The team's topological analysis of the grid structure reveals that its integrity may depend on protecting a few key elements, even though the system has been designed to withstand the random loss of generators or substations. According to the group’s analysis, major disruption can result from loss of as few as two percent of the grid's substations, says Albert, whose research team includes Istvan Albert, research associate in the Bioinformatics Consulting Center at Penn State, and Gary L. Nakardo as the National renewable Energy Laboratory.
Future additions to the system, say the researchers, should consider the effect of new nodes on relieving strain on key nodes. Albert says they know how defects can propagate through the system. The team has identified parts of the system that need to be improved because they are not redundant and he says they can show which substations need to be protected from failure in order to avoid widespread system failure.
The researchers constructed a model of the entire transmission grid with over 14,000 "nodes," including generators, transmission substations, and distribution substations, and over 19,000 "edges," corresponding to the high voltage transmission lines that carry power between the nodes. They measured the importance of each substation node based on its "load," or the number of shortest paths between other nodes that pass through it.
One implication of the research is that identification of strategic points in the grid system can enhance defense against interruptions, whether by equipment failure, natural disasters, or human activity. A major blackout that affected the northeastern part of the country during the summer of 2003 was caused by failures in the grid.
The researchers explain that while 40 percent of the nodes had a load below one thousand, the analysis identified one percent of the nodes — approximately 140 — that have a load higher than one million. This high degree of connectiveness in the grid system allows power to be transmitted over long distances, but it also allows local disturbances to propagate across the grid.
There are systems to protect the nodes from overload, and in general these systems to a good job of protecting the nodes.
According to the research model, a loss of only 4 percent of the 10,287 transmission substations results in a 60 percent loss of connectivity. During a cascading failure, in which the high-load substations fail in sequence, the model shows that the loss of only 2 percent of the nodes causes a catastrophic failure of the entire system.
This vulnerability is an inherent part of the existing system, say the authors. If the power grid were highly redundant, however, the loss of a small number of nodes should not cause power loss because the system reroutes through alternative paths. Possible remediation schemes include increased redundancy focused on key substations and transmission lines, or more distributed generation, which would decrease the load on these key points.
The study, titled "Structural Vulnerability of the North American Power Grid," was published in a recent issue of the journal Physical Review.
