IEEE Electrification - September 2020 - 22

Note that system
inertia is affected by
many other factors,
but the increase in
inverter-based
generation plays a
major role in causing
lower system inertia.

A declining trend of the minimum
system inertia has been observed in
the Electric Reliability Council of
Texas (ERCOT) system in recent years,
particularly since 2015 (Figure 6).
These minimum inertias were
observed late at night when loads
were low and wind generation was
high. In Figure 6, the minimum inertias are normalized by the system
load observed at the time. This declining trend in inertia generally correlates to ERCOT's wind generation
increase from 11,065 MW in 2013 to
23,860 MW in 2019. Note that system
inertia is affected by many other factors, but the increase in inverter-based generation plays a
major role in causing lower system inertia. Ireland, the
United Kingdom, and South Australia have also experienced low-inertia challenges because of high percentages
of inverter-based generation.
The reduced mechanical inertia has caused significant
reliability challenges for the power system community.
Lower inertia means faster dynamics because the intrinsic
dynamics of the power system is less affected by the
heavy mechanical inertia but more dominated by the
electrical inertia from the inverters. This becomes a challenge if the speed of the controls cannot catch up with the
speed of the intrinsic system dynamics. However, as long
as the speed of the controls matches the speed of the
intrinsic system dynamics, the power system can respond
much faster to external disturbances and can perform
better than its conventional heavy-inertia counterpart.
This presents a significant opportunity for using the highspeed control capabilities of inverters to achieve more
responsive and optimal performance of the power system.

The Impact of Active Participation
of Many Small Devices
Distributed energy resources (DERs) in power systems
have been growing and will continue to grow worldwide,
and most of them-rooftop PV generation, active loads, EV

Inertia (GWs)

180
170
160
150
140
130
120

2013 2014 2015 2016 2017 2018 2019
Year

Figure 6. ERCOT inertia (normalized by load) decreasing as renewable power sources increase (data source: ERCOT, 2020).

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2020

charging, and distributed battery storage-will be inverter-based. This
unprecedented number of small
resources is a significant contrast to
the few larger resources in conventional power systems. Managing
many small resources is a fundamental challenge. Utilities have reported
cases in which conservation voltage
reduction programs did not achieve
the desired reduction of power consumption because of distributed PV
generation being controlled as a current source.
Another issue is that these small
resources are mainly behind the meter
and not visible for system operation. Although these
resources are actively participating in system dynamics,
they are often not actively used to improve the system's
dynamic performance. If properly metered by sensors and
controlled by inverters, these active DERs can enable a
power system to be more adaptive and scalable than conventional large power plants. For example, rooftop PVs can
operate at various scales as they could adaptively combine
into different groups when facing cyber and physical disturbances. They can also potentially help the system recover quickly from outages, compared to using just
conventional large-scale blackstart power plants.

Current Approaches to Mitigating
the Impact of Low Inertia
One direct consequence of low inertia is the frequency
deviating quickly from its nominal value when a disturbance such as generation tripping occurs. Such frequency
deviations would cause a high rate of change of frequency
(ROCOF). Then, depending on the setting, a high ROCOF
could cause nonsynchronous generators to trip because of
their loss-of-mains protection. During the 9 August 2019
power outage in the 60-GW U.K. system, 350-430 MW of
distributed generation tripped off unnecessarily because of
a high ROCOF (>0.125 Hz/s threshold in their protection settings) due to light system inertia prior to the outage. In
addition, a large frequency drop could cause underfrequency-load-shedding relays to trip and result in unnecessary
load loss, which has been observed in South Australia and
Nordic systems.
Most current approaches are passively accommodating
inverter-based generation by using other resources in the
power system instead of the fast control capabilities of
power electronics. One approach is to keep synchronous
condensers online to enforce minimum inertia limits, in
combination with forecasting and assessing inertias for realtime operation. Ireland, the United Kingdom, and South
Australia have used synchronous condensers for maintaining acceptable inertia levels in their power systems. And Ireland, the United Kingdom, South Australia, and ERCOT all

IEEE Electrification - September 2020

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