IEEE Power & Energy Magazine - July/August 2020 - 25

The analysis identified the need for seven new spare transformers located strategically at six substations and a congestion risk exposure of US$74 million annually that will be
mitigated by the deployment of the backup transformers. The
PRA also revealed that spares would increase the acceptable
risk limit for transformer units in operation, extending their
service lives. As of 2018, our system remains adequately mitigated with the current spare transformer unit population. System planners and transmission owners gain invaluable insight
from this process. Knowing and understanding risk helps
to proactively and economically address aging transformer
infrastructure. An additional analysis has enabled stakeholders to plan proactive transformer replacements, spare transformer purchases, and the optimal location of spares.

Cascading-Event Analysis Tool
At its most fundamental level, a cascading tree evaluates an
extreme event that encompasses a risk that may, after some
number of additional cascading events, lead to a system collapse (i.e., a blackout). Major blackouts are usually caused by
low-probability, high-consequence events. Since the attacks
of 9/11, the power industry has taken a closer look at system
contingencies that are driven not only by naturally occurring
events but human-made threats as well, including
✔ cyberattacks
✔ physical attacks
✔ electromagnetic pulses
✔ the loss of interdependent systems
✔ severe terrestrial weather
✔ earthquakes
✔ geomagnetic disturbances.

Lost Substation (N-k)

Any such initial precipitating event could cause one or more
transmission line overloads (on common rights-of-way), transformer overloads, substation losses, generator undervoltages,
or load undervoltage conditions, among others.
The high-voltage transmission system is planned to be
capable of withstanding a significant outage of one or a few
critical pieces of equipment. However, these planning criteria
do not assess what would happen to the system should a significant disruption of many pieces of equipment occur at once
or in quick succession, as might be triggered by an extreme
weather event or deliberate attack. Developed with a transmission owner, Dominion Virginia Power (DVP), cascading trees
(Figure 4) assess the probability and consequences of cascading outages in electric systems. A cascading tree is the set of
all likely cascading paths; these, in turn, describe a sequence of
potential cascading outages that could reasonably be expected.
These possible outages are then classified as bounded or
blown up, that is, whether the propagation of a disturbance
can be confined to a certain area or if the exact extent of
the cascading cannot be determined. The initial N-k event
equates to the complete loss of a substation and transmission
facilities. Cascading trees quantify the probability of cascading and the extent of associated consequences, leading
to a natural ranking of substations. Substations and transmission facilities can then be grouped into different tiers,
each having a different priority and discrete set of mitigation actions. DVP has used this methodology to identify and
rank critical substations. Ideally, the best way to protect a
critical substation is not to have one. However, once these
critical substations are identified and prioritized, transmission upgrades are designed and integrated into the system to

Pseudostable

Cascading

Bounded

Collapse

No Cascading

Collapse
Stable

More

High Voltage

Lost Substation (N-k)

Low Voltage

Pseudostable

Cascading

Bounded

Collapse

No Cascading

Collapse

Less

Stable

figure 4. Example images from the cascading-tree assessment.
july/august 2020

ieee power & energy magazine

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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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IEEE Power & Energy Magazine - July/August 2020 - Cover3
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