Everyone wants reliable power.
And one reason why today’s grid is so reliable is its inertia, which basically helps the
whole system keep running even when a power plant fails.
But the grid is evolving to include ever-higher levels of solar and wind—which don’t provide inertia.
Should we all panic?
No.
Here’s why.
You don’t have to be able to cite Newton’s Laws of Physics to understand inertia.
Anyone who has driven a car or ridden a bike is familiar with it.
It’s the tendency of an object in motion to remain in motion—and it keeps your vehicle
moving forward when you stop pressing on the gas pedal, or keeps your bike from falling
over if you stop pedaling.
Spinning objects, like wheels or power plant turbines, have rotational inertia.
And this inertia is useful whether driving a car or operating the power grid.
Here’s how it works.
The grid consists of hundred of generators rotating in lock step, each synchronized to
a grid frequency of 60 cycles per second in the U.S.
This frequency is used as a measure of the health of the grid.
During normal operation, the supply of power from all the generators equals the demand
for electricity, and the frequency remains nearly constant.
But just like how 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 instantly.
But the demand for electricity hasn’t changed.
The lights and air conditioners and TVs will continue to extract the same amount of power
from the system.
As this energy is extracted from the inertia of the spinning generators, the grid will
begin to slow down, so the frequency will drop.
Devices called governors located on the generators detect these changes in frequency.
They measure how fast the generators are spinning, and tell power plants to speed up or slow down.
This is essentially the “cruise control” for the power grid.
But it takes time—up to several seconds—for all these levers and valves to work and the
power plant to increase output.
That’s where inertia comes in.
It gives the system time for all these mechanical systems to react to an emergency—while keeping
the lights on.
So that’s how inertia works to make the current grid reliable—but how we operate
that grid is changing as more renewables are added to it.
We can still get inertia from renewables that use traditional generators, including geothermal,
hydropower, biomass, and concentrating solar power.
But variable-generation renewables like wind and solar PV don’t use traditional generators—instead,
they use inverters, which have electronic controls that do not provide inherent inertial response.
And when we add inverter-based renewables and start turning traditional generators off,
we have less inertia on the system.
Does this mean doom for the grid?!
No!
Grid operators have figured out how to address this without affecting reliability.
If we replace the slow mechanical systems in conventional generators with something
faster, we simply don’t need as much inertia.
Fast frequency response replaces some of the mechanical processes with electronic sensors
that that can quickly measure frequency and respond in fractions of a second.
This response can be derived from non-critical loads that sense low frequency and disconnect
in less than half a second.
You can also get fast frequency response from renewables
by controlling the output of wind and solar plants, or extracting the stored kinetic energy
from rotating wind turbines.
Many storage technologies—like batteries—can also provide very rapid frequency response.
And when we say fast frequency response, we mean fast: wind plants can respond 10 times
faster than traditional generators, and solar plants can be more than 50 times faster.
And these aren’t just lab experiments.
This is already happening in real systems.
In Texas, wind plants have been required to provide frequency-responsive services for years.
And since 2018, it’s been a requirement for all new utility-scale wind and solar plants in the U.S.
So we don’t need to panic: there are many solutions to help wind and solar “play nice”
with the grid—even without traditional inertia.
Although how far we can go with inverter-based resources remains an open question, researchers
at NREL and around the world are exploring this challenge, and ways to make our grid
more reliable, stable, and cost-effective as it continues to transform.