glenmorangie,
Your comment is missing.... All I'm seeing is that you quoted ratm.
ratm,
This is the last time. Let's say a machine has 4% Droop (typical of GE-design heavy duty gas turbines as supplied from the "factory"). TNR is the turbine speed reference--this is the command that the operator (almost universally unknowingly) changes when increasing or decreasing the load on the machine while watching the MW value (either on a meter or on the HMI display). TNH is the actual turbine speed.
A GE-design Frame 9FA heavy duty gas turbine is directly coupled to the synchronous generator. That means that as the turbine speed changes so does the generator speed. AND, as the generator speed changes--so does the turbine speed. One speed can't change unless the other speed changes. Full stop. Period.
For AC power systems, generator speed and frequency are directly related. As generator speed increases, generator frequency increases; and as generator speed decreases generator frequency decreases. Think of the generator speed and generator frequency being directly coupled--like the turbine and generator are (directly coupled).
When a synchronous generator and it's prime mover are synchronized to a grid with other generators and their prime movers, there are HUGE magnetic forces at work inside the generator (between the magnetic field of the generator rotor and a magnetic field on the generator stator that is a function of the current and frequency and voltage flowing in the generator stator windings. The magnetic field on the stator APPEARS to rotate around the stator, and the magnetic field of the stator (that is apparently rotating) grabs the magnetic field of the generator rotor and LOCKS the generator rotor into a speed that is directly related to the frequency of the grid. This means that EVERY generator synchronized to a grid rotates at a speed that is directly related to the frequency of the grid. No single generator, or even groups of generators, synchronized to a grid can spin at any speed other than the speed that is directly related to the frequency of the grid. This means that for a GE-design Frame 9FA heavy duty gas turbine-generator, the speed will be 3000 RPM for a grid operating at 50 Hz. NO MATTER HOW MUCH, OR HOW LITTLE, FUEL IS BEING ADMITTED INTO THE TURBINE'S COMBUSTORS!!! One can put more fuel into the turbine combustors to try to make it spin faster but it is NOT EVER going to spin faster than 3000 RPM when the grid frequency is stable at 50.0 Hz. Full stop. Period.
So, what happens when more fuel is admitted into the turbine's combustors--trying to make it spin faster??? Well, the generator--being unable to spin faster (because the grid frequency is keeping the speed at 3000 RPM as long as the grid frequency is stable at 50.0 Hz)--does what generators do: It converts the extra torque being produced by the increased fuel flowing into the turbine's combustors into amperes. And, as the amperes increase, the watts (MW) being produced by the generator increases! That's what generators do--they convert torque into amperes. Reduce the fuel flowing into the combustors and the opposite happens--the turbine would try to slow down, but the generator, being synchronized to a grid operating at a stable 50.0 Hz is going to keep that generator and the turbine that's directly coupled to it spinning at 3000 RPM, but because the decrease in fuel flow means the torque being produced by the generator will decrease, which means the amperes being produced by the generator will decrease, which means the watts (MW) being produced by the generator decreases.
Now, when the operator wants to make more MW, what does the operator do? Manually, the operator would click on the RAISE SPEED/LOAD target on the HMI. See that SPEED/LOAD part of the text? When the generator is NOT synchronized to the grid if the operator clicked on RAISE SPEED/LOAD the turbine and generator speed would increase above its present level. If the generator is synchronized to the grid, the turbine speed would try to increase--but that generator, being synchronized to a grid operating at a stable 50 Hz--will keep the speed constant at 3000 RPM, and the extra fuel that is trying to make the turbine and generator speed increase gets converted to amperes by the generator. And that makes the MW value the operator is watching increase--which is what the operator wanted to do. (The same thing happens in a round-about way when the operator is using Pre-Selected Load Control and increases the Pre-Selected Load Control Setpoint--the same thing that happens with the operator manually clicks on RAISE SPEED/LOAD happens when the Pre-Selected Load Control Setpoint is increased.)
