every spinning generator has stored energy in the rotor's inertia. Some turbine-generators have flywheels on the shaft to increase the inertia and decrease cha

every spinning generator has stored energy in the rotor's inertia. Some turbine-generators have flywheels on the shaft to increase the inertia and decrease changes in speed that happen when there is a mismatch in load (power out) and generation (power in). When there is a mismatch in load and generation on the grid every single generator slows down or speeds up. you can monitor this by measuring the frequency of the voltage anywhere on the grid, it will be the identical measurement at all times (except where DC interconnects are involved). Every generator should have a 5% speed-droop characteristic where a 5% change in speed results in a 100% change in output. So if you were running your hydro plant at 80% output with 60 Hz speed setpoint, and some other generator tripped resulting in more load than generation, and the frequency on the grid dips to 59.9 Hz (a 0.1% change), your hydro plant would increase the output by 1/5% * 0.1% = 2%, to 82%, as would every other generation source that can control its throttle, and once again generation and load are matched, but the frequency is now 59.9. A couple of special power plants (running in Automatic Generation Control (AGC) mode, have control systems with a big picture view of large portions of the grid) will increase their output to restore the lost generation, raising the frequency back to 60.0, and re-balancing whatever power flows were scheduled across the various transmission interties for that hour. Any source where you can control the throttle should have this 5% droop characteristic. wind turbines and solar distributed generation without batteries can't provide it, so not only do we need more dispatchable resources like gas, hydro, etc to be able to quickly make up the variations in wind and solar output, but the conventional plants either can't run as close to 100% output or have to have increased droop to make up for the sources that have zero droop.