How does the electrical grid cope with small fluctuations like flipping a light switch?

Question

The operation of an electrical grid is a balancing act – you have to produce *exactly* the same amount of energy as is needed, otherwise bad things happen. I don’t understand how does this rule apply on anything other than the largest scale of things.

I understand that *in general* you can predict a higher load on the grid during Monday evening when everyone is at home, and plan your energy production accordingly. But a power station can’t predict smaller load fluctuations like if I decide to turn on or off my TV at this very moment.

So, how does the electrical grid cope with unpredictable load that differs from the planned & expected one?

In: Engineering

Answer 1

There is a “gross level” balancing act happening. But, you are speaking of a small instantaneous change. I think we can agree the generating facilities cannot change *power* output instantly – most being some form of mechanical thing with momentum.

However, roughly, we can think of *power* as :

*Volts x Amps = Power*

So, in a given instant, if *power* is constant (since the generating systems cannot instantly change), then the other two variables are what change. In particular, adding a load to a system draws more *Amps* from the system. If *Power* is to remain constant, then *Voltage* must decrease in the instant that more *Amps* are consumed.

Therefore, adding a load to a system creates a *voltage drop* in the system. All other active devices in the system see this voltage drop and thus receive slightly less power in that instant. The “new” active device is effectively “stealing” power from all other devices in the system in that instant. If the new load is tiny (like a 100w light bulb in a house), then the impact is effectively not noticeable. Also, the effect impacts devices closest to the load more. So, turning on a heavy appliance in a house may cause lights to dim in that house (voltage drop), but not in adjacent houses.

Ultimately, if enough loads are turned on, the loads are “felt” mechanically at the generators in the form of additional rotational resistance inside the generators. This force tries to slow the rotation of the generators, but governors in the generators are designed to keep them rotating at a constant RPM. The governors signal the power plant to ramp-up “steam” to the generator to meet the load.

There are also inductors (transformers) and capacitors in the electrical transmission and distribution systems that can “store” power that smooth-out any sort of minor disturbance.