IEEE Electrification - March 2021 - 90

High-voltage ac and high-voltage dc transmission technology options can be used, depending on many factors,
such as distances between islands, water depths, projected interconnection capacities, costs, and reliability considerations. AC and dc interconnections will also have
different implications for system stability: a dc interconnection is similar to an inverter-based plant from a system stability perspective; an ac interconnection
effectively pools the inertia of the two regions, and its
impedance affects the system stability.

Conclusions
Summary of Lessons Learned
One of the key lessons for island grids that may eventually
have high levels of wind and PV generation is that it is
wise to consider the potential future states of the system
when specifying performance requirements for new generation: generation installed today will need to be capable
of supporting the needs of the system in 10 or 20 years. As
a baseline, all new generation should be required to be
capable of riding through a wide range of voltage and frequency events, including very fast rates of frequency
change, jumps in the phase angle, transient overvoltages,
and other events. In addition, operators may consider the
participation of IBRs in providing grid-stabilizing functions
and services, such as frequency response, FFR, voltage regulation, and fast voltage support. As peak IBR generation
reaches high levels (perhaps 75% and above), the use of
grid-forming controls should be considered, especially as
this technology matures. The use of synchronous condensers for various purposes (system strength, fault current, and inertia) should also be considered.
The draft IEEE P2800 standard contains guidance for
the minimum performance requirements for transmission-connected IBRs (see " IEEE P2800-Draft Standard
for Interconnection and Interoperability of InverterBased Resources Interconnecting With Associated Transmission Electric Power Systems: Draft 5.0 " in the " For
Further Reading " section). IEEE Standard 1547-2018 contains similar guidance for distribution-connected
resources (see " IEEE 1547-2018-IEEE Standard for Interconnection and Interoperability of Distributed Energy
Resources With Associated Electric Power Systems Interfaces " in the " For Further Reading " section). These standards (and similar ones worldwide) represent the
collective wisdom from various IBR deployments,
including those on island systems and hence serve as
good starting points for performance requirements;
however, because of the unique characteristics of island
power systems, it is important to also consider whether
additional requirements may be needed (for example
increased rate-of-change-of-frequency ride-through
capability and improved control stability in high-impedance conditions). It is feasible to operate island power systems with very high instantaneous levels of wind and PV

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generation. Operation with 100% instantaneous inverter-based generation has been demonstrated for small
islands (a few megawatts) and likely will be demonstrated
in the coming years for large islands (hundreds of megawatts) once significant ongoing studies and development
projects are complete.

Open Research Questions
Open research questions concerning the integration of
very high instantaneous levels of wind, PVs, and BESSs in
island power systems include the following:
xx
When can traditional positive sequence transient
analysis models be used, and when do we need to
transition to EMT simulations?
xx
What constraints and requirements can be placed on
IBRs to ensure that they are stable in a given set of
high-IBR operating conditions? How can emerging
impedance-based stability analysis tools best be leveraged to help ensure system stability?
xx
Given the great flexibility of IBR controls, what are the
best practices, especially when handling physical limits on current and power?
xx
In very high-IBR (low-inertia) power systems, some
frequency stabilization is needed. Can this best
be achieved by adding synchronous condensers,
by deploying FFR in IBRs, or through a combination
of both?
xx
How should IBRs best be synchronized with the rest
of the power system, and how does the answer to
that question depend on the system details? For
example, when is it OK to use PLL-based grid-following inverters, and when do we need grid-forming
inverters? Should other means of synchronization be
considered in some scenarios?
xx
What should be the ratio of voltage-controlled
resources (conventional generators, grid-forming
inverters, and synchronous condensers) to currentcontrolled resources (grid-following inverters) in a
system for ensuring stability, and what are the driving factors?
xx
Large numbers of highly distributed IBRs connected to
the distribution system are intractable to model in full
detail, so how can we gain confidence in the system
stability? Should distribution-connected inverters be
required to operate stably at some maximum expected level of grid impedance?
xx
How should compliance with standards and codes be
verified, especially for large IBRs that cannot easily be
lab tested? What should be the roles of testing versus
modeling versus field monitoring? How should IBR
models be validated?
We do not necessarily expect all the answers to these
questions to come through research and analysis. As with
the initial deployment of interconnected power systems
more than 100 years ago, comfort with increasing levels of
IBRs will also be gained gradually through field experience.
IEEE Electrification - March 2021

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