We have two gas turbine generators in droop mode when they are connected to a public grid.

If a grid main breaker opened or tripped, large one change the conrol mode to isochronous mode and the other one stays in droop mode.

I have a question.
When a grid main breaker tripped, which generator maintain the frequency ?
Which generator control the megawatts output from the load changes ?

 

jackson,

You neglected to say if the site uses any kind of power management system (sometimes called a PMS) or some kind of "load sharing" control system. The explanation below presumes NO power management system OR "load sharing" system--just a unit operating in Isochronous Speed Control and a unit operating in Droop speed control, with no external system trying to adjust load/frequency, just a conscious, experienced, well-trained operator who is focused and paying attention to the island load and the loads on the two machines.

The Isochronous machine will control the frequency, and it will do so by varying its load as the load changes. So, if the Isoch GT is rated for 25 MW and is carrying (producing) 14 MW when operating in island mode with the Droop machine, running at 4 MW out of a rating of 10 MW, if the load on the island decreases by 2 MW the effect on the island will be that the frequency will start to increase. The Isoch unit will respond to the frequency increase and reduce its load by 2 MW to keep the island frequency at setpoint. The Droop machine will keep running at 4 MW.

Let's say the island load in this example has increased, over time to approximately 27 MW. The Isoch machine would be carrying 23 MW and the Droop machine would still be carrying 4 MW (the presumption is no changes were made from the above paragraph--except the total island load increased to 27 MW). Let's also say the island load has the potential to increase above 30 MW, which would mean if nothing were done to the current operating condition that the Isoch machine would hit its maximum output when its load went above 25 MW, and the island frequency would start to decrease below setpoint. So, the conscious, experienced and well-trained operator would increase the load on the Droop machine by let's say 5 MW (from 4 MW to 9 MW). This would cause the island frequency to start to increase--but the Droop machine would reduce it's load by the same amount--5 MW--to keep the island frequency at setpoint. So, the Isoch machine would be at 18 MW, the Droop machine would be at 9 MW, and if the island load did increase to 30 MW (from 27 MW), the Isoch machine would increase it's load to 21 MW to keep the island frequency at rated, while the Droop machine remained at 9 MW.

The Isoch machine is the machine that maintains frequency--and it does so by adjusting its load, automatically, to respond to the changes in the island load. BUT, it can only do so if the load on the Isoch machine doesn't exceed the Isoch machine's maximum rating OR the load on the Isoch machine doesn't go below 0 MW. And, the ONLY way to keep the load on the Isoch machine within its operating limits is to change the load on the Droop machine. One CANNOT change load manually on the Isoch machine.

Again, the explanation above presumes NO power management system or "load sharing" system is in use at the plant, just a human operator paying attention to the system frequency and the loads on the machines.

Hope this helps!!!

 

Hello CSA thanks for the reply but I did not understand this part
QUOTE="CSA, post: 208388, member: 189"]


Let's say the island load in this example has increased, over time to approximately 27 MW. The Isoch machine would be carrying 23 MW and the Droop machine would still be carrying 4 MW (the presumption is no changes were made from the above paragraph--except the total island load increased to 27 MW). Let's also say the island load has the potential to increase above 30 MW, which would mean if nothing were done to the current operating condition that the Isoch machine would hit its maximum output when its load went above 25 MW, and the island frequency would start to decrease below setpoint. So, the conscious, experienced and well-trained operator would increase the load on the Droop machine by let's say 5 MW (from 4 MW to 9 MW). This would cause the island frequency to start to increase--but the Droop machine would reduce it's load by the same amount--5 MW--to keep the island frequency at setpoint. So, the Isoch machine would be at 18 MW, the Droop machine would be at 9 MW, and if the island load did increase to 30 MW (from 27 MW), the Isoch machine would increase it's load to 21 MW to keep the island frequency at rated, while the Droop machine remained at 9 MW.


Hope this helps!!!
[/QUOTE]

If I increase the load in the droop operating gas turbine from 4mw to 9mw than the frequency of that gas turbine is going to go lower,no?

