How do we change the power of an electric generator without changing the frequency?

Let's suppose we have a gas turbine which is connected to an electric generator.Now it says that we can increase the power of the electric generator,but to increase that power we would need to increase the rotation of the gas turbine,but increasing the rotation we would change the frequence of the electricity we are generating.So how do we increase electric power with the same RPM and the same frequency?
 
I may have missed something obvious here - if so other members will correct me ....

In contract meetings with mechanical engineers, many solutions would compare with a motor car: and would suggest the same here.
The battery would sit there until it goes dark - the lights are turned on, and to charge the battery we have an alternator; but because the supply is DC the alternator frequency does not matter. We could do a modern comparison with an electric vehicle (ev) except the battery charger usually takes a 'mains' supply; likewise the supply is DC the supply frequency does not matter.

We now have a scenario of a diesel or gas turbine (gt) driving an AC generator. The regulator of the diesel or gt maintains constant speed to give us constant frequency (of 50 or 60Hz). Application of load on the generator would tend to slow it's speed, but is corrected by the regulator. Increase in loading would obviously increase power to the 'genset' until reaching generator full power specification or overload trip, hopefully both being one of the same.
Likewise decreasing the load would increase supply frequency but is compensated for by the regulator.

Is this a too simplistic explanation of part of AC theory ??
 
I may have missed something obvious here - if so other members will correct me ....

In contract meetings with mechanical engineers, many solutions would compare with a motor car: and would suggest the same here.
The battery would sit there until it goes dark - the lights are turned on, and to charge the battery we have an alternator; but because the supply is DC the alternator frequency does not matter. We could do a modern comparison with an electric vehicle (ev) except the battery charger usually takes a 'mains' supply; likewise the supply is DC the supply frequency does not matter.

We now have a scenario of a diesel or gas turbine (gt) driving an AC generator. The regulator of the diesel or gt maintains constant speed to give us constant frequency (of 50 or 60Hz). Application of load on the generator would tend to slow it's speed, but is corrected by the regulator. Increase in loading would obviously increase power to the 'genset' until reaching generator full power specification or overload trip, hopefully both being one of the same.
Likewise decreasing the load would increase supply frequency but is compensated for by the regulator.

Is this a too simplistic explanation of part of AC theory ??
Hello,thanks for your reply,it makes sense!But I have a question,for example I have a load of 2,5MW connected to the electrical generator,suddenly this load changes,lets say to 5MW,is the gas turbine able to keep with this change,cause now we have to give double the power,(mechanically the gas turbine is able to give this type of power),so the question is can the gas turbine react so swiftly as to double the power maintaining the same speed for the LP shaft,if a double shaft gas turbine or the HP shaft if it is single shaft turbine which generates electricity?
 
There have been a LOT of posts on here about load sharing and droop. A key point is whether you're talking about an island operation or if it's connected to an "infinite" grid. If it's an "infinite" grid, your turbine will not change speed. It CAN'T change speed.

It looks like it's an island system. If that's the case, it will depend on how fast the governor control can react to a sudden change in load. I'm by no means a power generation guy (I have some experience with a diesel-electric propulsion system on an oilfield service vessel) but that seems like a large change to me and I would not be at all surprised to see a noticeable speed fluctuation. Whether it's "too much" fluctuation depends on the turbine's specs and the loads it's supplying.
 
Following Points need to be looked at:
1. "to increase that power we would need to increase the rotation of the gas turbine"

This is true only when you are transmitting mechanical power.
Like stepping on the gas of car, engine rpm increases hence the power.
Or the case when a compressor is coupled to a gas turbine when the load on the compressor increases the RPM needs to go up as well.

When it comes to Electro-Magnetic Energy: The Load (MW of Generation) is the RESISTANCE to the rotation of the shaft. For a given generator the speed needs to be constant (eg. 1500 RPM for 4 pole Generator in a 50Hz system), as the load increases so does the resistance to rotation of the generator shaft hence you need to burn more fuel to keep the generator running at the same speed.

More practical way of imagining it is try cycling, at a constant speed. Now try to maintain the same speed uphill. You need to spend more energy for keeping it at same speed.


2. "Likewise decreasing the load would increase supply frequency but is compensated for by the regulator."
This relates to Droop control Characteristic and is a totally different thing.

3. "can the gas turbine react so swiftly as to double the power maintaining the same speed"
The answer is yes! The gas supply pressure is way above what is required at the burner of a GT.

Should such a situation occur the Fuel valves open(very quickly) and boost the generation with a minimum drop in frequency. Every Generator has a loading rate X kW/s that can give you a idea about how quickly can the generator react.
 
Following Points need to be looked at:
1. "to increase that power we would need to increase the rotation of the gas turbine"

This is true only when you are transmitting mechanical power.
Like stepping on the gas of car, engine rpm increases hence the power.
Or the case when a compressor is coupled to a gas turbine when the load on the compressor increases the RPM needs to go up as well.

