres of the outermost wind turbines of the

wind farm in Q3 and Q4 of 2006.

JUL AUG SEP OKT NOV DEC

type 0 6 type 0 4 type 0 4 type 0 48 type 1 4 type 1 1



type 3 13 type 3 7 type 3 10 type 3 171 type 4 1

type 4 3 type 4 26 type 1 1






type 4 4 type 5 2 type 3 5 type 2 267


type 1 1 type 1 9


type 3 8 type 2 28

type 4 1 type 3 2

type 0 6 type 0 1

type 1 16

type 2 561

type 3 15

type 4 2

type 0 1

type 0 13 type 0=isolated point, no lightning, e.g. communication intrusionstype 1 46 type 1=start cloud-cloud dischargetype 2 984 type 2=next point in cloud-cloud dischargetype 3 44 type 3=end cloud-cloud dischargetype 4 24 type 4=ground stroke type 5 8 type 5=return stroke

type 1 5

type 2 43

type 3 8

type 0 1

type 1 5

type 2 163

type 3 4

type 4 4

type 0 1

type 1 6

type 2 93

type 3 6

datum : 20060811

datum : 20060814

datum : 20060705

datum : 20060722

datum : 20060725

datum : 20060829

datum : 20060914

datum : 20060929

datum : 20060930

datum : 20060817

datum : 20060820

datum : 20060825

datum : 20060828

datum : 20060801

datum : 20060802

datum : 20061001

datum : 20061002

datum : 20061006

datum : 20061023

datum : 20061207

datum : 20061118

datum : 20061129

datum : 20061204

datum : 20061206

-33- OWEZ_R_113_20090803

Appendix I page 2


wind farm in 2007.

JAN FEB MAA APR MEI JUN JUL AUG SEP OKT NOV DEC

type 1 4 type 1 1 type 1 1 type 1 2 type 2 1 type 0 1 type 1 1 type 1 2


type 3 5 type 3 1 type 3 1 type 3 1 type 4 2 type 2 22 type 3 2

type 2 36





type 4 33 type 4 4


type 2 27


type 1 13 type 1 95

type 0 1 type 2 157 type 2 1658


type 3 2 type 4 14

type 0 3 type 5 4

type 0 6 type 1 15





type 4 2

type 0 443

type 1 592

type 2 1754

type 3 610

type 4 27

type 5 16

type 0 3

type 1 4

type 2 59

type 3 3

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 dischargetype 3=end cloud-cloud discharge type 0 1

type 4=ground stroke type 5=return stroke type 0 1

datum : 20071201

datum : 20071207

datum : 20070723

datum : 20070730

datum : 20070822 datum : 20070917

datum : 20070715

datum : 20070716

datum : 20070720

datum : 20070722

datum : 20070918

datum : 20070925

datum : 20070627

datum : 20070703

datum : 20070704

datum : 20070709

datum : 20070507 datum : 20070607

datum : 20070608

datum : 20070620

datum : 20070118

datum : 20070121

datum : 20070207

datum : 20070228

datum : 20070101

datum : 20070110

datum : 20070111

-34- OWEZ_R_113_20090803

Appendix I page 3


wind farm in 2008.

FEB

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

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

type 0 6 type 2 39 type 3 1 type 2 512 type 2 232 type 2 62 type 3 1 type 3 16 type 2 180 type 1 6


type 2 261 type 0 2 type 4 1 type 1 45 type 4 1 type 4 1 type 2 3 type 0 1 type 3 6






type 4 1 type 4 8


type 1 3 type 1 13



type 4 1 type 4 3

type 2 29

type 1 1

type 2 7 type 0 3

type 3 1 type 1 17

type 2 246

type 0 1 type 3 13

type 1 3 type 4 2

type 3 3

type 0 1

type 0 8

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 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

NOV DECOKTMAA APR MEI JUN JUL AUG SEP20081122

20081123

20081201

20081204

20081003

20081017

20081027

20081028

20080814

20080823

20080904

20080911

20080807

20080808

20080812

20080813

20080726

20080728

20080731

2008080120080702

20080708

20080712

20080719

20080531 20080601

20080602

20080605

20080324

20080331

20080408

20080423

JAN2008032120080201

20080301

owez r 113 20090803 lightning - noordzeewind · analysis and evaluation on collected lightning...

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