Crystal Clear CSAamit0612,
While a sizable portion of original posters to this site provide feedback on their issues, a larger number do not. Many are just hoping for an exact answer to their problem so they can go back to whatever they do when they are getting paid to troubleshoot problems or operate power generation equipment. I subscribe to the theory of, "If you give a man a fish you feed him for a day; if you teach him to fish you feed him for life." In other words, I'm trying to use some of these questions as a teaching tool to teach people how power generation works so that they become better at their jobs since so many people don't have or don't receive much, if any, training in power generation fundamentals or principles.
Good on you, mate, for using the 'Search' feature of Control.com (I'm presuming you used the 'Search' feature; I hope you used the 'Search' feature!). And, I hope you're looking for some basic operational and fundamental information as you try to solve your problem; it comes with the response--free of charge!
What part of:
"So, when a generator--and its prime mover--are tripped by reverse power that means the prime mover is not providing or producing sufficient torque to keep its generator spinning at its rated speed and the other generators it is synchronized tothe gridwill provide amperes to the generator, causing it to act like a motor, and spin the prime mover.
You need to understand why the prime mover isn't producing sufficient torque to keep the generator spinning at its rated speed."
didn't you understand?
Are you operating or troubleshooting a diesel gen-set? If not, what is the prime mover--a steam turbine? A gasoline engine? A hydro turbine? A combustion turbine?
If you have a diesel gen-set, then it's not getting enough fuel for some reason. If the load is dropping before the reverse power trip then the problem is likely dirty fuel filters or low fuel level or some kind of mechanical problem with the fuel rack slipping.
If the diesel control gets signals from another, external control system, the external control system might be not working properly. Or the wiring between the gen-set and the external control system might be bad.
When you want help with a problem, you need to provide more details than, "Help; it keeps tripping on reverse power." Here are a few ideas if you really want help:
What is going on when it trips?
Is there a large load change in the facility the gen-set is powering? (It's presumed the gen-set is powering some facility/load which is not being powered by a large grid/utility at the time of the reverse power tripping; if that's not correct, please tell us.)
Is the frequency of the system going high before the reverse power trip? (Or is the frequency going low?)
What mode is the diesel controller operating in: Droop or Isochronous speed control?
How many other gen-sets is the diesel synchronized with? (It's presumed the gen-set with the reverse power tripping problem is NOT synchronized to a large, "infinite" grid; if that's not correct, please tell us if it is.)
Are the other gen-sets significantly larger than the one that's having problems (higher rated power)?
What is the setting of the reverse power relay on the gen-set that keeps tripping?
Help us to help you!
duongtl,
You see the response(s) to the questions. (There weren't any.) Don't know if that means it was clear to amit0612 or intimidating (I hope the former; but I suspect the latter).
Anyway, we do what we can. "Feedback is the most important contribution!" (c) here at Control.com. It's what really sets this forum apart from many other similar/wannabe forums on the World Wide Web.
Thank you for your, also (feedback, that is).
I wish I knew how to get people to understand that motors are machines that convert amperes to torque. And generators are machines that convert torque to amperes. And wires are the way generators are connected to motors. The prime movers that supply torque to the generators are actually doing the work which is seemingly done by the motors. But, people just can't get their heads and hands and arms around this fact. Whether is AC motors and generators, or DC motors and generators--it's all the same.
The basics are very simple; the actual implementation isn't that much more difficult. It's just that it's been explained so poorly in many texts and technical reference books and manuals. On an AC power system of any size speed and frequency are directly related. These texts that say (and this is a direct quote): "As the load on the generator increases, the speed decreases." Since speed and frequency are directly related, and most AC power systems at least TRY to run at a constant frequency the statement is incorrect, misleading and without context. It's just plain wrong--without context, which is NEVER supplied, implied or explained.
Anyway, we do what we can here. And we continue to keep trying.
And we do really love feedback. So, thank you.
You see the response(s) to the questions. (There weren't any.) Don't know if that means it was clear to amit0612 or intimidating (I hope the former; but I suspect the latter).
Anyway, we do what we can. "Feedback is the most important contribution!" (c) here at Control.com. It's what really sets this forum apart from many other similar/wannabe forums on the World Wide Web.
Thank you for your, also (feedback, that is).
