How does a doubly-fed induction generator (DFIG) actively contribute to grid stability during voltage dips, beyond merely riding through the fault?

A doubly-fed induction generator (DFIG) actively contributes to grid stability during voltage dips by providing dynamic voltage support and damping oscillations, going beyond simply riding through the fault. Riding through the fault simply means the generator remains connected to the grid during the voltage dip. A DFIG is connected to the grid through its stator winding directly, and its rotor winding is connected through a power electronic converter, typically a back-to-back PWM (Pulse Width Modulation) converter. This converter allows for independent control of active and reactive power. During a voltage dip, which is a temporary reduction in grid voltage, the DFIG can rapidly inject reactive power into the grid. Reactive power is necessary to support the voltage level; injecting it helps to counteract the voltage dip and prevent it from becoming more severe or cascading into a larger system disturbance. The DFIG's converter actively regulates the reactive power output, dynamically adjusting it based on the severity of the voltage dip. This active control is crucial because a simple induction generator would passively draw reactive power from the grid during a voltage dip, exacerbating the problem. The converter also allows the DFIG to control its active power output during the voltage dip. By reducing its active power output, the DFIG can lessen the stress on the grid and help prevent frequency instability. The reduced active power also limits the current flowing through the generator, protecting the converter from overcurrent damage. Further, the DFIG can damp oscillations in the grid voltage and frequency. These oscillations can occur after a disturbance and can threaten grid stability if they are not quickly damped. The DFIG's converter can inject active or reactive power in a manner that opposes these oscillations, helping them to subside more quickly. For example, if the grid frequency starts to oscillate, the DFIG can inject active power that is out of phase with the oscillation, effectively absorbing energy from the oscillation and reducing its amplitude. Finally, some DFIG control strategies also allow for voltage angle control. This capability allows for precise management of power flow, ensuring stable and predictable responses during grid disturbances. In summary, the DFIG actively controls both active and reactive power through its power electronic converter, providing dynamic voltage support, damping oscillations, and enhancing overall grid stability during voltage dips, exceeding the performance of a standard induction generator.