The thermal proﬁle of power switching devices under steady-state wind speeds is an important tool which can be used either as a loading indicator for certain converter topologies or as a lookup table for the lifetime estimation. It was found that wind speed variations will lead to severe thermal excursion of some power switching devices in the given 3L-NPC wind power converter. Particularly, the clamping diode and outer switch suffer from more adverse thermal excursion than the other devices. The grid codes even under normal operation may change the delivered reactive power of wind power converters and thereby have impacts to the converter efﬁciency as well as thermal stress distribution of power devices.
By circulating the reactive power among paralleled converters in a wind farm or multi-cell converter system, it is possible to control the junction temperature and relive the thermal excursion in most stressed devices under wind gust operation, leading to higher reliability of the converter, while the increased thermal stresses to the other devices or paralleled converters are still acceptable.