Caracterización de la electrificación de tormentas para su aplicación en procesos de sistemas de alerta: el caso de aerogeneradores y otras infraestructuras elevadas

Pla de Doctorats Industrials de la Generalitat de Catalunya. Tesi defensada en modalitat per compendi de publicacions.In an increasingly technological society,vulnerability to the effects of lightning is also increasing. The implementation of monitoring and surveillance systems in real time of lightning activity can greatly improve the issuance of warnings and the actions to be carried out during storm episodes, especially if it is combined with other detection systems such as meteorological radar, images of satellite, etc. The electrical industry has a special exposure to this type of damage with a special focus on wind turbines. This doctoral thesis analyses the information on the morphology of storm clouds and the intensity of precipitation with meteorological radar, as well as the detection of electrical sources with lightning locating systems. This makes it possible to detect the most common meteorological conditions and characteristics that allow their anticipation, the detection of the first lightning strike and to generate warnings. First, an introduction summarises this tesis scope and objectives, followed by the state of the art to be able to monitor the work carried out. The second part includes the articles that make up this thesis by compendium. In the first article, meteorological indicators are proposed for the detection of electrical activity. With data from the radiosonde of Barcelona, a climatology of the isotherms of - 10°C and - 40°C (temperature range where it is estimated that the different charge regions of a storm cloud exist) is carried out, evaluating their seasonality. The meteorological radar provides the location and intensity of precipitation of storms. The TOP-12 and TOP-35 products (maximum height reaching the reflectivity level of 12 and 35 dBZ) are postulated as indicators of the presence of lightning activity when they reach the isotherm heights of - 40°C and - 10°C, respectively. Finally, the height of the - 10°C isotherm is evaluated during the winter, highlighting the possible implication of affecting elevated infrastructures. The second article analyses what relationships are observed between the possible electrical structures of a storm with their electrical activity. With the implementation of the Lightning Mapping Array in the Ebro Delta area, the electrical structure is established by determining the initiations of each flash. Lightning strikes generally originate at the interface of different regions of opposite charge in the storm cloud, and their detection allows to determine the heights of these regions. The most common electrical structures found are the classic tripolar, one with an upper dipole in which the lower positive charge region does not exist or is very weak, and a lower dipole, with a dominant lower positive region. The latter shows the lowest cloud-toground discharge ratio, and the tripolar structure is the most favorable. The last article has a more practical and applied focus. Being an emerging field, and with plans to increase wind farms in Catalonia, it is of special interest for the wind industry. The meteorological conditions in situations of discharges in wind turbines are evaluated. The frequency of these effects is not directly related to the annual distribution of cloud-to-ground activity, which are much more frequent in warm periods, but rather they occur in colder periods, mostly from November to April. The classic tripolar structure is the most frequently detected in these episodes, with a lower positive charge region close to the surface. Next, the general conclusions and future work perspectives are presented, as well as the articles and conference presentations derived from this doctoral period. It concludes with the presentation of other related works developed during this doctoral studies. Finally, acknowledges the Industrial Doctorate Program (PDI - Project DI 59/2015) and its applied approach for supporting this thesis work.En una sociedad cada vez más tecnológica, la vulnerabilidad a los efectos del rayo también va en aumento. La implementación de sistemas de seguimiento y vigilancia en tiempo real de actividad de rayos puede mejorar enormemente la emisión de avisos y las actuaciones a llevar a cabo durante episodios de tormenta, especialmente si se combina con otros sistemas de detección como el radar meteorológico, imágenes de satélite, etc. La industria eléctrica tiene una especial exposición a este tipo de daños, con un especial enfoque hacia los aerogeneradores. Esta tesis doctoral analiza la información de la morfología de las nubes de tormenta y la intensidad de precipitación con el radar meteorológico, así como la detección de señales eléctricas con sistemas de localización de rayos. Esto permite detectar las condiciones y características meteorológicas más comunes que permiten su anticipación, la detección del primer rayo y poder generar avisos. En primer lugar, se realiza una introducción enmarcando la situación y objetivos de esta tesis, seguido del estado del arte para poder hacer un seguimiento del trabajo. La segunda parte comprende los artículos que integran esta tesis por compendio. En el primer artículo se establecen indicadores meteorológicos para la detección de actividad eléctrica. Con datos del radiosondeo de Barcelona, se realiza una climatología de las isotermas de - 10°C y - 40°C (rango de temperaturas donde se estima que existen las distintas regiones de carga de una nube de tormenta), evaluando su estacionalidad. El radar meteorológico proporciona la localización e intensidad de precipitación de las tormentas. Los productos TOP-12 y TOP-35 (altura máxima que alcanza el nivel de reflectividad de 12 y 35 dBZ) se postulan como indicadores de presencia de electricidad atmosférica cuando alcanzan las alturas de las isotermas de - 40°C y - 10°C, respectivamente. Finalmente, se evalúa la altura de la isoterma de - 10°C durante el invierno, destacando la posible implicación con afectaciones a infraestructuras elevadas. El segundo artículo analiza qué relaciones se observan entre las posibles estructuras eléctricas de una tormenta con su actividad eléctrica. Con la implementación del Lightning Mapping Array en la zona del Delta del Ebro, se establece la estructura eléctrica a partir de la determinación de los inicios de cada rayo. Los rayos se originan generalmente en la interfase de distintas regiones de carga opuesta de la nube de tormenta, y su detección permite determinar las alturas de estas regiones. Las estructuras eléctricas mayoritariamente encontradas han sido la clásica tripolar, una con dipolo superior en que la región de carga positiva inferior no existe o es muy débil, y una de dipolo inferior, con región positiva inferior dominante. Esta última presenta la menor ratio de descargas nube-tierra, y la estructura tripolar resulta la más favorecedora. El último artículo, con una voluntad más práctica y aplicada, de especial interés para la indrustria eólica, siendo un campo emergente, y con planes de incremento de parques eólicos en Catalunya, evalúa las condiciones meteorológicas en situaciones de descargas en aerogeneradores. Se detecta que la frecuencia de estas afectaciones no va directamente relacionada con la distribución anual de los rayos nube-tierra, mucho más frecuentes en periodos cálidos, sino que suceden en periodos más fríos, mayormente de noviembre a abril. La estructura tripolar clásica es la más frecuentemente detectada en estos episodios, con una región de carga positiva inferior cercana a la superficie. Seguidamente, se presentan las conclusiones generales y unas perspectivas de trabajo futuro, así como los artículos y presentaciones en congresos derivados de este periodo doctoral. Se concluye con la presentación de otros trabajos realizados durante esta tesis doctoral. Finalmente, agradecer el Programa de Doctorat Industrial (PDI - Project DI 59/2015) dentro del cual esta tesis ha sido enmarcada, destacando así el enfoque aplicado de esta.Postprint (published version

