IEEE Electrification - December 2021 - 9

Renewable generation-predominantly wind and solar
photovoltaic (PV) resources in North America-is inherently
variable and uncertain, as the prime mover (e.g.,
wind speed and solar irradiance) is not constant or guaranteed.
These types of resources have significantly
grown in terms of their share of the generation mix
across North America. As the percentage of renewables
increases, the variability of the overall BPS rises. One
widely known result of high renewable penetration is the
infamous California Independent System Operator
(CAISO) " duck curve " (see Figure 1). Solar PV output
grows in the morning when the load is ramping up and
drops off in the afternoon when the load is still high; this
results in severe net load ramps (the net load is the difference
between the system load and the sum of wind
and solar generation) that have to be managed by other
flexible generating resources. BESSs are poised to be a
solution to support balancing variability and provide
flexibility to the BPS. In the " duck curve " example, BESSs
can decrease the need for ramping from traditional rotating
generation. This lessens wear on traditional generation
assets and the risk of import or export violations
between regions.
There is a long history of BESSs in the United States;
however, the industry has experienced a wave of BESS
projects seeking interconnection to the BPS during the
past five years. As expected, with the versatile nature of
BESSs, these projects are being utilized for many applications.
Let us review the following primary applications for
BPS-connected BESSs.
x Provision of frequency support: This involves the
charging and discharging of active power to regulate
the grid frequency, providing fast frequency
response (FFR), primary frequency response (PFR),
and secondary frequency response; it is also
called regulation.
■ The FFR is a subset of frequency support in which
a BESS must rapidly respond, for example, in
~250 ms, to frequency deviations.
x Provision of voltage and reactive power support: This
relates to injecting or absorbing reactive power to
regulate the grid voltage. Projects with reactive
power as a main application are sized much differently,
as they must consider reactive power capability
first.
x Energy arbitrage: This is the process of discharging
active power during peak consumption and recharging
during times of abundant generation.
x Participation in capacity market: This application
offers BESSs to a capacity market. Projects vary based
on market specifics, but a general " rule of thumb " is
4-h BESSs.
x Firming renewable output: This concerns discharging
active power when the output from solar is low and
charging when solar output is abundant. This is typically
done when there is cloud cover.
x Providing or enabling black-start capability: This
involves using a BESS as a black-start unit in small
island and microgrid applications as well as to start
larger synchronous generators.
While each of these applications has unique behavior,
it is common for BESS projects to include multiple
functions. Applications may dictate how transmission
planners model and study BESS projects. Alterations
after construction may have negative impacts on battery
chemistry and lifecycles and trigger a restudy, so
careful consideration of a resource's uses should occur
prior to selecting a technology. Now, let us say that a
developer or owner needs to come back and change
something from the initial selection. Augmentation is
the ability to combine BESS technologies. BESS plant
age and expansion drives the need for it. Battery technology
has changed and will continue to, and the same
is true for inverter technology. DC-dc converters facilitate
connecting battery systems of differing dc
IEEE Electrification Magazine / DECEMBER 2021
9
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IEEE Electrification - December 2021

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