Voltage Dip Induced Frequency Dips for Power Systems with High Shares of Wind Energy

In order to limit drivetrain mechanical stress, wind turbine generators typically implement a delayed active power recovery following a voltage dip, which may result in a substantial reduction in system frequency, if many wind farms adopt a similar strategy, due to the resulting generation-demand imbalance. Therefore, based on a modified IEEE 39-bus system, the impact of active and reactive current priority strategies, and various reactive current loop controls for wind turbine generators on voltage dip induced frequency dips (VDIFDs) are examined. The effectiveness of local voltage control with reactive current priority is validated. In addition, if synchronous-based generation is displaced by grid-forming converters (GFMs), the frequency dips are less severe and the post-fault frequency recovers more quickly. However, due to reduced overcurrent capability, the GFM virtual angle control must be carefully designed to avoid transient instability, by, for example, reducing the droop gain. Finally, if DC-link voltage control and maximum power point tracking control for the grid-side and machine-side converters are switched (to simplify fault ride through implementation) care is needed to avoid large post-fault over-frequency transients for VDIFD events.