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The identification of the most vulnerable points on a given structure to be struck by lightning is an important issue on the design of a reliable lightning protection system. Traditionally, these lightning strike points are identified using the rolling sphere method, through an empirical correlation with the prospective peak return stroke current. However, field observations in Kuala Lumpur and Singapore have shown that the points where lightning flashes strike buildings also depend on the height and geometry of the structure. Since a lightning strike point is believed to be the place on a grounded structure where a propagating upward leader is first initiated, a physical leader inception model is used here to estimate the background electric field required to initiate a stable upward leader from the corners of some complex buildings. The computed location of the points from where leaders are incepted are compared with the damaged points on buildings struck by lightning. The observed lightning strike points coincide rather well with the corners of the buildings which are characterized by lower leader inception electric fields. Furthermore, it is found that the geometry of the buildings significantly influences the conditions necessary to initiate upward leaders and, therefore, the location of the most likely strike points.

Electric field measurements above ground have shown that the space charge layer created by corona at ground level shields the background electric field produced by the thundercloud. Therefore it is expected that this space charge layer can also influence the conditions required to initiate upward lightning from tall objects. For this reason, a numerical model that describes the evolution of the main electrical parameters below a thunderstorm is used to compute the space charge layer development. The time variation of the electric field measured at 600 m above ground during the 1989 rocket triggered lightning experiment at the Kennedy Space Center (Florida) is used to drive the model. The obtained space charge density profiles are used to compute the conditions required to initiate stable upward lightning positive leaders from tall towers. Corona at the tip of the tower is neglected. It is found that the space charge layer significantly affects the critical thundercloud electric fields required to initiate upward lightning leaders from tall objects. The neutral aerosol particle concentration is observed to have a significant influence on the space charge density profiles and the critical thundercloud electric fields, whereas the corona current density does not considerably affect the results for the cases considered in the analysis. It is found that a lower thundercloud electric field is required to trigger a lightning flash from a tall tower or other tall slender grounded structure in the case of sites with a high neutral aerosol particle concentration, like polluted areas or coastal regions.

The evaluation of the upward connecting leader inception from a grounded structure has generally been performed neglecting the effect of the propagation of the downward stepped leader. Nevertheless, field observations suggest that the space charge produced by streamer corona and aborted upward leaders during the approach of the downward lightning leader can influence significantly the initiation of stable upward positive leaders. Thus, a physical leader inception model is developed, which takes into account the electric field variations produced by the descending leader during the process of inception. Also, it accounts for the shielding effect produced by streamer corona and unstable leaders formed before the stable leader inception takes place. The model is validated by comparing its predictions with the results obtained in long gap experiments and in an altitude triggered lightning experiment. The model is then used to estimate the leader inception conditions for free standing rods as a function of tip radius and height. It is found that the rod radius slightly affects the height of the downward leader tip necessary to initiate upward leaders. Only an improvement of about 10% on the lightning attractiveness can be reached by using lightning rods with an optimum radius. Based on the obtained results, the field observations of competing lightning rods are explained. Furthermore, the influence of the average stepped leader velocity on the inception of positive upward leaders is evaluated. The results obtained show that the rate of change of the background electric field produced by a downward leader descent largely influences the conditions necessary for upward leader initiation. Estimations of the leader inception conditions for the upper and lower limit of the measured values of the average downward lightning leader velocity differ by more than 80%. In addition, the striking distances calculated taking into account the temporal change of the background field are s

The knowledge of the initiation and propagation of an upward moving connecting leader in the presence of a downward moving lightning stepped leader is a must in the determination of the lateral attraction distance of a lightning flash by any grounded structure. Even though different models that simulate this phenomenon are available in the literature, they do not take into account the latest developments in the physics of leader discharges. The leader model proposed here simulates the advancement of positive upward leaders by appealing to the presently understood physics of that process. The model properly simulates the upward continuous progression of the positive connecting leaders from its inception to the final connection with the downward stepped leader (final jump). Thus, the main physical properties of upward leaders, namely the charge per unit length, the injected current, the channel gradient and the leader velocity are self-consistently obtained. The obtained results are compared with an altitude triggered lightning experiment and there is good agreement between the model predictions and the measured leader current and the experimentally inferred spatial and temporal location of the final jump. It is also found that the usual assumption of constant charge per unit length, based on laboratory experiments, is not valid for lightning upward connecting leaders.

AE42A-05

QC 20120224

QCR 20160516

QCR 20160516

Field observations have shown that the frequency of dangerous lightning events to wind turbines, calculated 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 off shore 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 show that only a small fraction of damages observed in the analysed parks can be attributed to downward lightning. It is also estimated that only a small fraction (less than 19%) of all active thunderstorms in the area of the analysed parks can generate sufficiently high thundercloud fields to self-initiate upward lightning. Furthermore, it is shown that upward flashes can betriggered even under low thundercloud 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 casestudies., QC 20161201

This paper presents an experimental study on second mode positive streamers propagating along mineral-oil/solid interfaces. The inception and propagation of these streamers is investigated with different impregnated solids (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) polyvinylidene fluoride (PVDF), two papers referred to as kraft paper and a kraft fibril paper, made from cellulosic micro and nano fibrils, a lignin-free paper and a paper with high lignin content referred to as k107 kraft paper). Streamers are initiated in a point-plane configuration under step voltages with 35 ns rise time. The radius of the tip is 2.9 μm and the solid is installed in an inclined position in close contact to the point electrode. Shadowgraphs, charge and light recording of the streamers are reported for each case. Furthermore, estimations of the streamer stopping length, velocity, current and average charge are reported. It is found that the streamer inception is influenced by the solid interface indicating that the inception process is not only conditioned by the field at the tip but also by the interface. A time delay is observed before the initiation of the streamer and probably correlated with the initiation process and formation of the gaseous phase. Additionally, the threshold propagation voltage of the second mode streamers at mineral-oil/solid interfaces is shown to be independent of the interface. It is also shown that the different characteristics of streamers propagating along the tested interfaces cannot be fully explained by the capacitive coupling effect due to permittivity mismatch. Thus, it is suggested that the characteristics of streamers propagating near interfaces is affected by other properties of the solid such as chemical composition, wettability and surface roughness., QC 20170901

This paper presents an experimental study on the inception of first mode negative streamers at different mineral-oil/solid interfaces. This study is performed with a point-plane configuration immersed in mineral oil (point cathode). The mineral-oil/solid interface is done by assembling a solid in an inclined position into the point-plane gap. The solid is in contact (or in the proximity) with the point electrode tip. The tested solids are a kraft paper, a paper made from cellulosic micro and nano fibrils and different polymeric films (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)). These solids have different relative permittivity and structural composition. It is found that the streamer inception voltage is statistically similar for the cases with solids with higher permittivity than mineral oil. The streamer inception voltage for the case without any solid barrier is also statistically similar to the cases with solids with higher permittivity than mineral oil. It is also found that the inception voltage is higher for streamers initiated at permittivity-matched interfaces (cases with LDPE and PTFE). Additionally, the influence of the spatial limitation with the solid surface to the volume where the streamer initiation process takes is performed with PTFE by varying the distance between the point electrode and the solid surface. It is shown that the streamer inception voltage depends on the distance between the point electrode and the solid surface. Furthermore, It is observed that the streamer inception voltage is also influenced when the distance between the point electrode and the surface of the PTFE is several micrometers (twenty times longer than the penetration depth of the avalanche responsible of streamer initiation). Additionally, it is also shown that a recently-proposed streamer inception criterion is unsuitable to predict the streamer inception conditions close to perm, QC 20170901