4
This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.
3 How Does Inertia Help Maintain Reliability in
Today’s Grid?
Inertia is a useful property whether one is riding a bicycle or operating a power grid. On a bicycle,
inertia gives the rider a chance to stop pedaling and coast without falling over. In the grid, it gives
the system operator a chance to respond to power plant failures (called contingencies). This is
because inertia resists changes in frequency, giving other systems time to respond and rebalance
supply and demand.
Grid operators in the United States may incur financial
penalties if they do not maintain systems to keep
frequency within tight tolerances.
3
One of the challenges
facing grid operators is maintaining grid frequency
following a contingency event in which a large power
plant (or associated transmission line) fails, meaning its
supply of power is removed from the system very rapidly
and without warning. The resulting imbalance will result
in the grid slowing down and the frequency starting a
steady decline, similar to how a vehicle slows when a
driver eases up on the gas pedal.
When the grid frequency falls below a predefined level
(59.5 Hz in most of the United States and 59.3 Hz in
Texas), a portion of the customer load is disconnected
(NERC 2019a). Such disconnections are known as
underfrequency load shedding (UFLS) and are used to
balance the remaining load with the remaining
generation. A similar disconnection of generation occurs
if the frequency exceeds a certain limit (e.g., 60.5 Hz in
the Western Interconnection, or WI). The small change
in frequency that results in UFLS is less than a 1%
change from the base frequency. For instance, if the
same principle were applied to a vehicle, it would mean
driving 60.5 mph in a 60-mph zone would result in a
speeding ticket.
3.1 Primary Frequency Response: Cruise Control for the Grid
To maintain grid frequency and avoid UFLS, grid operators use a variety of processes that can
respond to events that might change frequency (Denholm, Sun, and Mai 2019). One of these
key processes is primary frequency response (PFR). PFR detects changes in frequency and
automatically—without action from the system operator—adjusts operations of online generators
to maintain frequency within the desired range.
3
Compliance is monitored and enforced in the United States by the North American Electric Reliability Corporation
(NERC). https://www.nerc.com/Pages/default.aspx
At a Glance: Inertia’s Role in
Maintaining Grid Frequency
Grid frequency is used as an
indicator of significant changes in
either supply or demand. During
normal grid operation, the supply of
power from all the generators equals
the demand for electricity, and the
frequency remains constant. But just
as a vehicle slows when you take
your foot off the gas, if there is a loss
of a power plant, the supply of power
will drop almost instantaneously.
However, the demand for electricity
has not changed, so the same
amount of power will still be extracted
from the system.
That is where inertia comes in.
Stored energy is extracted from the
inertia of the spinning generators and
can temporarily make up for the lost
generator. This action will slow down
the generators. Although it cannot be
sustained for more than a few
seconds, it provides time for the
mechanical systems in the grid to
detect the imbalance (as reflected in
declining frequency) and tell power
plants to speed up (or slow down).