When it comes to Electro-Magnetic Energy: The Load (MW of Generation) is the RESISTANCE to the rotation of the shaft. For a given generator the speed needs to be constant (eg. 1500 RPM for 4 pole Generator in a 50Hz system), as the load increases so does the resistance to rotation of the generator shaft hence you need to burn more fuel to keep the generator running at the same speed.

More practical way of imagining it is try cycling, at a constant speed. Now try to maintain the same speed uphill. You need to spend more energy for keeping it at same speed.


2. "Likewise decreasing the load would increase supply frequency but is compensated for by the regulator."
This relates to Droop control Characteristic and is a totally different thing.

3. "can the gas turbine react so swiftly as to double the power maintaining the same speed"
The answer is yes! The gas supply pressure is way above what is required at the burner of a GT.

Should such a situation occur the Fuel valves open(very quickly) and boost the generation with a minimum drop in frequency. Every Generator has a loading rate X kW/s that can give you a idea about how quickly can the generator react.
Thank you,is there any book which explains these things,maybe in a very simplistic manner?
 
Hello,thanks for your reply,it makes sense!But I have a question,for example I have a load of 2,5MW connected to the electrical generator,suddenly this load changes,lets say to 5MW,is the gas turbine able to keep with this change,cause now we have to give double the power,(mechanically the gas turbine is able to give this type of power),so the question is can the gas turbine react so swiftly as to double the power maintaining the same speed for the LP shaft,if a double shaft gas turbine or the HP shaft if it is single shaft turbine which generates electricity?
When load increases from 2.5 MW to 5 MW, speed/ frequency starts falling and control system responds to increase fuel input so that generation can increase to 5 MW to match that of new load. At settling time speed/frequency however does not return to predisturbance value due to "droop" characterstic. This assumes GT has sufficient capacity.
 
When load increases from 2.5 MW to 5 MW, speed/ frequency starts falling and control system responds to increase fuel input so that generation can increase to 5 MW to match that of new load. At settling time speed/frequency however does not return to predisturbance value due to "droop" characterstic. This assumes GT has sufficient capacity.
ok,it makes sense,but for a small amount of time (when the gas turbine is sending enough fuel to the combustion chamber to go from 2,5MW to 5MW) the frequency is not going to be ideal (by that I mean not 50 Hz,or not ideal in the sense that it is not constant it is changing) ,and we cannot connect that generator to the cirucit,no?
 
Let's suppose we have a gas turbine which is connected to an electric generator. Now it says that we can increase the power of the electric generator, but to increase that power we would need to increase the rotation of the gas turbine, but increasing the rotation we would change the frequence of the electricity we are generating. So how do we increase electric power with the same RPM and the same frequency?
If your generator is connected to a electrical grid, typical one generator alone can't change the frequency of that grid (The typical electrical grids are very stiff) starting with that basic concept (frequency will not change); what will change as soon as you push more fuel into your GT "nonetheless you are "trying" to change the frequency" but it will not do nothing (or will be neglected), this extra power and fuel added to your unit will be converted into MW, by advancing into the generator the angle "electromagnetic" between the rotor and stator, and with that you will push more power out of it without frequency change (remember that a coupled generator into a typical electrical grid the rotation speed of that generator is dictated by the grid frequency, and not the opposite, a generator alone by it self will not determine the frequency system grid pace).
Now if your grid is very small and very weak, and if that generator could take part in changing the frequency (when you change the fuel input) if the frequency changes means that you can be unbalanced your your loads (more Pgen than Pload will make frequency go up, and vice versa).
For the case you are disconnected from any grid GCB open, this means that you can change freely the speed of your GT and Generator, but also there is no Power being produced.
 
ok,it makes sense,but for a small amount of time (when the gas turbine is sending enough fuel to the combustion chamber to go from 2,5MW to 5MW) the frequency is not going to be ideal (by that I mean not 50 Hz,or not ideal in the sense that it is not constant it is changing) ,and we cannot connect that generator to the cirucit,no?
Generator will continue to be connected to load. Turbine control can take care. In the rare event when control fails, under frequency relays etc can act and protect machine.
 
The way it was explained to me:
The gas turbine is rotating an electromagnet, producing a rotating flux field. The current in the stator is also producing a rotating flux field. The resulting rotating field is the vector sum of the two. The angular difference of the two fields is referred as the torque angle. The are locked in place magnetically as one pushes on the other. When fuel is added to the gas turbine, more torque is created making the field flux push harder against the stator flux. This changes the torque angle and the resultant flux thus changing the power generated.
I look at it as if we are pushing on a brick wall which is leaning towards us. If one on the people holding up the wall steps away and I want to push harder to prevent the wall from falling, I move my feet back to get a better angle of attack.
 
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