2018

Becerra M., M. Long, W. Schulz, R. Thottappillil:
On the estimation of the lightning incidence to offshore wind farms

Electric Power Systems Research 157, 211–22, 2018

Field observations have shown that the frequency of dangerous lightning events to wind turbines, caculated according to the IEC standard 61400-24:2010, is grossly underestimated. This paper intends to critically revisit the evaluation of the incidence of downward lightning as well as self-initiated and other-triggered upward flashes to offshore wind power plants. Three different farms are used as case studies.The conditions for interception of stepped leaders in downward lightning and the initiation of upward lightning is evaluated with the Self-consistent Leader Inception and Propagation Model (SLIM). The analysis shows that only a small fraction of damages observed in the analysed farms can be attributed to downward lightning. It is also estimated that only a small fraction (less than 19%) of all active thunder-storms in the area of the analysed farms can generate sufficiently high thundercloud fields to self-initiate upward lightning. Furthermore, it is shown that upward flashes can be triggered even under low thunder-cloud fields once a sufficiently high electric field change is generated by a nearby lightning event. Despite of the uncertainties in the incidence evaluation, it is shown that upward flashes triggered by nearby positive cloud-to-ground flashes produce most of the dangerous lightning events to the case studies.

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Hettiarachchi P., V. Cooray, G. Diendorfer, H. Pichler, J. Dwyer:
X-rays observations at Gaisberg Tower

Atmosphere, 9, 20; doi:10.3390/atmos9010020, 2018

We report the occurrence of X-rays at ground level due to cloud-to-ground flashes of upward-initiated lightning from Gaisberg Tower, in Austria, which is located at an altitude of 1300 m. This is the first observation of X-ray emissions from upward lightning from a tower top located at high altitude. Measurements were carried out using scintillation detectors installed close to the tower top in two phases from 2011 to 2015. X-rays were recorded in three subsequent strokes of three flashes out of the total of 108 flashes recorded in the system during both phases. In contrast to the observations from downward natural or triggered lightning, X-rays were observed only within 10 µs before the subsequent return stroke. This shows that X-rays were emitted when the dart leader was in the vicinity of the tower top, hence during the most intense phase of the dart leader. Both the detected energy and the fluence of X-rays are far lower compared to X-rays from downward natural or rocket-triggered lightning. In addition to the above 108 flashes, an interesting observation of X-rays produced by a nearby downward flash is also presented. The shorter length of dart-leader channels in Gaisberg is suggested as a possible cause of this apparently weaker X-ray production.

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Birkl J., T. Böhm, G. Diendorfer, F. Heidler, C. Paul, H. Pichler:
Measurement of lightning currents on high structures and wind turbines

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

This article deals with the measurement of lightning current on high structures and, in particular, wind turbines. It offers a synopsis of the special lightning current characteristics for high building structures. The special requirements for lightning current measurements on high objects and, especially, wind turbines, are derived from this. It presents a measurement system for determining lightning currents in wind turbines based on Rogowski coils. In order to verify this lightning current recording unit, comparison measurements were carried out at the lightning measurement stations Gaisberg (Austria) and Peissenberg (Germany). First measurements have also been carried out on wind turbines.

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Birkl J., A. Kern, G. Diendorfer, S. Thern:
Extremely high lightning currents

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018  

Lightning flashes are still a major cause of injury, fire, mechanical destruction and, above all, surges. Time and again there are reports of extremely high lightning currents which, of course, can cause considerable damage and destruction. In some cases, peak values of over 300 kA are mentioned. This throws up questions because the “classical” lightning statistics (e.g., CIGRE and IEC [9, 11]) do not recognize such values. These extreme lightning currents are, as a rule, identified using the data provided by lightning detection systems.
This article will examine such extreme lightning currents. The necessary fundamentals of lightning detection will be explored as well as the limits when verifying extreme values. “Classical” lightning statistics and further studies of extreme lightning currents will be discussed.

