IEEE Power & Energy Magazine - May/June 2017 - 92

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figure 5. A typical power system control center. (Photo by George Loehr, used
with permission.)

operations. the initiating event was
the sudden loss of five system elements.
Power systems in virtually all developed countries are designed and
operated on the basis of a principle
called worst single contingency or first
contingency design. this means that the
single most severe event that can occur
will be absorbed by the grid without
any negative consequences-no overloads, low voltages, cascading outages, instability, loss of customer load,
or blackouts. it's sometimes referred to as
the "n-1" design. that was true in 1965,
and it's still true today. that november
blackout was caused by the sudden, almost simultaneous, loss of five system
elements, an event that far exceeded the
standards used to plan and operate interconnected power systems.
A question is sometimes raised
about the use of deterministic criteria
for transmission reliability rather than
probabilistic criteria, as has been the
approach used in resource adequacy
studies for many years. the reason is
that transmission reliability is several
orders of magnitude more complex than
resource adequacy. A simple explanation is that, in assessing transmission,
one faces the mathematical problem of
multiplying a number that approaches
infinity (the number of contingencies
that could occur) by a number that approaches zero (the probability of each

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possible contingency). there have been
many attempts to develop probabilistic
methodologies for transmission reliability, and research continues. this is
becoming increasingly important today
with the development of variable energy resources, such as wind power and
photovoltaics. nevertheless, historical
deterministic methods have served the
industry well.
On the night of the 1965 blackout,
it took only about 10 s for the system
to go unstable. Obviously, that's far too
rapidly for human operator intervention from even the most state-of-the-art
control center; a photograph of a typical
such control center is shown in figure 5.
Could an automated system be developed to detect an imminent outage and
take preemptive action? theoretically,
yes, and a lot of research work is being
done. the problem is more difficult
than it would appear, though. the main
challenge isn't the speed required; it's
the need to avoid initiating a mitigation scheme at times when it wouldn't
be required. the problem isn't only to
design a protective system that will operate when you want it to; it's preventing it
from operating when you don't want it to.
the results of this and other studies were documented in a three-volume
report, "Prevention of Power failures,"
published by the fPC in July 1967. not
only was it an in-depth and accurate

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2017