Educational Insights Into Short-Circuit Calculations: Iterative Methods For Inverter-Based Fault Response

Short-circuit current contribution of synchronous generators is a well understood topic with fundamental tools that have been established to facilitate the calculation of balanced and unbalanced fault currents. For protection coordination, the short-circuit current calculations are based upon the fundamental frequency in the steady-state RMS phasor domain. Inverter-based resources exhibit fault current characteristics that are controlled by the inverter controller. Moreover, fault currents are subject to current limits, which makes the fault response nonlinear and therefore requires an iterative solution for calculating the short-circuit current. Phasor domain models utilizing an iterative short-circuit solution have been created by EPRI and implemented into commercially available short-circuit programs. This paper presents a review of short-circuit calculations, demonstrating the iterative process required to calculate the fault current. The paper starts with a simple network composed entirely of synchronous generators, and then repeats the example with the addition of an inverter-based resource.

This paper does not propose any solutions, rather it is intended to provide a review and education for engineers on iterative methods for short-circuit calculations required to simulate inverter-based resources in the steady-state RMS phasor domain.