Conclusions
The thermal performance of power switching devices is important for the modern wind power converter system. It is found that, the thermal-related characteristics of the three dominant power switching devices in wind power applications are quite different.
For all of the power switching device solutions in the 3L-NPC grid side inverter, the outer switches Tout, clamping diodes Dnpc and inner switch Tin are likely to become the most stressed devices under various important operation modes of the wind turbines, therefore, they are critical components for the 3L-NPC wind power converter; while the performance of outer freewheeling diode Dout and inner freewheeling diode Din is less important because they are barely used.
Regarding the most stressed devices Tout, Tin and Dnpc, three device solutions show quite different loading behaviors because of different power loss and thermal impedance characteristics: for the IGCT solution, the switching loss in the diode is much larger because of the normally used clamping circuit, this disadvantage may lead to much higher junction temperature in Dnpc in comparison with the other two IGBT solutions. The IGBT press-pack solution tends to have larger switching loss in the switch, however, because of smaller thermal resistance, it shows similar junction temperature level in the switches as the IGCT solution, but with much better thermal performance in the clamping diode. IGBT module solution shows the best loss performances among the three device solutions, but due to much large thermal resistance, especially from case to heat sink, the junction temperature level is generally high. It is noted that the IGBT module solution may result in a large component counts in 10 MW power conversion system, which may be unpreferable in the wind power application.
Finally it is worth to mention that, the paralleling of power switching devices may change the loading profile significantly and have strong impacts on the loss, thermal, cost, and power density performances of the converter, thereby, the parallel numbers of power switching devices should be carefully evaluated in the design process.