  In this paper, we study numerically the behavior of the closest unstable
equilibrium point (UEP) on the stability boundary of a stable equilibrium point
(SEP) of a post-switching power system along a P-V curve. Using the
structure-preserving model of the WSCC 9-bus 3-machine system, we show that
along the load curve, the closest UEP can switch to a new UEP. We also show
that the stability region of the post-switching SEP can expand and contract as
the load moves towards the nose point of the P-V curve. Our numerical results
also show the impact of the direction of movement of the closest UEP on the
size and shape of the stability region of a SEP.