I wish I knew how to get people to understand that motors are machines that convert amperes to torque. And generators are machines that convert torque to amperes. And wires are the way generators are connected to motors. The prime movers that supply torque to the generators are actually doing the work which is seemingly done by the motors. But, people just can't get their heads and hands and arms around this fact. Whether is AC motors and generators, or DC motors and generators--it's all the same.
The basics are very simple; the actual implementation isn't that much more difficult. It's just that it's been explained so poorly in many texts and technical reference books and manuals. On an AC power system of any size speed and frequency are directly related. These texts that say (and this is a direct quote): "As the load on the generator increases, the speed decreases." Since speed and frequency are directly related, and most AC power systems at least TRY to run at a constant frequency the statement is incorrect, misleading and without context. It's just plain wrong--without context, which is NEVER supplied, implied or explained.
Anyway, we do what we can here. And we continue to keep trying.
And we do really love feedback. So, thank you.
Thank you so much for your time.
In your operator example I've few doubts.
Lets say the machine is at bseload now frequency decreases. The governor is already wide opened. So mass flow cannot be increase. Now the additional MW will come from the stored rotational energy hence rpm drops MW increases BUT how this effect performance of Gas turbine and generator, are there any limitations other than under frequency. What will be the effect on efficiency as per my understanding i think efficiency will increase as MW increases and heat input is same.
If all the generators connected to any bus are at baseload and load increases then as per my understanding the only way to restore rated frequency 50/60 Hz is to shed load.
Syed Ahmed,
Please refer to your Oxfords English Dictionary for the definition of ‘doubt.’ In my opinion, it’s a very ugly word.
Why would gas turbine performance be an issue when grid frequency is not normal?
Gas turbines are mass flow machines. That means the more mass (of air and fuel) that can flow through the turbine the more power it can produce. When a GE-design heavy duty gas turbine is operating at Base load the Mark* is dumping as much fuel as possible into the machine trying to keep the exhaust temperature at the exhaust temperature limit.
Now, if the grid frequency decreases while operating at Base Load the axial compressor speed decreases. This does two things. It reduces the mass flow of air through the machine—which, by itself, reduces the power output of the machine. If the fuel flow didn’t change when the grid frequency decreased but the air flow through the machine decreases that would cause the exhaust temperature to increase above the exhaust temperature limit. But, the Mark* won’t let that happen—so it reduces the fuel to keep the exhaust temperature from exceeding the maximum limit. So—the power output of the machine will decrease due to both of those factors.
This is a dirty little secret of gas turbines. When you need power most during a grid frequency disturbance and a gas turbine is operating at Base Load it’s NOT normally capable of producing more power AND it will actually lose power output. There are special control schemes which can be implemented in the Mark* which will “over fire” the gas turbine to increase power output during a grid frequency decrease. BUT, this is stressful on the turbine hot gas path components—but if the power is absolutely necessary then the turbine be damned.
As to your second paragraph, you are correct. This essentially happened on the Malaysian peninsula—the load on the system was more than the generation on the system could provide and most of the generation was from gas turbines and most of those were operating at Base Load. And because the system didn’t shed load fast enough the peninsula basically went black. (There was also a sketchy ring grid which should have been upgraded/modified years before, but hadn’t been.) But it’s unreasonable to expect that gas turbines will even maintain power output while operating at Base Load during a grid frequency decrease. Further, it’s not a well-recognized fact that gas turbine power output will decrease when operating at Base Load during a grid frequency decrease. The fact that the grid operators did not have sufficient “spinning reserve” (another name for machines NOT generating at rated output and capable of responding to a typical overloaded grid situation) was yet another contributing factor to the blackout.
I don’t always state that prime movers only behave as I describe IF they aren’t operating at rated power output (Base Load for gas turbines)—because it adds to the confusion and complexity of the explanation of Droop Speed Control—which is difficult enough for most people without adding more constraint and conditions. Perhaps I will develop a standard disclosure for my explanations that states this ONLY applies to machines NOT running at rated power output. I have done that on occasion, but not always.
Hope this clarifies the explanations. And allays your concerns. I don’t intend to mislead—ever-/but I FREQUENTLY run up against the Control.com 10,000 character limit, and I try not to add to the confusion if possible.
Please refer to your Oxfords English Dictionary for the definition of ‘doubt.’ In my opinion, it’s a very ugly word.
Why would gas turbine performance be an issue when grid frequency is not normal?
