Estimation of the Developed Overvoltages at the Entrance of a HV/MV Substation

External overvoltages can cause several damages to a HV/MV substation, leading to insulation breakdowns. The appropriate protection of the substation against external overvoltages is critical, in order to ensure the efficient, reliable and safe operation of the system. Shielding wires, mast and surge arresters are the main parts of a lightning protection system. The current PhD project deals with the effective lightning protection of HV/MV substations, examining the impact of various factors on the magnitude of the developed overvoltages at different positions of the system. The influence of the equivalent circuit model of the various components to the calculated overvoltages is also examined. Moreover, the substation outage rate due to lightning strokes is calculated, considering shielding failures and back-flashovers. In addition, special issues are discussed, i.e. induced overvoltages and installation of arresters in parallel. In comparison with other studies, main contribution of the thesis is that does not focus only on the role of the grounding resistance, but examines the dominant influence of other parameters; the improvement of the lightning performance of the substation can be achieved by the appropriate adjustment of various factors of the system. The current work highlights also the advantages and drawbacks of each equivalent circuit model, providing a guide to other researchers in order to select the appropriate models. As far as the risk assessment analysis is concerned, innovation of the performed study constitutes the inclusion of the arresters‟ failure rate to the total substation failure rate, since their possible failure results in malfunction of the nominal operation of the system. The induced overvoltages arriving at the entrance of the substation are also calculated, examining the role of the lightning hit position, the waveform of the lightning current, emphasizing to the role of the installation position of the arresters. Finally, the need for good matching of the voltage – current characteristics of the arresters is revealed, otherwise, the expected equal sharing of the injected lightning current will not be achieved. The current Thesis indicates that the combination of the arresters in parallel does not influence the expected overvoltages, but has to do mainly with the absorbed energy by the arresters