What are the implications of using a doubly-fed induction generator (DFIG) versus a permanent magnet synchronous generator (PMSG) in an offshore wind turbine's performance during grid faults?

During grid faults, such as voltage dips or short circuits, the performance of an offshore wind turbine is significantly influenced by whether it uses a Doubly-Fed Induction Generator (DFIG) or a Permanent Magnet Synchronous Generator (PMSG). A DFIG has its stator winding directly connected to the grid and its rotor winding connected through a power converter, typically rated for about 25-30% of the generator's total power. This partial power converter allows the DFIG to operate at variable speeds, improving energy capture efficiency. However, during a grid fault, the voltage dip can cause a large inrush of current from the grid to the DFIG's stator. This high current can damage the power converter, potentially leading to a turbine shutdown. To protect the converter, a crowbar circuit is often used. The crowbar shorts the rotor circuit, effectively disconnecting the converter from the grid and protecting it from overcurrent. While this protects the converter, it also means the DFIG loses its ability to control reactive power injection during the fault. Reactive power support is crucial for grid stability during faults; without it, the voltage dip can worsen and spread, potentially leading to wider grid instability. A PMSG, on the other hand, has its entire output power processed through a full-scale power converter. This allows the PMSG to provide much better grid fault ride-through (FRT) capabilities. During a grid fault, the full-scale converter can actively control both active and reactive power output, supporting grid voltage and frequency. The PMSG can quickly inject reactive power into the grid to help stabilize the voltage, preventing further voltage dips and contributing to overall grid stability. Because the PMSG is fully decoupled from the grid through its converter, it is less susceptible to the large inrush currents that affect DFIGs. While the full-scale converter is more expensive and can have slightly lower efficiency than the DFIG's partial converter, its superior FRT capability makes PMSG turbines more desirable in locations with stringent grid codes or weak grid connections, which are common considerations for large offshore wind farms. In summary, DFIGs are more vulnerable to grid faults and rely on protection mechanisms that limit reactive power support, while PMSGs offer enhanced fault ride-through capability due to their full-scale converters, allowing for better reactive power control and grid stabilization during disturbances.