Spinning reserve,System inertia,Frequency variation

In simple terms,,

In order to control or change the frequency of a large system grid, more or less generation (power) must be added or removed from the system. The rate of speed change to the grid is proportional to the amount of energy change and the inertia of the system. This includes the inertia of ALL the rotating mass in the power grid. The rate of speed change response is determined by the amount of power change and the total inertia of the grid.

Think of your car going down the highway at a constant speed (Hz) and you want to accelerate to pass. You step on the pedal and give it more gas (power) but the weight and mass of your car causes a delay in speed change. You have to overcome the inertia to make the speed change (Hz per second).

In system operation, this is a constant normally calculated continuously by system computers and the energy has to be provided by the spinning reserve.

Once a system slows down (around 58 cycles) the energy required to restore it becomes very high and there comes a point where system recovery is impossible unless load is reduced. (load Shedding). At lower speeds your system is still trying to do the same work. The voltage has dropped so the current must go up.

As I remember the factors include is something like {Hz per Megawatt per minute}

Trivia: At midnight each night the power companies make a system time correction for the clocks in your house. This calculation and power correction is generally done over a 2 hour period so your clock will wake you up on time the next morning.

Source.. My association with the New York Power Pool in Guilderland New York many years ago.