Conclusion
The reactive current injection requirements for full-scale wind power converter during LVRT will impose some power switching devices (especially the diodes) with even larger stress than the rated normal operation condition.
The device loading of the grid side converter under balanced LVRT changes dramatically under different grid voltage dips. When the grid voltage is below 0.5 p.u., 100 % rated reactive current is needed, the amplitude and position of the current is kept fixed and only the conduction loss is changed in the power devices. While when the grid voltage is above 0.5 p.u., both switching loss and conduction loss are changed dramatically in the power devices because the grid code allows some room and flexibility for the active current, which is related to the wind speed as well as to the pitch angle/rotation speed control strategies for wind turbines during LVRT.
When undergoing single phase unbalanced grid fault, it is found that the device loading among the three phases of converter is asymmetrical for all of the interested topologies. And it is also found that both the three-level and five-level H-bridge topologies show more potential to reduce and more equally distribute the stress in the power switching devices, compared to the well-known three-level neutral point clamped topology under various LVRT conditions.