Projected grid-forming control for current-limiting of power converters

Sustainability and resilience concerns have motivated an unprecedented transformation of the electric power systems towards massive integration of renewable generation interfaced by power electronics. In this context, voltage source converters using grid-forming control are envisioned to provide services that so far have been provided by synchronous machines. In contrast to synchronous machines, voltage source converters are subject to stringent overcurrent limits. By exploiting the inherent time-scale separation between the inner control loops, the grid-forming reference dynamics (i.e., droop control, virtual oscillator control, etc and the transmission network, we highlight the well-known fact that the grid-forming reference dynamics need to be restricted to limit the converter output current without compromising stability. Next, we propose to formalize the problem of limiting the output current of a grid-forming converter by projecting the grid-forming reference dynamics onto a constraint on the output current. The main contribution is a projected droop controller that can be implemented using only local measurements. Moreover, we link the results to current limiting approaches using virtual impedance. Finally, we use a high-fidelity simulation to show that projected droop control outperforms virtual impedance current limiting.