I assume you're talking about a synchronous generator. It must remain sync'd to grid. If grid frequency is constant, machine speed constant. Google synchronous generator.
It's the nature of a synchronous machine that the rotor and stator magnetic fields stay in synchronicity (as long as the rotor field strength doesn't drop so low the machine slips a pole). If the torque provided by the prime mover changes due to a change in steam flow to the turbine, the angle between the magnetic fields of the stator and rotor will change, but they stay synchronized. Since power equals torque times speed, with a change in torque and constant rotor speed, the power output changes.
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
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The worm model of generation:
Every generator in the world has the same two knobs:
1. Throttle: This controls the driver power output
2. Field Current: This controls the magnitude of the spinning magnetic field inside the stator.
The knobs cause different generator actions depending on if the generator is islanded or paralleled.
Islanded: The load is what it is, a combination of reactance and resistance.
If one opens the throttle, the driver output goes up. The load didn’t change, so generator speeds up – the frequency goes up, until the load accepts the additional power. (Yes, the voltage will increase a bit – ignore for now)
If one turns up the field current, the spinning magnetic field increases. This raises the voltage. (Yes, the gen will slow down a bit – ignore for now)
Basic principle for islanded: Throttle controls frequency, Field current controls voltage
Paralleled: Consider the bus voltage and frequency are fixed by the other generation on-line. Perhaps not exactly true, but close enough for this model
If one turns up the throttle, the driver is putting out more power, but the generator can’t speed up. The generator exports the power to the bus. The load has not changed, so consider the other generators back down and put out less power.
If one turns up the field current, the voltage can’t go up. The generator is over excited, and will export vars. Again, the load didn’t change, so the other generation has to back down and produce less vars.
Basic principle for Parallel operation: The throttle controls KW sharing, the field current controls KVAR sharing.
Yes this is simplistic – But correct for understanding why the generator reacts differently under island or parallel.
If you generate more than your load, the frequency MUST increase.
NOW...the change in speed is propertional to the excess generation versus the total load. so if your unit load increase is a very small percent of the total system, the change in speed will be very small. so you will need a few decimal places on your frequency meter to see the effect
I think both explanations happen, just not like you think.
If a generator is in sync. with the grid (any grid), then increasing the steam will increase the speed of the generator as well as the grid. With this increase in speed, the grid will offer an increase in power requirements. So the increase in load (power delivered) on the generator is proof that you have increased the speed of the grid, or someone else has decreased there output on there generator. But the standards state the grid frequency should remain at XX. So very quickly as the frequency rises, someone has the duty to decrease steam on there generator to bring the frequency back to the normal value.
So yes you do increase the speed as you increase the steam, but someone else will attempt to reduce there steam to keep the system frequency at normal. So you won't notice an increase in speed no matter how much steam you apply, as long as you are in sync. with your generator.
Or if the grid is large enough, the speed increase may not be large enough for anyone to notice (like pissing in the ocean, you don't see the water level rise). When they say a power grid supports 18 TW of generation, it can be assumed the load can reach about 80% of that or 14 TW. How many Giga watts can you generate to speed up the grid?
Spring "S" tension represents the power drawn or given, alfa is the torque angle.
Both rotor and stator fields turn at same speed but rotor "leads" the stator or "lags" if it works as motor like in the figure.
if it leads or lags too much will lose synchronism with stator field, and will do a mess (I don´t have the feeling of what exactly happens in that situation, apart from all the overcurrent protections tripping I suppouse)
Maybe I should say it this way: As you leap off a lader, you fall to the earth. The earth falls twords your gravitional pull also. But being that the earth is much more massive than you, the movement of the earth twards you is so small that it can't be measured.
xj is also correct, in that the magnetic field angle will change. Up until you reach pull out torque, at which point I don't want to be around.
Adding a little to xj's diagram, increasing the field current effectively makes the spring more powerful, while weakening the filed makes the spring less powerful. I'm sure you can visualise what happens with a more or less powerful spring.
This is the first time I've seen it sketched although it's an analogy I've used plenty times - the date on the diagram just shows there is nothing much new in the world!
Isn't this beginning to look a little like the beer/froth thing?
The name synchronous says it all - in step with each other. Motor is in step with grid. And grid doesn't change speed. Hence the generator doesn't either. The torque changes and the pole angle increases. Up to out-of-sync. And then the speed changes. But not in normal operation.
The mechanism behind has been mentioned and well explained by some and less well by some, more verbose, posters. An explanation that involves physiscs and math is probably beyond the OP and therefore not of much use. Electricpete gave an adequate explanation 6 Sep 12 10:39.
The analogy posted by xj has proven useful to plant operators who are principally mechanical or process guys trying to better understand how the machine they are controlling operates. But I agree than analogies only go so far before they grow so complex that they become confusing.
Agree. The mechanical analogy has also been used at the Royal School of Technology, KTH, in Stockholm. A "friction disk" was added to explain induction motors. It seems to have worked quite well. Several good engineers use this model by V. Török, also for quantative calculations.
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