Voltage dip at maximum angular swing in the context of direct stability analysis

The critical clearing time of transient stability analysis represents maximal fault duration beyond which the system loses synchronism. In many practical situations utilities are concerned with post-fault transient voltage dips below certain specified values. For most cases dips below 0.8 p.u. will occur for clearing times which are shorter than this critical clearing time. Normally, the maximum voltage dip will occur at the point of peak angle swing. Since, in the context of direct stability analysis, the peak angular swing occurs at the point where the transient potential energy is also at its maximum, it follows that lowest voltage dips will occur at the maximum of this potential energy. In the approach proposed, transient potential energy is maximized along a linear trajectory connecting the clearing angle to the unstable equilibrium point. This approach is verified against time-domain simulations. Subsequently, it is shown that VAr capacitive support can improve on the voltage dip condition. A fast approximate approach to voltage dip computations is shown to be justified, leading the way to a direct stability program based on voltage dip considerations