  This paper studies the consequences of a human-initiated targeted attack on
the national electric power system. We consider two kinds of attacks: ($i$) an
attack by an adversary that uses a tactical weapon and destroys a large part of
the grid, by physically targeting a large geographic region; ($ii$) a targeted
attack by an adversary that takes out a small number of critical components in
the network simultaneously. Our analysis uses ($i$) a realistic representation
of the underlying power grid, including the topology, the control and
protection components, ($ii$) a realistic representation of the targeted attack
scenario, and ($iii$) a dynamic stability analysis, that goes beyond
traditional work comprising structural and linear flow analysis. Such realistic
analysis is expensive, but critical since it can capture cascading failures
that result from transient instabilities introduced due to the attack. Our
model acknowledges the presence of hidden failures in the protection systems
resulting in relay misoperations. We analyze the extent of cascading outages
for different levels of hidden failures. Our results show that: ($i$) the power
grid is vulnerable to both these attacks, ($ii$) the tactical attack has
significant social, economic and health damage but need not result in a
regional cascade; on the contrary the targeted attack can cause significant
cascade and lead to power outage over a large region. Our work shows the
necessity to harden the power grid not just to cyber-attacks but also to
physical attacks. Furthermore, we show that realistic representations and
analysis can lead to fundamentally new insights that simplified models are
unlikely to capture. Finally, the methods and results help us identify critical
elements in the grid; the system can then be hardened in a more precise manner
to reduce the vulnerabilities.
