IEEE Power & Energy Magazine - July/August 2020 - 65

Cascading blackouts are the oldest major unsolved technical
problem in power system engineering, highly visible since
the Northeast Blackout of 1965 in North America.
ending with generators and customers being disconnected.
Before cascading starts, three factors are always present:
failures happen in the ancillary elements mentioned previously, the system is fully operational though under stress,
and the system is large. We do not treat the important, but
different, problem of blackouts caused by widespread physical damage from external causes.
Our new metrics come from network and power system
theory. They quantify stress, or the susceptibility of a system
to cascading failures. The metrics use a new network, based
on known engineering principles, to show how failures propagate through a system. In contrast, ordinary models measure how power flows through a system.
We describe research conducted by the University of Utah
and the Western Electricity Coordinating Council (WECC).
The use of real data on full-scale systems, including data
on a preblackout state, is an unusual and valuable feature
of our studies. The stress metrics were calculated for a very
large system [the Western Interconnection (WI) of North
America] and a large section of this system [Southwestern
WI (SW WI)] in a wide range of conditions. Even for a very
large system, the computations are easily performed. The
metrics agree with informal stress judgments by experienced
utility engineers. For the SW WI just before a major blackout, the stress measured by the metrics was very high. The
metrics flagged areas and facilities involved in the cascading
as being particularly vulnerable or critical in the preblackout
state. The computation of these metrics in real time and in
power system planning is a promising tool for forestalling
future cascading blackouts.

The Importance of Cascading Blackouts
A cascading blackout is an uncontrolled, unexpected chain
of cause-and-effect failures in hardware used for generating
and carrying current. The event interrupts bulk power service over a large area. Cascading blackouts matter because it
can take days to restore the system. Meanwhile, many functions of society are disrupted, with losses sometimes reaching billions of dollars.

Typical Cascading Blackouts
On 9 November 1965, a famous cascading blackout occurred
with relatively heavy flows on five essentially parallel
230-kV lines between Niagara Falls and Toronto, Canada.
Unbeknownst to the operators, backup relays set for other
purposes would disconnect or trip the lines well below their
capabilities. A relay tripped one line. Power flows on the
july/august 2020

other lines increased instantly to compensate. They tripped
too, separating New York from most of Ontario. The demand
and generation in each of the two separated systems were
suddenly out of balance. The systems could not absorb their
imbalances and blacked out.
On 14 August 2003, precursor events in Ohio and nearby
areas of the Eastern Interconnection of North America
included procedural failures in two control centers and computer failures as well as generation and transmission mishaps. Three 345-kV lines failed between 15:05 and 15:46
Eastern Daylight Time, the first at only 44% of its rating.
Although not overloaded, they were loaded more than usual
due to earlier events. One by one, the lines heated and sagged
into trees that were taller than they should have been, due
to poor vegetation management by the owner. Each contact
took a line out of service, increasing the heating and sagging
of the other lines.
A relay interpreted high current and low voltage as a
short circuit at 16:06 and tripped a fourth 345-kV line.
The system could not survive this and changed from "slow
cascading" to "fast cascading," creating electric islands
with unsustainable demand-supply imbalances and leaving millions without power. This blackout cost the United
States between US$4 billion and US$10 billion. Manufacturing shipments in Ontario were down CAD$2.3 billion
that month.
On 28 September 2003, at 3:00 a.m. Sunday (an hour, day,
and month of low demand), Italy was importing 6,400 MW,
300 MW more than what was contracted from France,
mostly via Switzerland, in part to fill its pumped-hydro
plants. A heavily loaded 380-kV Swiss line got hot, sagged
into a tree, flashed, and stayed open. Swiss dispatchers asked
Italian dispatchers to reduce their imports by 300 MW to
comply with the schedule. This solution did not work, as the
schedule was premised on all lines being in service. The
Swiss could have asked the Italians to stop pumping their
hydro storage plants but did not. A second line sagged on
to a tree and tripped at 3:25 a.m. Within seconds, all lines
into Italy tripped, isolating Italy with less online supply than
demand. In 3 min, all of mainland Italy blacked out. The
rest of Europe had more supply than demand, but this much
larger system recovered from an imbalance that was too
much for Italy. The official Union for the Coordination of
Transmission of Electricity report on the event noted that the
system was N−1 compliant before the first outage because
corrective actions were available and there was time to take
them should the outage occur.
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IEEE Power & Energy Magazine - July/August 2020

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2020

Contents
IEEE Power & Energy Magazine - July/August 2020 - Cover1
IEEE Power & Energy Magazine - July/August 2020 - Cover2
IEEE Power & Energy Magazine - July/August 2020 - Contents
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