IEEE Power & Energy Magazine - July/August 2020 - 67
Multiple Contingency Studies
Long-standing, incorrect assumptions can hinder the understanding of cascading blackouts. For instance, several studies on cascading have assumed that power systems are generally operated to be N−1 secure; therefore, most cascading
has been triggered by multiple independent outages. Thus it
is thought necessary to consider daunting numbers of simultaneous combinations of two outages.
In fact, multiple independent NERC-criterion outages
have directly caused few, if any, cascading blackouts. In the
8 September 2011 blackout (described later in this article),
the system was clearly not N−1 compliant. The systems were
N−1 compliant before both 2003 blackouts, and the operators apparently thought their system was compliant in 1965.
But in each of these three events, a single outage, abetted or
triggered by failures in ancillary elements, triggered one or
more secondary outages and cascaded to the blackouts.
A multiorganization committee chaired by N. Bhatt studied multiple contingency chains (two simultaneous independent outages causing overloads and further outages).
Their summer peak model of the Eastern Interconnection
of North America consisted of approximately 50,000 buses
and 65,000 branches. A list of roughly 250 single contingencies was considered. More than 31,000 double-contingency
pairs were created from this list and simulated. Only 953 of
these double-contingency events led to overloads. For most of
the 953 overloads, there were no further overloads once the
overloaded branches were tripped. Only 38 of the 953 led
to cascading. This approach, its statistical analysis, and its
conclusions parallel and differ from ours in important ways.
The NERC Criteria May Not Be Sufficient
to Prevent Cascading Events
After nearly 40 years of cascading failures, p. 23 of the
postmortem report, produced ad hoc under the direction
of the U.S. Secretary of Energy and the Minister of Natural Resources Canada, on the 14 August 2003 U.S./Canada
blackout reached a momentous conclusion. At 15:05, before
the slow cascading of the four 345-kV lines that ended in
fast cascading, "FirstEnergy's ... system [at the heart of the
cascading] was electrically secure and was able to withstand
the occurrence of any one of more than 800 contingencies ...
[T]he system was electrically ... in compliance with NERC's
operating policies. [But] there was clear evidence that the ...
area was highly vulnerable ..." In fact, the system did not
withstand the 15:05 contingency (at 44% of the line's rating)
but cascaded slowly, as line by line the increased loading,
line temperatures, and sagging into trees proceeded.
There is an important fine point here. In 2003, Ohio's
FirstEnergy was in compliance with NERC's criteria but was
still vulnerable. How can this be? The postmortem report
explains: satisfying the criteria means that "while it was possible to operate the system securely," there was no guarantee
that this would happen. In particular, failures in control and
protection devices, or in practices and procedures, can cause
july/august 2020
cascading whether the system is NERC compliant or not (see
Table 1). The 2003 Italian blackout described previously is a
supporting example. In both blackouts, there were sufficient
such failures.
System Model and Necessary
Conditions for Cascading
Our recounting of cascading events suggests a new conceptual model of the bulk power system, which includes three
intertwined elements: current producing and transmitting
equipment, control and protection devices, and practices
and procedures. These elements, like Borromean rings (Figure 1), form an inseparable system. But if any of the three
rings breaks and is removed, the other two fall apart. This
also holds for the power system. Most of the emphasis has
been placed on studying current-producing and transmitting
equipment. Nonetheless, history shows that failures in the
two ancillary elements and system stress are necessary contributors to cascading blackouts. The three elements interact
table 1. The three conditions needed for cascading
blackouts to occur.
1
Failure in
Control and
protection
equipment
or
Practices and
procedures
2
When the system is stressed
3
The system is large
figure 1. Borromean rings with soap film. (Source: Ken
Brakke, Susquehanna University; used with permission.)
ieee power & energy magazine
67
IEEE Power & Energy Magazine - July/August 2020
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