how does a sync. generator get slower as load increases? what makes it to slow down?

 

It won't slow down. Sync. generator is called that way because it works in parallelism with the grid. It will rotate exact the same frequency as the grid.

If you increase the load, it means that new fuel must be added (or steam, if steam turbine is a prime mover). You should look on the problem the other way round.

If you're adding more fuel or generating more steam, then generator will not rotate faster, it will continue to rotate at speed which matches the grid frequency. Newly added fuel will increase the torque which will increase the power output.

 

All of the discussion below presumes that the synchronous generators are driven by prime movers that are mechanically coupled to the generators. There are some turbines which exhaust into a "free turbine" which is coupled to a generator, but the main turbine's speed changes with load, but those are not being discussed or described here since they are not common (they do exist, but they are not common).

One of the most important aspects of an AC (Alternating Current) system is the stability of the frequency.

The frequency of a synchronous generator is directly proportional to the speed of the generator rotor, which is driven by a prime mover. The formula is:

F = (P * N) / 120

Where F = Frequency (in Hz)
P = Number of poles of the generator
N = RPM of the generator rotor

When a synchronous generator is being operated in parallel with other synchronous generators on an electrical system, or grid, which is supplying loads (motors, lights, computers, etc.) the frequency of the grid should be fairly stable, which according to the formula above means the speed of the generator rotor and the prime mover driving the generator rotor should be fairly stable.

The name 'synchronous' means that no generator (and therefore the prime mover driving the generator) can go faster or slower than any other generator with which it is paralleled. The magnetic fields of the generators are locked into synchronism with the apparently rotating magnetic fields produced by the circulation of current through the armature windings of the generator. All of the generators connected to a grid see the same frequency, so their generator rotors are locked into synchronism with each other as defined by the formula above. Two-pole generators on a 50 Hz system will run at 3000 RPM; four-pole generators on a 60 Hz system will run at 1800 RPM. In lock-step; in synchronism.

So, when multiple generators are being operated simultaneously and are connected in parallel with each other to drive loads that, in total, are larger than any single generator could provide then the entity which has responsibility for maintaining the grid frequency is supposed to be managing the system to maintain a stable grid frequency, which, implies stable generator (and prime mover) speed.

Maintaining stable grid frequency means controlling the amount of generation so that it matches the amount of load, anticipating load changes during the day (and night) and being able to respond to problems (loss of load "blocks"; tripped generators; etc.). If the entity responsible for maintaining grid frequency is doing their job properly, then as a synchronous generator is loaded there should be no appreciable change in speed.

Now, there is another case where a single synchronous generator is supplying a small load, and perhaps there are one or two other generators connected in parallel with that one generator. In that case, one of the prime movers driving one of the generators is usually being operated in Isochronous speed control mode, which means that the load of the unit (the generator and the prime mover) is controlled a function of frequency (speed), to automatically maintain a constant and stable frequency of the system. Any other generator connected to that system will be operating at the same frequency (speed) as the one whose prime mover governor is being operated in Isochronous speed control mode.

If the Isochronous speed control mode is not tuned properly or the system can't respond to load changed quickly enough then the frequency will not be stable.

There are some places where all the generators on a small electrical grid or system are operated in Droop speed control and in that case the generation must be manually controlled to maintain the frequency. In some cases, there is an external control system providing signals to the prime movers to do the frequency control; some work better than others.

So, if you are witness to a synchronous generator that is directly coupled to a prime mover and it slows down when it is loaded, then there is a problem somewhere in the system. Either the grid operator is not maintaining frequency properly or the prime mover's governor is not working properly (not likely if the generator is connected to a large grid in parallel with many other generators), or the load exceeds the capability of the prime mover driving the generator, or you have one of the few prime movers that is not directly coupled to the generator rotor. But something is amiss, or there is something about the generator you are describing which we do not know.

Finally, most everyone has ridden a bicycle. To ride a bicycle at a constant speed on a flat road requires some concentration on the part of the rider (the "control system", the governor, for the bicycle) to provide a constant amount of torque to the bicycle's crank. Now, if you're riding at a relatively slow speed and you start to ride up a hill (which can be considered a "load") then to maintain that same speed one has to produce more torque.

If one is riding very fast, producing nearly as much torque as one can produce, and starts to ride up a hill then it might not be possible to produce any more torque in order to maintain a constant speed and in that case the bicycle would start to slow down. In this case, the rider is "overloaded" and can't produce enough torque to maintain a constant speed.

The same thing can happen to a synchronous generator driving a load, or to many synchronous generators all driving a very large load: If the amount of the load exceeds the amount of generation capability, then the grid frequency will decrease. In other words, if all the generators are producing maximum output and the load increases there is no more capability on the part of any of the generators to produce any more torque and the frequency will start to decrease, which means the speed of all the generators will start to decrease.

Always think of the bicycle riding at a constant speed. It takes a constant amount of torque as long as the road is flat. If the rider tries to carry some baggage or has to ride up a hill, it requires more torque to maintain the same speed. A synchronous generator is no different. If the load (baggage or steepness of the hill) exceeds the capacity of the prime mover (the bicycle rider), then the generator (and its prime mover, and the bicycle and its rider) will slow down. If there's no more torque available there's no more torque available.

But it's difficult to conceive of a synchronous generator that is mechanically coupled to a prime mover that slows down when it is loaded if it is being operated in parallel with other generators and the system is being operated properly.

Hope this helps! This topic has been covered many times on control.com; usually under the heading of droop speed control. The 'Search' feature of control.com requires some learning, but it is very fast and very powerful. (It does have a Help display!) Don't be afraid to try multiple search terms/words and different combinations of terms and words in your endeavor to find information. Remember: Everyone doesn't always use the same words/terms to describe the same phenomena.

 

Like CSA has explained, you need to distinguish between droop and isochronous speed control. Droop is somewhat hard to understand. Basically that is feature of turbine governor system. It is a straight line on frequency vs load characteristic. Imagine that your machine is running on the full load and suddenly it trips (goes to no load condition). In such case, it will accelerate and thus increase the speed.

Now you can find the droop on the following way:
Let's say prime mover speed is 60Hz and when turbine trips it increases speed to 61.8 Hz. In that case, droop is: (61.8-60)*100/60 = 3%.

In such case, it is said that droop is 3%.

That is at least how it is explained to me. In this example, I can find the following problem: if generator trips, turbine will increase the speed until overspeed system trips the turbine.

I think that this can be measured only in case when turbine and generator trips together, but in that case I don't think that droop can be calculated well. In a conclusion, droop definition is a theoretical but cannot be determined in such way.

I agree that in general sync generator will rotate the same speed as grid, but I don't think that is the case when there is sudden change in load. For example, if turbine control valves suddenly change their position (reduce), load will change, but also there will be a change in the turbine speed (although a small one).

I think that this concept is not fully understood. I too, have problems with it. Droop is feature that belongs to the governing system. If governor is mechanical, then droop is constant. If governor is electrical, than, the droop can be adjusted. In any case, there must be some droop, otherwise, system can become unstable.

Anyway, when there is a sudden increase in load, at first there would be a small speed decrease, but governor will add more steam (for steam turbine) and speed will be restored but not to the same vale as before. This offset is cause by droop.

Isochronous speed control is present only when there is a small island or grid is weak (not infinity bus). Ischronous control mode will always restore the speed in case of load change.

Hope this helps a bit.