IEEE Power & Energy Magazine - November/December 2021 - 31

and solar output across a wide region, and correlations
between weather events and load requirements.
* Modular technology: Wind, solar photovoltaic, and
battery storage are highly modular. Unlike a fossil
generator, which can lose hundreds or even thousands
of megawatts of capacity to a single failure, a
modular resource will likely lose only a small portion
during an outage. This modularity requires less
emphasis on discrete generator forced outages and
shifts the analysis to evaluate the likelihood of correlated
events and common mode failures.
* Climate change: Conventional RA analysis relied
solely on the historical observed weather data. A
changing climate is leading to changes in weather,
temperature, and extreme events. The use of historical
data to characterize load and renewable resources
may not be appropriate for gauging future risks
with climate change.
Two recent events underscore the importance of modernizing
our thinking on RA. The first occurred in California
in August 2020, when a heat storm event resulted in two
separate days of involuntary rolling outages across the
system. The second occurred in Texas in February 2021,
when extreme winter weather resulted in extreme electricity
demand while also causing natural gas fuel supply
shortages, low wind output, and widespread equipment
failures across all generation types. These events serve as
a reminder of the importance of reliable electricity supply
for public health and safety and highlight the importance
of updating RA methods for a changing resource mix and
extreme events.
Reviewing the August 2020 events, the former CEO of
California Independent System Operator (CAISO), summarized
the changing needs of RA analysis by stating:
There doesn't have to be a tradeoff between reliability and
decarbonization. What caused the [August blackouts]
was a lack of putting all the pieces together. You have to
rethink these old ways of doing things, and I think that's
what didn't happen. [...] The RA program in California is
now not matched up with the realities of working through
a renewable-based system, and in a nutshell, needs to
be redesigned.
The events that transpired in California were a planning
failure-not a failure or limitation of the grid's ability to transition
to a decarbonized energy mix. In that example, planning
entities were not properly evaluating all hours of the year and
failed to mitigate the changing periods of risk shifting to later
in the evening or the potential impacts of geographically widespread
heatwaves.
An Opportunity to Rethink Old Ways
The transforming resource mix is an opportunity to update
RA methods for a modern power system-one characterized
by increasing shares of variable renewables, energy storage,
and load flexibility.
november/december 2021
The transition also allows reconsidering the data on
which we rely for the analysis. While we can certainly learn
from past trends, history is not always the best guide for
future planning, especially when the landscape is changing
rapidly. Our energy systems now have a fundamentally
different resource mix, one with significantly less operational
experience. Periods of risk in the past may no longer
apply. Grid planning should consider that extreme events
are changing in severity and frequency. Planners need to
identify those extreme events for which we are unprepared,
quantify probabilities, and proactively identify and evaluate
mitigation strategies.
Redefining RA will require the industry to take a fresh
look at reliability planning. Planners and policy makers will
need to define what reliability means in their region and
determine how we measure risk, and what level of risk is
acceptable-without being confined by limitations in the
modeling tools and methods we have at our disposal today.
New RA concepts outlined in this article are not meant to be
prescriptive, but rather provide a flexible framework, principles,
and foundation that can be applied to unique situations
and adapted in a regional context.
Six Principles of RA for
Modern Power Systems
To overcome these challenges, let us go back to the first principles
while taking a fresh look at RA by asking few simple questions.
If we started from scratch, without 100 years of power
system planning and conventional approaches, how would we
evaluate RA for modern power systems? How should risk and
reliability be evaluated in a power system with large shares of
wind, solar, storage, and load flexibility? The following six
principles of RA may provide answers to the above questions
for modern power systems.
Principle 1: Quantifying Size, Frequency, and
Duration of Capacity Shortfalls is Critical to
Finding the Right Resource Solutions
As the power system's resource mix changes, RA metrics
need to transform as well. The conventional RA metric, loss
of load expectation (LOLE), quantifies the expected number
of events when capacity is insufficient to meet the load. A
common reliability criterion is one day of outage in 10 years
or often simplified to 0.1 days per year LOLE.
But LOLE is an opaque metric when used in isolation. It
only provides a measure of the average number of shortfalls
over a study period and does not characterize the magnitude
or duration of specific outage events. It does a poor job at differentiating
shortfalls, which can vary considerably and have
an unequal impact on consumers and require different mitigation
options. For example, a shortfall of 1% of load for 10 h is
measured the same way as a shortfall of 10% of load for 10 h.
In addition, there is very little consistency, as different planners
in different regions interpret the criteria differently and each
region has different institutional and regulatory requirements.
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IEEE Power & Energy Magazine - November/December 2021

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