Gas turbines are mass flow machines. That means the more mass (of air and fuel) that can flow through the turbine the more power it can produce. When a GE-design heavy duty gas turbine is operating at Base load the Mark* is dumping as much fuel as possible into the machine trying to keep the exhaust temperature at the exhaust temperature limit.
Now, if the grid frequency decreases while operating at Base Load the axial compressor speed decreases. This does two things. It reduces the mass flow of air through the machine—which, by itself, reduces the power output of the machine. If the fuel flow didn’t change when the grid frequency decreased but the air flow through the machine decreases that would cause the exhaust temperature to increase above the exhaust temperature limit. But, the Mark* won’t let that happen—so it reduces the fuel to keep the exhaust temperature from exceeding the maximum limit. So—the power output of the machine will decrease due to both of those factors.
This is a dirty little secret of gas turbines. When you need power most during a grid frequency disturbance and a gas turbine is operating at Base Load it’s NOT normally capable of producing more power AND it will actually lose power output. There are special control schemes which can be implemented in the Mark* which will “over fire” the gas turbine to increase power output during a grid frequency decrease. BUT, this is stressful on the turbine hot gas path components—but if the power is absolutely necessary then the turbine be damned.
As to your second paragraph, you are correct. This essentially happened on the Malaysian peninsula—the load on the system was more than the generation on the system could provide and most of the generation was from gas turbines and most of those were operating at Base Load. And because the system didn’t shed load fast enough the peninsula basically went black. (There was also a sketchy ring grid which should have been upgraded/modified years before, but hadn’t been.) But it’s unreasonable to expect that gas turbines will even maintain power output while operating at Base Load during a grid frequency decrease. Further, it’s not a well-recognized fact that gas turbine power output will decrease when operating at Base Load during a grid frequency decrease. The fact that the grid operators did not have sufficient “spinning reserve” (another name for machines NOT generating at rated output and capable of responding to a typical overloaded grid situation) was yet another contributing factor to the blackout.
I don’t always state that prime movers only behave as I describe IF they aren’t operating at rated power output (Base Load for gas turbines)—because it adds to the confusion and complexity of the explanation of Droop Speed Control—which is difficult enough for most people without adding more constraint and conditions. Perhaps I will develop a standard disclosure for my explanations that states this ONLY applies to machines NOT running at rated power output. I have done that on occasion, but not always.
Hope this clarifies the explanations. And allays your concerns. I don’t intend to mislead—ever-/but I FREQUENTLY run up against the Control.com 10,000 character limit, and I try not to add to the confusion if possible.
Dear CSA
Thank you so much for your valuable time that you've given to me. I apologize if you are hurt from that word.
In Base load operation when frequency is decreasing the machine mass flow decreases. So at base load when freq decreases to avoid high exhaust temp governor must reduce fuel flow.
But in your example when load is 10 MW and operator went for cup of coffee and 5 MW of load is added , as speed reference is unchanged so as per you the extra energy is supplied from stored rotational kinetic energy in shaft. So the machine in your example is not Gas turbine?
How it is possible that in one case the machine is supplying more MW from stored rotational energy without doing anything to governor but on the other side machine net output reduces as freq decreases.
Thank you so much for your valuable time that you've given to me. I apologize if you are hurt from that word.
In Base load operation when frequency is decreasing the machine mass flow decreases. So at base load when freq decreases to avoid high exhaust temp governor must reduce fuel flow.
But in your example when load is 10 MW and operator went for cup of coffee and 5 MW of load is added , as speed reference is unchanged so as per you the extra energy is supplied from stored rotational kinetic energy in shaft. So the machine in your example is not Gas turbine?
How it is possible that in one case the machine is supplying more MW from stored rotational energy without doing anything to governor but on the other side machine net output reduces as freq decreases.
Syed Ahmed,
In my example, the machine was rated at 20 MW—so it was operating at 50% of rated. When the extra load was sensed because the speed decreased that changed the error between the speed reference (which DIDN’T change!) and the actual speed (which DID change). The error wasn’t enough to do anything more than increase the load—Droop Speed Control is, after all, simple proportional control—it doesn’t correct the process variable (speed; frequency) it just changes the fuel flow-rate in proportion to the change in the error between the reference and actual speeds.
When the gas turbine is producing as much power as it can and the load increases, the turbine control system can’t increase the fuel flow so the stored rotational energy gets converted into load and in this case without some intervention (load shedding; QUICKLY increasing fuel flow(s) on other unit(s)—the grid is going to go into a death spiral.
