A: For power systems without IBR, it was found that for many cases short circuit calculations could be calculated with reasonable accuracy by using a linear approximation. This linear approximation formed the basis of short circuit calculations for a long time. However, these linear models do not apply as well to IBR devices.
The linearized short circuit model traditionally used for conventional generators does not work well for IBRs because the current contribution to faults from IBR is algorithm-controlled, except for a brief subtransient period. To achieve a more reasonable approximation for the IBR fault response, nonlinear equations must be introduced into the short circuit solution.