  Power system N-x contingency analysis has inherent challenges due to its
combinatorial characteristic where outages grow exponentially with the increase
of x and N. To address these challenges, this paper proposes a method that
utilizes Line Outage Distribution Factors (LODFs) and group betweenness
centrality to identify subsets of critical branches. The proposed LODF metrics
are used to select the high-impact branches. Based on each selected branch, the
approach constructs the subgraph with parameters of distance and search level,
while using branches' LODF metrics as the weights. A key innovation of this
work is the use of the distance and search level parameters, which allow the
subgraph to identify the most coupled critical elements that may be far away
from a selected branch. The proposed approach is validated using the 200- and
500-bus test cases, and results show that the proposed approach can identify
multiple N-x contingencies that cause violations.
