IEEE Power & Energy Magazine - July/August 2020 - 75

operators and planners to quantify and analyze a system's
stress and forestall cascading.
How could diagnosing and reducing stress have prevented previous cascading failures? The SW WI on 8 September 2011 was highly stressed, approaching the HS 2016
base case, which is extreme. It is not clear that the operators knew this. They might have taken precautionary actions
identified in the postmortem report. The metrics identified
the vulnerable and critical areas and branches. Knowing that
the Hassayampa-North Gila line was highly critical today,
operators could have postponed the complicated switching
and other sensitive operations at North Gila. Increasing generation or reducing demand in the Imperial Valley, as well
as other available options, would have reduced the vulnerability and criticality and could have prevented the blackout.
Nonetheless, we believe that the real solution will not
be to present the operators with 126 vulnerable branches or
183 critical ones (Table 7), which they will have to do something about in real time. These metrics are just very good
indicators of stress. We believe that the power system of the
future will not be made more robust by patches but by being
planned and operated in ways not currently recognized that
are inherently less stressful and without increasing its reliance on inherently failure-prone ancillaries.

Satisfying NERC Criteria
Today's NERC criteria have not proved sufficient for preventing all cascading events, but they are the law in the
United States, and similar criteria are applied elsewhere.
The following stress metrics can be used:
✔ as a screening tool for identifying certain contingencies that are worth examining for possible cascading
✔ to provide documentation for ignoring others
✔ to apply the NERC and other criteria more effectively
in other ways.

Summary and Conclusions
We have presented advances on the prevention of cascading blackouts, the oldest major unsolved technical problem
in power systems. We introduced a new cascading failure
network. Four metrics quantify stress, a necessary condition
for cascading to occur. The network and metrics are new
applications of well-known and accepted tools. We studied the WI of North America using utility data for different seasons and demands. The metrics are consistent with
intuitive assessments of stress for this system. The metrics,
applied to the pre-event state of a major cascading blackout,
showed that the system was highly stressed, especially in
the areas and circuit elements where cascading developed.
The metrics are practical measures of stress that can be rapidly computed, even for large systems. The proper use of
these metrics will identify a system's susceptibility to cascading, beyond pinpointing the most vulnerable and critical
branches and areas, and should help spot incipient cascading
before it occurs. The metrics provide a way to meet NERC
july/august 2020

planning study requirements more fully and point to ways to
make fully functioning and undamaged systems inherently
more immune to cascading blackouts.

Acknowledgments
We gratefully acknowledge the management and engineers
of the WECC for providing financial support, technical assistance, and sensitive data and for allowing the results to be
published. Their commitment to fighting blackouts is evident. We especially salute, with gratitude, the contributions
of Donald G. Davies, former WECC chief senior engineer.
The conclusions are our own, but the WI study would not
have been possible without the support of the WECC.

For Further Reading
P. Pourbeik, P. S. Kundur, and C. W. Taylor, "The anatomy
of a power grid blackout," IEEE Power Energy Mag., vol.
4, no. 5, pp. 22-29, Sept./Oct. 2006. doi: 10.1109/MPAE.
2006.1687814.
G. P. Loehr, "The 'good' blackout: The Northeast power
failure of 9 November 1965," IEEE Power Energy Mag., vol.
15, no. 3, pp. 84-96, May-June 2017. doi: 10.1109/MPE.
2017.2659379.
U.S.-Canada Power System Outage Task Force, "Final
report on the August 14, 2003 Blackout in the United States
and Canada: Causes and recommendations," U.S. Environmental Protection Agency, Washington, D.C., Apr. 2004.
[Online]. Available: https://www3.epa.gov/region1/npdes/
merrimackstation/pdfs/ar/AR-1165.pdf
J. Chen, J. D. Thorp, and I. Dobson, "Cascading dynamics
and mitigation assessment in power system disturbances via a
hidden failure model," Int. J. Elect. Power Energy Syst., vol. 27,
no. 4, pp. 318-326, 2005. doi: 10.1016/j.ijepes.2004.12.003.
N. Bhatt et al., "Assessing vulnerability to cascading outages," in Proc. IEEE/PES Power Systems Conf. and Expo.,
Mar. 2009, pp. 1-9. doi: 10.1109/PSCE.2009.4840032.
M. E. J. Newman, "The structure and function of complex networks," SIAM Rev., vol. 45, no. 2, pp. 167-256, 2003.
doi: 10.1137/S003614450342480.
H. M. Merrill and J. W. Feltes, "Cascading blackouts:
Stress, vulnerability, and criticality," Merrill Energy LLC,
Salt Lake City, UT, white paper, Sept. 2016. Accessed on:
May 6, 2020. [Online]. Available: http://www.merrillenergy
.com/Cascading%20Blackouts%20-%20Merrill%20&%20
Feltes.pdf
M. A. Hossain, H. M. Merrill, and M. Bodson, "Evaluation of metrics of susceptibility to cascading blackouts," in
Proc. Power & Energy Conf., 2017, pp. 321-325.

Biographies
Hyde M. Merrill is with the University of Utah, Salt Lake City.
Md Abid Hossain is with the University of Utah, Salt
Lake City.
Marc Bodson is with the University of Utah, Salt Lake City.
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https://www3.epa.gov/region1/npdes/merrimackstation/pdfs/ar/AR-1165.pdf https://www3.epa.gov/region1/npdes/merrimackstation/pdfs/ar/AR-1165.pdf http://www.merrillenergy.com/Cascading%20Blackouts%20-%20Merrill%20&%20Feltes.pdf http://www.merrillenergy.com/Cascading%20Blackouts%20-%20Merrill%20&%20Feltes.pdf http://www.merrillenergy.com/Cascading%20Blackouts%20-%20Merrill%20&%20Feltes.pdf

IEEE Power & Energy Magazine - July/August 2020

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