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.