Unbalanced Grid Fault Ride-Through Control for a Wind Turbine Inverter

Large variable speed wind turbines with fully rated, direct-in-line converters are increasingly used as silicon cost continues to decline. While grid codes require wind turbines to ride-through grid faults, including unbalanced faults, voltage source inverters interfaced to the grid are inherently sensitive to any unbalance in the a.c. voltage. This paper presents an unbalanced grid fault ride-through control method for such a wind turbine configuration. The method can be easily integrated into the power tracking control needed during normal operation. It is shown that the major constraint is the 2^nd order harmonic to be absorbed by the d.c. link capacitor and that the control effectiveness depends on the voltage and current capabilities of the semiconductor devices in the inverter. Simulation and experimental results are used to validate the proposed control method and to identify the power limit that is necessary during ride-through.