Without sufficient synchronous grid inertia, the grid becomes unstable and a blackout occurs.

Inertia refers to a system’s capability to resist change. For a power grid, greater synchronous inertia confers greater ability to resist frequency changes. //

In contrast to gigantic 2,256 megawatt nuclear power plants such as Diablo Canyon Power Plant (DCPP) near San Luis Obispo, California which provide very large amounts of synchronous grid inertia, so-called inverter-based resources (IBRs) such as solar powered generators, wind power generators, and batteries supply negligible amounts of synchronous grid inertia. //

Prior to the introduction of significant penetrations of IBRs, each power grid's synchronous generators (coal and natural gas-fired generators, large hydroelectric dams, geothermal plants, and nuclear power plants) had sufficient synchronous grid inertia to assure power grid stability. The synchronous generators have a large amount of rotational inertia as a consequence of having massive rotating turbines and massive rotating generator rotors. (See photograph below.)

As a simplified example, each of the pair of DCPP’s generators have rotating components which weigh in excess of a million pounds (500 tons.) DCPP’s turbines rotate 30 times per second. The rotating magnetic field induces the 60 cycle per second (Hertz) AC voltage (25,000 Volts) and AC current (45,120 Amperes) in the stator windings of each unit. In response to perturbations in grid frequency, the rotational kinetic energy can be instantaneously converted to changes in the output power of the generator which tend to stabilize the generator’s output frequency and voltage.