Hi. I am hoping someone can answer a question that I have that I have been unable to find the answer to. How can you calculate the amount of torque (or power in watts) needed to twist or deflect the shaft of a gas turbine generator that is locked to the grid so that power can be generated?
Hi. I'm hoping someone on this forum can
- Thread starter katratzi
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Hi
You better speak about generator mechanical/elctrical torque/power than gas turbine to be synchronized to a grid ..4
Dont you know that before to calculate Electrical torque /power one should calculate mechanical torque as a function of speed ...
Here is the equation :
Cheers
James
Have a good read on this article it can help !
The answer is inside:
https://www.theengineeringknowledge.com/power-and-torque-in-synchronous-generator/
The answer is inside:
https://www.theengineeringknowledge.com/power-and-torque-in-synchronous-generator/
ControlsGuy25,
I was so hoping to learn a lot from the article--but it contains too much maths for me to be useful (in explaining what happens in a generator).
When a gas turbine is driving a generator it's usually a synchronous generator. And a synchronous generator is designed to produce electricity and power at a certain frequency, either 50 Hz or 60 Hz, usually. The gas turbine section of the machine sends torque to two places: the axial compressor (and any mechanical loads of the turbine and compressor, such as pumps driven by a gearbox, for example) and the generator. The basic rule of thumb is that when the gas turbine is operating at rated power output (the TURBINE's rated power output--NOT the generator's) approximately two (2) of every three (3) horsepower produced by the turbine section is going to drive the axial compressor, and one (1) of every three horsepower is left over to drive the generator, which converts the power (transmitted from the gas turbine in the form of torque through the load shaft/coupling) to electrical power--specifically, amperes.
BUT, to produce the electrical power the generator must be spinning at it's rated frequency--and THAT requires power to be developed by the turbine just to drive the axial compressor and to maintain rated generator frequency, which is directly proportional to speed.
If the generator is synchronized to a grid with other generators and their prime movers and the synchroscope being used for synchronization is moving very slowly (usually in a clock-wise direction) when the generator breaker actually closes and the generator output terminals are connected to the grid the amount of power being produced by the generator is very small, sometimes on the order of just a few kW. If you are watching the turbine speed very closely during synchronization you will see that if the grid frequency is nominally at 100.0% the turbine speed, if the synchroscope is spinning slowly in the clockwise direction, will be just slightly higher--say, at 100.08% or 100.10% or something like that. (Some gas turbine OEMs use 100.3% as the target speed for a machine being automatically synchronized, and in this case the synchroscope is spinning pretty quickly in the clockwise direction!). AND, when the generator breaker closes the speed of the turbine and generator slows IMMEDIATELY to 100.0% (or whatever the grid frequency is, and in our example we said it was (nominally) at 100.0%). That little bit of fuel that was causing the speed of the turbine-generator to be just slightly greater than grid frequency gets converted to electrical power--amperes, specifically--when the generator breaker closes. That's called positive power--flowing "out onto" the grid. And, it's happening because, usually, when the unit is synchronized to a grid the incoming generator (the one being synchronized to the grid) is made to be just a little faster than grid frequency by flowing a little more fuel than is required just to maintain the same frequency (speed) as the grid. And, the turbine control system maintains that little bit of extra fuel flow when the generator breaker closes--precisely to make sure that positive power (amperes) flow "out onto" the grid when the generator breaker closes and the unit is then synchronized to the grid. And, again--when the generator breaker closes it forces the generator and the prime mover driving it (a gas turbine in this example) to slow down to be EXACTLY equal to the speed which is proportional to the grid frequency. But, because there was a little extra fuel flowing at the time the generator breaker closed, and that fuel flow-rate is maintained after the generator breaker closes that results in a positive power output.
NOW, if you could watch the shaft coupling the turbine and generator when the generator breaker closed you would see it twist ever so slightly--because of the positive power flowing out of the generator because of the little extra fuel flowing into the turbine which was making the turbine-generator speed just a little higher than the speed proportional to the grid frequency. When the grid forces the generator--and its prime mover--to slow down to match grid frequency speed, that extra fuel causes the coupling shaft to twist a little bit because the turbine is TRYING to drive the generator faster (as fast as the turbine-generator was spinning during synchronization) but IT CAN'T--the grid is holding the speed equal to the speed proportional to grid frequency.
Now, if you DECREASED the fuel flow-rate very slightly, you would see the power output of the generator (the amperes) move towards zero, and maybe even just a little less than zero if you decreased it just a little too much. At this point, if the amperes are exactly zero and the generator breaker is closed the load shaft coupling the turbine and generator would have NO twist at all. Because the amount of torque being transmitted from the turbine to the generator is zero.