That’s why well-regulated grids keep a certain number of units at part load—less than their rated loads—so that if the grid frequency decreases either because load increases (and the machines already at rated power output can’t produce any more power!—OR because generators trip and that amount of generation is lost) the units at part load will increase their power output—QUICKLY, too!—to help support the grid and hopefully keep it from going into that death spiral that ends in blackout.
That difference between the actual load of the units running at Part Load (less than rated load) and their rated loads is often referred to as “spinning reserve.” It’s an amount of available power, “in reserve,” that is immediately available in the case of overload to support grid stability and prevent the decay of grid frequency from resulting in blackout.
Hope this helps.
In my example, the machine was rated at 20 MW—so it was operating at 50% of rated. When the extra load was sensed because the speed decreased that changed the error between the speed reference (which DIDN’T change!) and the actual speed (which DID change). The error wasn’t enough to do anything more than increase the load—Droop Speed Control is, after all, simple proportional control—it doesn’t correct the process variable (speed; frequency) it just changes the fuel flow-rate in proportion to the change in the error between the reference and actual speeds.
When the gas turbine is producing as much power as it can and the load increases, the turbine control system can’t increase the fuel flow so the stored rotational energy gets converted into load and in this case without some intervention (load shedding; QUICKLY increasing fuel flow(s) on other unit(s)—the grid is going to go into a death spiral.
That’s why well-regulated grids keep a certain number of units at part load—less than their rated loads—so that if the grid frequency decreases either because load increases (and the machines already at rated power output can’t produce any more power!—OR because generators trip and that amount of generation is lost) the units at part load will increase their power output—QUICKLY, too!—to help support the grid and hopefully keep it from going into that death spiral that ends in blackout.
That difference between the actual load of the units running at Part Load (less than rated load) and their rated loads is often referred to as “spinning reserve.” It’s an amount of available power, “in reserve,” that is immediately available in the case of overload to support grid stability and prevent the decay of grid frequency from resulting in blackout.
Hope this helps.
Dear CSA,
I am very thankful to your time that you spend here giving us help. You've once said somewhere that if you feed someone fish he will ask for fish but if you teach him fishing he will not ask for food.
So i am asking about fishing , not fish.
I am trying to understand that in Gas turbine when load was 10 MW 50% . Speed Ref 102% . Operator went for tea. Now someone starts pump of 5 MW. Now he didn't change speed ref to 103% . So during this period fuel is same as before.
Now to supply extra load the turbo gen uses its rotational energy in supplying that extra load.
But as fuel flow is constant but speed reduction will reduce air flow so with reduction of air flow the mass flow will decrease which will decrease output.
Syed Ahmed,
There are many differences in operation between Part load and Base Load. Usually,at Part Load the IGVs are not fully open and the air flow through the machine is not at rated. At this point it’s mostly producing power—at rated speed—from fuel.
When the unit in the example—regardless of the type of prime mover—was producing 10 MW of its 20 MW rated power output at rated speed—if the machine had a 4% droop characteristic the speed reference would have been 102% and the actual speed would have been 100%—for a speed error of 2%, which equates to 50% load for a machine with 4% droop (which for a machine rated at 20 MW is equal to 10 MW).
When the load increased to 15 MW, the speed reference remained at 102% (because the operator was getting his cuppa) but the actual speed decreased to 99%. 102%-99%=3%—which is equivalent to the amount of fuel required to produce 75% of rated power which is 15 MW for a machine with a 4% droop characteristic for a machine rated at 20 MW at any speed. The change in speed in this case was limited by the amount of fuel flow-rate increase which was necessary to produce the extra 5 MW but not enough to maintain or return to rated speed. That difference is the amount of rotational energy that was lost when the extra 5 MW was added to the system. Without the extra fuel flow-rate caused by the increase in the speed error the frequency would have continued to decrease because the extra load wasn’t being supplied and eventually the frequency of the grid would have dropped enough to cause the under-frequency relay to trip the unit.
When the operator returned to find the extra load AND the decrease in frequency he started increasing the speed reference which increased fuel flow-rate which increased the actual speed. So as the speed reference increased the speed error did not change but the actual speed DID change, but the load remained at 15 MW. When the speed reference reached 103% the actual speed reached 100%—for a speed error of 3%, which is equal to 75% of rated at rated speed power for a machine with a 4% droop characteristic, which is 15 MW for a machine with a rated power output of 20 MW.