Thunderstorm characteristics favouring downward and upward lightning to wind turbines

Meteorological conditions and thunderstorm characteristics related to lightning threats to wind turbines are discussed in this paper. Due to the rotating blades, wind turbines may be regarded peculiar tall objects, more susceptible to lightning strikes than other tall man-made structures. In the present study, Lightning Mapping Array and weather radar observations allowed to draw a clear picture of the thunderstorm characteristics leading to lightning strokes to wind turbines, in a coastal area of the Mediterranean basin. Results showed that lightning threats to wind turbines tend to occur during transitional periods (spring and autumn), although the main thunderstorm activity concentrates in the warm summer months. Thunderstorms with downward strokes to wind turbines presented particular features, like a limited vertical development and a dominant lower positive charge layer. Downward cloud-to-ground strokes hitting wind turbines were mainly of negative polarity and with peak currents above the average. On the other hand, conditions for self-initiated upwards from wind turbines resemble those reported in Japan and the U.S winter thunderstorms, with low-cloud based large electrified stratiform regions. These particular conditions, leading to lightning threats to wind turbines, should be properly included in lightning protection standards.Peer ReviewedPostprint (author's final draft

Charge structure of two tropical thunderstorms in Colombia

Charge structure derived from lightning leader development of tropical thunderstorms comprising equatorial latitudes of < ±10° has not been investigated yet. In this work, using a Lightning Mapping Array (LMA) installed in northern Colombia, the charge structure, lightning leader initiations and the cloud-to-ground strokes rates of two thunderstorms have been analyzed. Additionally, radar information is also included. The identification of the charge regions has been obtained by analyzing the propagation of lightning leader developments. Flashes initiate between 4 and 15 km altitude. High initiation rates are different in the two storms. In one case the high rates are found between 8 and 13 km. In the other these are found between 10 and 15 km. The storms show typical tripolar structure where the upper positive charge is present at 10 to ~15 km, the mid-level negative charge is found between 6 and 9 km and the lower positive charge between 4 and 6 km altitude. Intracloud lightning flashes with inverted polarity have been identified for short period. In other periods, screening layer flashes have been detected at 14-15 km. The overall results show that the charge structures in the two Colombian storms are similar to the ones reported in North-Central Florida, but with a significant difference being the more than 2 km higher initiation altitude in Colombia. The vertical configuration of the charge regions and the leader development of these thunderstorms might help to explain the occurrence of Terrestrial Gamma-ray Flashes in tropical thunderstormsPeer ReviewedPostprint (author's final draft