Now, if you INCREASED the fuel flow-rate you would see the amperes being produced by the generator would start to increase. You would also see the power (kW; MW) increase. IF you could see the load coupling shaft between the turbine and the generator it would be twisting--because the turbine is TRYING to make the generator shaft spin faster than speed proportional to grid frequency BUT the grid WILL NOT permit that to happen. As you increase the fuel flow-rate to the gas turbine and the gas turbine TRIES to spin the generator rotor (shaft) faster, the twist on the load coupling shaft will increase, BUT the generator rotor (shaft)--and the prime mover shaft--will remain at the speed that's proportional to the frequency of the grid.
I have seen videos of a line painted on the load coupling shaft between a prime mover and a synchronous generator that is illuminated by a stroboscope as the shaft is spinning. As the load on the turbine-generator is increased when the generator is synchronized to a grid (which is really the ONLY time the load on the turbine-generator can be increased...!) the line on the shaft "bends" or twists. That's often called load angle and I think that's what you are referring to. MOST generators and their prime movers have NO way of measuring the torque or the load angle or the amount of twist on the load coupling shaft. It's all happening, but it's not being measured. You can be assured that it's all been calculated during the design phase of the equipment, because if the load coupling shaft isn't strong enough to withstand the twist being applied by the prime mover to the generator it will break!
But, again, it DOES happen. If you are pedaling a bicycle at a constant rate on a flat road, and you increase the force you are applying to the pedals and the cranks the SAME THING happens. If you could see it, the crank actually "bends" or "twists" as you increase the force being applied to the pedals which in turn increases the speed of the bicycle. As the bicycle speed increases and stabilizes, the bending of the crank actually lessens a little, but there's still a slight bending action.
If your bicycle starts up an incline and starts to slow, if you want to maintain a constant speed, you will have to increase the pressure you are applying to the pedals/crank. And that will cause the crank to bend a little more because it's requiring more force (torque) to maintain the same speed while riding up the incline.
A prime mover driving a generator (sending torque to a generator) has to send more torque than is required to maintain rated frequency (the same frequency as the grid) in order for the generator to produce power. The more torque, the more power. This power--if the unit WERE NOT synchronized to the grid--would cause the generator shaft (rotor) speed to increase. BUT when the generator is synchronized to a grid THAT CAN'T happen!!! The grid keeps the generator shaft (rotor) spinning at a speed that is directly proportional to grid frequency--NO MATTER HOW HARD THE PRIME MOVER TRIES TO MAKE IT SPIN FASTER!!! And, that force, trying to make the generator shaft (rotor) spin faster, is causing the load coupling shaft to twist. More force, more twist. Less force, less twist. It's that simple.
A lot of textbooks and references seem to explain the amount of power being produced as being a function of the twist being applied. Maybe the maths make it seem that way, maybe it really IS that way. I don't know. In my mind, the resulting twisting of the load coupling shaft is the RESULT of trying to make the generator shaft (rotor) spin faster than the grid will allow it to spin. The amount of the twist (it seems to me) is also a function of the metal the load coupling shaft is made of. And, that means, it's not going to be EXACTLY the same for every single load coupling shaft on the same prime mover-generator combination for the same load on the generator.
So, all you ivory tower types can talk all you want about load angle and torque twist. Me? I'm a technician. I understand that there will be twisting action--but the ONLY control I have over the twisting action is the fuel "knob" of a gas turbine. Pure and simple. AND, there's not even a meter or any kind of measurement of load angle or twist on the load coupling shaft--it's all just a calculation.
Hope this helps!
I was so hoping to learn a lot from the article--but it contains too much maths for me to be useful (in explaining what happens in a generator).
When a gas turbine is driving a generator it's usually a synchronous generator. And a synchronous generator is designed to produce electricity and power at a certain frequency, either 50 Hz or 60 Hz, usually. The gas turbine section of the machine sends torque to two places: the axial compressor (and any mechanical loads of the turbine and compressor, such as pumps driven by a gearbox, for example) and the generator. The basic rule of thumb is that when the gas turbine is operating at rated power output (the TURBINE's rated power output--NOT the generator's) approximately two (2) of every three (3) horsepower produced by the turbine section is going to drive the axial compressor, and one (1) of every three horsepower is left over to drive the generator, which converts the power (transmitted from the gas turbine in the form of torque through the load shaft/coupling) to electrical power--specifically, amperes.
BUT, to produce the electrical power the generator must be spinning at it's rated frequency--and THAT requires power to be developed by the turbine just to drive the axial compressor and to maintain rated generator frequency, which is directly proportional to speed.