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Hunt H.G.P., K.J. Nixon, I.R. Jandrell, G. Diendorfer, W. Schulz:
Students’ t-distribution modelling of LLS location errors

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

This paper investigates the statistical distribution that best describes Lightning Location System location errors. It is generally assumed that the bivariate Gaussian distribution is a good description but the presence of outliers calls for a heaviertailed distribution. The bivariate Students’ t-distribution is proposed. Using Maximum Likelihood Estimation, Mahalanobis quality-of-fit tests and ground-truth events to the Brixton tower, South Africa and Gaisberg tower, Austria, we are able to show that the bivariate Students’ t-distribution is a better fit than the bivariate Gaussian distribution.

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Keul A., G. Diendorfer:
Assessment of ball lightning cases by correlated LLS data

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

Ball lightning (BL) research lacks instrumental records and field experiments. Today, widely available networks for lightning detection offer a chance to assess alleged BL reports with located lightning and its physical parameters. This assessment was realized with BL case data and EUCLID lightning data for Germany, Austria, the Czech Republic, and Switzerland. According to the initial flash hypothesis, the electromagnetic pulse of a detected stroke near the geocoded BL position could be the triggering event of the BL process. The lightning location system (LLS) provides stroke location, time, polarity (positive/negative), and peak current in kA. In this assessment, 34 BL case reports over the period from 1994 to 2016 suggested a physical link of the stroke with maximum peak current in kA to the subsequent BL process. No electromagnetic field values for individual strokes can be computed with the kA peak current. As a first step in this analysis, geocoded distances of less than 1 km between stroke location and BL observation site were called „close event“ (CE) and distances 1-10 km „distant event“ (DE). For cloud-ground (CG) strokes the stroke-to-BL distance values were obtained. LLS-provided coordinates for intra-cloud (CC) strokes were also used to obtain a distance, although CC do not have a defined striking point. 28 BL cases occurred in the summer months, 6 cases in winter. Their time pattern follows the diurnal thunderstorm frequency in Central Europe. 19 cases were from Germany, 10 from Austria, 3 from the Czech Republic and 2 from Switzerland. 19 of the correlated EUCLID strokes were positive and 15 negative. Positive strokes ranged from 4 to 370 kA, negative from -3 to -37 kA. 28 were classified by the LLS as CG, 6 as CC strokes. The mean value of the semi-major axis of the EUCLID location confidence ellipse was 0.45 km. The BL-to- stroke distance in km for CG events had a mean of 0.42 km and a range of 0.0-0.8 km for CE cases, and a mean of 5.7 km and a range of 1.4-10 km for DE cases. The case statistics of this sample showed smaller BL-stroke distances for higher kA peak currents for both negative and positive CG events. This result leads to the interpretation that most DE strokes are uncorrelated with BL since their physical effects are marginal. Replications with more BL cases will show whether the pattern holds.

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Mostajabi A., A. Sunjerga, M. Azadifar, A. Smorgonskiy, M. Rubinstein, F. Rachidi. G. Diendorfer:
On the Impact of Meteorological Conditions on the Initiation of Upward Lightning Flashes from Tall Structures

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

Upward lightning flashes that originate from tall grounded structures have been classified as either ‘self-triggered’
or ‘other-triggered’ based on preceding lightning events in their vicinity, i.e. strokes or pulses from cloud-to-ground (CG) or intracloud (IC) discharges occurring around the tower. A total of 165 upward flashes recorded between March 2015 to July 2016 at the Säntis Tower in the eastern Swiss Alps are analyzed considering the related meteorological conditions. The results are compared to similar previous studies to reveal some general ideas on how meteorological conditions influence the initiation of selfand other- triggered upward flashes.

