This report presents new concept for modelling of high-voltage and extra-high voltage transmission systems, which provide higher accuracy for modelling of underground cables. The growing worldwide demand for electricity influences the transmission system in a way that requires the upgrade and construction of new transmission lines. Also, the high degree of urbanization, the growth of the cities and the social discontent with living nearby overhead lines leads to gradual transition from overhead lines to underground cables at all voltage levels. This paper presents the discrepancies between the real-world situation and the mathematical model due to the specific operation, construction and environment of the cable lines.

In the recent years, many electric power companies have introduced cable lines for high and extra-high voltage and have studied the effect of their implementation on their power grids. The transmission network is dynamic and it is constantly evolving, as the underground cable lines can be used for its expansion, as well as for replacing the existing overhead transmission lines (OHL) in highly urbanized places. The report presents the necessary approaches in the study of transition processes in cable electric lines for the influence on power system in the process of their operation.