Thunderstorm charge structures favouring cloud-to-ground lightning

Thunderstorm electrical structures favouring cloud-to-ground lightning were investigated through a Lightning Mapping Array (LMA), an accurate three-dimensional lightning location system that allows inferring the heights of the regions of charge. The present study focused on classical, convective-scale thunderstorms, aiming to shed new light on how the charge structure affects lightning production, especially the cloud-to-ground fraction, including flash rate and polarity. Results showed that lightning flashes mainly initiate at two levels, around -41 °C (9,150 m MSL) and around -7 °C height (4,730 m MSL). These initiation levels, located between the dominant positive and negative charge regions, allowed to define three main charge structures: an upper dipole (positive above negative), a classical tripole and a lower dipole (negative above positive). Several differences were found between the three categories in terms of the cloud-to-ground lightning production: (i) the classical tripole structure is the one presenting a higher cloud-to-ground flash rate (5.2 flashes·min-1); (ii) in terms of intensity, the presence of an upper positive charge region is more relevant than a lower positive below the main mid negative; (iii) conversely, the lower positive favours higher cloud-to-ground peak currents; (iv) A higher upper positive charge region favours a higher cloud-to-ground rate.Peer ReviewedPostprint (author's final draft

Seasonal variations on the conditions required for the lightning production

Given the growing concern on lightning threats outside the main warm-season, the objective of this study is to get further insight on the seasonal variations of the necessary conditions required for the production of lightning. In this regard, the study aims to find a basic indicator of cloud electrification that could be useful as an all-year-round robust predictor to warn about lightning threats. To this end, a large dataset of weather radar data products, total lightning observations and radiosounding isotherm heights have been used. According to previous studies, the radar storm height is tightly correlated with the total lightning flash rate. A fifth order relationship fits from spring to autumn and a third order power law in winter. In spite of the good correlation between the radar storm height and the total lightning flash rate, the vertical development alone may be insufficient as a basic indicator for thunderstorm conditions. Alternatively, two different predictors for the lightning onset have been analysed: 12 dBZ radar reflectivity echoes reaching the -40 °C isotherm height and 35 dBZ reflectivity echoes reaching the -10 °C. Results show that the most suitable all-year-round predictor is the TOP-35 above -10 °C.Peer ReviewedPostprint (updated version

Thunderstorm characteristics favouring downward and upward lightning to wind turbines

Meteorological conditions and thunderstorm characteristics related to lightning threats to wind turbines are discussed in this paper. Due to the rotating blades, wind turbines may be regarded peculiar tall objects, more susceptible to lightning strikes than other tall man-made structures. In the present study, Lightning Mapping Array and weather radar observations allowed to draw a clear picture of the thunderstorm characteristics leading to lightning strokes to wind turbines, in a coastal area of the Mediterranean basin. Results showed that lightning threats to wind turbines tend to occur during transitional periods (spring and autumn), although the main thunderstorm activity concentrates in the warm summer months. Thunderstorms with downward strokes to wind turbines presented particular features, like a limited vertical development and a dominant lower positive charge layer. Downward cloud-to-ground strokes hitting wind turbines were mainly of negative polarity and with peak currents above the average. On the other hand, conditions for self-initiated upwards from wind turbines resemble those reported in Japan and the U.S winter thunderstorms, with low-cloud based large electrified stratiform regions. These particular conditions, leading to lightning threats to wind turbines, should be properly included in lightning protection standards.Peer Reviewe

Lightning mapping observations of downward lightning flashes to wind turbines

Negative downward leaders that produced lightning strokes to wind turbines are identified by means of the Lightning Mapping Array data of the Ebro Valley Laboratory (NE Spain). Four cases are analyzed together with weather radar imagery. All flashes hitting wind turbines were originated in small convective cells with moderate development. Notwithstanding the moderate convection, all cases occurred under what can be called “out of season conditions”, where the ‘‘charging zone’’ is located closer to the ground and may favour downward leaders to tall structures. Cloud charge structures showed negative region from 3 km to more than 5 km with a low positive charge below (not always detectable)

Charge structure of two tropical thunderstorms in Colombia

Charge structure derived from lightning leader development of tropical thunderstorms comprising equatorial latitudes of < ±10° has not been investigated yet. In this work, using a Lightning Mapping Array (LMA) installed in northern Colombia, the charge structure, lightning leader initiations and the cloud-to-ground strokes rates of two thunderstorms have been analyzed. Additionally, radar information is also included. The identification of the charge regions has been obtained by analyzing the propagation of lightning leader developments. Flashes initiate between 4 and 15 km altitude. High initiation rates are different in the two storms. In one case the high rates are found between 8 and 13 km. In the other these are found between 10 and 15 km. The storms show typical tripolar structure where the upper positive charge is present at 10 to ~15 km, the mid-level negative charge is found between 6 and 9 km and the lower positive charge between 4 and 6 km altitude. Intracloud lightning flashes with inverted polarity have been identified for short period. In other periods, screening layer flashes have been detected at 14-15 km. The overall results show that the charge structures in the two Colombian storms are similar to the ones reported in North-Central Florida, but with a significant difference being the more than 2 km higher initiation altitude in Colombia. The vertical configuration of the charge regions and the leader development of these thunderstorms might help to explain the occurrence of Terrestrial Gamma-ray Flashes in tropical thunderstormsPeer Reviewe