Improvement in critical clearing time calculation by a new equation of motion in the on-fault condition

In the stability analysis of power systems, the influence of the abrupt output change on the electromagnetic torque T
em
in the on-fault condition is conventionally neglected. A straight and rigorous method to derive the corresponding approximated expression of T
em
, strictly based on the generally accepted Park-equations and known parameterratios, is presented. The resulting improvement in the accuracy of the calculations of the critical clearing time is shown to be highly significant.