Unbalanced-grid-fault ride-through control for a wind turbine invertor

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 ac 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 second-order harmonic to be absorbed by the dc-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.