Online Computation of Short-Circuit Ratio Incorporating Transient Generator Equivalent Model

Online system strength assessment is getting attention recently due to the growing integration of inverter-based resources (IBRs) and the challenges IBRs face in weak grids. Calculating short-circuit ratio (SCR), a metric for system strength assessment, actually requires finding the fault current at the buses of interest using measurements of the current healthy system. Calculation of the exact short-circuit current incorporating the dynamic models of synchronous generators can be computationally intensive for large systems. Traditionally an approximate generator equivalent circuit model having a voltage source in series with its direct-axis transient reactance is used. However, this approach results in overestimation of SCR values and any operational or control decision taken based on such overestimated SCR values can be detrimental to the system. This article proposes an accurate equivalent circuit model that represents generators during the transient state immediately after a short-circuit and a computationally less intensive method to compute the fault current, and hence the SCR values, using the measurements of the healthy system. Tested on multiple IEEE test systems, the method is found to be more accurate than the traditional method and computationally much less intensive than the exact method especially for large systems. Additionally, the influence of generator parameters and load on the equivalent circuit parameters is analysed.