Cascading failures in complex infrastructure systems

This paper studies the effect of cascading failures in the risk and reliability assessment of complex infrastructure systems. Conventional reliability assessment for these systems is limited to finding paths between predefined components and does not include the effect of increased flow demand or flow capacity. Network flows are associated with congestion-based disruptions which can worsen path-based predictions of performance. In this research, overloads due to cascading failures are modeled with a tolerance parameter α that measures network element flow capacity relative to flow demands in practical power transmission systems. Natural hazards and malevolent targeted disruptions constitute the triggering events that evolve into widespread failures due to flow redistribution. It is observed that improvements in network component tolerance alone do not ensure system robustness or protection against disproportionate cascading failures. Topological changes are needed to increase cascading robustness at realistic tolerance levels. Interestingly, targeted topological disruptions of a small fraction of network components can affect system-level performance more severely than earthquake or lightning events that trigger similar fractions of element failure. Also, regardless of the nature of the hazards, once the triggering events that disrupt the networks under investigation occur, the additional loss of performance due to cascading failures can be orders of magnitude larger than the initial loss of performance. These results reinforce the notion that managing the risk of network unavailability requires a combination of redundant topology, increased flow carrying capacity, and other non-conventional consequence reduction strategies, such as layout homogenization and the deliberate inclusion of weak links for network islanding. Furthermore, accepted ideas that rare loss of performance events occur exponentially less frequent as the performance reduction intensifies contrast with more frequent network vulnerabilities that result from initial hazard-induced failures and subsequent cascading-induced failure effects. These compound hazard-cascading detrimental effects can have profound implications on infrastructure failure prevention strategies.