Power Systems - Inertia- Big Iron Rolling

First off, I want to acknowledge the prior work of Gene Nelson, the GreenNUKE, in the area of inertia. If you have not read his work, follow the link below. Gene says to make sure to read the notes and comments too. Gene’s tireless work to push nuclear energy back to the forefront is much appreciated

Why is Grid Inertia Important?

So, inertia, how many of you have pushed a car, bike, wagon, any rolling thing, and then had to stop it. That push, the resistance you feel when trying to both start and stop that object is inertia. It is the inherent property to change its state unless acted upon by an external force.  Inertia in motion has two major components, mass and speed. So, to demonstrate mass, let’s assume you are sitting at a stoplight, and you get hit from the rear by a vehicle going 15 mph. Would you prefer a Smart Car or a Freight Train? The answer is obvious because of mass. Now let’s look at speed. Would you prefer to be hit by that Smart Car if it was moving at 15 mph, or 115 mph? Again, the answer is obvious.

Inertia is a fundamental component of a stable power system. It is the combined rotating inertia of all the magnetically coupled rotating machinery on the Interconnection, including rotating loads (synchronous motors), that contributes to maintain stable frequency. Let’s take a look at a couple of snapshots of typical frequency deviation events caused by a loss of major generation on Interconnections. The frequency change from A to B is entirely resisted by the inertia of the system. The inertia change from B to C is resisted by inertia plus some governor action, you can see the change in the slope at about 59.95 on the left chart. The frequency recovery C to D occurs because of governor action on the power system (right chart). If nothing else happened the Interconnection would stabilize at that point, but it does not. Automatic Generation Control (AGC), takes over and restores the Interconnection to pre-event frequency.

When we talk about rotating equipment, we talk about the “moment of inertia”. The formula to determine the energy given up by a rotating machine based on its moment of inertia is;

E=½Iω²

where:

• E is the energy in joules

• II is the moment of inertia of the rotating object

• ω\omega is the angular velocity in radians per second

Yup, I am not going to try to calculate it either! But what we learn if we dig into the formula is the speed of rotation, the mass of the object, and the distance of the mass from the center of rotation are important. This is why large high speed steam turbines (3600 rpm) are so important to the stability of the power system. We only find these large, high speed turbines in select places, large steam plants and large nuclear plants. Predominantly most of those large steam plants are coal fired, upwards of 1500MW in a single unit. With nuclear steam turbines, 1200 MW is a very common size. The issue is these large steam turbines are disappearing from our Interconnections at an alarming rate.

The other important source of inertia to our Interconnections are large hydroelectric units. These units win on mass. The 805MW Grand Coolee unit 24 rotates at only 85.7 rpm. However, the rotating mass weighs 2138 US tons. More importantly, the rotor has the most mass at 1480 US tons, most of that weight is in the pole pieces that sit at the exterior of the rotor increasing the rotor's angular velocity. On a MW per MW basis a hydro unit is likely a better source of inertial energy, but they are relatively rare. They also spend much of their time offline to preserve the reservoir storage.

So, what is happening to our inertia? It would be nice to blame it all on renewables, but unfortunately, they can’t take all the blame. The proliferation of new combined cycle plants is a major contributor. Combined cycle plants are several generators working together to make a larger output, usually two gas turbines to one steam turbine. This makes all of the units individually relatively small and light. In addition, gas turbines are very light by design to begin with. So, CC plants do not offer nearly the system inertia as older steam plants. Renewables of course offer virtually no inertia except artificial inertia from batteries. I will cover more about renewables later.

That about covers it, so far we have pulled together magnets, frequency, governors, and inertia. With each article I have reached back and made the connection to demonstrate how all the pieces fit together. Next, we will get into the actual day to day control of frequency on the Interconnection, how it’s done, and …. MORE MATH!! Yeah!