And what, exactly is happening when RAISE SPEED/LOAD is being used (either manually or by Pre-Selected Load Control)? The turbine speed reference, TNR, is being increased. The turbine is being told to spin faster--but the generator, being synchronized to a grid operating stably at 50.0 Hz, is NOT GOING TO ALLOW the turbine or generator to spin any faster than 3000 RPM. Full stop. Period. This means the actual turbine speed is NOT CHANGING. TNH is NOT CHANGING!!!
So, when the operator clicks on RAISE SPEED/LOAD or increases the Pre-Select Load Control Setpoint when Pre-Select Load Control is active, what happens is that TNR increases--but TNH does not. If we subtract TNH from TNR, this means the difference (the "error") between TNR and TNH increases. And it's this increase in the difference (the "error") between TNR and TNH that ultimately causes the amount of fuel flowing into the turbine to increase. Which TRIES to increase the turbine-generator speed--BUT the generator won't let the speed increase because the grid frequency is stable (or at least it should be!). And that extra fuel becomes amperes at the generator output and that causes the MW being produced by the generator to increase.
When the operator raises load (watching the MW value), what is really happening is the turbine and generator are being told to increase speed. BUT, we know from more than 100 years of experience that when a synchronous generator and its directly coupled prime mover are synchronized to a grid with other generators and prime movers that it's speed CANNOT increase. And we use that knowledge to change the fuel flowing into the turbine's combustors.
The Mark* uses the difference between TNR and TNH to determine how much fuel to put into the turbine's combustors. If we increase TNR--but we know that TNH CAN'T change when the unit is synchronized to the grid--that increased difference can be used to increase the amount of fuel flowing into the turbine's combustors. Decrease TNR, when TNH can't change, and the difference between the two decreases--and that causes the fuel flowing into the combustors to decrease. BUT the ACTUAL SPEED (TNH) doesn't change (as long as the grid frequency is stable).
This is Droop Speed Control, and this is how the machine operates when at rated speed (which means rated frequency) BELOW BASE LOAD. Full stop. Period. Now, most GE-design heavy duty gas turbines are supplied with 4% Droop--which means that when the difference between TNR and TNH gets to 4%, the unit will be at or very near rated power output (the rating of the turbine--NOT the generator!!!). This also means that when the difference between TNR and TNH is 1%, the output of the generator will be approximately 25% of gas turbine nameplate rating. For a difference of 1.5%, thegenerator output will be approximately 37.5% of gas turbine nameplate rating. When the difference is 3%, the generator output will be at approximately 75% of gas turbine nameplate rating. THIS PRESUMEs THE AMBIENT CONDITIONS ARE NEAR TO THOSE STATED ON THE GAS TURBINE NAMEPLATE, THE AXIAL COMPRESSOR INLET FILTERS ARE CLEAN, THE AXIAL COMPRESSOR IS CLEAN AND CLEARANCES ARE WITHIN NORMAL TOLERANCE, THE FUEL IS AS PER SPECIFICATION, THE GAS TURBINE BACK-PRESSURE IS WITHIN SPECIFICATION--AND THE GRID FREQUENCY IS AT OR VERY NEAR RATED.
Now, from the graph, the turbine-generator speed IS NOT stable at 100.0 %--so HOW CAN THE FREQUENCY BE ROCK-SOLID AT 50.00 Hz??? It can't, that's how.
There are other things wrong with the graph. Where did you get a load of 90 MW for a TNR of 101.47? Let's just say that the machine in the graph has 4% Droop (because the load at TNR=104% equals 220 MW). That would mean that for a TNR of 101.47% the load would be approximately 80.85 MW ((1.47/4)*220). That's NOT what the graph shows, either. We don't know what the gas turbine nameplate rating is, what the conditions were at the time the graph was run, or what the Droop is on the machine in the graph.
The error is going to change as TNR changes (presuming grid frequency is relatively stable--and it is somewhat relatively stable in the graph--it's just NOT as stable as the frequency is--which can't be right). TNH doesn't change--as long as grid frequency is relatively stable. And, as the difference (the "error") between TNR and TNH changes then the load (DW) is going to change. That's it. Full stop. Period.
