This presentation will provide an update on our current project: designing a cooler for a high power (5 MW) MVDC converter for offshore wind turbines applications. A number of constraints are imposed, mainly related to a limited volume, environmental, safety and health regulations, and of course cooling performance. Indeed, as we presented last year (ATW 2017), the silicon carbide power semiconductors used in this converter should operate at a junction temperature lower than 100 °C for better efficiency. Of the number of solutions existing in the literature, we chose a biphasic heat transfer system for a number of reasons (a short overview of the cooling methods will be given, along with a justification of the choice). In particular, it can fit in a smaller volume, since cooling with latent heat can require as little as 10 times less mass than that cooling with specific heat. Other advantages may include setting an almost constant temperature on the power electronic modules (lower thermal cycling increases the lifetime of the power modules), or a totally passive cooling system (requiring no external energy to operate, and no maintenance). Fluid selection is very important since it dictates the performances of the cooling system. There are a number of criteria that must be addressed: the fluid must be good thermodynamically, it must respect environmental, safety and health regulations, and be chemically inert with most materials. We investigated more than 40 fluid formulations (covering many families), and found that 2 are particularly attractive. The presentation will describe the selection process (including the assessment of the thermodynamic properties), and give the list of the fluids, along with their properties. However, it is found that in many case, not all properties are given by the fluid manufacturers or the literature. Therefore, we will conclude by presenting a test system (under development) which will allow for the measurement of these properties.

M. MOUSTAID, Vincent PLATEL, Cyril BUTTAY