Construction And Optimization Of Phase Change Cooling Platform For Simulated Converter Valve Operating Conditions

The rapid development of HVDC transmission technology puts forward higher requirement for the capacity and reliability of the converter valve in the transmission system,and the converter valve is composed of thousands of power semiconductor devices(thyristors and IGBTs)to realize function of rectification and inverter.Due to the existence of its own impedance,these high-power semiconductor devices produce serious power consumption and heating problems in the process of carrying high voltage and big current.In addition,these devices are extremely sensitive to temperature,so in order to avoid being burned out,it is necessary to transfer the heat outside in time.Currently,the converter valve is usuallly cooled by active water cooling.However,due to facing complicated electromagnetic environment and a crucible of heat,the converter valve cooling system is large and complicated,and the stability and reliability of its operation is greatly reduced.Besides,the problem that the impurities are easily blocked and the pipeline is easy to leak often happens,and its heat dissipation capability also reach its heat dissipation limit.In order to fundamentally solve those problems in the cooling system of converter valve,this paper proposes a new and efficient phase change cooling scheme.Phase change cooling technology takes the advantage of the huge phase change latent heat carried by phase-to-phase conversion and has the advantage of fast and efficient energy transfer,which has attracted the attention of many researchers.Firstly,from the perspective of the two-phase flow theory model,this paper chooses the combination of the Mixture method and the Lee model source term,and uses the Mixture method to deal with the flow problem of the two-phase flow,and uses the Lee model source term to solve the problems of heat transfer and mass transfer between the two phases in the phase change process.Combined with the valve string structure of the converter valve and the application experience of phase change cooling technology in other fields,a phase change cooling test platform is built.The empirical coefficient in Lee model is modified based on the test results.The simulation and test results under different power are coMPared and analyzed,which proves the accuracy and rationality of the established semi-empirical and semi-theoretical model.Based on the comprehensive analysis of the calculation and test results of the forth,the FC-72 medium with lower saturation temperature is selected as the new phase change cooling medium.The heat dissipation effect of the three heat sink structures is analyzed through three perspectives of the uniformity of the surface temperature and the proportion of the gas phase,and the convective heat transfer coefficient.After coMParing and analyzing the heat dissipation effects of the three heat sink structures,a structure which is more favorable for the phase change process is selected.Based on the optimized structural model,the influence of the boundary conditions of the heat sink inlet on the heat dissipation effect is analyzed by changing the inlet temperature and inlet flow velocity of the heat sink,which provides a guiding plan for future engineering applications.In addition,according to the face that condition of power fluctuation often occurs during the operation of the power device,the heat dissipation response performance of the heat sink structure under different powers is also simulated.The calculation results show that the heat sink structure can meet the demand of different operating conditions of the power device.The fifth chapter summarizes the innovations and shortcomings of this paper,as well as the direction that needs to be improved in the next step,which lays a foundation for the team's subsequent research on phase change heat dissipation of power devices.