IEEE Power & Energy Magazine - July/August 2020 - 73

a RankV $ 1, double the SW WI average. Six IID branches,
including the Coachella transformers, were vulnerable for
two or more single outages. The cascading began in Arizona
(area 1), where many critical SW WI branches were found,
including the Hassayampa-North Gila line. (Arizona and the
IID were named in the postmortem report. The other areas
may not be named.)
Area 7, also highlighted, was highly vulnerable and was
the most critical in SW WI. An area being highly stressed
does not mean that cascading will start there or will even
occur. The cascading did not start in area 7. Stress is a necessary, but not sufficient, condition for cascading to occur-
the necessary procedures failure occurred in areas 1 and 2
(see Table 1).

threshold easily can be found empirically when analyzing a system's metrics.
Planners and operators could identify the high-vulnerability branches and look for ways to reduce vulnerability, as
higher stress means a higher risk of cascading. They could
also study the highly critical branches: some outages could
cause especially high postoutage flows. A few of these outages are serious for a different reason, as they may affect
many other branches. Other useful ways to look at the metrics are illustrated in the next section.

Preblackout Stress Metrics: 8 September 2011
The cascading blackout of 8 September 2011 began with a
switching error in the North Gila substation. A technician
carrying out a complex series of operations was distracted
momentarily and incorrectly marked his checklist. His outof-sequence next operation tripped the 500-kV HassayampaNorth Gila line, which had tripped frequently and recently without cascading. But on 8 September, the SW WI was
highly stressed (Table 6). This time, the 500-kV outage caused
both Coachella 230-kV/92-kV transformers of the Imperial
Irrigation District (IID) to overload and trip. The single-outage event became a three-outage event, and further cascading blacked out much of the SW WI.
The WECC created a reconstruction of the system state
before the blackout. We computed the metrics for this preblackout state. See Table 6 and for more detail Table 7, where
each row corresponds to an area within the SW WI. The SW
WI precascading stress (the 8 September 2011 row in Table 6
and the last row in Table 7) was high compared to the 2016
stresses and second only to the HS 2016 case. In Table 7,
the columns have exactly the same meaning as explained for
Table 6. The rows (areas) where the stress was highest are
highlighted: 7.5% of the IID branches (Table 7, area 2) had

Very Large System Study
The 2016 operations planning model of all of the WI had
roughly 26,400 branches. In a further study, essentially all
of the nonradial branches in the 21 areas making up the WI
were outaged one by one and monitored for each outage. (In
contrast, normal planning and operating studies are done for
smaller subsystems.) Stressed areas, especially, were identified, and the analysis led to remarkable observations specific to very large systems.
Regarding vulnerability, three large areas had almost
half of all the WI's branches with postoutage flows that
exceeded the branch ratings (a RankV> 100% of ratings).
The areas represented a large cluster of vulnerability,
although their percentage of vulnerable branches was typical of the entire WI.
Regarding criticality, two of the three aforementioned
areas had relatively typical criticality metrics. The third had
455 branch outages with a RankC greater than the monitored
line ratings. Almost one-third of the critical branches in the

table 7. The precascading metrics in seven SW WI areas (upper rows) and their totals (bottom row) for
8 September 2011. The number of stressed branches and the percent of all the branches.
Vulnerability

Criticality

SW WI Areas

RankV ≥ 1
(DegreeV ≥ 1)

DegreeV ≥ 2

RankC ≥ 1
(DegreeC ≥ 1)

DegreeC ≥ 2

Branches Per Area

1

39

2.5%

25 1.6%

73 4.7%

7

0.5%

1,542

2

10

7.5%

6

4.5%

7

5.2%

1

0.7%

134

3

0

0%

0

0%

1

0.4%

1

0.4%

272

4

4

1.6%

2

0.8%

4

1.6%

0

0%

255

5

6

1.2%

2

0.4%

14 2.8%

0

0%

506

6

6

1.5%

1

0.3%

11 2.8%

4

1%

397

7

62

5.9%

28 2.6%

75 7.1%

15 1.4%

1,057

Total

126 3.1%

63 1.5%

183 4.5%

27 0.7%

4,096

Thresholds = 100% of ratings for degree metrics.
A few branches are in two or more areas. They are counted in each area but just once in the bottom-row totals.
The highest-stress metrics (by number and %) are highlighted in green.
july/august 2020

