Thermal cycling evaluation for DFIG wind turbine power converter based on joint modelling

The wind turbine power converter is a critical system component due to relatively higher failure rates compared with the other sub-assemblies. The lifetime of a power device in the converter is closely related to its mean temperature and its variations. In this paper, the power device thermal cycling is analyzed with a multi-physical model developed in PSCAD, which includes detailed doubly-fed induction generator wind turbine modeling, and an electrothermal network of the semiconductors. The power loss obtained from PSCAD is verified with that from the software SemiSel. SemiSel however only shows average temperatures, where as the new model shown here offers much higher fidelity since it can show time-based variation. A discussion and analysis of various methods to implement the electro-thermal model is undertaken. It is shown with the new model that a large temperature swing occurs for the rotor-side converter at the synchronous operating point due to very low frequency currents. This may cause serious stress and uneven heat distributions among the semiconductors. Therefore, working near this point should be avoided during normal operation in order to increase the inverter lifetime. This integrated model can also be used to evaluate the effects of wind speed variations, different power module sizes and cooling systems.