How much over-current capability is needed for grid-forming converters?

A quantitative study of the short-term overcurrent capability required by grid-forming (GFM) inverters is presented. Through theoretical modeling, fault simulation, and economic assessment, it is shown that, in a typical power system, setting the overcurrent threshold Ith between 2.0 and 3.0 p.u. enables the inverter to maintain voltage-source operation under most fault conditions, accommodate a 5° phase jump, and ensure low-voltage ride-through functionality. Introducing a fixed virtual impedance of 0.1 p.u. reduces the required capability to approximately 1.5 p.u. For applications subject to larger disturbances, such as a 60° phase jump or a three-phase fault proximal to the GFM inverter, the threshold may need to exceed 3.0 p.u., or an adaptive virtual impedance scheme may be adopted to relax this requirement. Economic analysis under current inertia-market conditions (e.g., in the UK) confirms that the incremental investment to enhance overcurrent capability is both technically justified and economically viable.