This thesis has been conducted in cooperation with the Norwegian transmission system operator, Statnett, and their research on two new interconnectors linking Norway to England and Germany. The work presented in this thesis has considered one of these interconnectors, with a third terminal connected offshore to allow for integration of offshore wind power and oil platforms, as shown in fig.~ref{fig:theGrid}.Investigation of the voltage source converters (VSCs) control ability as well as operational aspects of multi-terminal DC-grids has been performed. Identified as the key control variable of a DC-grid is the DC-voltage serving as a measure of power balance, since an unbalance will affect the DC-voltage through charging or discharging of the DC-capacitances. Utilizing VSCs allowing a fast and accurate control of the DC-voltage can lead to a stable operation of a DC-grid.The three-terminal structure, fig.~ref{fig:theGrid}, was implemented in the simulation tool SIMPOW and connected to a 35-node AC-grid model representing the Nordic grid. The simulation model was used for analyzes of the DC-grid operation and its affect on the connected AC-grids. The converter station connected to the Nordic grid were applied with DC-voltage control, implying that it will act as the power balancing unit of the DC-grid. The simulations performed show that the control structure implemented could assure a stable DC-grid operation within 0.3s of a fault. It identified the need for a fast response to changes in the DC-voltage as the influence of the other connected AC-grids were directly linked to DC-voltage oscillations. Simulations were also conducted to study the advantage of applying AC-voltage control compared to reactive power for the converter connected to the Nordic grid. It was identified that an increased stability of the Nordic grid could be assured during faults in the grid itself. This also affected the DC-grid operation through an increased capability of power exchange with the Nordic grid.