The work of this thesis focuses on the experimental development of a passive and biphasic cooling system to cool down power electronics. Some of those modules are mean voltage power converters developed by Supergrid Institute. Some constraints have been imposed upstream to this project. While some limitations are of the technological nature, other constraints are related mainly to the environmental and biological aspects. The first part of this thesis was to find a suitable cooling system that could be adapted to the project specifications. After some research, we decided to build a loop thermosiphon filled with NOVEC 649.

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.