Your comment is missing.... All I'm seeing is that you quoted ratm.
ratm,
This is the last time. Let's say a machine has 4% Droop (typical of GE-design heavy duty gas turbines as supplied from the "factory"). TNR is the turbine speed reference--this is the command that the operator (almost universally unknowingly) changes when increasing or decreasing the load on the machine while watching the MW value (either on a meter or on the HMI display). TNH is the actual turbine speed.
A GE-design Frame 9FA heavy duty gas turbine is directly coupled to the synchronous generator. That means that as the turbine speed changes so does the generator speed. AND, as the generator speed changes--so does the turbine speed. One speed can't change unless the other speed changes. Full stop. Period.
For AC power systems, generator speed and frequency are directly related. As generator speed increases, generator frequency increases; and as generator speed decreases generator frequency decreases. Think of the generator speed and generator frequency being directly coupled--like the turbine and generator are (directly coupled).
When a synchronous generator and it's prime mover are synchronized to a grid with other generators and their prime movers, there are HUGE magnetic forces at work inside the generator (between the magnetic field of the generator rotor and a magnetic field on the generator stator that is a function of the current and frequency and voltage flowing in the generator stator windings. The magnetic field on the stator APPEARS to rotate around the stator, and the magnetic field of the stator (that is apparently rotating) grabs the magnetic field of the generator rotor and LOCKS the generator rotor into a speed that is directly related to the frequency of the grid. This means that EVERY generator synchronized to a grid rotates at a speed that is directly related to the frequency of the grid. No single generator, or even groups of generators, synchronized to a grid can spin at any speed other than the speed that is directly related to the frequency of the grid. This means that for a GE-design Frame 9FA heavy duty gas turbine-generator, the speed will be 3000 RPM for a grid operating at 50 Hz. NO MATTER HOW MUCH, OR HOW LITTLE, FUEL IS BEING ADMITTED INTO THE TURBINE'S COMBUSTORS!!! One can put more fuel into the turbine combustors to try to make it spin faster but it is NOT EVER going to spin faster than 3000 RPM when the grid frequency is stable at 50.0 Hz. Full stop. Period.
So, what happens when more fuel is admitted into the turbine's combustors--trying to make it spin faster??? Well, the generator--being unable to spin faster (because the grid frequency is keeping the speed at 3000 RPM as long as the grid frequency is stable at 50.0 Hz)--does what generators do: It converts the extra torque being produced by the increased fuel flowing into the turbine's combustors into amperes. And, as the amperes increase, the watts (MW) being produced by the generator increases! That's what generators do--they convert torque into amperes. Reduce the fuel flowing into the combustors and the opposite happens--the turbine would try to slow down, but the generator, being synchronized to a grid operating at a stable 50.0 Hz is going to keep that generator and the turbine that's directly coupled to it spinning at 3000 RPM, but because the decrease in fuel flow means the torque being produced by the generator will decrease, which means the amperes being produced by the generator will decrease, which means the watts (MW) being produced by the generator decreases.
Now, when the operator wants to make more MW, what does the operator do? Manually, the operator would click on the RAISE SPEED/LOAD target on the HMI. See that SPEED/LOAD part of the text? When the generator is NOT synchronized to the grid if the operator clicked on RAISE SPEED/LOAD the turbine and generator speed would increase above its present level. If the generator is synchronized to the grid, the turbine speed would try to increase--but that generator, being synchronized to a grid operating at a stable 50 Hz--will keep the speed constant at 3000 RPM, and the extra fuel that is trying to make the turbine and generator speed increase gets converted to amperes by the generator. And that makes the MW value the operator is watching increase--which is what the operator wanted to do. (The same thing happens in a round-about way when the operator is using Pre-Selected Load Control and increases the Pre-Selected Load Control Setpoint--the same thing that happens with the operator manually clicks on RAISE SPEED/LOAD happens when the Pre-Selected Load Control Setpoint is increased.)
