IEEE Power & Energy Magazine - November/December 2021 - 26

on frequency. However, there are pitfalls since the inverter
response is entirely dictated by controls. There may be unexpected
behaviors that can trigger interactions. The ability of
GFM devices to respond effectively to disturbances and provide
a stabilizing influence is limited by equipment capability
and grid constraints.
Medium Islands
Medium island systems (roughly several gigawatts), such as
Ireland, include tightly integrated networks. Here, frequency
stability could be more pressing than voltage stability and
system strength. Such systems with high IBR penetration
may also experience voltage-induced frequency dips due to
wider-area low-voltage propagation during faults. This phenomenon
leads to additional frequency stability concerns, as
it may increase the size of the largest contingency. In Ireland,
75% IBR penetration [considering high-voltage dc (HVdc)
imports and exports] has been achieved by keeping six to
eight large synchronous generators online and introducing
new services. These generators can manage issues, including
high-wind ramps and a high RoCoF (with key concerns
around loss-of-mains protection), and achieve fast frequency
response (FFR). However, to operate the system close to
100%, low system strength becomes the main challenge.
Large Islands
For large island systems (roughly several tens of gigawatts), stability
concerns depend on the size of the greatest contingency
relative to the system size, geographic dispersion of IBRs, and
distance from areas of IBR concentration to load centers. In
tightly integrated large island systems (for example, Great Britain),
frequency stability is a pressing concern. For such systems,
inertia limits are enforced, and FFR services are introduced. In
systems such as Texas, mainland Australia, and remote parts
of Great Britain, abundant wind and solar IBRs are in areas
far from load centers and synchronous generation. This initially
results in voltage stability and low-system-strength concerns in
IBR-rich regions. These areas of concern then gradually expand
as IBRs grow. Further challenges are encountered due to weak
interconnections within the system, where a normally interconnected
network may split into several electrical islands. In areas
with a high GFL IBR concentration, there is increasing complexity
and a stronger impact of outages on system stability. In
Texas, transmission constraints are increasingly set by stability
considerations. As a result, transfer limits vary with network
topology, synchronous generation online, and IBR output.
Some of the solutions that have been implemented in large
islanded systems, such as maintaining minimum inertia,
introducing FFR products, GFL IBR tuning, and the addition
of SynCons, have already been discussed in this article. Work
is ongoing to better understand and utilize the capabilities provided
by GFM devices. Consider the following examples:
✔ In Australia, studies are being carried out to determine
the extent to which GFM inverters can provide
system strength to support GFL IBRs.
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ieee power & energy magazine
✔ In Great Britain, nonmandatory GFM specifications
for power sources (including IBRs and synchronous
machines) are being developed along with market
solutions for the implementation of GFM services.
These are focused mainly on three aspects: instantaneous
active power injection or absorption during frequency
events, instantaneous active power injection in
response to voltage phase angle jumps, and instantaneous
active power to damp voltage oscillations. The
work has gradually concluded that phase jump active
power is the most critical contribution to system stability
due to its contribution to synchronizing torque.
Geographically Large
Interconnected Systems
Continental-scale systems (several hundreds of gigawatts),
such as the entire Continental Europe synchronous area, the
Eastern and Western Interconnections in North America,
and the national grid in China, are quite resilient. For them,
operation at or near 100% IBR penetration is not envisioned
in the near term, yet it could occur in the future. However,
as IBR penetration increases, problems faced by smaller
systems may manifest themselves at a larger scale. Even if
the systemwide inertia and strength are relatively high, IBRs
are often concentrated in localized weak grids where voltage
and control stability issues may occur.
One concern is that part of the system with high IBRs may
separate from the rest of the interconnected grid. Such a split
is more likely to occur across highly loaded weak transmission
corridors. Power exports and imports between market
zones within the interconnected synchronous system before
the split become power imbalances for each subsystem after
the separation, resulting in rapid frequency excursions. Without
effective countermeasures, frequency limits would be
exceeded, additional generators would be disconnected, and
a system blackout would be inevitable. This is increasingly
a concern for Continental Europe under high cross-border
trade and IBR penetration. Necessary instantaneous reserves
can be provided by GFM IBRs and supplemented by FFR to
ensure that subsystems remain stable after separation.
Another emerging issue for systems with long, heavily
loaded transmission corridors is maintaining steady-state
and dynamic voltage stability. Besides SynCons, static synchronous
compensators are increasingly used for dynamic
reactive support. German system operators have identified
a need for an additional 23-28 GVA-reactive of controllable
reactive power compensation devices during the next
decade. HVdc connections are another solution to bridge
long distances in meshed and congested ac networks. The
German grid development plan identifies 4,000 km of
HVdc circuits. To overcome stability problems, static synchronous
compensators and HVdc links may need to be
equipped with GFM capabilities that include a sufficient
energy buffer to support the grid during and after significant
disturbances.
november/december 2021
IEEE Power & Energy Magazine - November/December 2021

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