If you knew what the actual fuel flow-rate actually was at rated speed while producing 10 MW of a 20 MW rating and then knew the actual flow-rate at 15 MW of a 20 MW rating at 99.5% of rated speed and then knew what the actual fuel flow-rate was at 15 MW of a 20 MW rating at 100% of rated speed you would see that the actual fuel flow-rate was slightly lower at 99.5% of rated speed while producing 15 of 20 MW than it was at rated speed when producing 15 of 20 MW. The difference in fuel flow-rate at the same power output BUT at two different speeds is the rotational energy (if you want to call it that) that is needed to produce the same power (15 MW) at rated speed (100% speed; 100% frequency).
When a GE-design heavy duty gas turbine is truly operating at Base Load and the load on the machine increases it cannot increase the fuel flow-rate—it’s already at the maximum fuel flow-rate possible. So the speed decreases because the load increased—same as happens at Part Load!—but this time there’s no extra fuel to actually supply the increased load so there’s nothing to stop the speed from decreasing. Worse, the air flow-rate decreases as the speed decreases which, one, causes the mass flow-rate to decrease, and, two, causes the exhaust temperature to increase which means the turbine control has to, at some point, start decreasing the fuel flow-rate even more to not exceed the maximum allowable exhaust temperature. Meanwhile with nothing to stop the speed from decreasing the frequency continues to decrease and the under-frequency relay will eventually trip the machine.
Yes, the air flow-rate decreases when the speed decreases while at Part Load but the fuel flow-rate increases (which is part of the total mass flow-rate through the machine!) and the machine is capable, in this case, of supplying the additional load AND that helps to “arrest” (stop) the speed from continuing to decrease. Is the speed decrease EXACTLY 0.5% in real life? Is the increase in load EXACTLY 5 MW? Yes—because that’s what the additional pumps require. But the increase in fuel flow-rate because of the increase in speed error was enough to stabilize the system until the operator returned with his cuppa and took action to return the speed (frequency) to normal by returning the actual speed to normal by restoring the rotational energy lost as the result of the speed decrease in the first place when the load was added.
Whew! That fish is fighting!!! But we managed to reel it in without breaking the fishing line or the fishing pole. We will eat again another day!
Yes, the air flow decreases when the speed decreases at any load—but all the other conditions are not the same at the two loads in this example when the speed changed. The IGV angle is different, for one. The ability to increase fuel flow-rate is different, for two. I don’t know if the percentage of air flow change is the same or not at the different IGV angles (I suspect not; axial compressors are unique devices with some very wild maths associated with operation).
I sincerely hope this helps. I am out of ways to describe this. Perhaps if my math skills were better it might help, but then a lot of readers I’m trying to reach would just roll their eyes back in their heads and stop reading and go find an interesting YouTube video to watch. Even this little bit of maths is more than most want to consider; you’re an exception to the readership, Syed Ahmed. And I appreciate the questions. Don’t over-think or over-complicate the the explanation. Try to understand how the two situations are different. I suspect you have been reading/studying some texts or references about grid stability and Droop Speed Control and are trying to relate gas turbines to this and possibly in light of some event or events that occurred somewhere. This is not rocket science, but it does require thought and contemplation and reasoning. In other words, critical thinking—which, because it’s hard (critical thinking, that is) is disappearing from the world and is sorely needed especially in today’s world. Don’t give up. It took me more than thirty years to get to the point where I am at—and sometimes I even have questions. I read a LOT of documents and texts/references that were very confusing and even misleading. But, it’s all part of finding the thing which makes sense for you—and when you find it you will say, “Oh yeah—I knew that!” Because it will seem like you’ve known this all along and it makes perfect sense at that point. (From personal experience, questions will still enter your thinking from time to time—it’s natural.) And, maybe my explanations are still not enough for you to achieve your “AHA!!!” moment about this particular—and very important—aspect of Droop Speed Control as it relates to gas turbines. Don’t stop looking/reading if not. Because I can tell you—when you do reach that understanding of this topic it will feel SOOOO GOOD! It’s one of those things that is very poorly understood in the power generation industry but that most people don’t think about and when they do it can seem so difficult because a lot of the available documentation is really very poor and even misleading. (Have a read of the definition of Droop Speed Control in Wikipedia. THAT will really blow your mind!)
