IEEE Electrification - December 2021 - 12

these evaluations provide assurance that long-duration
systems have proper fuse coordination and the ability to
withstand design faults. Once the preceding are obtained,
a developer can confidently begin building an online sample
of how the controllers will interface (Figure 5).
The following discussion of inverter-based resources
applies to BESSs, as well. Inverter-based resources are
available as grid-following (GFL) and grid-forming (GFM)
devices. GFL inverters are controlled as current sources,
and GFM variants are managed as voltage sources. Historically,
inverters have been introduced to the BPS in GFL
mode. They operate as a current source with synchronization
to the grid, enabling them to achieve requested set
points faster, as they directly control the current. GFL
inverters that provide grid services, such as frequency
support, voltage support, and solar plant ramp rate control,
are sometimes called grid supporting.
GFM inverters have risen from microgrid applications
as well as the black-starting of critical grid infrastructure,
and they are currently being considered for providing inertia,
improving the voltage stability of weak grids, and
black-start applications. In GFM mode, inverters can control
active and reactive power by directly adjusting the
output voltage, similar to synchronous generators. In the
future, the industry will look to have GFM inverter capability
lead the transition to an inverter-based grid.
Hybrid Plant Design Considerations
One of the most common forms of hybrid plants in the
BPS is a solar and BESS facility. The surge of solar-plusstorage
stems from BESSs being eligible for investment
tax credits if >75% of their charging energy comes from
solar during a recapture period of ~5 years. Hybrid plants
can be built using an ac-coupled or a dc-coupled configuration.
Each of these can make a portion of the solar energy
dispatchable.
AC-Coupled Systems
In this configuration, BESSs and solar are connected on
the low-voltage side of the generator step-up transformer.
BESSs are typically located close to substations to reduce
line losses. They are integrated in nearly the same way as
stand-alone projects and can function independently if
needed. Augmenting new batteries is simpler and the preferred
option if a solar project is being retrofitted with
BESSs. Another advantage of this design is the metering
convenience for the separate market participation of the
two resources. However, the major disadvantage is the
loss of energy on the dc side of the solar inverters, due to
the dc overbuild of a solar array. An example of this configuration
is in Figure 6.
DC-Coupled Systems
This configuration utilizes dc-dc converters to couple battery
containers to solar arrays on the inverters' dc bus.
BESSs have access to the dc-clipped energy that can be
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IEEE Electrification Magazine / DECEMBER 2021
captured in the batteries, which otherwise would be lost.
Close coupling (as shown in Figure 7) of solar and batteries
results in higher efficiency and increased generation compared
to ac-coupled systems. Augmentation and metering,
however, are more challenging, and developers need
to plan for both.
Navigating the Interconnection Process
For a new developer, and sometimes even an experienced
one, navigating the interconnection process is like stepping
into a maze. You know where the exit is, but uncertainties
and variations along the way may lead to a dead
end. Every year, utilities receive hundreds of new interconnection
requests. Most are for renewables, BESSs, and
hybrid generating facilities, and only a small number successfully
complete the process and get developed. Interconnection
is initiated by developers. They choose the
location, technology and size of generating facilities.
Transmission planners respond to requests by performing
planning studies. Developers modify their requests (and
even cancel projects) based on the findings.
Initiating an Interconnection Request
Some planning authorities provide information to guide
developers toward areas where there is abundant transmission
capacity and help them avoid locations where
expensive upgrades are needed. However, transmission
capacity does not always align with the abundance of
renewable resources. Developers often find themselves in
wind- and solar-rich areas that have limited transmission.
Alth ough adding BESSs could improve the situation,
expensive upgrades are often necessary even for hybrid
interconnection requests. Decisions about who utilizes
limited capacity and gets to the finish line are outcomes
of joint efforts between developers and planning authorities.
Some utilities have proactive planning processes that
allocate available capacity to the best standing request.
Others rely on passive procedures that wait for inviable
requests to be withdrawn.
Stand-alone storage in high load and congestion areas
can provide additional benefits to the grid through various
services, such as peak load reduction, voltage support, grid
strengthening, and frequency response. Unlike frequency
regulation, these do not result in battery charging during
peak loads. It should also be noted that in high-density
areas, transmission and distribution upgrades may not be
feasible, and appropriate storage use may be the most
cost-effective solution for rate payers. A collaborative
approach between developers and transmission planning
authorities to determine use cases is needed to ensure
that infeasible and unnecessarily expensive upgrades are
not imposed.
Interconnection Study Process
Most utilities in the United States follow Federal Energy
Regulatory Commission (FERC) pro forma large-generation
IEEE Electrification - December 2021

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