Transient Response of Grounding Systems Caused by Lightning: Modelling and Experiments

Abstract: In order to achieve better lightning protection and electromagnetic compatibility (EMC) requirements, the needs for a proper grounding system and the knowledge of its transient behaviour become crucial.The present work is focused towards developing engineering models for transient analysis of grounding system with sufficient accuracy and simplicity for lightning studies. Firstly, the conventional uniform transmission line approach for a single grounding conductor is modified and extended to grounding grids. Secondly, in order to overcome the drawbacks of all the existing transmission line approaches, for the first time, a non-uniform transmission line approach is developed for modelling the transient behaviour of different types of grounding systems. The important feature of such an approach is in its capability to include the electromagnetic couplings between different parts of the grounding system using space and time dependent per-unit length parameters.High voltages and currents induced in the grounding systems due to lightning always produce ionization in the soil. This phenomenon should be included during the transient analysis of grounding systems. In the present work, an improved soil ionization model including residual resistivity in ionization region is developed. The fact that there exists residual resistivity in ionization region (7 % of the original soil resistivity) can be proved by the experiments reported in the literature and the experiments carried out at the high voltage lab of Uppsala University. The advantage of including residual resistivity is that the beneficial influence of soil ionization in reducing the potential rise of grounding system will not be overestimated, especially in high resistivity soil.Finally, the transmission line approaches are adopted for studying the response of grounding systems due to lightning for different applications. These are, influence of soil parameters on the transient behaviour of grounding systems, transient analysis of grounding structures in stratified soils, investigation of the validity of existing definitions for effective length/area of different grounding structures, current distribution in the shields of under ground cables associated with communication tower, and influence of insulator flashover and soil ionization around the pole footing on surge propagation in Swedish railway system.