Anyway, I need to get some food and some sleep. I’ve been waiting for about 11 hours today for a machine to be started and there isn’t enough lunar radiation for that to happen now. (The moon dipped below the horizon hours ago—and lunar radiation is necessary for a start-up on projects/jobs that are poorly run and people are screaming at each other because the schedule said the unit was supposed to start two or three days ago but the mechanical department was late in reassembling the machine nobody adjusted the schedule to reflect the added time—they just all think even though the reassembly was two or three days late the schedule will miraculously get reset to the stupid condition it was written as, and the controls guy doesn’t need sleep for two days and can live on crappy pizza, chips and soda and bad coffee with powdered creamer). I’m (the controls guy) and I’m going to the hotel. If I only get one or two hours’ sleep, at least I won’t have to listen to idjits screaming at each other about the schedule. My boss will get yet another phone call telling him I wasn’t on site when they wanted to (finally) start the machine, but they won’t say their incompetence and pi**-poor planning led to me being on site waiting for 28 out of the last 38 hours-/but they will REALLY scream when they get the bill for all my time that they demanded I be on site (and for which my boss says I have to change them for), listening to screaming idjits. At least I have written my report (which they won’t like either). But my ears need a break and I need sleep.
Blessed day.
There are many differences in operation between Part load and Base Load. Usually,at Part Load the IGVs are not fully open and the air flow through the machine is not at rated. At this point it’s mostly producing power—at rated speed—from fuel.
When the unit in the example—regardless of the type of prime mover—was producing 10 MW of its 20 MW rated power output at rated speed—if the machine had a 4% droop characteristic the speed reference would have been 102% and the actual speed would have been 100%—for a speed error of 2%, which equates to 50% load for a machine with 4% droop (which for a machine rated at 20 MW is equal to 10 MW).
When the load increased to 15 MW, the speed reference remained at 102% (because the operator was getting his cuppa) but the actual speed decreased to 99%. 102%-99%=3%—which is equivalent to the amount of fuel required to produce 75% of rated power which is 15 MW for a machine with a 4% droop characteristic for a machine rated at 20 MW at any speed. The change in speed in this case was limited by the amount of fuel flow-rate increase which was necessary to produce the extra 5 MW but not enough to maintain or return to rated speed. That difference is the amount of rotational energy that was lost when the extra 5 MW was added to the system. Without the extra fuel flow-rate caused by the increase in the speed error the frequency would have continued to decrease because the extra load wasn’t being supplied and eventually the frequency of the grid would have dropped enough to cause the under-frequency relay to trip the unit.
When the operator returned to find the extra load AND the decrease in frequency he started increasing the speed reference which increased fuel flow-rate which increased the actual speed. So as the speed reference increased the speed error did not change but the actual speed DID change, but the load remained at 15 MW. When the speed reference reached 103% the actual speed reached 100%—for a speed error of 3%, which is equal to 75% of rated at rated speed power for a machine with a 4% droop characteristic, which is 15 MW for a machine with a rated power output of 20 MW.
If you knew what the actual fuel flow-rate actually was at rated speed while producing 10 MW of a 20 MW rating and then knew the actual flow-rate at 15 MW of a 20 MW rating at 99.5% of rated speed and then knew what the actual fuel flow-rate was at 15 MW of a 20 MW rating at 100% of rated speed you would see that the actual fuel flow-rate was slightly lower at 99.5% of rated speed while producing 15 of 20 MW than it was at rated speed when producing 15 of 20 MW. The difference in fuel flow-rate at the same power output BUT at two different speeds is the rotational energy (if you want to call it that) that is needed to produce the same power (15 MW) at rated speed (100% speed; 100% frequency).
When a GE-design heavy duty gas turbine is truly operating at Base Load and the load on the machine increases it cannot increase the fuel flow-rate—it’s already at the maximum fuel flow-rate possible. So the speed decreases because the load increased—same as happens at Part Load!—but this time there’s no extra fuel to actually supply the increased load so there’s nothing to stop the speed from decreasing. Worse, the air flow-rate decreases as the speed decreases which, one, causes the mass flow-rate to decrease, and, two, causes the exhaust temperature to increase which means the turbine control has to, at some point, start decreasing the fuel flow-rate even more to not exceed the maximum allowable exhaust temperature. Meanwhile with nothing to stop the speed from decreasing the frequency continues to decrease and the under-frequency relay will eventually trip the machine.
