Transient behaviour of a “mixed” overhead-cable EHV line under lightning events

Recently, in many countries have been undergone or are planning links with AC extra-high voltage (EHV) underground cables. Cable lines offer the possibility of a reduced environment impact and for this reason they are often the only alternative for the reinforcement or the expansion of power system accepted by public opinion. The coexistence of overhead and cable line in the so called ¿mixed¿ lines, however, raises several questions regarding their behaviour both in steady state and in transient. The purpose of this work is to investigate the transient behaviour of a mixed overhead-cable EHV line under lightning events, describing the effects of the following overvoltages on the system. The mixed line analyzed is a known case found in literature: the results obtained by the model are assumed as term of comparison with the known case. The model thus validated is used for further analysis, taking into account different configuration of the mixed system. The events subsequent lightning may be different: the study considers the shielding failure and the direct stroke on tower, with the possibility of back flashover along insulators. The analysis, carried out in PSCAD/EMTDC environment, has shown that shielding failure does not represent a critical event to mixed line and back flashover occurs for fast waveshape and high value of peak current.