  The frequent occurrences of cascading failures in power grids have been
receiving continuous attention in recent years. An urgent task for us is to
understand the cascading failure vulnerability of power grids against various
kinds of attacks. We consider a cost restrained hybrid attack problem in power
grids, in which both nodes and links are targeted with a limited total attack
cost. We propose an attack centrality metric for a component (node or link)
based on the consequence and cost of the removal of the component. Depending on
the width of cascading failures considered, the attack centrality can be a
local or global attack centrality. With the attack centrality, we further
provide a greedy hybrid attack, and an optimal hybrid attack with the Particle
Swarm Optimization (PSO) framework. Simulation results on IEEE bus test data
show that the optimal hybrid attack is more efficient than the greedy hybrid
attack. Furthermore, we find counterintuitively that the local centrality based
algorithms are better than the global centrality based ones when the cost
constraint is considered in the attack problem.
