Efficiency improvement is one of the most important aspects of engineering and especially important in the field of energy production. In the past decades, energy was mostly produced by fossil based technologies involving turbomachines, and the efficiency of these machines nearly quadrupled since the introduction of the first economically viable gas turbines. The progress continues, as there are still areas where improvement can be made. Such area is the High Pressure Turbine stage (HPT), which influences the flow characteristics and losses downstream, which this thesis will examine in more detail. In the open literature it can be found that one of the areas with potential for progress is the external cooling of the nozzle guide vanes (NGV) of the HPT stage. However not many studies go towards supersonic exit velocities even though that is the most common trend followed by the industry these days. The external cooling allows the turbine entry temperature (TET) to go beyond the melting point of the blade material thus increase Carnot efficiency but in the meantime influences the flow characteristics and losses. To understand these influences of the cooling, experiments in an annular sector cascade (ASC) were conducted with exit velocities from Mach 0.95 to 1.2 without and with cooling applied. The findings of the experiments are believed to help the more detailed understanding of the flow behaviour at high exit velocities. When comparing the corresponding runs in the two cases it became obvious that with cooling applied the deviation of the exit flow angle is generally smaller than in the uncooled case. This might be a highly important design feature for designers to work with. From the available data it was concluded that the total pressure distribution across the span is not significantly affected with the introduction of cooling.