Nonlinear Network Equivalents of Systems with Inverter Based Resources to Study Unbalanced Faults in Steady State

This paper proposes a method to obtain steady state short circuit network equivalents of systems with Inverter-Based Resources (IBRs) in the positive and negative sequence domains. Such an equivalent model improves the efficiency of short circuit simulation studies by reducing the computational burden while preserving simulation accuracy. The proposed network equivalent in the positive sequence domain consists of a Thevenin equivalent circuit in parallel with a voltage dependent nonlinear current source to account for the combination of conventional synchronous generators and IBRs. The representation in the negative sequence system depends on the fault-ride-through requirements, IBR type, and IBR control scheme. The proposed network equivalents in the positive and negative sequence domains are shown to successfully reproduce the fault response of a detailed system at a given point of interconnection with an external network.