Generator Load 6

[Parabola37]

I have a question that I get all different answers for.Here it is:
Steam driven generators.I completely understand the concept of what happens when you admit more steam to produce load.(Control valves open wider etc....)The turbine is locked in at 3600 rpm from the grid.What actually happens that the generator produces more load? Is it that the field gets stronger and produces more resistance?Does the phase angle change?

 

[abeltio]

this posting should more appropriately go to Electric Power Engineering.
nevertheless, i will attempt an explanation in layman's terms...

the unit has different ways to operate.

1. isochronous: the unit will maintain the speed no matter what load is demanded - týpical form: island mode operation.

2. droop control: the unit reacts delivering more load when the grid frequency drops, and viceversa unloads when the frequency rises

3. partial pre-selected load: the unit maintains a certain load independent of the grid requirements

4. base load: the unit is at its nominal load (cannot admit any more steam) and will follow frequency changes.
Power output = torque x speed
torque being constant.

The operator can raise the load with the governor only if the turbine did not reach its base load (is at a partial load) - and there is load available in the grid.
how does the generator accept more load? it raises the excitation current.
so, if there is no more load available in the grid... and the operator keeps raising with the governor the following will happen:
if there is no ISOCHRONOUS DRIVER in the grid...
the grid frequency and voltage will increase.

if there is an isochronous driver, this driver will unload to keep the frequency constant.

to understand the effect on voltage it is easier to understand it when some of the generators trip off-line...
there are only some generators left, which cannot supply all the load required... the frequency drops (it actually works like a brake on the generator) and the voltage drops (lights go dimmer) - you've surely seen this before.

hope this helps.

saludos.
a.

 

[ScottyUK]

Think of the generator and grid as being locked together as they rotate. When the generator is delivering power it is trying to accelerate and the grid provides a restraining torque. I've used the following analogy: it isn't perfect but it is easy to visualise.

Consider the grid and the generator as two rotating shafts having a mechanical coupling which comprises of two rotating discs linked by rubber band. The rubber band represents the magnetic field of the rotor. When the generator load is increased the rubber band stretches and the generator pulls slightly ahead in phase relative to the grid. This phase angle represents the load angle of the generator. If the generator delivers too much torque the rubber band will break, which equates to the generator breaking synch with the grid. In the event of the generator's prime mover failing the generator will adopt a negative load angle and the generator will slip in phase so it lags behind the grid in phase and the rubber band is taut once again.


Hope this helps a little.


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Sometimes I only open my mouth to swap feet...

 

[waross]

We answered a similar question regarding wind generation.
You may wish to look at thread238-171826
respectfully