Also what happens when there is a PMS?

 

You caught me.

There is a typo (like some other contributors to Control.com, I am not my own best proofreader).

"This would cause the island frequency to start to increase--but the Droop Isoch machine would reduce its load by the same amount--5 MW--to keep the island frequency at setpoint."

When two or more synchronous generators are synchronized with each other NO generator can have a different frequency than the other(s). Full stop. Period.

As was written (without typos) the Isoch machine will sense any change in frequency--whether by a Droop machine changing the load it is carrying or motors or other electrical devices starting or stopping--and adjust its fuel flow-rate to maintain the system frequency setpoint for ALL THE MACHINES SYNCHRONIZED TOGETHER ON THE SYSTEM/GRID.

Droop machines don't attempt to control frequency--they will respond to frequency disturbances that the system experiences, but they do not automatically attempt to return the system frequency to normal; that's the function of an Isoch machine, or machines being operated with a PMS (Power Management System) or a trained and conscious operator.

When there's a PMS, the configuration and programming of the PMS--if its configured and programmed to do so--could adjust the load(s) of one (or more) machine to attempt to keep from exceeding the maximum load of any machine which is synchronized to the system from exceeding the load limit or dropping below 0 MW. Some PMSs can do this; some can't; most don't do it very well (at least in the beginning, and without a lot of trial-and-error tuning). Some PMSs are primarily load-shedding systems which try to keep a system running by disconnecting block(s) of load when the system is over-loaded (which can happen in unusual circumstances, or when one or more machines suddenly trip and can't be quickly restarted and re-synchronized). There's usually some kind of control matrix in a PMS that which operators have to choose the machine which is the primary frequency control unit, and the standby frequency control machine, etc. Many operators are not properly trained to understand this aspect of system operation, nor do they have a good grasp of what the PMS does and how it does it. (This goes for the PMS programmer(s) as well; programmable controllers are wonderful things--but ONLY when the programmer understands the process, and system frequency control is not widely understood or explained in most textbooks and references.)

That's about it. Remember: EVERY synchronous generator synchronized to (operating in parallel with) to a system or grid runs at the same frequency as all the other machines synchronized to the system. Full stop. Period. No matter if its two machines or two hundred machines. The system frequency controls the speed of all synchronous generators--and their prime movers--through magnetic forces inside the generator which lock the generator rotors into a speed that is directly related to system frequency (F=((P*N)/120) is the formula.)

Sometimes, if there is a large discrepancy between the size (power rating) of one machine versus another machine (say, a 750 kW machine and a 30 MW machine are synchronized together (with no other machines)) the smaller machine may not be able to have much, if any, effect on the system frequency (the speed of the larger machine). This is kind of rare, but it does happen.

 

Hello,

@nikidi.control , Please keep track of your prefixes and units.
m = mili = X0.001
M = Mega = X 1000000

Also have a look at the attached PDF.

Bertus

 

You caught me.

There is a typo (like some other contributors to Control.com, I am not my own best proofreader).

"This would cause the island frequency to start to increase--but the Droop Isoch machine would reduce its load by the same amount--5 MW--to keep the island frequency at setpoint."

When two or more synchronous generators are synchronized with each other NO generator can have a different frequency than the other(s). Full stop. Period.

As was written (without typos) the Isoch machine will sense any change in frequency--whether by a Droop machine changing the load it is carrying or motors or other electrical devices starting or stopping--and adjust its fuel flow-rate to maintain the system frequency setpoint for ALL THE MACHINES SYNCHRONIZED TOGETHER ON THE SYSTEM/GRID.

Droop machines don't attempt to control frequency--they will respond to frequency disturbances that the system experiences, but they do not automatically attempt to return the system frequency to normal; that's the function of an Isoch machine, or machines being operated with a PMS (Power Management System) or a trained and conscious operator.

