  For a power system operating in the vicinity of the power transfer limit of
its transmission system, effect of stochastic fluctuations of power loads can
become critical as a sufficiently strong such fluctuation may activate voltage
instability and lead to a large scale collapse of the system. Considering the
effect of these stochastic fluctuations near a codimension 1 saddle-node
bifurcation, we explicitly calculate the autocorrelation function of the state
vector and show how its behavior explains the phenomenon of critical
slowing-down often observed for power systems on the threshold of blackout. We
also estimate the collapse probability/mean clearing time for the power system
and construct a new indicator function signaling the proximity to a large scale
collapse. The new indicator function is easy to estimate in real time using PMU
data feeds as well as SCADA information about fluctuations of power load on the
nodes of the power grid. We discuss control strategies leading to the
minimization of the collapse probability.