If the generator is synchronized to a grid with other generators and their prime movers and the synchroscope being used for synchronization is moving very slowly (usually in a clock-wise direction) when the generator breaker actually closes and the generator output terminals are connected to the grid the amount of power being produced by the generator is very small, sometimes on the order of just a few kW. If you are watching the turbine speed very closely during synchronization you will see that if the grid frequency is nominally at 100.0% the turbine speed, if the synchroscope is spinning slowly in the clockwise direction, will be just slightly higher--say, at 100.08% or 100.10% or something like that. (Some gas turbine OEMs use 100.3% as the target speed for a machine being automatically synchronized, and in this case the synchroscope is spinning pretty quickly in the clockwise direction!). AND, when the generator breaker closes the speed of the turbine and generator slows IMMEDIATELY to 100.0% (or whatever the grid frequency is, and in our example we said it was (nominally) at 100.0%). That little bit of fuel that was causing the speed of the turbine-generator to be just slightly greater than grid frequency gets converted to electrical power--amperes, specifically--when the generator breaker closes. That's called positive power--flowing "out onto" the grid. And, it's happening because, usually, when the unit is synchronized to a grid the incoming generator (the one being synchronized to the grid) is made to be just a little faster than grid frequency by flowing a little more fuel than is required just to maintain the same frequency (speed) as the grid. And, the turbine control system maintains that little bit of extra fuel flow when the generator breaker closes--precisely to make sure that positive power (amperes) flow "out onto" the grid when the generator breaker closes and the unit is then synchronized to the grid. And, again--when the generator breaker closes it forces the generator and the prime mover driving it (a gas turbine in this example) to slow down to be EXACTLY equal to the speed which is proportional to the grid frequency. But, because there was a little extra fuel flowing at the time the generator breaker closed, and that fuel flow-rate is maintained after the generator breaker closes that results in a positive power output.
NOW, if you could watch the shaft coupling the turbine and generator when the generator breaker closed you would see it twist ever so slightly--because of the positive power flowing out of the generator because of the little extra fuel flowing into the turbine which was making the turbine-generator speed just a little higher than the speed proportional to the grid frequency. When the grid forces the generator--and its prime mover--to slow down to match grid frequency speed, that extra fuel causes the coupling shaft to twist a little bit because the turbine is TRYING to drive the generator faster (as fast as the turbine-generator was spinning during synchronization) but IT CAN'T--the grid is holding the speed equal to the speed proportional to grid frequency.
Now, if you DECREASED the fuel flow-rate very slightly, you would see the power output of the generator (the amperes) move towards zero, and maybe even just a little less than zero if you decreased it just a little too much. At this point, if the amperes are exactly zero and the generator breaker is closed the load shaft coupling the turbine and generator would have NO twist at all. Because the amount of torque being transmitted from the turbine to the generator is zero.
Now, if you INCREASED the fuel flow-rate you would see the amperes being produced by the generator would start to increase. You would also see the power (kW; MW) increase. IF you could see the load coupling shaft between the turbine and the generator it would be twisting--because the turbine is TRYING to make the generator shaft spin faster than speed proportional to grid frequency BUT the grid WILL NOT permit that to happen. As you increase the fuel flow-rate to the gas turbine and the gas turbine TRIES to spin the generator rotor (shaft) faster, the twist on the load coupling shaft will increase, BUT the generator rotor (shaft)--and the prime mover shaft--will remain at the speed that's proportional to the frequency of the grid.
I have seen videos of a line painted on the load coupling shaft between a prime mover and a synchronous generator that is illuminated by a stroboscope as the shaft is spinning. As the load on the turbine-generator is increased when the generator is synchronized to a grid (which is really the ONLY time the load on the turbine-generator can be increased...!) the line on the shaft "bends" or twists. That's often called load angle and I think that's what you are referring to. MOST generators and their prime movers have NO way of measuring the torque or the load angle or the amount of twist on the load coupling shaft. It's all happening, but it's not being measured. You can be assured that it's all been calculated during the design phase of the equipment, because if the load coupling shaft isn't strong enough to withstand the twist being applied by the prime mover to the generator it will break!
But, again, it DOES happen. If you are pedaling a bicycle at a constant rate on a flat road, and you increase the force you are applying to the pedals and the cranks the SAME THING happens. If you could see it, the crank actually "bends" or "twists" as you increase the force being applied to the pedals which in turn increases the speed of the bicycle. As the bicycle speed increases and stabilizes, the bending of the crank actually lessens a little, but there's still a slight bending action.
If your bicycle starts up an incline and starts to slow, if you want to maintain a constant speed, you will have to increase the pressure you are applying to the pedals/crank. And that will cause the crank to bend a little more because it's requiring more force (torque) to maintain the same speed while riding up the incline.
