In order to include large-scale renewable sources into the electrical system and to transport high amounts of energy through long distances, the actual AC grid must be upgraded. HVDC transmission grids appear as a promising solution to upgrade the system and answer correctly the future needs and requirements. The development of such grids can be done following two different approaches. For one side, a DC system designed totally from zero following a standardization of HVDC technology, and for the other side, an incremental evolution using the existing HVDC lines. The second approach seems more reasonable due to the reutilization of infrastructure, the inconvenient is that the technology used on each existing line is different. Thus, their interconnection will require DC¬DC converters as interface elements. These structures allow the interconnection of different HVDC schemes and offer more functionalities than only DC voltage adaptation like power flow control and protection.