  We study the evolution of fast blackout cascades in the model of the Polish
(transmission) power grid (2700 nodes and 3504 transmission lines). The cascade
is initiated by a sufficiently severe initial contingency tripping. It
propagates via sequential trippings of many more overheated lines, islanding
loads and generators and eventually arriving at a fixed point with the
surviving part of the system being power-flow-balanced and the rest of the
system being outaged. Utilizing an improved form of the quasi-static model for
cascade propagation introduced in our earlier study (Statistical Classification
of Cascading Failures in Power Grids, IEEE PES GM 2011), we analyze how the
severity of the cascade depends on the order of tripping overheated lines. Our
main observation is that the order of tripping has a tremendous effect on the
size of the resulting outage. Finding the "best" tripping, defined as causing
the least damage, constitutes a difficult dynamical optimization problem, whose
solution is most likely computationally infeasible. Instead, here we study
performance of a number of natural heuristics, resolving the next switching
decision based on the current state of the grid. Overall, we conclude that
controlled intentional tripping is advantageous in the situation of a fast
developing extreme emergency, as it provides significant mitigation of the
resulting damage.
