Vulnerability of U.S.
power grid centers on key junctions
University Park, Pa. -- Vulnerabilities
inadvertently 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.
The team's
topological analysis of the grid structure reveals that,
although the system has been designed to withstand the random
loss of generators or substations, its integrity may depend on
protecting a few key elements.
"Our analysis indicates
that 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. Nakarado at the National Renewable Energy Laboratory.
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 during the summer of 2003,
incur tremendous economic, public-health and security
risks.
The study, titled "Structural Vulnerability of
the North American Power Grid," was published in a recent
issue of the journal Physical Review E. 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.
"While 40 percent of the nodes had a load below
one thousand, the analysis identified 1 percent of the
nodes-approximately 140-that have a load higher than one
million," Albert says.
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, 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," says Albert. "What this
model really looks at is the effect of losing a number of
nodes in a short period."
If the nodes are removed
randomly, the effect on the system is roughly proportional to
the number of generators or substations removed. However, the
grid quickly becomes disconnected when the high-load
transmission substations are selectively removed from the
system-if the nodes that have the highest load are removed
first, followed progressively by the nodes with successively
lower loads.
According to the 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.
The authors
point out that this vulnerability is an inherent part of the
existing system. 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.
"Future additions to the system
should consider the effect of the new nodes on relieving
strain on key nodes," Albert says. "From this model, we know
how defects can propagate through the system, we have
identified parts of the system that need to be improved
because they are not redundant, and we can show which
substations need to be protected from failure in order to
avoid widespread system failure. These are considerations that
could help guide energy policy decisions."
This
research was funded in part by the Midwest Research
Institute.
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Contact
Reka
Albert
rza1@psu.edu
814-863-5346
|
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Contact
Barbara
Kennedy
science@psu.edu
www.science.psu.edu
814-863-4682
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