Inertia — Small-signal Stability Nexus in Grid-forming Inverters

As rotating fossil-fuel-based synchronous generators (SGs) are phased out in line with decarbonization goals, grid-forming inverters—operating as virtual SGs—are expected to be integrated into future power systems. However, to date, there exist no standards on operating grids with both conventional and virtual SGs as their relative shares vary in the future. This work studies the small-signal stability of the grid in such a scenario, considering the example of the Greater London distribution system. Results show that when both power-sharing and system-level-inertia constraints are imposed, instabilities could arise for even high levels of inertia, depending on the number/location of generators and the overall share of inverter-based generation. This points to the need for decentralized stabilization strategies for the inverters, one that must be scalable to the grid-level. One such strategy based on virtual impedance control is proposed here and its effectiveness illustrated for the Greater London case study.