Modern transmission power networks consist mainly of High Voltage overhead lines, since the later have proven their high operational reliability. However, technological developments have made it possible to use High Voltage cables, not only in the distribution grids, but also at higher transmission voltage levels. Thus, more mixed configurations, consisting of overhead lines and underground cables will take place in the near future. The Dutch transmission system operator, TenneT TSO, is a pioneer at this field, since it will install the longest 380 kV cable through the “Randstad 380 kV” project. On the other hand, mixed transmission systems bring new challenges at the field of the Power System protection because of their complexity. Consequently, the protection schemes need to be developed in order to retain their fast and reliable performance. Modern numerical protective relays are used, which can combine various operations within the same device, i.e. differential protection, distance protection etc. Before installing the protective relays, firstly their correct operation needs to be verified. For that reason, specific testing setups and programs are used to simulate possible transient fault conditions that can occur in a power system. This Thesis Project focuses on the differential protection of the 380 kV XLPE cable, which is installed at the South Transmission Ring of the Randstad area in the Netherlands. A number of simulations were carried out to verify the correct operation of the protection scheme, when faults occur within the High Voltage cable and at other points of the system. Important conclusions were reached, with respect to the correct response of the developed differential relay model and its operating time, when the protection current transformers are not driven into saturation. Moreover, recommendations for future research are given regarding the implementation of the differential protection model to the other parts of the studied configuration together with the distance protection scheme. The electromagnetic transient program ATP has been used to model the mixed configuration between the Maasvlakte and Westerlee substations of the Randstad’s South Ring.