A prime mover driving a generator (sending torque to a generator) has to send more torque than is required to maintain rated frequency (the same frequency as the grid) in order for the generator to produce power. The more torque, the more power. This power--if the unit WERE NOT synchronized to the grid--would cause the generator shaft (rotor) speed to increase. BUT when the generator is synchronized to a grid THAT CAN'T happen!!! The grid keeps the generator shaft (rotor) spinning at a speed that is directly proportional to grid frequency--NO MATTER HOW HARD THE PRIME MOVER TRIES TO MAKE IT SPIN FASTER!!! And, that force, trying to make the generator shaft (rotor) spin faster, is causing the load coupling shaft to twist. More force, more twist. Less force, less twist. It's that simple.
A lot of textbooks and references seem to explain the amount of power being produced as being a function of the twist being applied. Maybe the maths make it seem that way, maybe it really IS that way. I don't know. In my mind, the resulting twisting of the load coupling shaft is the RESULT of trying to make the generator shaft (rotor) spin faster than the grid will allow it to spin. The amount of the twist (it seems to me) is also a function of the metal the load coupling shaft is made of. And, that means, it's not going to be EXACTLY the same for every single load coupling shaft on the same prime mover-generator combination for the same load on the generator.
So, all you ivory tower types can talk all you want about load angle and torque twist. Me? I'm a technician. I understand that there will be twisting action--but the ONLY control I have over the twisting action is the fuel "knob" of a gas turbine. Pure and simple. AND, there's not even a meter or any kind of measurement of load angle or twist on the load coupling shaft--it's all just a calculation.
Hope this helps!
CSA
That link was shared here to give an idea about calculations on Power vs torque for a synchronous generator...
No problemo ! we all happy to learn from each other & give a good support to people as oftenly on this wonderful forum...
Stay blessed.
James
That link was shared here to give an idea about calculations on Power vs torque for a synchronous generator...
No problemo ! we all happy to learn from each other & give a good support to people as oftenly on this wonderful forum...
Stay blessed.
James
Again I am speechless to your insightful, simple explanation CSA. and Thanks again ControlGuy25 for your link.
To try to add few humble explanation to this topic and marry both technical and math :
In fact my self I called it the magic of the magnetic forces. After synchronization stator and rotor fields are locked to each other. They are strongly coupled to each other. In fact, the rotor field "pulls" the field of the stator (the network). with an angel δ between both called load/Torque angle (after synchronization and if no power output is drawn from the generator the angle = 0) Generator would be floating on the line.
If the turbine or prime mover power increases slightly, the imbalance increases the energy in the rotor, and this causes the rotor field to speed up very slightly (Bnet lags Brotor) in other words, the torque applied is greater than the torque induced. As a result, the rotor field moves a little ahead of the stator field, and the angle between the two fields increases. In turn, the electrical power delivered to the generator increases and the balance is restored (torque applied equals torque induced). In the new balance condition, the rotor goes back to synchronous speed (due to induced torque) Tind =K BR Bnet sinδ but at an increased angle δ.
In CSA words : the generator converts the extra torque into amperes (or watts) which is how the generator output increases.
View attachment 1701
The same thing applies when unloading the generator, if the prime mover power decreases, this causes the rotor field to speed down. As a result, the angle between the two fields decreases. hence, the electrical power delivered the generator decrease.
Again I am grateful to be part of this knowledge exchange which is very insightful.
Regards
To try to add few humble explanation to this topic and marry both technical and math :
In fact my self I called it the magic of the magnetic forces. After synchronization stator and rotor fields are locked to each other. They are strongly coupled to each other. In fact, the rotor field "pulls" the field of the stator (the network). with an angel δ between both called load/Torque angle (after synchronization and if no power output is drawn from the generator the angle = 0) Generator would be floating on the line.
If the turbine or prime mover power increases slightly, the imbalance increases the energy in the rotor, and this causes the rotor field to speed up very slightly (Bnet lags Brotor) in other words, the torque applied is greater than the torque induced. As a result, the rotor field moves a little ahead of the stator field, and the angle between the two fields increases. In turn, the electrical power delivered to the generator increases and the balance is restored (torque applied equals torque induced). In the new balance condition, the rotor goes back to synchronous speed (due to induced torque) Tind =K BR Bnet sinδ but at an increased angle δ.
In CSA words : the generator converts the extra torque into amperes (or watts) which is how the generator output increases.
View attachment 1701
The same thing applies when unloading the generator, if the prime mover power decreases, this causes the rotor field to speed down. As a result, the angle between the two fields decreases. hence, the electrical power delivered the generator decrease.
Again I am grateful to be part of this knowledge exchange which is very insightful.
Regards
