IEEE Electrification - December 2021 - 17

While these are normally available
from wind and solar only when they
are generating power into the grid,
BESSs can deliver them in charging
and discharging modes and event at
zero active power output. However,
the specifics of BESSs produce additional
considerations.
Systems dominated by synchronous
machines exhibit an initial inertial
response followed by a slower
turbine governor response or PFR.
While BESSs do not have physical
inertia like synchronous machines
do, their controls can be tuned to provide
suitably fast energy injection-
FFR-such that the initial rate of change of frequency and
frequency nadir in low-inertia systems improve. This
aspect of battery storage helps improve system reliability.
Note that voltages should be closely monitored, as highspeed
active power responses can cause high-speed voltage
fluctuations, especially in weak grid conditions.
Another function of a traditional synchronous generator
is the ability to dampen power system oscillations.
Many synchronous generators are equipped with power
system stabilizers (PSSs) that dampen system oscillations,
typically in the range of 0.2-2 Hz. As these resources
become increasingly scarce, there is a growing need for
oscillation damping support in certain parts of the BPS.
For example, in the Texas Panhandle, which is dominated
by renewable generation, synchronous generators may be
off-line during high renewable output, which could lead to
insufficient damping. Currently, for ERCOT, renewable
generation resources and BESSs are not required to provide
damping support, and synchronous condensers used
to increase system strength in the Panhandle are not
equipped with PSSs. A 2019 ERCOT study identified oscillatory
behavior of around 1.8 Hz between synchronous
condensers in the region and synchronous generators in
other areas. System planners may identify a need for this
type of control as the penetration of inverter-based
resources continues to increase and define requirements
for inverter-based resources to provide power oscillation
damping. BESSs are well suited to this application.
Care should be taken regarding the SOC since it affects
the ability of a BESS to provide energy and essential reliability
services to the grid at any given time. In many
cases, a BESS may have SOC limits that are tighter than
0-100% for the battery life span and other equipment and
performance considerations. These restrictions should be
taken into account while evaluating the ability of a BESS
to provide energy and stability functions.
Grid Forming and Black Starts
BESSs have unique abilities to effectively deploy GFM
technology that helps improve BPS reliability in high
While BESSs do not
have physical inertia
like synchronous
machines do, their
controls can be
tuned to provide
suitably fast energy
injection.
penetrations of inverter-based
resources. Key aspects that enable
this include the availability of an
energy buffer against generation
and load imbalances, low communication
latency between different
controller layers, and robust dc voltage
that enables the synthesis of an
ac voltage for a wide variety of system
conditions. In grids where system
strength and other stability
issues are of concern, BESSs may be
required to support reliable BPS
operation. System planners should
develop interconnection requirements
and practices as needed to
integrate GFM technology into the planning process,
especially in areas where stability is a worry.
GFM BESSs may have the ability to form and sustain
their own electrical island if they are designed and designated
as part of a black-start cranking path. This may
require new control topologies and modifications to inverter
settings. Black-start conditions may cause large power
and voltage swings that must be reliably controlled and
withstood. For a BESS to operate as a black-start resource,
energy availability assurance is required. This includes a
specific period for restoration activities and a requirement
that energy be available 24/7. At this time, it is unlikely that
most legacy GFL BESSs can support system restoration
activities as stand-alone resources; however, they may be
used to start subsequent solar, wind, and synchronous
machine plants. GFM capability from BESSs is low-hanging
fruit compared to other inverter-based resources and
should be considered by system planners and developers
to better position themselves for the future.
Resource Adequacy
With BESSs having typical durations of around 4 h, their
role in supporting long-term energy adequacy and assurance
is limited. That said, BESSs with 4 h of duration and
longer are participating in capacity markets and remunerated
for their contribution to resource adequacy. Storage
for resource adequacy is an active area of research. Findings
from the National Renewable Energy Laboratory, Electric
Power Research Institute, and LBNL show that BESS
contributions to resource adequacy is affected by energy
capacity (megawatt-hours) and intended uses. For example,
BESSs intended solely for frequency response may not
be available to provide energy during peak load conditions.
This is, for example, the case for most BESSs in ERCOT.
The contribution of storage to resource adequacy is
affected by the width of the net load peak and is system
specific. As more storage is added to a system, the width
of the net load peak increases, and contributions from
BESSs with durations shorter than that width will be
reduced, as they need to provide energy through a
IEEE Electrification Magazine / DECEMBER 2021
17
IEEE Electrification - December 2021

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