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

#### nikidi.control

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?

#### oneye14

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 ??

#### nikidi.control

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?

#### joseph_e2

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.

#### salz

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.

#### nikidi.control

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?

#### salz

This was all from experience!

#### drmsrmurty

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.

#### nikidi.control

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?

#### FarOut

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.

#### drmsrmurty

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.

#### MWO

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.

#### Amitbajpayee

The power output of an electric generator is proportional to both the rotational speed (RPM) of the generator and the strength of the magnetic field within the generator. Change the power output without changing the frequency, there are a few different ways that this can be accomplished:
1. Adjust the magnetic field strength: The power output of a generator is directly proportional to the strength of the magnetic field within the generator. By adjusting the field strength, the generator's power output can be increased or decreased without affecting the frequency of the output.
2. Adjust the load on the generator: The power output of a generator is also directly proportional to the amount of load that is placed on it. By adjusting the load on the generator, its power output can be increased or decreased without affecting the frequency of the output.
3. Adjust the voltage regulator: The voltage regulator in a generator controls the voltage output of the generator. By adjusting the voltage regulator, the generator's power output can be increased or decreased without affecting the frequency of the output.

#### drmsrmurty

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.
A Gas turbine generator (GTG) is either synchronized to a grid or connected an isolated load.
Under grid connected operation, generator output is gradually increased by increasing the fuel flow till rated value. Once synchronized to a grid , frequency (or speed) does not change.
If generator is connected to an isolated load, (usually at minimum load) load is gradually increased and speed/ frequency tries to fall which is sensed by the governor and fuel flow is increased. This closed loop action goes on till generation matches the load value.
This change in frequency from rated value (say 50 Hz) is part of normal control. The steady state frequency is 50 Hz. In reality, in a grid, there will be small load variations continuously and frequency also changes. When the frequency deviations are beyond a threhold value ("dead band") governor acts.
You may please go through some standard power system text books (like Electric Energy Systems Theory by Elegerd, Power system analysis by Grainger and Stevenson) also to understand swing equation and frequency control.

#### WTF?

Amitbajpayee,

Synchronous generators are devices that convert mechanical power (torque) into electrical power (watts, kW, MW). Synchronous generators generally operate at a relatively constant terminal voltage and a varying current (amperes). Three-phase power (real power--watts, kW, MW) can be calculated by multiplying the terminal voltage times the amperes times the square root of three times the power factor of the load being supplied (carried) by the generator. This means that to change the real power output of a synchronous generator it's necessary to change the amount of mechanical power (torque) being applied to the generator rotor, which changes the amperes being produced by the generator which changes the power output (real power--watts, kW, MW) of the generator.

Most synchronous generators have a range of allowable generator terminal voltage--usually plus or minus 5% of nameplate rated terminal voltage. That means that under normal operating conditions changing the generator terminal voltage will only change the power output (presuming the amperes (mechanical power input--torque) stays constant by as much as 5% of rated.

What really happens, though, when generator excitation is varied to change generator terminal voltage is that the reactive current of the generator changes--the VAr flow, or kVAr, or MVAr, but not so much the real power output--because the mechanical power input to the generator (torque) is most likely remaining relatively constant when the excitation is being varied to change the reactive current of the generator.

And drmsmurty clarified very well the rest of what I was going to say. Typically a prime mover and the generator it is driving connected to a grid of any size in parallel with other generators and their prime movers is operated in Droop Speed Control mode (typically, but especially for large and very large (so-called "infinite" grids). A prime mover and generator that is independently powering a very small grid/system, or is only connected to one or two other prime movers and their generators, is operated in Isochronous Speed Control mode--and that is the mode that causes the prime mover to adjust its mechanical power output (torque input) to the synchronous generator as motors and refrigerators and tea kettles and lights and computers and computer monitors are turned on and off causing the load being carried by the grid/system to change--but in an effort to keep the grid/system frequency at nominal/rated the Isochronous machine quickly and automatically changes the prime mover's mechanical power output (torque input) to the synchronous generator. (Larger grids/systems with many prime movers and generators don't typically have an Isochronous machine--it's the grid/system operator's responsibility to vary one or more prime movers' outputs to maintain frequency (because Droop machines don't really care about maintaing grid frequency--only the grid load). But, the topic of Droop Speed Control, and to an extent, Isochronous Speed Control, has been covered in tens of threads on Control.com (so often in fact that sometimes it seems like the name of this forum should be SpeedControl.com). ;-)

Anyway, what you wrote is pretty much what most textbooks and reference books on the subject of prime movers and synchronous generators say--because most of the people who write those tomes have little or no actual operating experience of real-world power systems, just mathematical "understanding" of basic fundamentals which aren't exactly how machine work in reality. In other words, an "ivory tower" view of the concept, not a down-to-earth, practical view and understanding.

#### ZubairKhan

Changing the power output of an electric generator without changing the frequency can be achieved by adjusting the voltage and current supplied to the generator, either by controlling the load or by using a voltage regulator.

#### WTF?

ZubairKhan,

What kind of generator, and on which planet in which solar system?

Do you have actual operating experience with AC synchronous generators (alternators) driven by steam or combustion turbines or reciprocating engines? (On planet Earth in our solar system?)

How much would one have to increase the voltage and current supplied to the generator [rotor] of a synchronous generator (alternator) to raise the output of a machine rated at 25 MW from 5 MW to 20 MW?

#### WTF?

Oh, ZubairKhan, where does the current and voltage supplied come from? Current times voltage is Watts/kW/MW—power. What is the source of that power (current and voltage)?

If what you say is true, why then is a prime mover required to operate a generator? Why not just supply current and voltage to the load without a prime mover and generator—since electric loads require current and voltage?

Thank you.

#### ZubairKhan

Thank you for your response! My post was referring to AC synchronous generators on planet Earth in our solar system, particularly those driven by steam or combustion turbines or reciprocating engines.

Regarding your question about increasing the power output of a 25 MW generator from 5 MW to 20 MW, it would depend on the specific characteristics of the generator and the load being driven. In general, increasing the voltage and current supplied to the generator's rotor can increase its output, but this must be done within safe operating limits and with proper regulation to avoid damaging the generator or the connected load.

Adjusting the voltage and current can be accomplished through various means, such as using voltage regulators or controlling the load on the generator. The exact method used would depend on the specifics of the generator and the load.

It's important to note that making significant changes to the power output of a generator should only be done with proper training and experience, as there can be significant risks involved.