Lightning electromagnetic field coupling to overhead transmission line networks and to buried cables

Lightning-induced voltages in power and communication systems are nowadays one of the main causes of power quality and electromagnetic compatibility. In recent years, due to the increasing demand by customers for good quality in the power supply along with the widespread use of sensitive devices connected to distribution lines, the protection against lightning-induced disturbances became of primary importance. As a consequence, for a correct protection and insulation coordination, the accurate estimation of lightning-induced overvoltages is essential. In addition, sensitive electronic components used in power and communication systems may suffer logic upset or damage at significantly lower levels of induced electromagnetic interferences. As a result, the evaluation of lightning-induced disturbances in underground cables has recently gained more interest compared to the past. The major aim of the present thesis is the development of models and computer codes, allowing the computation of the voltages and currents induced by an external transient electromagnetic excitation, especially due to lightning discharge, along realistic -and hence complex- transmission line networks and in buried cables. After a brief introduction on lightning phenomenology, Chapter 2 presents an overview of lightning return stroke modeling and methods for the calculations of lightning-generated electromagnetic fields above and inside the ground. We show in this chapter, that the simplified expression recently proposed by Cooray for the calculation of electric field penetrating the ground yields very good approximations to the exact numerical solutions, for distances as close as 100 m. The main original contributions of this thesis are presented in Chapters 3 through 5. They consist of theoretical and experimental work as follows. In Chapter 3, based on the Agrawal et al. coupling model, extended to the case of a multiconductor line and interfaced with EMTP program, a new software tool called LIOV-EMTP96, developed in the framework of a collaboration with the University of Bologna, is described. This software tool is able to analyze the response of complex distribution systems to indirect lightning. We carried out several experimental campaigns to test and validate the LIOV-EMTP96 code. First of all, we used the NEMP simulator of the Swiss Federal Institute of Technology (SEMIRAMIS) to illuminate a reduced-scale model of a multiconductor line. Then, we used a more complex 27-line reduced-scale network illuminated by the VERIFY NEMP simulator belonging to the Swiss Defence Procurement Agency (Spiez). And, finally, we used experimental data obtained by means of artificially-initiated lightning in 2002 and 2003 on an experimental distribution line at the International Center for Lightning Research and Testing, Camp Blanding, Florida. Calculations performed with the developed program have been tested versus the obtained experimental results, and a very good agreement between the