Stability enhancement strategies for a 100% grid-forming and grid-following converter-based Irish power system

With increasing shares of wind and/or solar power in many power systems, the possibility of a 100% power converter-based system becomes more likely. Consequently, the dynamic response of the Irish transmission systems with 100% (grid-following and grid-forming) power converters under 3-phase faults is investigated for 100% converter urban and remote scenarios. Time-domain simulations show that when active or reactive current prioritisation current saturation controls are applied, grid-forming converters can introduce large, high-frequency LC resonance oscillations, but a scaling-down current saturation approach can help to mitigate such problems as it generates smoother current references. Virtual impedance current limits are most effective at reducing oscillations, but the initial fault current can be transiently high. Furthermore, freezing the virtual angular speed for a grid-forming converter under either current saturation or virtual impedance current limitation approaches can enhance transient stability during faults. Finally, with modified controls applied to the grid-following converters, the grid-forming requirement can be reduced from approximately 40% to less than 30%, with the Remote Irish grid remaining robust against bolted 3-phase faults, and oscillations quickly damped out during and post fault.