ieee power & energy magazine

73



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
IEEE Power & Energy Magazine - July/August 2020 - 2
IEEE Power & Energy Magazine - July/August 2020 - 3
IEEE Power & Energy Magazine - July/August 2020 - 4
IEEE Power & Energy Magazine - July/August 2020 - 5
IEEE Power & Energy Magazine - July/August 2020 - 6
IEEE Power & Energy Magazine - July/August 2020 - 7
IEEE Power & Energy Magazine - July/August 2020 - 8
IEEE Power & Energy Magazine - July/August 2020 - 9
IEEE Power & Energy Magazine - July/August 2020 - 10
IEEE Power & Energy Magazine - July/August 2020 - 11
IEEE Power & Energy Magazine - July/August 2020 - 12
IEEE Power & Energy Magazine - July/August 2020 - 13
IEEE Power & Energy Magazine - July/August 2020 - 14
IEEE Power & Energy Magazine - July/August 2020 - 15
IEEE Power & Energy Magazine - July/August 2020 - 16
IEEE Power & Energy Magazine - July/August 2020 - 17
IEEE Power & Energy Magazine - July/August 2020 - 18
IEEE Power & Energy Magazine - July/August 2020 - 19
IEEE Power & Energy Magazine - July/August 2020 - 20
IEEE Power & Energy Magazine - July/August 2020 - 21
IEEE Power & Energy Magazine - July/August 2020 - 22
IEEE Power & Energy Magazine - July/August 2020 - 23
IEEE Power & Energy Magazine - July/August 2020 - 24
IEEE Power & Energy Magazine - July/August 2020 - 25
IEEE Power & Energy Magazine - July/August 2020 - 26
IEEE Power & Energy Magazine - July/August 2020 - 27
IEEE Power & Energy Magazine - July/August 2020 - 28
IEEE Power & Energy Magazine - July/August 2020 - 29
IEEE Power & Energy Magazine - July/August 2020 - 30
IEEE Power & Energy Magazine - July/August 2020 - 31
IEEE Power & Energy Magazine - July/August 2020 - 32
IEEE Power & Energy Magazine - July/August 2020 - 33
IEEE Power & Energy Magazine - July/August 2020 - 34
IEEE Power & Energy Magazine - July/August 2020 - 35
IEEE Power & Energy Magazine - July/August 2020 - 36
IEEE Power & Energy Magazine - July/August 2020 - 37
IEEE Power & Energy Magazine - July/August 2020 - 38
IEEE Power & Energy Magazine - July/August 2020 - 39
IEEE Power & Energy Magazine - July/August 2020 - 40
IEEE Power & Energy Magazine - July/August 2020 - 41
IEEE Power & Energy Magazine - July/August 2020 - 42
IEEE Power & Energy Magazine - July/August 2020 - 43
IEEE Power & Energy Magazine - July/August 2020 - 44
IEEE Power & Energy Magazine - July/August 2020 - 45
IEEE Power & Energy Magazine - July/August 2020 - 46
IEEE Power & Energy Magazine - July/August 2020 - 47
IEEE Power & Energy Magazine - July/August 2020 - 48
IEEE Power & Energy Magazine - July/August 2020 - 49
IEEE Power & Energy Magazine - July/August 2020 - 50
IEEE Power & Energy Magazine - July/August 2020 - 51
IEEE Power & Energy Magazine - July/August 2020 - 52
IEEE Power & Energy Magazine - July/August 2020 - 53
IEEE Power & Energy Magazine - July/August 2020 - 54
IEEE Power & Energy Magazine - July/August 2020 - 55
IEEE Power & Energy Magazine - July/August 2020 - 56
IEEE Power & Energy Magazine - July/August 2020 - 57
IEEE Power & Energy Magazine - July/August 2020 - 58
IEEE Power & Energy Magazine - July/August 2020 - 59
IEEE Power & Energy Magazine - July/August 2020 - 60
IEEE Power & Energy Magazine - July/August 2020 - 61
IEEE Power & Energy Magazine - July/August 2020 - 62
IEEE Power & Energy Magazine - July/August 2020 - 63
IEEE Power & Energy Magazine - July/August 2020 - 64
IEEE Power & Energy Magazine - July/August 2020 - 65
IEEE Power & Energy Magazine - July/August 2020 - 66
IEEE Power & Energy Magazine - July/August 2020 - 67
IEEE Power & Energy Magazine - July/August 2020 - 68
IEEE Power & Energy Magazine - July/August 2020 - 69
IEEE Power & Energy Magazine - July/August 2020 - 70
IEEE Power & Energy Magazine - July/August 2020 - 71
IEEE Power & Energy Magazine - July/August 2020 - 72
IEEE Power & Energy Magazine - July/August 2020 - 73
IEEE Power & Energy Magazine - July/August 2020 - 74
IEEE Power & Energy Magazine - July/August 2020 - 75
IEEE Power & Energy Magazine - July/August 2020 - 76
IEEE Power & Energy Magazine - July/August 2020 - 77
IEEE Power & Energy Magazine - July/August 2020 - 78
IEEE Power & Energy Magazine - July/August 2020 - 79
IEEE Power & Energy Magazine - July/August 2020 - 80
IEEE Power & Energy Magazine - July/August 2020 - 81
IEEE Power & Energy Magazine - July/August 2020 - 82
IEEE Power & Energy Magazine - July/August 2020 - 83
IEEE Power & Energy Magazine - July/August 2020 - 84
IEEE Power & Energy Magazine - July/August 2020 - 85
IEEE Power & Energy Magazine - July/August 2020 - 86
IEEE Power & Energy Magazine - July/August 2020 - 87
IEEE Power & Energy Magazine - July/August 2020 - 88
IEEE Power & Energy Magazine - July/August 2020 - 89
IEEE Power & Energy Magazine - July/August 2020 - 90
IEEE Power & Energy Magazine - July/August 2020 - 91
IEEE Power & Energy Magazine - July/August 2020 - 92
IEEE Power & Energy Magazine - July/August 2020 - Cover3
IEEE Power & Energy Magazine - July/August 2020 - Cover4
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091020
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070820
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050620
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030420
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010220
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111219
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091019
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070819
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050619
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030419
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010219
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111218
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091018
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070818
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050618
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030418
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010218
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111217
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091017
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070817
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050617
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030417
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010217
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111216
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091016
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070816
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050616
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030416
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010216
https://www.nxtbook.com/nxtbooks/ieee/powerenergy_010216
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111215
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091015
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070815
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050615
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030415
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010215
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111214
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091014
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070814
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050614
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030414
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010214
https://www.nxtbookmedia.com