IEEE Electrification - September 2022 - 56

inertia. This means that as more renewables are added to
the grid, there is less inertia. With less inertia, there is
more instability, and this is the problem operators across
the globe are wrestling with right now. The good news is
that there are new digital solutions hitting the market
that can guarantee stable inertia on the renewable grid.
Power companies across the globe are using these solutions
to rapidly scale renewable energy sources and safely
scale down fossil fuel energy sources. In this article, we
explore the inertia challenge, and we delve into the
results of trials we have run using new digital solutions
that power companies are using
across the globe to fuel their transition
to net-zero carbon.
The Role of Inertia in
System Stability
What is inertia exactly? Let's go back
to physics 101. According to Newton's
first law of motion, an object in
motion stays in motion with the
same speed unless it is acted upon. It
is this motion that is inertia, which
means anything with moving parts
has inertia. What does this have to do
with the energy grid?
Traditional electrical generators,
With less inertia,
there is more
instability, and this
is the problem
operators across the
globe are wrestling
with right now.
such as fossil, nuclear, or hydroelectric power plants, have
many parts that are continuously moving, and, because
they are moving, they naturally have inertia. In a power
grid, inertia is derived from hundreds or thousands of
these moving generators that are synchronized to the
same frequency. When you are riding a bike, inertia gives
you the chance to stop pedaling and coast for a few seconds
without falling. This is pretty much how inertia
works for a grid.
When grid frequency drops below a predefined level
(60 Hz in most of the United States) because of supply and
demand issues or a power failure (also known as contingencies),
inertia can temporarily make up for the power
lost in this frequency drop. This gives the systems at the
power plant time to detect and respond to the power failure
and, in turn, keeps the entire system stable.
If the grid frequency falls below a predefined level
(59.5 Hz in most of the United States), a portion of the customer
load is disconnected; this is what the industry calls
underfrequency load shedding (UFLS)-often referred to as
blackouts by consumers. UFLS is used to balance the
remaining load and keep the system stable.
Until recently, inertia was taken for granted, and it was
not a concept that was heavily explored. However, it has
become a prominent issue when it comes to renewable
energy sources that do not have naturally occurring inertia.
This is a huge challenge for grid and network operators
who want to rapidly make the transition to renewable
energy sources.
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IEEE Electrification Magazine / SEPTEMBER 2022
Primary Frequency Response Versus
Fast Frequency Response
Let's first talk about the primary frequency response (PFR)
that the traditional mainland power grid uses. Synchronous
generators still rely on mechanical systems to
change the supply of energy, such as increasing the flow
of fuel, steam, or water to generator turbines. After a contingency
event, the PFR detects a frequency change and
responds by increasing power from the remaining
generators to temporarily replace energy from the
failed generator. Inertia is in play in the few seconds it
takes between the contingency and
the mechanical response of the PFR
to kick in.
While inverter-based renewables
reduce the amount of inertia available
on the grid, they also replace some of
the mechanical processes of traditional
energy generation. Unlike synchronous
generators, renewables do not
rely on mechanical systems when
there is a contingency. As a consequence,
it is possible to measure and
respond to frequency drops in fractions
of a second-this is known as
fast frequency response (FFR). PFR can
be replaced, to a large extent, with FFR
from inverter-based renewable resources.
However, even with FFR, inertia is still necessary
for grid stability, especially when getting to a high
percentage of renewables on the grid. However, the
problem is that no one knows exactly how much inertia
sits on the renewable grid. While some utilities
may say they have a good enough idea of how much
inertia there is, when it comes to renewable grids, it is
imperative to have the most accurate data possible.
Without knowing this information, it is difficult for
grid operators to accurately plan ahead for contingencies
(Figure 1).
Transmission system operators (TSOs) currently rely on
inertia estimates to keep their systems stable. While estimations
worked in the past for traditional fossil-fueled
energy sources, they are becoming increasingly incorrect
because of the renewable energy sources being added into
the distribution network. Why are renewable energy
sources having this impact? Because the mission-critical
data TSOs need from renewable energy sources is invisible
to them.
To ensure there is enough inertia available when contingencies
arise, TSOs go back to what they know is a stable
source of energy-synchronous generators. Without
knowing how much inertia is necessary for contingencies,
TSOs take large margins to make sure the lights stay on.
However, as you can imagine, this is an expensive way to
run the grid, making renewables an expensive alternative
to synchronous energy sources.
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