Without sufficient synchronous grid inertia, the grid becomes unstable and a blackout occurs. The 28 April 2025 Iberian Peninsula Blackout underscored how important grid inertia is.
While inertia is a requirement, it is basically a frequency damper. When there is a large and sudden mismatch between supply and load, such as a large generator tripping, reserve power must be immediately available in addition to inertia to restore frequency. This is a function that only dispatchable sources can provide.
Yes. A large generator such as at Diablo Canyon has the ability to instantaneously add on the order of a gigawatt of energy to the grid - or absorb a large amount of energy until the slower-reacting grid elements can compensate. For California, there is no other means to accomplish this essential grid reliability function.
Nice article addressing another challenge of high renewable penetration. I thought one of the issues CAISO had was the tendency of IBR's to trip offline when there were frequency and or voltage fluctuations. Not sure if they're been inverter improvements to address.
Why is the term "inertia" used. Inertia is a property of something not moving with respect to the observer which resists the initiation of movement. Momentum is a property of something moving which resists slowing. The grid is a moving machine, its synchronous generators are certainly moving and the flow of electricity through the grid is also moving. Shouldn't the term be "momentum?
Dear Dennis: Thank you for your question. The first sentence in the article is, "Inertia refers to a system’s capability to resist change." That is the rigorous definition that covers both bodies at rest and bodies in motion. In the case of Diablo Canyon, each generator has about a million pounds (453,592.37 kg) spinning precisely at 1,800 RPM. That spinning mass is the reason why Diablo Canyon has the greatest amount of synchronous grid inertia of any California power plant. Furthermore, energy storage is proportional to the rotational velocity **squared**. As a result, the million pounds of rotating components in a Helms Pumped Storage motor-generator which spin at 1/5 the speed of Diablo Canyon (720 RPM) store only 1/25 of the energy.
Inertia is the *property* of matter that causes an object to resist changes in its state of motion. An object at rest tends to remain at rest, and an object in motion tends to continue moving with the same speed and direction unless acted upon by a net external force. Mass is the quantitative measure of inertia: the greater the mass, the more difficult it is to change the object’s motion.
Momentum is a *measure* of how much motion an object has. It depends on both the amount of matter in the object and how fast it is moving. Momentum describes how difficult it is to stop, redirect, or otherwise change the motion of an object. In modern physics, momentum is a conserved quantity, meaning that the total momentum of an isolated system remains constant unless acted upon by external forces.
The analogy is quite direct. In an AC power grid, “grid inertia” refers to the tendency of the system to resist changes in its frequency, in much the same way that a massive object resists changes in its velocity.
When engineers speak about “grid inertia,” they are usually referring to the fact that the rotating machines possess substantial angular momentum and kinetic energy. When a disturbance occurs, that stored motion cannot change instantly, so the machines continue spinning and temporarily inject or absorb energy, slowing the rate of frequency change.
A good question, and hopefully this isn't overkill, and Gene gave a great answer. But here's a little extra info. "Inertia" is a generic (qualitative) term that means resisting a change (originally in Newton resisting a change in uniform motion - and being at rest is a special case of uniform motion). So we use "grid inertia" in the general sense of the grid resisting shocks to the system allowing it to maintain constant frequency.
"Momentum" is a quantitative measure that is the product of mass and velocity in linear motion (it gets a little more complicated in rotational motion as Gene aluded too; you have to calculate the moment of inertia and multiply that by the angular velocity).
So the usage boils down to speaking qualitatively (inertia) rather than quantitatively (momentum).
While inertia is a requirement, it is basically a frequency damper. When there is a large and sudden mismatch between supply and load, such as a large generator tripping, reserve power must be immediately available in addition to inertia to restore frequency. This is a function that only dispatchable sources can provide.
Yes. A large generator such as at Diablo Canyon has the ability to instantaneously add on the order of a gigawatt of energy to the grid - or absorb a large amount of energy until the slower-reacting grid elements can compensate. For California, there is no other means to accomplish this essential grid reliability function.
Nice article addressing another challenge of high renewable penetration. I thought one of the issues CAISO had was the tendency of IBR's to trip offline when there were frequency and or voltage fluctuations. Not sure if they're been inverter improvements to address.
You are correct regarding one of the weaknesses of IBRs. To learn more, I suggest searching for the phrases, "Blue Cut Fire" and "Inverter." Here is a 2017 NERC report: https://www.nerc.com/globalassets/our-work/reports/event-reports/1200_mw_fault_induced_solar_photovoltaic_resource_interruption_final.pdf
Why is the term "inertia" used. Inertia is a property of something not moving with respect to the observer which resists the initiation of movement. Momentum is a property of something moving which resists slowing. The grid is a moving machine, its synchronous generators are certainly moving and the flow of electricity through the grid is also moving. Shouldn't the term be "momentum?
Dear Dennis: Thank you for your question. The first sentence in the article is, "Inertia refers to a system’s capability to resist change." That is the rigorous definition that covers both bodies at rest and bodies in motion. In the case of Diablo Canyon, each generator has about a million pounds (453,592.37 kg) spinning precisely at 1,800 RPM. That spinning mass is the reason why Diablo Canyon has the greatest amount of synchronous grid inertia of any California power plant. Furthermore, energy storage is proportional to the rotational velocity **squared**. As a result, the million pounds of rotating components in a Helms Pumped Storage motor-generator which spin at 1/5 the speed of Diablo Canyon (720 RPM) store only 1/25 of the energy.
Inertia is the *property* of matter that causes an object to resist changes in its state of motion. An object at rest tends to remain at rest, and an object in motion tends to continue moving with the same speed and direction unless acted upon by a net external force. Mass is the quantitative measure of inertia: the greater the mass, the more difficult it is to change the object’s motion.
Momentum is a *measure* of how much motion an object has. It depends on both the amount of matter in the object and how fast it is moving. Momentum describes how difficult it is to stop, redirect, or otherwise change the motion of an object. In modern physics, momentum is a conserved quantity, meaning that the total momentum of an isolated system remains constant unless acted upon by external forces.
The analogy is quite direct. In an AC power grid, “grid inertia” refers to the tendency of the system to resist changes in its frequency, in much the same way that a massive object resists changes in its velocity.
When engineers speak about “grid inertia,” they are usually referring to the fact that the rotating machines possess substantial angular momentum and kinetic energy. When a disturbance occurs, that stored motion cannot change instantly, so the machines continue spinning and temporarily inject or absorb energy, slowing the rate of frequency change.
Well stated!
A good question, and hopefully this isn't overkill, and Gene gave a great answer. But here's a little extra info. "Inertia" is a generic (qualitative) term that means resisting a change (originally in Newton resisting a change in uniform motion - and being at rest is a special case of uniform motion). So we use "grid inertia" in the general sense of the grid resisting shocks to the system allowing it to maintain constant frequency.
"Momentum" is a quantitative measure that is the product of mass and velocity in linear motion (it gets a little more complicated in rotational motion as Gene aluded too; you have to calculate the moment of inertia and multiply that by the angular velocity).
So the usage boils down to speaking qualitatively (inertia) rather than quantitatively (momentum).