Inertia – the Hidden Asset Provided by Old Generators

Inertia is a concept that is rarely talked about in the electric grid, but it is one that plays an important role and will need to be well managed in any transition to clean energy.

When I mention power system inertia, I generally hear the same story –– “we have done things this way for more than 100 years – and there is no reason for change…” While “human inertia” plays a role in many companies, it is not the same inertia that I am about to address.

The power system has relied on central generation for more than 100 years to deliver power. All classic power system generators turn at a synchronous speed, and while that speed may be different for each generator, the result is the same.  Each generator MUST create 60 electrical cycles in every second and generators may create a differing number of cycles per rotation, resulting in different synchronous speeds. Speeding or slowing the generator results in more or fewer cycles in a second.  All generators in a single interconnected area are essentially “locked in step” together, so they slow down or speed up together.  

Generators typically have large rotors, and these may weigh up to 1,000 tons in large machines. These spinning rotors store kinetic energy in their rotational speed.  Any change in speed will change the amount of kinetic energy stored.

If you were to examine a generator spinning, delivering no load, and only enough power applied to the turbine to make it continue to rotate at the synchronous speed, and you suddenly applied a large load to the machine.  The system would slow down and the increased power to the load would be taken from the kinetic energy stored as the generator rotor at the synchronous speed.

A generator with a large storage capacity would see the speed decline slowly, while one with a small storage capacity would slow down much more quickly.

There is a non-intuitive aspect to inertia in generators. Hydro generators that have very heavy rotors, (1,000 tons or more) may have much less inertia than a high-speed lightweight generator used in a steam turbine (coal fired or nuclear generating station).  This occurs because the energy stored in the rotor is determined by the equation shown

Where I is the inertia constant (a number that is related to the weight and structure of the rotor) and ω is the speed of rotation of the generator. Stored energy increases with the square of the rotational speed.

Consider two generators. A hydro generator, 100 RPM/1,000 tons and a steam turbine generator, 3,600 RPM/15 tons.  The steam turbine machine, with far less weight, has almost 20x the stored energy in the heavier hydro machine.  This is because of the significant difference in the speed of rotation.  Coal-fired and nuclear plants, which operate at either 1,800 or 3,600 RPM are the largest source of inertia in the grid.

Why is inertia important? The space station is entirely powered with solar energy, and there is no inertia at all, so why is inertia important for terrestrial based systems?

System inertia provides protection for steam turbines.  These machines are powered by steam that is “blasted” at the turbine blades to deliver rotational power.  The angle at which the steam hits these blades is carefully adjusted to transfer maximum steam power to mechanical power.  If the rotating speed of the turbine, which is directly coupled to the generator is decreased, the angle at which the steam hits the turbine blades changes, and the blades may experience a phenomenon known to aircraft pilots, a stall. The smooth flow around the blade becomes turbulent, energy transfer is reduced, and vibration from the turbulent flow can cause damage to the turbine.

Steam turbines must be protected. If there is a large loss of generation on an interconnection, the speed of rotation of all generators falls as the generators each give up some of their stored energy to meet the increased demand.  This occurs in a fraction of a second after the loss of capacity.  Once the frequency decline is detected, governor action increases turbine and generated power, but this occurs slowly after about 0.5 seconds.  That first half second is critical. The decline in frequency will continue at a slowing rate, until governor action on all generators creates enough new capacity to offset the loss that started the problem. System inertia throttles the rate of change of frequency (RoCoF).  High inertia results in a lower RoCoF.

If frequency declines to a level that may result in turbine damage, the machine is tripped offline with the result that the RoCoF increases and all steam turbine generators then trip and the systems rapidly collapse into a dark and quiet place.

As the world transitions off fossil fuels, system inertia is expected to decline.  As inertia declines, the RoCoF after a loss increase, and halting the decline becomes more difficult.  As long as there generating systems that are sensitive to lower frequencies on the grid, the problem will remain and potentially grow.  Norway recently announced a new record low level of inertia as their renewable generation has increased.  Norway operates a system that is based largely on hydro and nuclear, and it would be interesting to understand the steps that have been taken to manage any significant generation loss.  France, with its extensive nuclear system may become a significant inertia source for Europe.

North American utilities have implemented extensive autonomous load shedding systems to halt a decline that may be a threat, as a total collapse may take many days to recover.

Inertia is important and will continue to have a critical role until a time there are no generators that are sensitive to under frequency situations.

Control system companies may have opportunities to create synthetic inertia, using fast storage to support the system by slowing frequency decay in the short first period after a significant loss of generation capacity.

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