IEEE Electrification - March 2021 - 79

IBRs have fast and
complex control
dynamics, resulting
in stability problems
across a broader
frequency range.

high--resolution field data, as seen in
Figure 3 (see Kenyon et al. 2021); the
damped subsynchronous oscillation
in the field data is believed to be due
to an electromechanical mode that
no longer exists in today's system.
This model is now being used to simulate the minimal-inertia system
dynamics, with the new inverterbased PV-BESS plants expected to
come online between 2022 and 2024.

Stability-Enhancing Controls
and Services for Island Grids

0.5

-0.5

-1

Voltage (kV)

1.1
Time (s)
(a)

1.2

-10

-20

1.1
Time (s)
(c)

1.2

Measurement
PSSE
PSCAD
60.4
60.2
1

-20

Stability Controls

IBRs have been shown to be capable of
providing grid services that have typically been delivered by synchronous
generators, including primary frequency response (PFR), secondary
frequency regulation, and voltage regulation, including in island grids, such
as those in Puerto Rico (see Gevorgian and O'Neill 2016)
and Kauai. These services are chiefly supplied by utilityscale generation, except in the case of PFR, which has
been required, even from small customer-owned DERs, in
Hawaii since 2017 (see Hoke et al. 2018). Responding to
underfrequency, of course, requires reserves, which for PV
and wind means either operating below the maximum
available power or including energy storage.
In addition to conventional grid services, IBRs can provide a range of other services to stabilize the grid voltage
and frequency. In particular, the class of grid services
known as fast frequency response (FFR) involves rapidly
injecting power into the grid in response to frequency disturbances (or absorbing power, in the case of overfrequency) (see " Fast Frequency Response Concepts and Bulk
Power System Reliability Needs, " in For Further Reading).
FFR can take many forms, including but not limited to
the following:
xx
emulation of the conventional governor-proportional
droop response (i.e., PFR) but with a much faster

1.1
Time (s)
(b)

1.2

Frequency (Hz)

Current (kA)

When considering the stability-enhancing services IBRs
can provide, it is useful to differentiate between large utility-scale and smaller distribution-connected IBRs. While it
is technically possible for small-scale IBRs to provide
many of the same services that utility-scale IBRs can,
practical concerns about communications and monitoring
often limit what is requested from small IBRs. This may
change in the future as the ability of aggregations of small
distributed energy resources (DERs) to provide stability
services develops. The basic requirement for IBRs to contribute to power system stability is to remain connected
during a wide range of disturbances, including underfrequency, overfrequency, undervoltage, overvoltage, high
rates of change of frequency, fast changes in the voltage
phase angle, and so on. This is especially important for
island power systems, where the range of disturbances is
generally larger than on large interconnections. This
applies to both small- and large-scale IBRs and should be
applied not just to systems where IBR levels are currently

high but also to any system where IBR
levels may one day be high, to avoid
costly retrofits, such as those currently being considered in Hawaii.

a
b
c

60
59.8
59.6
59.4
59.2
59

1.1
1.2
Time (s)
(d)

8
10
Time (s)
(e)

Figure 3. Validating an EMT (PSCAD) model of Maui against field data for a 2 March 2017 fault followed by a generation trip event. (a) Measured
currents in line 200-3. (b) PSCAD-simulated currents in line 200-3. (c) Measured voltages at the Kahului 23-kV bus. (d) PSCAD-simulated voltages at the Kahului 23-kv bus. (e) System frequency, measured and simulated.

IEEE Electrific ation Magazine / MARCH 2 0 2 1

IEEE Electrification - March 2021

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