U.S. power grid at risk

VULNERABILITIES MISTAKENLY built into the U.S. power grid, which is one of the most complex systems ever constructed, have been identified by a research team lead by Reka Albert, assistant professor of physics at Penn State University.

The team's analysis of the grid structure reveals that the system design allows for it to withstand the random loss of generators or substations.

The system's integrity may depend on protecting a few key elements.

"Our analysis indicates that major disruption can result from loss of as few as 2% of the grid's substations," said Albert. 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. Major blackouts caused by failures in the grid, such as the one that affected the northeastern part of the country last summer, incur tremendous economic, public health, and security risks.

The researchers constructed a model of the entire transmission grid with more, than 14,000 "nodes," including generators, transmission substations, and distribution substations, and more than 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. "While 40% of the nodes had a load below one thousand, the analysis identified 1% of the nodes-approximately 140-that have a load higher than one million/'Albert said.

This high degree of connectiveness in the grid system allows power to transmit over long distances, but it also allows local disturbances to propagate across the grid. "There are systems to protect the nodes from overload, such as a controlled shutdown to take a substation out if it overloads or to shut off a generator. In general, these systems do a good job of protecting the nodes," Albert said. "What this model really looks at is the effect of losing a number of nodes in a short period."

If the nodes go out randomly, the effect on the system is roughly proportional to the number of generators or substations removed. However, the grid quickly disconnects when the high-load transmission substations are selectively removed from the system-if the nodes that have the highest load go out first, followed progressively by the nodes with successively lower loads. According to the model, a loss of only 4% of the 10,287 transmission substations results in a 60% 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% of the nodes causes a catastrophic failure of the entire system.

Copyright Instrument Society of America Oct 2004
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