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The growing sensibility to environment issues contributed in recent years to the erection of barriers in the construction of new overhead transmission lines. One of the solutions to overcome these problems was the use of cable lines for high (HV) and extra-high (EHV) voltage systems. However, the coexistence of overhead and cable lines in transmission systems, in the so called Â¿mixedÂ¿ lines, has raised a number of issues regarding the behaviour of such systems both in transient and steady-state. In particular, the definition of transient stability is necessary for a system to describe the behaviour respect to events aimed at altering its equilibrium point. The purpose of this study was to investigate the transient stability of single machine infinite bus (SMIB) system containing a mixed overhead-cable line affected by fault: the critical clearing time (CCT) was assumed as stability index. The study has taken under investigation three different cases and was developed through a sensitivity analysis, varying the most significant system parameters. The analysis, based on the time domain method, has shown that mixed line with reactive shunt compensation has practically the same performance of traditional overhead line. However, it was observed that an increase in compensation degree could improve system transient stability, especially for long cable lines.