Yes, the air flow-rate decreases when the speed decreases while at Part Load but the fuel flow-rate increases (which is part of the total mass flow-rate through the machine!) and the machine is capable, in this case, of supplying the additional load AND that helps to “arrest” (stop) the speed from continuing to decrease. Is the speed decrease EXACTLY 0.5% in real life? Is the increase in load EXACTLY 5 MW? Yes—because that’s what the additional pumps require. But the increase in fuel flow-rate because of the increase in speed error was enough to stabilize the system until the operator returned with his cuppa and took action to return the speed (frequency) to normal by returning the actual speed to normal by restoring the rotational energy lost as the result of the speed decrease in the first place when the load was added.
Whew! That fish is fighting!!! But we managed to reel it in without breaking the fishing line or the fishing pole. We will eat again another day!
Yes, the air flow decreases when the speed decreases at any load—but all the other conditions are not the same at the two loads in this example when the speed changed. The IGV angle is different, for one. The ability to increase fuel flow-rate is different, for two. I don’t know if the percentage of air flow change is the same or not at the different IGV angles (I suspect not; axial compressors are unique devices with some very wild maths associated with operation).
I sincerely hope this helps. I am out of ways to describe this. Perhaps if my math skills were better it might help, but then a lot of readers I’m trying to reach would just roll their eyes back in their heads and stop reading and go find an interesting YouTube video to watch. Even this little bit of maths is more than most want to consider; you’re an exception to the readership, Syed Ahmed. And I appreciate the questions. Don’t over-think or over-complicate the the explanation. Try to understand how the two situations are different. I suspect you have been reading/studying some texts or references about grid stability and Droop Speed Control and are trying to relate gas turbines to this and possibly in light of some event or events that occurred somewhere. This is not rocket science, but it does require thought and contemplation and reasoning. In other words, critical thinking—which, because it’s hard (critical thinking, that is) is disappearing from the world and is sorely needed especially in today’s world. Don’t give up. It took me more than thirty years to get to the point where I am at—and sometimes I even have questions. I read a LOT of documents and texts/references that were very confusing and even misleading. But, it’s all part of finding the thing which makes sense for you—and when you find it you will say, “Oh yeah—I knew that!” Because it will seem like you’ve known this all along and it makes perfect sense at that point. (From personal experience, questions will still enter your thinking from time to time—it’s natural.) And, maybe my explanations are still not enough for you to achieve your “AHA!!!” moment about this particular—and very important—aspect of Droop Speed Control as it relates to gas turbines. Don’t stop looking/reading if not. Because I can tell you—when you do reach that understanding of this topic it will feel SOOOO GOOD! It’s one of those things that is very poorly understood in the power generation industry but that most people don’t think about and when they do it can seem so difficult because a lot of the available documentation is really very poor and even misleading. (Have a read of the definition of Droop Speed Control in Wikipedia. THAT will really blow your mind!)
Anyway, I need to get some food and some sleep. I’ve been waiting for about 11 hours today for a machine to be started and there isn’t enough lunar radiation for that to happen now. (The moon dipped below the horizon hours ago—and lunar radiation is necessary for a start-up on projects/jobs that are poorly run and people are screaming at each other because the schedule said the unit was supposed to start two or three days ago but the mechanical department was late in reassembling the machine nobody adjusted the schedule to reflect the added time—they just all think even though the reassembly was two or three days late the schedule will miraculously get reset to the stupid condition it was written as, and the controls guy doesn’t need sleep for two days and can live on crappy pizza, chips and soda and bad coffee with powdered creamer). I’m (the controls guy) and I’m going to the hotel. If I only get one or two hours’ sleep, at least I won’t have to listen to idjits screaming at each other about the schedule. My boss will get yet another phone call telling him I wasn’t on site when they wanted to (finally) start the machine, but they won’t say their incompetence and pi**-poor planning led to me being on site waiting for 28 out of the last 38 hours-/but they will REALLY scream when they get the bill for all my time that they demanded I be on site (and for which my boss says I have to change them for), listening to screaming idjits. At least I have written my report (which they won’t like either). But my ears need a break and I need sleep.
Blessed day.