When there's a PMS, the configuration and programming of the PMS--if its configured and programmed to do so--could adjust the load(s) of one (or more) machine to attempt to keep from exceeding the maximum load of any machine which is synchronized to the system from exceeding the load limit or dropping below 0 MW. Some PMSs can do this; some can't; most don't do it very well (at least in the beginning, and without a lot of trial-and-error tuning). Some PMSs are primarily load-shedding systems which try to keep a system running by disconnecting block(s) of load when the system is over-loaded (which can happen in unusual circumstances, or when one or more machines suddenly trip and can't be quickly restarted and re-synchronized). There's usually some kind of control matrix in a PMS that which operators have to choose the machine which is the primary frequency control unit, and the standby frequency control machine, etc. Many operators are not properly trained to understand this aspect of system operation, nor do they have a good grasp of what the PMS does and how it does it. (This goes for the PMS programmer(s) as well; programmable controllers are wonderful things--but ONLY when the programmer understands the process, and system frequency control is not widely understood or explained in most textbooks and references.)

That's about it. Remember: EVERY synchronous generator synchronized to (operating in parallel with) to a system or grid runs at the same frequency as all the other machines synchronized to the system. Full stop. Period. No matter if its two machines or two hundred machines. The system frequency controls the speed of all synchronous generators--and their prime movers--through magnetic forces inside the generator which lock the generator rotors into a speed that is directly related to system frequency (F=((P*N)/120) is the formula.)

Sometimes, if there is a large discrepancy between the size (power rating) of one machine versus another machine (say, a 750 kW machine and a 30 MW machine are synchronized together (with no other machines)) the smaller machine may not be able to have much, if any, effect on the system frequency (the speed of the larger machine). This is kind of rare, but it does happen.

thank you very much

 

jackson,

You neglected to say if the site uses any kind of power management system (sometimes called a PMS) or some kind of "load sharing" control system. The explanation below presumes NO power management system OR "load sharing" system--just a unit operating in Isochronous Speed Control and a unit operating in Droop speed control, with no external system trying to adjust load/frequency, just a conscious, experienced, well-trained operator who is focused and paying attention to the island load and the loads on the two machines.

The Isochronous machine will control the frequency, and it will do so by varying its load as the load changes. So, if the Isoch GT is rated for 25 MW and is carrying (producing) 14 MW when operating in island mode with the Droop machine, running at 4 MW out of a rating of 10 MW, if the load on the island decreases by 2 MW the effect on the island will be that the frequency will start to increase. The Isoch unit will respond to the frequency increase and reduce its load by 2 MW to keep the island frequency at setpoint. The Droop machine will keep running at 4 MW.

Let's say the island load in this example has increased, over time to approximately 27 MW. The Isoch machine would be carrying 23 MW and the Droop machine would still be carrying 4 MW (the presumption is no changes were made from the above paragraph--except the total island load increased to 27 MW). Let's also say the island load has the potential to increase above 30 MW, which would mean if nothing were done to the current operating condition that the Isoch machine would hit its maximum output when its load went above 25 MW, and the island frequency would start to decrease below setpoint. So, the conscious, experienced and well-trained operator would increase the load on the Droop machine by let's say 5 MW (from 4 MW to 9 MW). This would cause the island frequency to start to increase--but the Droop machine would reduce it's load by the same amount--5 MW--to keep the island frequency at setpoint. So, the Isoch machine would be at 18 MW, the Droop machine would be at 9 MW, and if the island load did increase to 30 MW (from 27 MW), the Isoch machine would increase it's load to 21 MW to keep the island frequency at rated, while the Droop machine remained at 9 MW.

The Isoch machine is the machine that maintains frequency--and it does so by adjusting its load, automatically, to respond to the changes in the island load. BUT, it can only do so if the load on the Isoch machine doesn't exceed the Isoch machine's maximum rating OR the load on the Isoch machine doesn't go below 0 MW. And, the ONLY way to keep the load on the Isoch machine within its operating limits is to change the load on the Droop machine. One CANNOT change load manually on the Isoch machine.

