1.- Increase the Moment of Inertia of the grid

Because generation and demand on an electric grid have to be balanced, power plants with rotating units have a speed governor.

The speed governor tries to maintain a stable grid frequency.

At a given moment, if the electric demand raises, the “extra” energy needed will be drained from the kinetic energy of all the rotating units connected to the grid.

As units slow down, the grid frequency will also start to fall. Every speed governor will send a “more power needed” signal to its unit to stop the frequency from falling.

How fast the speed drops, depends on the sum of the moments of inertia of all the units connected to the grid.

If the number of units connected to the grid is big enough, you won’t need a flywheel, but if you are on an island or small grid, they would definitely help you to maintain the stability of the grid, as the governors could not be fast enough.

2.- Reduce Overspeed

When a unit is generating, the mechanical energy that the runner supplies goes through the generator into the grid.

If the unit trips, it will stop sending energy to the grid, but until the governor reduces the water flow, the runner will still continue supplying that much energy.

If energy can not go into the grid, where will it go? In fact, it will increase the kinetic energy of the unit by increasing its speed. The acceleration of the unit will depend again on the moment of inertia.

A flywheel can reduce the over a speed of the unit by increasing the moment of inertia, which is a good thing for the mechanical stress on the unit.

It is usually used in small units as big units usually have big moments of inertia.