IEEE Electrification - September 2020 - 23

enforced minimum inertia limits, at which a system can be
reliably operated with current frequency control practices
for their respective systems. EirGrid's minimum inertia limit
was 23,000 MW-seconds for the whole Ireland island at a
system capacity of 6.5 GW in 2018. Basically, these
approaches are trying to keep the heavy-inertia nature of
the power system the same as it has been in the past.
An emerging way of actively utilizing the inverter capabilities for mitigating low-inertia impact is to make the
inverters provide reserve support through the resources
behind the inverters-a concept called "synthetic inertia."
Under this concept, when the system frequency drops, the
inverter would increase its active power output and help to
arrest the frequency drop. This way, the system would see
an equivalent inertia from this inverter, hence synthetic
inertia. From a system perspective, this synthetic-inertia
approach makes inverter-based resources essentially
behave like conventional generators, and thus the equivalent inertia of the system remains at an acceptable level.
Hydro Quebec has included synthetic inertia capabilities
as part of its interconnection requirements for wind generation resources. The wind turbines would convert some
of their kinetic energy to electric power output during a
frequency drop event, and then the depleted kinetic energy would be restored during the frequency recovery phase.
The actual implementation of this synthetic inertia
approach depends on the operating conditions of the
wind turbine and the system and needs careful planning
and coordination. This approach might be a good intermediate step for utilizing the inverters' control capabilities. But making the low-inertia system behave like a
heavy-inertia system does not fully utilize the fast control
capabilities of inverters. Current research on grid-forming
inverters is exploring ways to achieve fast arresting and
recovering from frequency drops. Grid-forming inverters
can also provide other control functions such as voltage
control and oscillation damping control. Such fast control
functions would truly take advantage of the fast inverter
controls and make the power system more responsive in
improving its frequency performance.
On the DER side, inverters are mostly not utilized for
system reliability and resilience, but they have fast control
capabilities. Inverters associated with rooftop PVs do perform the basic function of converting the dc power to ac
power for energy use. However, they do not usually provide stability support for frequency or voltage control. As a
result, when they lose the connection to the main grid,
these distributed inverters have to shut off and remain
offline until the grid recovers, as observed in the "Public
Safety Power Shutoffs (PSPS)" implementation in California. The PSPS is for avoiding wildfire hazards under high
wind and dry weather conditions.
For example, in October 2019, Pacific Gas & Electric deenergized lines for more than two million customers in its
service territory in Northern and Central California. However, these distributed inverters, combined with energy

storage, can do better to maintain standalone rooftop PV
operations and can be organized into a local microgrid to
supply resilient power to a larger community. The gridforming inverter research mentioned earlier aims to enable
individual inverters to provide frequency and voltage support. Beyond individual inverters, there would need to be a
control structure that enables the many distributed inverters to work concertedly for system-level performance.

Power Electronics Is Enabling a Responsive,
Adaptive, and Scalable Power System
As mentioned previously, inverter-based generation in
power systems is generally either not controlled for the
support of system operation, or it is controlled to mimic
conventional generators to fit the power system of the past
century. This may work fine as an intermediate step when
the use of power-electronics-connected resources is still at
a low level. In future scenarios with ubiquitous power electronics, the inverters offer opportunities to achieve faster
control with multiple functions for both large and small
resources. We should fully utilize these control capabilities
and help enable the power system to perform at its full
potential as a responsive, adaptive, and scalable system.

Fast Control of Individual Inverters
Because of the high-speed switching within the power
electronic modules, inverters can respond very quickly
to system changes, being limited by how fast the sources behind the inverters can respond. To illustrate the
different control performance of different inertia scenarios, the responses of three generators to a hypothetical fault situation are simulated, and the results
are shown in Figure 7. These three generators are a

Terminal
G

System
(a)
Terminal
System

(b)
Terminal
System

±
(c)

Figure 7. Three generation scenarios with a (a) hydro generator, (b)
Type 3 wind generator, and (c) PV generator.

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IEEE Electrification - September 2020

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