| Supported by Hebei Natural Science Foundation under grant No. E2005000561,The overvoltage on tower and surge impedance is analyzed on numericalelectromagnetic method when the transmission line is struck by lightning and the circuitmodel of the tower is researched in this paper. The main contributions are as follows:1 The search methods of surge impedance are generalized and electromagnetic fieldapproach is used to analyze the transient surge response when the transmission line isstruck by lightning.2 Tested is developed and the calculated result from NEC is proved to beunaccurate and the result gained by HIFREQ in CDEGS is very accurate.3 More practical as the soil and the grounding system of the tower is considered andcompared in the results computed by HIFREQ, respectively.4 The impedance functions are obtained based electromagnetic field approach andapproximated by the Vector Fitting. The circuit model of the impedance functions isestablished by the network synthesis.5 With this model the lightning surge performance of a power transmission line canbe analyzed more accurately by EMTP or other methods.
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