Again, the explanation above presumes NO power management system or "load sharing" system is in use at the plant, just a human operator paying attention to the system frequency and the loads on the machines.

Hope this helps!!!

I have another question,if the operator increased as you say " Let's also say the island load has the potential to increase above 30 MW, which would mean if nothing were done to the current operating condition that the Isoch machine would hit its maximum output when its load went above 25 MW, and the island frequency would start to decrease below setpoint. So, the conscious, experienced and well-trained operator would increase the load on the Droop machine by let's say 5 MW (from 4 MW to 9 MW). This would cause the island frequency to start to increase--but the Droop machine would reduce it's load by the same amount--5 MW--to keep the island frequency at setpoint. So, the Isoch machine would be at 18 MW, the Droop ISOCH machine would be at 9 MW, and if the island load did increase to 30 MW (from 27 MW), the Isoch machine would increase it's load to 21 MW to keep the island frequency at rated, while the Droop machine remained at 9 MW."
If the trained operator increased the power of the turbine operating in droop then in that case the frequency of the grid would change and it would decrease because the turbine operating in droop mode increased it's power, and it would have the same frrequency of the turbine running in isochronous mode because for the gas turbine being operated in droop mode it would give us the same frequency of the ISOCH gas turbine only if it was operating at 4MW so if I am operating a droop and an isochronous gas turbine,should not the droop reemain at a fixed power output so that way it's frequency does not change?

 

WTF?

Are you suggesting, nikidi.control, that the two machines in this example —one operating in Droop Speed Control mode and the other operating in Isochronous Speed Control mode—could run at two different frequencies?

If the electrical load on the system (the total watts/kW/MW of all the electric motors and lights and electric tea kettles and computers and computer monitors and televisions and cellphone chargers) is stable during any period and someone increases the load being carried (powered) by the Droop machine the immediate effect of the increased torque being produced by the Droop machine’s prime mover will be to increase the frequency of the system. BUT the Isoch machine will sense the increase in frequency and reduce its torque and the amount of the system load it is carrying (powering) to maintain the system frequency at its normal value. The electrical load is not changing—only the amount of load being provided by each of the two machines is changing. The two machines, synchronized with each other, act as one machine to supply the electrical power required by the system at the rated frequency. And because the system is an AC (alternating current) system the frequency of the system is critical to the stability of the system. If the amount of torque being produced by the prime movers driving the generators synchronized together on the system is more or less than the amount of torque required by the loads on the system then the frequency of the system will be higher or lower than the rated system frequency. The job of the governor of the Isoch machine’s prime mover is to adjust the energy flow-rate into the prime mover to keep the system frequency at rated (50- or 60 Hertz). The Isoch machine’s prime mover governor senses changes in speed—which is directly related to system frequency—and changes the load being carried by the Isoch machine to maintain system frequency as the system load changes OR the amount of generation being carried by the Droop machine changes.

An AC power system produces electrical power at a constant frequency. It does so by changing the amount of generation as the amount of the system load changes OR the amount of generation changes (the amount of power being produced by the Droop machines synchronized to the AC power system). If nothing or no one adjusts the amount of generation as the amount of load changes OR the amount of generation changes (a Droop machine is synchronized to the system and its load is being increased OR a machine suddenly trips (is removed from the system) the effect will be the frequency of the system will change.

An AC power system can produce the electrical power required by the loads on the system at any frequency—but the electric motors receiving power from the system will not operate at their normal speed (and, in general, the largest group of loads on a system are electric motors driving water pumps and air conditioning compressors and fans and factories and refineries and refrigerators and fans, etc.) and for them to operate properly and efficiently they need a constant and stable AC system frequency.

It’s like a train carrying passengers and/or freight; if it doesn’t move at a relatively constant average speed then the train won’t be on schedule and people and won’t arrive on time (or they’ll be early). The passengers and freight will arrive, but they won’t be on schedule and that can create lots of knock-on effects (problems).