Syed Ahmed,
You wrote:
“…Lets say the machine is at bseload now frequency decreases. The governor is already wide opened. So mass flow cannot be increase. Now the additional MW will come from the stored rotational energy hence rpm drops MW increases …”
This is the problem—there will be no “additional MW” from the machine. The fuel cannot be increased. The gas turbine cannot actually increase its power output. It’s NOT operating on Droop Speed Control; it’s operating on CPD-biased exhaust temperature control. And as the gas turbine axial compressor speed decreases so does the mass flow through the machine, AND as the air flow decreases the exhaust temperature would tend to increase. And the Mark* will have to reduce fuel flow to limit the exhaust temperature. So, power output from fuel flow will decrease.
Any rotational energy is not available because speed, fuel and air flow are being reduced.
ONLY when the unit is at Part Load AND the amount of the load change on the grid doesn’t exceed the rated power output or cause the power output to go negative (reverse power) will the machine be able to handle the load change. It has to be at Part Load with “room to maneuver.”
When it is at Part Load AND has room to maneuver will the machine be able to respond as necessary.
I really am out of ideas to help with your understanding. I don’t know where you have found this “stored rotational energy” concept but it is not universally applicable. Droop Speed Control and COD-biased exhaust temperature control are not the same type of control and switching between them once the unit is truly operating on COD-biased exhaust temperature control is not instantaneous. The Mark* runs Droop Speed Control up and out of the way of interfering with CPD-biased exhaust temperature control. When the error between speed reference and actual speed isn’t able to change fuel flow Droop Speed Control is rendered ineffective.
By the way, just because a machine is at Part Load DOES NOT mean that it can properly respond to frequency disturbances. If the machine is operating on Pre-Selected Load Control then it WILL NOT properly respond to grid frequency disturbances (UNLESS PFR (Primary Frequency Control) is enabled and active; isn’t this FUN??!!??!?!).
There are so many exceptions to what one would think would be rules it’s VERY DIFFICULT to try to explain and account for every possibility. But, I keep trying.
But this is the “real world.”
You wrote:
“…Lets say the machine is at bseload now frequency decreases. The governor is already wide opened. So mass flow cannot be increase. Now the additional MW will come from the stored rotational energy hence rpm drops MW increases …”
This is the problem—there will be no “additional MW” from the machine. The fuel cannot be increased. The gas turbine cannot actually increase its power output. It’s NOT operating on Droop Speed Control; it’s operating on CPD-biased exhaust temperature control. And as the gas turbine axial compressor speed decreases so does the mass flow through the machine, AND as the air flow decreases the exhaust temperature would tend to increase. And the Mark* will have to reduce fuel flow to limit the exhaust temperature. So, power output from fuel flow will decrease.
Any rotational energy is not available because speed, fuel and air flow are being reduced.
ONLY when the unit is at Part Load AND the amount of the load change on the grid doesn’t exceed the rated power output or cause the power output to go negative (reverse power) will the machine be able to handle the load change. It has to be at Part Load with “room to maneuver.”
When it is at Part Load AND has room to maneuver will the machine be able to respond as necessary.
I really am out of ideas to help with your understanding. I don’t know where you have found this “stored rotational energy” concept but it is not universally applicable. Droop Speed Control and COD-biased exhaust temperature control are not the same type of control and switching between them once the unit is truly operating on COD-biased exhaust temperature control is not instantaneous. The Mark* runs Droop Speed Control up and out of the way of interfering with CPD-biased exhaust temperature control. When the error between speed reference and actual speed isn’t able to change fuel flow Droop Speed Control is rendered ineffective.
By the way, just because a machine is at Part Load DOES NOT mean that it can properly respond to frequency disturbances. If the machine is operating on Pre-Selected Load Control then it WILL NOT properly respond to grid frequency disturbances (UNLESS PFR (Primary Frequency Control) is enabled and active; isn’t this FUN??!!??!?!).
There are so many exceptions to what one would think would be rules it’s VERY DIFFICULT to try to explain and account for every possibility. But, I keep trying.
But this is the “real world.”
Hi Phil,
I have a question - I have 2 generators down due to revers current (down to earth).
Its 3 x free standing CAT 500kVA gen-sets feeding 2 camps. At first we thought its the inverter AC units causing the reverse current, so we changed some of them. But the problem is still there.
I don't know what ells to do
I have a question - I have 2 generators down due to revers current (down to earth).
Its 3 x free standing CAT 500kVA gen-sets feeding 2 camps. At first we thought its the inverter AC units causing the reverse current, so we changed some of them. But the problem is still there.
I don't know what ells to do
