RésuméL’article présente une analyse dans les domaines temporel et fréquentiel du champ électromagnétique généré par une décharge de foudre négative, calculé au niveau du sol. Cette analyse a été effectuée en se basant sur le modèle de la « ligne de transmission » modifié par les auteurs, qui donne des résultats concordant avec les données expérimentales. L’influence de plusieurs paramètres qui ont l’effet le plus important sur l’allure temporelle et le spectre d’amplitude du champ électromagnétique est mise en évidence. Ces paramètres sont : le temps de montée et la valeur de crête de l’impulsion de courant de foudre, sa vitesse de propagation, et le taux de décroissance de l’intensité de l’impulsion du courant qui se propage le long du canal. L’effet d’autres paramètres tels que la hauteur du canal de foudre et la distance de ce dernier au point d’observation est aussi analysé. Il est montré que la hauteur du canal de foudre influence très peu le champ électromagnétique. Deux expressions analytiques simplifiées du champ electromagnétique, employées pour le calcul des surtensions induites sur des lignes de transmission, sont examinées dans le but de discuter leurs limites de validité.AbstractThe paper presents an analysis in the time and in the frequency domain of the electromagnetic field originated by negative lightning return strokes, calculated at ground level. The analysis is performed on the basis of the « Transmission Line » model modified by the authors, which gives results in good agreement with experimental data. The influence of several parameters which deeply affect the time waveshape and the frequency spectra of the electromagnetic field has been investigated. These parameters are : the rise time and the peak value of the lightning current pulse, its velocity of propagation and the rate of decreasing of the intensity of the current pulse propagating along the channel. The influence of other parameters such as the lightning channel height and the distance of the observation point is also analyzed. It is shown that the height of the channel does not affect practically the electromagnetic field. Two simplified expressions of the electromagnetic field used for the calculation of overvoltages induced on transmission lines are discussed in order to assess their validity limits.
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