The Frequency of Large Blackouts in the United States
Paul Hines, Carnegie Mellon University, Department of
Engineering and Public Policy
15-March-2006
Motivation
- Industry,
Government, and Academia have invested heavily to improve grid reliability
in the United States.
- Has this effort
significantly affected reliability of the bulk electricity system in the United States?
Research hypothesis
- Controlling for
demand growth, the frequency of large blackouts in the United States has decreased between 1984 and the present.
Data source
- NERC Disturbance
Analysis Working Group (DAWG) event records for 1984—2002.
- 2003 NERC DAWG data
available from a committee report on the NERC web site.
- Supplementary data
for 2004, 2005 from the DoE Energy Information Agency (EIA) form 417 fillings.
- 879 events total.
Raw data
Data filtering procedure
- Removed extreme
events from the database to isolate events that were potentially
preventable, particularly cascading failures. Specifically:
- hurricanes
- earthquakes
- ice storms
- tornados
- In order to control
for population/demand growth, we normalize each event size (MW or
customers) by yearly nation-wide net generation or yearly population.
Thus, all blackout sizes are scaled to year-2000 MW, or year-2000
customers.
- Eliminated events
smaller than 400 MW or 100,000 customers, since only events larger than
400 MW must be reported to the Dept. of Energy.
- Removed data for
years 1998, 2004, 2005 for statistical accuracy. The NERC DAWG data shows
no events between 3/31/1998 and 12/08/1998, indicating a data
collection/reporting error. The years 2004, and 2005 come from the EIA,
not NERC, and are potentially biased.
- 167 events with
size ≥ 400 year-2000 MW.
- 137 events with
size ≥ 100,000 year-2000 customers.
Filtered results
Statistical tests
- Used two
statistical tests to evaluate our research hypothesis.
- Correlation test:
- evaluate the
hypothesis that there is a significant positive relationship between
blackout frequency and time;
- Kruskall-Wallis
(K-W) T-test
- tests the
hypothesis that the median blackout frequency has changed over time.
- We divide the data
at 1998 for this test. 1998 is 2 years after the 1996 blackouts,
allowing for changes to take effect. In 1998 both the CAISO and ISONE
are open, indicative of significant industry changes.
Results for events with size ≥ 100,000 year-2000
customers
Statistic
|
Value
|
Mean frequency (
size ≥ 100k Y2k customers ) for years 1984-1997
|
5.50
|
Mean frequency (
size ≥ 100k Y2k customers ) for years 1998-2005
|
15.50
|
Mean frequency (
size ≥ 100k Y2k customers ) for years 1999-2003
|
12.00
|
Significance
(P-Value) for the K-W t-test testing the difference between the years
(84-97) and (99-03).
|
0.009
|
Correlation
between year and frequency
|
0.33 (P=0.16)
|
The K-W test shows a
statistically significant increase in the frequency of large blackouts for the
most recent years. The correlation test shows a weak (not statistically
significant) correlation between years and frequency. Certainly the frequency
is not decreasing.
Results for events with size ≥ 400 year-2000 MW
Statistic
|
Value
|
Mean frequency (
size ≥ 400 Y2k MW ) for years 1984-1997
|
8.29
|
Mean frequency (
size ≥ 400 Y2k MW ) for years 1998-2005
|
11.00
|
Mean frequency (
size ≥ 400 Y2k MW ) for years 1999-2003
|
10.20
|
Significance
(P-Value) for the K-W t-test testing the difference between the years
(84-97) and (99-03).
|
0.4563
|
Correlation
between year and frequency
|
0.12 (P=0.62)
|
No statistically
significant increase or decrease is observable.
Potential explanations for the lack of improvement
- An increase in
stress on the transmission system due to industry restructuring
- Insufficient
investment in transmission infrastructure
- Inherent complexity
of the transmission system
- A lack
of coordinated, system-wide solutions to the blackout problem
- Failure
of the protection system to control cascading failures
Conclusions
- The frequency of
large blackouts in the United States is not decreasing, and may in fact be
increasing.
- More must be done
if we are to actually increase the reliability of the bulk electricity
system.
Acknowledgements
- The author wishes
to thank Prof. Jay Apt and Prof. Sarosh Talkudar for important
contributions to this work.
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