If the electrical power coming out of the socket on the wall isn’t at a constant frequency, bad things can start happening. Lights will be brighter or dimmer than normal; refrigerators won’t work properly or efficiently; chargers will excessively heat up; pumps won’t move fluids/gases at the required rate; the AC clock on the wall will be fast or slow.

All prime movers and the generators they drive need to act as a single prime mover and generator to provide power to the load of the AC power system—which is really a lot of “small” loads which all appear as one large load. Droop Speed Control allows multiple prime movers and the generators they drive to work stably together to act as one very large prime mover and generator. (THIS is one aspect of the load-sharing feature of Droop Speed But nowhere on Planet Earth.) The AC power system however needs either one Isoch machine to maintain the system frequency OR trained power system operators to anticipate and respond to system changes to maintain a stable system frequency so that everything runs properly and efficiently. On a small AC power system, a single Isochronous machine can do that—as long as the system operators understand the limits of the Isoch machine and act accordingly. In an ideal world, a PMS could do this, but the programming intelligence to do this entirely without human assistance or intervention isn’t quite ready.

Yet.

But nowhere on Planet Earth on any AC power system can any generator (or generators) run at any frequency other than the frequency of the entire AC power system it is (or they are) synchronized to. Full stop. Period. Not no how. Not no way. Not ever. Never.

If you are trying to describe something else, please try again. In this example of two machines synchronized together, one operating in Isoch mode and the other operating in Droop mode, they are both ALWAYS operating at the same frequency. The operator can ONLY change the load of the Droop machine, which will cause the load of the Isoch machine to change by an exactly equal amount—in order to maintain system frequency. If the operator tries to change the load of the Isoch machine using the Isoch machine’s governor controls all the operator will “succeed” in doing is changing the frequency of the system—AND of BOTH machines.

 

WTF?

Are you suggesting, nikidi.control, that the two machines in this example —one operating in Droop Speed Control mode and the other operating in Isochronous Speed Control mode—could run at two different frequencies?

If the electrical load on the system (the total watts/kW/MW of all the electric motors and lights and electric tea kettles and computers and computer monitors and televisions and cellphone chargers) is stable during any period and someone increases the load being carried (powered) by the Droop machine the immediate effect of the increased torque being produced by the Droop machine’s prime mover will be to increase the frequency of the system. BUT the Isoch machine will sense the increase in frequency and reduce its torque and the amount of the system load it is carrying (powering) to maintain the system frequency at its normal value. The electrical load is not changing—only the amount of load being provided by each of the two machines is changing. The two machines, synchronized with each other, act as one machine to supply the electrical power required by the system at the rated frequency. And because the system is an AC (alternating current) system the frequency of the system is critical to the stability of the system. If the amount of torque being produced by the prime movers driving the generators synchronized together on the system is more or less than the amount of torque required by the loads on the system then the frequency of the system will be higher or lower than the rated system frequency. The job of the governor of the Isoch machine’s prime mover is to adjust the energy flow-rate into the prime mover to keep the system frequency at rated (50- or 60 Hertz). The Isoch machine’s prime mover governor senses changes in speed—which is directly related to system frequency—and changes the load being carried by the Isoch machine to maintain system frequency as the system load changes OR the amount of generation being carried by the Droop machine changes.

An AC power system produces electrical power at a constant frequency. It does so by changing the amount of generation as the amount of the system load changes OR the amount of generation changes (the amount of power being produced by the Droop machines synchronized to the AC power system). If nothing or no one adjusts the amount of generation as the amount of load changes OR the amount of generation changes (a Droop machine is synchronized to the system and its load is being increased OR a machine suddenly trips (is removed from the system) the effect will be the frequency of the system will change.

An AC power system can produce the electrical power required by the loads on the system at any frequency—but the electric motors receiving power from the system will not operate at their normal speed (and, in general, the largest group of loads on a system are electric motors driving water pumps and air conditioning compressors and fans and factories and refineries and refrigerators and fans, etc.) and for them to operate properly and efficiently they need a constant and stable AC system frequency.

