Frequency Support from Variable Speed Wind Turbines: Secondary Frequency Drop Phenomenon and its Mitigation

Renewable energy sources such as wind and solar are increasingly being integrated into the grid, replacing conventional synchronous generators. This demands their participation in the provision of grid services such as frequency regulation. The potential of variable speed wind turbines (VSWT) in this respect has been well established. The transient power exchange during the frequency support (FS) phase causes the rotor speed of a VSWT to move in a direction away from its maximum power point (MPP), resulting in a reduction in wind energy capture potential. This, together with the subsequent need to recover the rotor speed after the FS phase, could result in a significant reduction in the power delivered from the wind farm (WF) to the grid compared to pre-disturbance conditions, leading to a secondary frequency drop (SFD) in the power system if not properly managed. A power system model with VSWTs providing frequency support is developed in this work. The root causes for the occurrence of an SFD are identified and possible solutions to mitigate or minimise its severity are explained. Finally, a power system level dynamic tuning method for the FS controller gains of the VSWTs is proposed as a measure to minimise the severity of a potential SFD in the power system during an under-frequency event, and its effectiveness is proven by time domain simulation results.