Renewable energy sources primarily employ power electronic converters to deliver their energy to the grid. These converter-based distributed generators (CBDGs) have a fault behaviour different from conventional generators, capable of compromising the reliability of traditional protection schemes. This paper investigates the fault behaviour of CBDGs by presenting both an analytical steady state model and a numerical transient simulation model developed in MATLAB/Simulink. Using these models, the impact of grid code requirements regarding reactive current injection during faults is analyzed. The results underline that different modelling techniques can support the design of grid codes.

Grid-forming (GFM) control offers promising performance features for inverter-based resources (IBRs) across scales. However, design, analysis, and benchmarking of GFM IBRs during unbalanced faults remains largely unexplored. In this paper, we outline a stationary-reference-frame nested-loop control architecture for GFM IBRs and integrate the same with novel current-limiting strategies. The architecture improves on virtual-impedance and current-reference-saturation limiting as well as state-of-the-art methods for control of voltage-source inverters. Electromagnetic-transient simulations for a modified IEEE 14-bus network validate salient features of the proposed control architectures. The proposed virtual-impedance limiter is shown to provide better voltage support during faults than the current-reference-saturation limiter (quantified via sequence voltages). On the other hand, the current-reference-saturation limiter offers better (and more accurate) fault-current contribution.

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