It’s like a train carrying passengers and/or freight; if it doesn’t move at a relatively constant average speed then the train won’t be on schedule and people and won’t arrive on time (or they’ll be early). The passengers and freight will arrive, but they won’t be on schedule and that can create lots of knock-on effects (problems).

If the electrical power coming out of the socket on the wall isn’t at a constant frequency, bad things can start happening. Lights will be brighter or dimmer than normal; refrigerators won’t work properly or efficiently; chargers will excessively heat up; pumps won’t move fluids/gases at the required rate; the AC clock on the wall will be fast or slow.

All prime movers and the generators they drive need to act as a single prime mover and generator to provide power to the load of the AC power system—which is really a lot of “small” loads which all appear as one large load. Droop Speed Control allows multiple prime movers and the generators they drive to work stably together to act as one very large prime mover and generator. (THIS is one aspect of the load-sharing feature of Droop Speed But nowhere on Planet Earth.) The AC power system however needs either one Isoch machine to maintain the system frequency OR trained power system operators to anticipate and respond to system changes to maintain a stable system frequency so that everything runs properly and efficiently. On a small AC power system, a single Isochronous machine can do that—as long as the system operators understand the limits of the Isoch machine and act accordingly. In an ideal world, a PMS could do this, but the programming intelligence to do this entirely without human assistance or intervention isn’t quite ready.

Yet.

But nowhere on Planet Earth on any AC power system can any generator (or generators) run at any frequency other than the frequency of the entire AC power system it is (or they are) synchronized to. Full stop. Period. Not no how. Not no way. Not ever. Never.

If you are trying to describe something else, please try again. In this example of two machines synchronized together, one operating in Isoch mode and the other operating in Droop mode, they are both ALWAYS operating at the same frequency. The operator can ONLY change the load of the Droop machine, which will cause the load of the Isoch machine to change by an exactly equal amount—in order to maintain system frequency. If the operator tries to change the load of the Isoch machine using the Isoch machine’s governor controls all the operator will “succeed” in doing is changing the frequency of the system—AND of BOTH machines.

What I am saying is that they should run at the same frequency otherwise we are going to have problems!What I am saying is that the load on the DROOP machine does not change in reality when the load requested by the system has increased ,what happens is that the ISOCH machine changes its load so that the frequency of the system stays the same,and since the DROOP can function at the given frequency of the system (let's say 50 herz) at a certain load (lets say 10 MW) it has to stay at that load and the ISOCH machine and only the ISOCH machine changes its load (staying at the same frequency) to give the system the right load so that we can stay at the same frequency.Is this what you are saying?

But now I have a question about an example,lets say we have two gas turbines,operating in parallel,connected to the same grid.The maximum output power these two gas turbines can dish out is 20MW for each of them cause they are identical machines.Now the system is operating with a machine in ISOCH mode at 50hz and the other one in DROOP.Now the DROOP machine can run at 50hz at a power of 10MW.So till' 30 MW we can run this way,we get at maximum 20MW from the ISOCH machine at 50hz and 10MW from the DROOP machine at 50hz.What if now the grid need a power of 35 MW,what would we do in that case?I cannot give more power from the ISOCH because that one can give us only 20MW,so I need to get 15 MW from the other gas turbine,the one operating in DROOP,but that one,if it changes its load,from 10MW to 15 MW it will change its frequency,so what does the conscious operator or the PMS do in this case?

 

You seem to be upset about something....

The conscious, well-trained and experienced operator would adjust the load on the Droop machine to keep the load on the Isoch machine from ever reaching its rated output (or zero) based on the anticipated load on the system. THAT'S what an experienced, well-trained and conscious operator would do.

A well-programmed PMS would do the same. But, even the best PMSs can't always anticipate load changes all the time and require manual intervention from an experienced, well-trained, conscious operator in order to keep the system running at the proper frequency. Automation is only so good--no matter what the salespeople say.

