Response of a grid forming wind farm to system events, and the impact of external and internal damping

Following from smaller-scale investigations of grid-forming converter control applied to wind turbines in 2017-2018, this study describes a much larger trial involving an entire wind farm, owned and operated by ScottishPower Renewables. To the authors' knowledge, this was the first UK converter-connected wind farm to operate in grid-forming mode, and the largest in the world to date. The 23-turbine, 69 MW farm ran in the grid-forming mode for 6 weeks, exploring inertia contributions between H = 0.2 s and H = 8 s. A number of unscheduled frequency disturbances occurred due to interconnector, combined cycle gas turbine (CCGT) and other trips, to which un-curtailed turbines were able to respond. In addition, several deliberate tests were carried out. The turbines were able to provide a stable and appropriate response at relatively high inertia levels to the frequency events commonly occurring today. The captured responses stimulated a debate as to whether external damping power might be required in a grid-forming converter, or whether internal damping is sufficient to allow stable and robust power-sharing with parallel devices in all grid event scenarios. Analysis in this study suggests that, practically, internal damping is probably appropriate, and that any deficiency in external damping power can be more than mitigated by reactance and/or droop-slope response-time management in the grid-forming converters.