Threshold virtual impedance current limiting design for grid-forming converters considering voltage vector location

Threshold virtual impedance (TVI) is a promising current limiting technology to ensure the hardware integrity of grid-forming converters (GFMs), as it maintains the voltage source characteristic behind an impedance. Existing TVI parameter design principles are based on limiting the GFM current to a maximum value when a bolted fault is applied at the point of common coupling (PCC). However, adopting existing TVI parameter design practices can cause the GFM current to exceed the maximum value after a fault is cleared, or under a large grid phase jump, since the angular difference between the GFM internal and PCC voltage vector can be large, causing the voltage across the virtual impedance and output transformer to exceed that assumed for TVI parameter design. Hence, the worst case design scenario is not during a fault, but during the subsequent recovery period, or under a large phase jump. Here, a new TVI parameter design process is proposed based on the GFM internal voltage vector being in anti-phase to the PCC voltage vector. However, the new approach should not be applied directly as the GFM current capacity may not be fully used. Hence, three alternative strategies are proposed, with a combined approach being recommended based on electromagnetic transient (EMT) simulations for a range of case studies.