IEEE Electrification - September 2022 - 52

1) a grid-forming inverter technology that does not
require synchronous generation to operate but that is
currently being developed and in commercial infancy
for kilowatt-scale, distribution-connected inverters
2) the addition of synchronous condensers, a mature
technology commercially available today
3) operational changes that commit additional synchronous
generators to maintain a minimum inertia level,
which would lead to solar curtailment.
Each of these mitigations comes at cost, and preference is
given to grid-forming inverter controls because they
would not require significant capital expenditures like
synchronous condensers, or increased fuel consumption
and curtailment like operational changes. However, a sensitivity
was evaluated to simulate the effects of grid stability
constraints. This provides a clear example that
reliability can be maintained even at very high-DER integration
if grid-forming technologies are not made available
and synchronous condensers are not installed. This is
especially useful for showing the effects of grid stability
constraints on near-term DER integration.
High penetrations of IBRs challenge grid stability
because they typically imply that there are relatively few
conventional synchronous-machine-based resources
online, which provide important stabilizing benefits to the
grid. This is often referred to as weak-grid or low-short circuit
ratio (SCR) applications. The primary stabilizing characteristic
of synchronous machine technologies is that
they provide short-term (fractions of a second) storage of
energy with a very high capability to release the energy
(maximum currents that are multiples of their rated currents).
The short-term energy reservoir in synchronous
machines comes in two forms: the rotational energy of
the spinning rotor and drivetrain and the magnetic field
energy in the steel core of the generator. The rotational
energy, typically described as inertia, acts to stabilize grid
frequency during sudden changes in the power balance
on the grid, like for loss-of-generation events. The magnetic
field energy helps to provide a constant " voltage
anchor " for the grid. Although not the focus of this study,
it is important to note that there are other challenges to
operating inverter-dominant grids; for instance, many
conventional transmission and distribution protection
schemes that expect fault currents to be several multiples
of nominal current may not operate as intended and
would need to be reconsidered.
Today's IBRs are designed to expect the grid to have
these characteristics of inertia and " voltage anchors, " and
therefore, they rely on a certain level of synchronous
machine technology to be connected to the grid with the
IBRs. If today's IBR are connected to a grid that does not
exhibit enough of these characteristics (i.e., because there
are too-few synchronous machines online), then disturbances
like a loss of generation will cause the grid to
" move " or change state too quickly for the IBR to respond
in a stabilizing way to support the grid. The result is typically
a disconnection of the IBR and a lack of support to
the grid that ends in a partial or complete blackout.
There are two general approaches for enabling very
high levels of IBRs on a grid. One approach is to improve
the design and behavior of the IBRs such that they provide
the inertia and " voltage anchor " characteristics that support
the grid similar to the way synchronous machines do.
This concept has been termed grid-forming inverter technology
by those in the industry. The second approach is to
maintain the inertia and " voltage anchor " characteristics
of the grid by keeping a sufficient number of synchronous
machines online. Both approaches are briefly discussed.
Grid-forming inverter technology is in its infancy as of
this publication. The primary thrust is in rewriting the
inverter's software-defined controls so that the inverter
provides the instantaneous active and reactive current
response. This response, in very short time frames, is similar
to the response of synchronous machines but not necessarily
the same as it is possible and advantageous to
omit some undesirable behaviors of synchronous
machines, such as the presence of an electromechanical
mode while preserving the desirable, stabilizing characteristics
of synchronous machines (see Miller and Richwine
2021). However, this task is not easy for inverter manufacturers.
Not only is it a fundamentally different control
strategy than what has typically been used, but the
response and therefore effectiveness on the grid, is still
subject to the inverter's hardware limitations in terms of
current-handling capability and access to short-term
energy reserves. On the grid operations and planning side,
there is the challenge of specifying the technical needs
from advanced, grid-forming IBRs to have a system that is
stable and can achieve higher levels of renewable penetration.
Inverter technology and application development is a
journey, as illustrated in Figure 9. It will not simply be flipping
a switch over to grid-forming inverter technology and
Grid-Penetration
Level
Grid-Forming Inverter Technology
Conventional Rotating Machine Technology
Grid Evolution in Time
Figure 9. The evolution of grid-stabilizing technologies.
52
IEEE Electrification Magazine / SEPTEMBER 2022
IEEE Electrification - September 2022

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