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Paul C., F. H. Heidler, W. Schulz:
Optical Lightning Measurement System and First Results

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018  

This paper introduces a newly developed optical lightning measurement system (OLMS). The OLMS is based on a video system using a camera with a 360° panorama view and a frame rate of 60 frames per second. The OLMS locates the lightning from the direction of the illuminated lightning channel and the travel time of the thunder sound. The analysis comprises 34 flashes containing 144 strokes, which occurred during the time period from 30th of July 2017 to 24th of August 2017. The results are compared to data obtained by the European lightning location network EUCLID. The comparison revealed a flash detection efficiency of 100 % and a stroke detection efficiency of 93.1 % for EUCLID. With EUCLID, the fraction of 92.5 % was classified correctly as cloud-to-ground lightning and the minor fraction of 7.5 % was miss-classified as cloud-to-cloud lightning. On average (arithmetic mean), the negative flashes contained 4.3 return strokes with 1.7 different striking points. For multi-stroke flashes with return strokes using the same lightning channel, EUCLID located the individual striking points with a median deviation of 57.3 m. The average striking distance was about 10 km. Due to the far distance the thunder sound could often not be separated from the thunder sound of other lightning. Therefore, the OLMS located the lightning with a relatively high deviation compared to the data of EUCLID. The arithmetic (geometric) mean value of the deviation was 1330 m (670 m).

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Schwalt L., S. Pack, W. Schulz:
LiOn ground truth data in correlation with ALDIS LLS detections

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

This paper presents the current analysis in the section of lightning research and observation in the Austrian Alps run by Graz University of Technology in correlation with Austrian Lightning Location System detections. The project “Lightning Observation in the Alps – LiOn” was created in 2017 at the Institute of High Voltage Engineering and System Performance at Graz University of Technology. Atmospheric discharges are observed at 19 different measurement locations in Austria by using a video and field recording system. These system consists of a high speed video camera (2000 frames per second) and a flat plate antenna to measure the electric field. The recorded data can be used to better understand the atmospheric discharges, especially in the alpine area. For this analysis a data set of the measurement periods 2015 and 2017 was used for validation of the Lightning Location System data of the Austrian Lightning Detection and Information System. The LLS performance parameter location accuracy and detection efficiency for these two years have been analyzed. Values for location accuracy are in the range of 111 m and 139 m for 2015 and 2017. For flash detection efficiency 96.08 % and 98.49 % and 85.60 % and 76.34 % for the stroke detection efficiency have been calculated for 2015 and 2017 respectively.

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Sunjerga A. , A. Mostajabi, F. Rachidi, N. Pineda, D. Romero, O. A. Van der Velde, J. Montanyà, M. Azadifar, M. Rubinstein, G. Diendorfer:
On the Classification of Self-Triggered versus Other-Triggered Lightning Flashes

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

We present in this paper lightning current measurements and LMA (Lightning Mapping Array) data associated with upward flashes observed at the Säntis Tower during Summer 2017. The LMA network consists of six stations located in the vicinity of the tower at distances ranging from 100 m to 11 km from it. 20 flashes simultaneously recorded by the current measurement system and the LMA are analyzed. Based on the lightning activity derived from the European Lightning Detection Network (EUCLID) in an area within 30 km from the tower and in a 5-second time window before the start of the flash, all the 20 flashes were classified as ‘self-triggered’ (ST). However, the investigations based on the LMA data reveal that 3 of the flashes were preceded by nearby activity and should be therefore classified as ‘other-triggered’ (OT) flashes. The results suggest that the number of OT flashes inferred from LLS data can be underestimated.

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Watanabe N., A. Nag, G. Diendorfer, H. Pichler, W. Schulz:
Current and Electric Field Changes Associated with the Initial Stage of Upward Lightning

