Must strictly abide operate specification, should stop work immediately and identify the reason when have a unusual phenomenon in working. Start work again after clearing of fault.

A direct method considering transient voltage dip is proposed to calculate the critical clearing time (CCT) ensuring that voltages of post fault power system are higher than limit.

So the traditional stability analysis methods cannot be applied to the new condition. This paper proposed a new method to compute the probability distribution of fault critical clearing time.