And what, exactly is happening when RAISE SPEED/LOAD is being used (either manually or by Pre-Selected Load Control)? The turbine speed reference, TNR, is being increased. The turbine is being told to spin faster--but the generator, being synchronized to a grid operating stably at 50.0 Hz, is NOT GOING TO ALLOW the turbine or generator to spin any faster than 3000 RPM. Full stop. Period. This means the actual turbine speed is NOT CHANGING. TNH is NOT CHANGING!!!
So, when the operator clicks on RAISE SPEED/LOAD or increases the Pre-Select Load Control Setpoint when Pre-Select Load Control is active, what happens is that TNR increases--but TNH does not. If we subtract TNH from TNR, this means the difference (the "error") between TNR and TNH increases. And it's this increase in the difference (the "error") between TNR and TNH that ultimately causes the amount of fuel flowing into the turbine to increase. Which TRIES to increase the turbine-generator speed--BUT the generator won't let the speed increase because the grid frequency is stable (or at least it should be!). And that extra fuel becomes amperes at the generator output and that causes the MW being produced by the generator to increase.
When the operator raises load (watching the MW value), what is really happening is the turbine and generator are being told to increase speed. BUT, we know from more than 100 years of experience that when a synchronous generator and its directly coupled prime mover are synchronized to a grid with other generators and prime movers that it's speed CANNOT increase. And we use that knowledge to change the fuel flowing into the turbine's combustors.
The Mark* uses the difference between TNR and TNH to determine how much fuel to put into the turbine's combustors. If we increase TNR--but we know that TNH CAN'T change when the unit is synchronized to the grid--that increased difference can be used to increase the amount of fuel flowing into the turbine's combustors. Decrease TNR, when TNH can't change, and the difference between the two decreases--and that causes the fuel flowing into the combustors to decrease. BUT the ACTUAL SPEED (TNH) doesn't change (as long as the grid frequency is stable).
This is Droop Speed Control, and this is how the machine operates when at rated speed (which means rated frequency) BELOW BASE LOAD. Full stop. Period. Now, most GE-design heavy duty gas turbines are supplied with 4% Droop--which means that when the difference between TNR and TNH gets to 4%, the unit will be at or very near rated power output (the rating of the turbine--NOT the generator!!!). This also means that when the difference between TNR and TNH is 1%, the output of the generator will be approximately 25% of gas turbine nameplate rating. For a difference of 1.5%, thegenerator output will be approximately 37.5% of gas turbine nameplate rating. When the difference is 3%, the generator output will be at approximately 75% of gas turbine nameplate rating. THIS PRESUMEs THE AMBIENT CONDITIONS ARE NEAR TO THOSE STATED ON THE GAS TURBINE NAMEPLATE, THE AXIAL COMPRESSOR INLET FILTERS ARE CLEAN, THE AXIAL COMPRESSOR IS CLEAN AND CLEARANCES ARE WITHIN NORMAL TOLERANCE, THE FUEL IS AS PER SPECIFICATION, THE GAS TURBINE BACK-PRESSURE IS WITHIN SPECIFICATION--AND THE GRID FREQUENCY IS AT OR VERY NEAR RATED.
Now, from the graph, the turbine-generator speed IS NOT stable at 100.0 %--so HOW CAN THE FREQUENCY BE ROCK-SOLID AT 50.00 Hz??? It can't, that's how.
There are other things wrong with the graph. Where did you get a load of 90 MW for a TNR of 101.47? Let's just say that the machine in the graph has 4% Droop (because the load at TNR=104% equals 220 MW). That would mean that for a TNR of 101.47% the load would be approximately 80.85 MW ((1.47/4)*220). That's NOT what the graph shows, either. We don't know what the gas turbine nameplate rating is, what the conditions were at the time the graph was run, or what the Droop is on the machine in the graph.
The error is going to change as TNR changes (presuming grid frequency is relatively stable--and it is somewhat relatively stable in the graph--it's just NOT as stable as the frequency is--which can't be right). TNH doesn't change--as long as grid frequency is relatively stable. And, as the difference (the "error") between TNR and TNH changes then the load (DW) is going to change. That's it. Full stop. Period.
