IEEE Power & Energy Magazine - July/August 2020 - 66

Instructive Blackouts That Did Not Cascade
On 11 April 1965, 37 Palm Sunday tornadoes in Ohio,
Michigan, and Indiana took 27 American Electric Power
Company transmission lines and two extra high voltage
(EHV) substations out of service. Customers on failed
radial lines lost power, but no cascading was reported.
The event occurred on a Sunday in a low-demand month.
The system was not designed to survive 29 contingencies; however, it was lightly loaded and not stressed by
any definition.
In January 1998, during a Quebec-New York ice storm
lasting several days, many overhead transmission and distribution lines in a large area were on the ground. The affected
region was blacked out, but cascading was not a factor. The
extent of the damage remained confined, credited to adequate automatic and manual responses.
Nonetheless, Hydro-Quebec may be classified as a smaller
system, and the loosely coupled Upstate New York area
involved was unquestionably small. In general, smaller
systems do not seem to be vulnerable to cascading blackouts. A representative of the Electric Reliability Council
of Texas (ERCOT) system told one of us that operation
had been sustained in extremis several times, sometimes
with controlled rolling blackouts, but without cascading.
ERCOT is connected only loosely to the two huge abutting Eastern Interconnection and WI of North America.

Large-Scale Blackouts Due
to Massive Destruction
Puerto Rico was blacked out by Hurricanes Irma and Maria,
which destroyed much of the island's electrical infrastructure in September 2017. Two other large-scale destruction
events were described in the previous section. Significant
work is being performed on this problem as discussed elsewhere in this issue of IEEE Power & Energy Magazine, in
part exploring the notion of resilience. Such blackouts are
caused by massive external events, unlike the cascading
failures referenced in this article. Major catastrophes cause
extensive physical damage, especially to the distribution system; any cascading is of secondary importance. The damage
can take weeks or years to repair. Cascading blackouts, in
contrast, are due to internal failures of a functioning system.
They mostly involve EHV/HV transmission, with little, if
any, physical damage. The system is usually restarted within
hours or days. The two problems have different causes,
effects, and solutions.

What Have We Accomplished
Since 1965?
NERC and the N−1 Criterion
After the 1965 blackout, the U.S.-Canadian power industry
created the National Electric Reliability Council (NERC)
(now the North American Electric Reliability Corporation) to improve reliability. NERC developed planning and
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operating criteria based on principles developed previously
by North American utilities, which did not fully consider
cascading. The details of these criteria have evolved while
remaining consistent in their basic concepts. With limited
exceptions, NERC standards mostly assume that ancillary
control and protective devices, and practices and procedures, work properly.
One key concept is that a system of "N" elements should
be designed and operated to withstand, without major
interruption of service, the unexpected loss (outage or
contingency) of any single element. This notion leads to
the "N−1 criterion," simple in principle but nontrivial to
apply. The elements modeled as at risk by the NERC criteria are mainly transmission hardware, with less emphasis
on generation.

Early Observations on Cascading
Vannevar Bush was a brilliant electrical engineer and chair
of the U.S. Office of Scientific Research and Development
during World War II. In a 1970 book, he blamed cascading blackouts on the enormous growth of power systems
and their interconnection over vast regions. He predicted
that there would be more blackouts based on engineering
common sense: the more complex something is, the more
likely it is to fail. Nonetheless, a common prescription
over most of 50 years has been that to prevent cascading
blackouts, power companies just need to adhere better to
the NERC criteria, maybe with some fine-tuning. There
has been little recognition that solving the problem would
require new thinking.

Hidden Failures
A NERC study found that 73.5% of significant cascading
events were caused or aggravated by unobservable "hidden
failures" of the protection system, including relays, breakers, and so on. Many researchers have responded with
interesting work. For example, Chen et al. created a probabilistic failure model, recognizing that heavily loaded systems are more prone to blackouts. They assumed that the
probability of a line tripping incorrectly is low when the
line is loaded below its rating. If the flow increases from
100 to 140% of line rating, their assumed probability of
incorrect trips increases linearly to one. They concluded
that increased spinning reserve, a more robust protection
system, and faster control actions will reduce the risk
of cascading.
The notion of hidden failures is so apt that we have
wrested it from the protection system context and applied it
to any unobservable element of the power system. These elements include the various triggering failures in the cascading blackouts described previously. Every blackout seems to
involve different hidden failures. Each failure seems simple,
but to instrument the system to detect them all is fundamentally impractical. The system is too complex, and there are
too many ways it can fail.
july/august 2020



IEEE Power & Energy Magazine - July/August 2020

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