nvestigators have traced the events that led to
Thursday's blackout to the failure of several high-voltage
transmission lines near Cleveland, officials said yesterday, but it
remained unclear how the problem spread from the Midwest to the
Power experts and companies that use large amounts of electricity
had warned for years that the nation's transmission grid was
seriously strained, especially in the Midwest. Some of those
warnings focused on Ohio.
For years, industry groups have complained of a rapid rise in the
number of times that transmission bottlenecks have led regulators to
restrict the flow of power around the region. A report by a group
called Industrial Energy Users-Ohio cited a nearly fivefold rise in
such incidents between 1999 and 2000, and called for changes to be
made in the system.
Reports by the North American Electric Reliability Council, which
is investigating the blackout, show that the sharp growth of such
problems has been limited almost entirely to the Midwest.
Yesterday, the council gave its first detailed timeline of
Thursday's events leading up to the huge power failures. The first
breakdown, said Michehl R. Gent, the group's president, occurred at
3:06 p.m., when a 345-kilovolt transmission line west of Cleveland
shut down for reasons that are still unknown.
Ordinarily, that would have no effect on service because the load
would be transferred to other lines nearby. But adding power to a
line makes it heat up, expand and sag. Twenty-six minutes after the
first line failed, Mr. Gent said, a second 345-kilovolt line in the
same area, probably one helping to carry the load from the first
failed line, sagged into a tree, causing it to shut down. Trees near
transmission lines are supposed to be kept cut back to prevent such
With the remaining lines in the Cleveland area becoming more
strained, three more failed over the next 34 minutes, the last at
4:06 p.m. Over the next few minutes, systems throughout the Eastern
United States and Canada began to see huge swings in voltage and in
the direction of power flow. More lines went down, and power plants
shut themselves off. At 4:11, the blackout began.
The first lines that failed are owned by the FirstEnergy Corporation, a Cleveland-area utility. Last
night, FirstEnergy released a statement acknowledging the breakdown
and suggesting that an alarm in its control system had failed. The
company did not respond to a reporter's request for more details.
The transmission lines in the Cleveland area are part of what the
power industry calls the Lake Erie Loop, a ring of lines in the
United States and Canada that have become a sort of electricity
interstate, moving vast amounts of power from one state to another.
On Thursday, wild fluctuations along the lines between Michigan and
New York through Ontario caused power plants to shut off, and the
In assessments of the reliability of the national grid, Mr. Gent
told reporters Friday, that loop "has always been a big, big
problem" in part because it is so heavily used.
Readings taken by a company that monitors electric flows for its
commercial customers suggest that the problems in Ohio might have
started the day before the blackout. Sensors used by SoftSwitching
Technologies, a Wisconsin company, show that its customers in Ohio
experienced frequent swings in voltage starting after 3 p.m. on
Wednesday, continuing into Thursday, according to a database
accessible on the company's Web site.
The readings show power lines dropping below their usual
operating voltage for a few seconds to several minutes, while the
flow to consumers in other parts of the country remained far more
stable. Small voltage swings are not uncommon, but they are usually
very mild and brief. These were sharper, more prolonged and much
more numerous than usual, and the database shows that some customers
lost power completely on Wednesday night.
"Instability in the voltage is something you really worry about,"
said Karl E. Stahlkopf, senior vice president of the Hawaiian Electric Company and a former vice president of the
Electric Power Research Institute in Palo Alto, Calif. "In the '96
outages in California, we had very large voltage swings, which
caused relays to trip, which caused plants to go offline, which
The company's database also shows Ohio consumers repeatedly
experiencing a much rarer condition: voltage running higher than
normal for a minute or more, from mid-June through late July.
Several experts said that such "overvoltage" can indicate
systemic trouble in the local grid, and that if it is severe enough,
it can damage power systems. But they also cautioned that it is not
clear whether any of these patterns were more widespread, or had
anything to do with the blackout.
Blackouts are often caused by summer heat, as peak demand strains
the system and equipment overheats. Mr. Gent and others have ruled
that out in this case, noting that neither temperatures nor demand
were terribly high on Thursday.
But a report by the Electric Power Research Institute proposes
that warm weather could have played an important role, though an
indirect one. When a problem causes voltage to drop, a lamp or a
television might dim, but when the voltage rises again, the
appliance returns to normal without a notable increase in power
Air-conditioners, though, work differently, and they account for
a huge proportion of summer electricity use. Air-conditioners have
electric motors that slow down when voltage drops. The motor's
response is to draw significantly more power, trying to resume its
The research institute's report proposes that that can lead to
damaging swings in voltage, and turn a slight drop into a much
steeper one that causes equipment to fail.