# Homework Help: Balance between generated and consumed power

1. Jul 31, 2013

### bd411

1. The problem statement, all variables and given/known data

Consider an AC electricity system operating with a balance between generated power and the consumed power. Explain what happens to the frequency of a system if a large generator is suddenly disconnected. Explain the role of the governors of the remaining generators in controlling the frequency.

2. Relevant equations

N/A

3. The attempt at a solution

Hello all :)

I am having some trouble understanding the relationship between generated power, consumed power and frequency of the system.

I understand that when generation exceeds demand the excess energy builds up in the inertia of the generators and the frequency rises. I'm looking for the oppositely equivalent statement, i.e. "when demand exceeds generation.....".

My best guess at a solution is that when demand exceeds generated power, it exerts some sort of torque on the rotor in the opposite direction to its inertia and therefore decreases the frequency ? Kind of like Lenz's law ?

As for the second part of the question, it is the role of the governors to switch in and out generators in order to keep the frequency at its nominal value, which is 50hz +/- 1% in the UK. This ensures there is a very low possibility of a system wide collapse.

Any help with this would be very greatly appreciated. Thank you in advance.

2. Aug 2, 2013

### CWatters

Think of each power plant as an engine (eg diesel or steam) that turns a generator at 50 or 60 revolutions per second (depending on your country). If you increase the load on the generator that will increase the load on the engine causing it to slow down. If the engine slows down the rpm of the generator will slow and so will the frequency of the grid.

Edit: It's not really correct to think of this in terms of inertia although clearly the rotating machinery does have inertia.

Last edited: Aug 2, 2013
3. Aug 5, 2013

### bd411

Thanks a a lot for your reply, it's made it much easier to visualise what's going on :)