Why do generators experience speed droop during peak demand in power systems?

[jonlg_uk]

Hi All, when a power system is in peak demand how come the generators that create the power experience a speed droop and hence a drop in frequency?

Can somebody please explain this to me in basic terms...

I thank you in advance

 

[vk6kro]

Generators get harder to turn as they deliver more power.

So, unless they get more power from the engine driving the generator, the speed would tend to decrease.

However this is only a momentary effect and automatic speed regulators act to apply more acceleration to the power unit and bring the generator back up to speed.

In power stations, these generators are large heavy machines and they do not really vary in speed in the short term.
The automatic controls sense the extra current and apply more power before the generator gets a chance to slow down.

 

[jonlg_uk]

Generators get harder to turn as they deliver more power.

Thanks for your help. Can you elaborate on this?

A 3ph generator is basically an induction motor with its rotor windings energized with a DC source, the prime mover is responsible for turning the rotor and the DC field created by the rotor induces an AC field in each of the coils, each AC wave is separated by 120 degrees. I know that much...but what is going on in terms of current and voltage that causes the generator to slow down in times of peak demand?

 

[vk6kro]

I guess it is easiest to explain in terms of Lenz's Law.

Imagine a coil of wire that is short circuited. Now approach it with a magnet.
It will generate a current into the short circuit to produce a field that opposes the movement of the magnet.
So the magnet gets harder to move.

This is what happens in a generator. As more current flows into the load, the field generated by the coils gets greater and this makes the generator harder to turn.

This also meets the Law of Conservation of Energy. If you could rotate a 100 KVA generator by pedalling a bicycle, then you would be getting a lot of power out without putting much power into the generator. Obviously, you can't do that.

 

[jonlg_uk]

I guess it is easiest to explain in terms of Lenz's Law.

Imagine a coil of wire that is short circuited. Now approach it with a magnet.
It will generate a current into the short circuit to produce a field that opposes the movement of the magnet.
So the magnet gets harder to move.

This is what happens in a generator. As more current flows into the load, the field generated by the coils gets greater and this makes the generator harder to turn.

This also meets the Law of Conservation of Energy. If you could rotate a 100 KVA generator by pedalling a bicycle, then you would be getting a lot of power out without putting much power into the generator. Obviously, you can't do that.

Thank you!...the greater the load,the more current flows in the circuit, the greater the magnetic "back" field generated, this magnetic field generated in the coils looks to oppose the magnetic field that created it in the stator...hence the rotor will slow down because the stator field is weakened by the field generated in the coils, is that correct?

 

[Mike_In_Plano]

Yep, that's pretty much it. The grids constantly adjust speed by supplying or removing steam as required.
If you trigger your scope on the AC line and watch a stable oscillator, you can watch the speed increase and decrease, though it's not a very pronounced effect.

An aside -
Years ago, I was contracted to design parts of an AC invertor, and one of the employees at the client was hostile. When I delivered the clock and it did not lock agree perfectly with the 60 Hz line, he argued it was proof of the clock's instability. Fortunately, I had a second board, and we connected them side-by-side. They tracked rock steady. This was my introduction that the line frequency did walk about a bit, though the utility always magaed to get the average frequncy correct or the AC clocks would be off.

-Mike

 

[jonlg_uk]

I guess it is easiest to explain in terms of Lenz's Law.

So the magnet gets harder to move.

Will the generator actually require greater torque to turn it now? if so why? Can you explain this in terms of magnetic fields generated due to currents varying.

Thank You.

 

[sophiecentaur]

Power = torque times angular speed

If the power you want to deliver is greater and the speed needs to be the same, the torque must be increased.
It's like driving on the flat and then coming to a hill - to keep the same speed, you need more right welly.