To address the key challenge of quantitatively characterizing the ability and conditions for grid-forming converters to maintain voltage source characteristics during fault conditions

this paper first proposes an analytical method for fault current calculation by decoupling the reactive power-voltage control loop from the electromagnetic transient behavior of the network. This method transforms the originally complex and analytically intractable second-order differential equations into two simple first-order differential equations that can be solved sequentially. Then

to quantify and characterize the capability and boundary conditions for grid-forming converters to maintain voltage source characteristics during faults

this paper defines a two-dimensional visualizable parameter feasible region with the fault voltage drop at the point of common coupling (UgF) and the virtual inductance (Lv) as key parameters. A practical characterization method for the parameter feasible region based on the boundaries and vertices is proposed for the first time. Specifically

the critical vertices of the UgF-Lv feasible region are determined based on the constraints of the maximum overcurrent capability of grid-forming converters and the internal potential saturation limit. Subsequently

the practical boundary of the UgF-Lv feasible region is characterized based on its near-linear geometric property. Finally

the effectiveness of the characterized parameter feasible region is validated through experiments on an RT-Lab-based hardware-in-the-loop experimental platform.