34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

We examine current and electric field waveforms for 58 negative upward flashes occurring in 2006-2014 initiated from the Gaisberg Tower located in Salzburg, Austria. Current was measured at the top of the tower using a 0.25 mΩ shunt. Electric field was measured simultaneously at near (170 m from the tower) and far (79 km from the tower in 2006 - 2007 and 109 km in 2008 - 2014) distances. The initial stage (IS) of these flashes comprised of relatively slowly varying “background” current (having durations ranging from 74 to 691 ms), with faster, more impulsive current variations (pulses having durations ranging from 4.0 μs to 29 ms) overlaid on this background current. In 45 of the 58 flashes (78%) the background continuing current was negative, and in the other 13 flashes (22%) it was bipolar. 1180 current pulses occurred during the IS of these 58 flashes, of which 709 (60%) were positive bipolar (positive initial polarity with a negative opposite polarity overshoot), 27 (2.3%) were positive unipolar (positive initial polarity with no opposite polarity overshoot), 435 (37%) were negative unipolar, and nine (0.76%) were negative bipolar pulses. The median peak current for the 718 bipolar pulses was 0.16 kA and that for the 462 unipolar pulses was 0.20 kA. The 1180 IS current pulses produced 341 detectable electric field change signatures at the near station and 109 at the far stations.

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Diendorfer G., H. Pichler, D. Lackner:
Self-initiated versus Nearby-lightning-triggered Upward Flashes at the Gaisberg Tower (2005 - 2015)

25th International Lightning Detection Conference and 7th International Lightning Meteorology Conference (ILDC/ILMC), Ft. Lauderdale, Florida, USA, 2018

Upward lightning initiated from tall objects is classified in two distinct groups. Self-initiated lightning occurs when the slow charge buildup produced by cloud electrification is sufficient to start the upward propagating leader. Nearby-lightning-triggered flashes are initiated by the rapid field changed caused by nearby lightning activity, either to ground or inter-cloud. In this paper we analyzed 307 upward flashes initiated from the Gaisberg Tower in Austria and based on measured electric fields at close distance to the tower 80 % were classified as self-initiated and 20 % as nearby-lightning-triggered. Some storms produced exclusively one type of discharges. In case of self-initiated lightning the median height of the -10°C isotherm was in the range of 3000 m, whereas in case of nearby-lightning-triggered lightning the isotherm was at a height of 4800 m.

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Mostajabi A., A. Smorgonskiy, F. Rachidi, M. Azadifar, M. Rubinstein, G. Diendorfer:
An Analysis of Lightning Activity in the Säntis Region through the Big Hiatus in Global Warming

25th International Lightning Detection Conference and 7th International Lightning Meteorology Conference (ILDC/ILMC), Ft. Lauderdale, Florida, USA, 2018

The response of local lightning to local surface temperature is examined near the Säntis mountain (2502 m ASL) in the eastern Swiss Alps during the first of the two hiatuses in global warming covering the time periods 1940-1972 and 1998-2014. The mountain summit was used since 1881 as a telegraph and meteorology station and since 1955, 3 different towers have been installed at the top of the mountain. In order to take the effect of the presence of a tower and its height into consideration, and also to be able to compare the trend of data inside and outside the hiatuses, the analysis period (1931-1994) is subdivided into four different time intervals. Depending on the availability of the data, the combination of surface air temperature and number of thunderstorm days obtained from MeteoSwiss were used to investigate the sensitivity of lightning activity to changes in surface air temperature at monthly and yearly timescales. The results show a clear difference between the hiatus and post-hiatus intervals and seem to support the idea that the lightning activity is correlated with the surface air temperature.

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Li D., M. Rubinstein, F. Rachidi, G. Diendorfer, W. Schulz:
Analysis of Location Accuracy of ToA-Based Lightning Location Systems in Mountainous Terrain

XVI International Conference on Atmospheric Electricity, Nara, Japan, 2018

This study is based on a three-dimensional (3D) full-wave FDTD approach with a topographic map including the Säntis Tower located in the Swiss Alps and the nearby sensor sites belonging to lightning location systems (LLSs). In the analysis, the considered seven sensors are divided into two different groups to represent different sensor geometrical distributions, and six different onset time estimation methods proposed in the literature are used to evaluate the time of arrival of a measured signal utilized by the ToA technique to determine the stroke location. It is found that the vertical electric fields are strongly affected by the presence of the mountainous terrain and the finite ground conductivity, and that the location error associated with the ToA technique highly depends on the used onset time estimation technique and the geometrical distribution of the sensor sites.

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