The neutral point clamping（NPC）three-level converters have broad application prospects with megawatt wind turbines.The power losses of the switching devices must be considered in such high-power applications.Practical loss-calculating formulas are derived based on the analysis of the conduction and switching principles of the converters.The difference between the motor-side power converter（MPC）and the grid-side power converter（GPC）in power losses distribution is identified.Combined with the characteristics of wind power，the influence of power fluctuation on efficiency is also analyzed.It is concluded that the efficiency has positive correlation with the converted power.Simulation results of a co-simulation platform using MATLAB/Simulink and GH Bladed show the validity of the analysis.