This paper presents a hierarchical control structure for Multi terminal HVDC grids. The hierarchical control structure is inspired by the three-layer control structure of large-scale AC power systems and it is divided into primary, secondary and tertiary control. This structure permits to create an easy interaction between power dispatch and the lower controllers. Moreover, a generalized voltage droop (GVD) characteristic is also proposed which facilitates control issues inside the Multi terminal HVDC grids. In the proposed control structure, the coefficients of the GVD characteristics are set based on the solution of a DC power flow algorithm, executed in the secondary control level.