  In modern power grids, a local failure or attack can trigger catastrophic
cascading failures, which make it challenging to assess the attack
vulnerability of power grids. In this Brief, we define the $K$-link attack
problem and study the attack vulnerability of power grids under cascading
failures. Particularly, we propose a link centrality measure based on both
topological and electrical properties of power grids. According to this
centrality, we propose a greedy attack algorithm and an optimal attack
algorithm. Simulation results on standard IEEE bus test data show that the
optimal attack is better than the greedy attack and the traditional PSO-based
attack in fracturing power grids. Moreover, the greedy attack has smaller
computational complexity than the optimal attack and the PSO-based attack with
an adequate attack efficiency. Our work helps to understand the vulnerability
of power grids and provides some clues for securing power grids.
