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What actually controls the speed when we synchronise a generator with the grid, previously say it was running at 3000rpm at no load, but at synchronisation the speed should go down but how does governing system controls the speed at that time?
Dear SirOh, nikkumnit,
You've fallen victim to the ivory-tower nonsense printed in a LOT of textbooks and references about Droop Speed Control and full load speed and no load speed.
There has been an awful lot written here on control.com--so much that sometimes it feels like it should be DroopSpeedControl.com. There is a helpful 'Search' function at the top of every control.com webpage that can be used unlock a plethora of information from previous posts.
But, I'm going to take a few minutes to try to directly answer your question.
A synchronous generator (more correctly called an alternator) and its prime mover must be synchronized when it is connected in parallel with any other synchronous generator and prime mover.
That means that the AC sine waves being produced by the "incoming" generator must be in phase with the AC sine waves being produced by all the other generators--which implies that the frequency of all of the other generators must be exactly the same <b>and</b> that the frequency of the incoming generator must be very nearly equal at the time of synchronization. (In fact it can be made to be exactly equal to the frequency of all of the other generators it is being synchronized to--but common practice is for the frequency to be just a little bit higher (a fraction of a Hz). However, once the generator breaker closes the speed is fixed by the frequency of the grid--read on.
Once all the permissives for synchronization have been met and the generator breaker is closed and there is current flowing in the generator stator, the magnetic field produced by the current flowing in the generator stator <b>locks the magnetic field of the generator rotor into synchronism.</b> This means that the generator rotor can't spin any faster or slower than synchronous speed (which for a two-pole generator synchronized to a grid operating at 50.0 Hz is 3000 RPM).
<b>Every synchronous generator synchronized to the same grid operates at its synchronous speed--and no faster nor slower <i>because of the magnetic forces at work inside every generator keeping the generator rotors locked into synchronism with the magnetic fields created by the current flowing in the stators of the generators which appear to rotate around the stator at a speed proportional to the frequency of the grid.</i></b>
That's what synchronism means--every generator rotor is locked into its synchronous speed with every other generator when multiple generators are operating in parallel (synchronized). One generator can't be running at 49.79 Hz, and another at 50.13 Hz, another at 48.6 Hz, another at 49.99 Hz, another at 50.0 Hz, another at 50.92 Hz--they ALL have to be operating at 50 Hz (or whatever the grid frequency is they are all connected to--nominally 50, or 60, Hz).
If any synchronous generator could operate at any speed while connected (synchronized) to a grid with other generators then why would it be necessary to synchronize the incoming generator? Why not just close the generator breaker regardless of the frequency or voltage differential and just start generating? The process of synchronization is rather involved and particular, and there's a reason for that and it doesn't change once the generator breaker closes. Not for any synchronous generator and its prime mover.
It's all about magnetism and the strengths of the magnetic fields inside the generators. There are "two" magnetic fields inside the synchronous generator--the one produced by the DC current flowing in the generator rotor windings, and the one produced by the AC current flowing in the generator stator windings. The one produced by the AC current flowing in the generator stator windings "appears" to rotate around the machine, and it keeps the rotating magnetic field of the generator rotor locked into synchronism with the apparently rotating magnetic fields of the stator preventing any single generator from running faster or slower than the speed corresponding to the frequency--when multiple generators are synchronized together.
So, it's <b>physically impossible</b> for a single synchronous generator and its prime mover to decrease (or increase) speed when it is synchronized with other synchronous generators and their prime movers. It just can't happen. Full stop. Period. (Under normal grid stability conditions.)
If a <b>single</b> generator and its prime mover was connected to a distribution network (a "grid") and no other synchronous generators were connected (synchronized) to that "grid" and the prime mover governor was operating in Droop Speed Control mode and had a droop setting of 4%, when the load was zero the machine (presuming it was a two-pole generator) would be operating at 3000 RPM to produce 50.0 Hz. As load was increased to 25% of the rating of the prime mover, the speed of the prime mover and generator would decrease to 99% of rated, and the frequency would drop to 49.5 Hz.
If the load were further increased to 75% of the rating of the prime mover, the speed of the generator and prime mover would decrease to 97% of rated, and the frequency would decrease to 48.5 Hz. But this <b>ONLY</b> happens when it's a single synchronous generator supplying a load and the generator's prime mover governor is operating in Droop Speed Control Mode.
Droop Speed Control is about how much the load changes--how much the energy flow-rate into the prime mover changes because load and energy flow-rate are directly proportional--when the error between the prime mover speed reference and the prime mover actual speed changes. It's a linear relationship and it can be explained using a simple y=mx+b function, where "x" is the error between the speed reference and the actual speed (Speed Reference minus Actual Speed in most cases).
So, those references and texts which say the speed decreases when the load increases are not being completely honest about the conditions when they say that. As you read on, you will see they are technically correct but they aren't properly stating all of the conditions for their charts and graphs.
In reality when multiple synchronous generators and their prime movers are synchronized together they operate as one "giant" synchronous generator and prime mover--again, because the magnetic forces inside each generator keep every generator spinning at its synchronous speed. And, all of the motors and televisions and lights and computers and computer monitors connected to the grid appears to be one "giant" load to the one generator and its prime mover.
When the amount of torque being provided by "the" prime mover is exactly equal to the amount required by the load and the frequency is at rated (50 Hz in your example) then the power provided by "the" generator is sufficient to supply "the" load at rated frequency.
Let's take a grid with many generators and prime movers supplying a load and the frequency is 50.0 Hz. Now, let's say that someone starts a group of very large AC motors (driving pumps moving water somewhere). And, further let's say that the energy flow-rates into all of the prime movers remained constant when the group of AC pump motors was started. The grid frequency would decrease in this case below 50.0 Hz--how much? By an amount equal to the relation of the increase in load to the total load on the grid.
It might be a lot, it might be very little--but the frequency (speed) <b>will</b> change. And, this DOES match what the texts and references say happens when load is increased on "a" synchronous generator with it's prime mover operating in Droop Speed Control Mode.
So, technically, the texts and references are correct, but the speed of one generator and prime mover doesn't change--the speed of all generators and their prime movers changes. And the speed changes as a function of the relative change in load to the total load.
Now, let's say someone synchronizes another generator to the grid and the load hasn't changed. As the new generator is loaded (the energy flow-rate into the generator's prime mover is increased) if nothing is done to any of the other generator prime movers synchronized to the grid the frequency of the grid will increase--because there's too much energy for the
AshutoshMendiratta,
There is a limit to the amount of torque the synchronous generator's prime mover can produce and transmit to the generator via the load coupling (and load gear box
Thanks alot Sir. You have explained to full satisfaction.
Next obvious question is if GTG is at 16 MW only, and we do not want to cross this limit to 18 MW or achieve base load, how to stop machine at 16 MW only. Suppose grid frequency is 48 Hz.
Thanks for extending support.