Most PMSs are PACs (Programmable Action Controllers) or PLCs (Programmable Logic Controllers) which can be used in warehouses and roofing factories and chicken processing plants. While there are some VERY knowledgeable and skilled PAC/PLC programmers those that understand AC power systems and frequency response under a variety of conditions, and that's where the problem begins: lack of knowledge and experience with AC power systems using multiple generator sets. This is where experienced, well-trained and conscious operators can help during commissioning and when problems arise--they understand how things should work and can explain (in some cases, anyway) to the programmers what happened and what didn't happen, so that the programming of the PMS can be improved and made more "intelligent" and reliable.

But, plant managers and owners (investors) just see dollar signs when automation salespeople are talking about reducing manpower and training of the operators with "smarter" automation systems. That's possible for a variety of applications, but AC power systems are not as well understood--or documented--as many people think they are. There are many textbooks and references and even control system manufacturer-produced documentation that are blatantly wrong about Droop speed control. (NO machine (prime mover and generator), when synchronized to a grid with other machines (prime movers and their generators), EVER changes speed when being loaded and unloaded, and yet that one of the most pervasive descriptions of Droop Speed Control--and while it may be true for a single generator and its prime mover operating in Droop Speed Control that is experiencing an increase in load on the system it is powering, it's NOT true for machines synchronized to a grid with other generators.

And an experience, well-trained and conscious operator would respond properly to raise the machine/system frequency back to normal.

I say "conscious" operator because they all believe that small, islanded power systems don't require much in the way of manual intervention--and, in fact, many of them are scared to death of losing their jobs if they make a manual change and a unit trips, or worse, the plant blacks out. They will eventually learn--after a few events, including one or three black-outs--that manual intervention is required sometimes, even during normal operation. It's NOT a set-it-and-forget-it thing. The operator(s) can't be reading the newspaper or surfing the Internet and expect the plant/system controls to do everything--even if the salesperson said it could. (You know how to tell when most salespeople are lying, right? When their lips are moving. And, especially when it comes to selling PMSs that someone else configures and programs and has to support.)

 

Actually, this is ANOTHER benefit of Droop Speed Control.

"So till' 30 MW we can run this way,we get at maximum 20MW from the ISOCH machine at 50hz and 10MW from the DROOP machine at 50hz.What if now the grid need a power of 35 MW,what would we do in that case?I cannot give more power from the ISOCH because that one can give us only 20MW,so I need to get 15 MW from the other gas turbine,the one operating in DROOP,but that one,if it changes its load,from 10MW to 15 MW it will change its frequency, ..."

If the Isoch machine is already at its rated output and the load on the system increases, the Droop machine WILL change its frequency AS IT increases the fuel flow-rate to accommodate the additional load the Isoch machine can't support. That's the secondary benefit of Droop Speed Control--grid support. If that didn't happen then the system would just continue to lose frequency in a death spiral until the system went black because of under-frequency protection. But, because the Droop Machine "picked up" the additional load--even though the system frequency decreased--the system stands a better chance of surviving the inattention of the operator(s) than it might otherwise have.

Again, the PMS or the operator(s), or the Operations Supervisor, should have noticed the Isoch machine was nearing its rated output and been proactive by loading the Droop machine to reduce the load on the Isoch machine so that it could respond to anticipated load changes. Experienced operators will know what to anticipate and when, in most cases anyway, so even if the PMS doesn't respond appropriately (and it might--this time; or, it might not...) the operator(s) (or the Operations Supervisor) should be paying attention (consciously) and taken appropriate action before the system frequency decreased.

The details of what would happen (if there were no PMS or it wasn't programmed to respond appropriately) would be that the operator would need to click on RAISE SPEED/LOAD until the system frequency went to normal, and then continue raising load to reduce the load on the Isoch machine. Again--attempting to lower load (in this case) directly on the Isoch machine would result in reducing the system frequency even more! Any manual load changes on the system in your example have to made with the Droop machine's controls; using the Isoch machine's controls to attempt to change its load will only result in changing the system frequency setpoint, and that's NOT what should happen.

When I say PMSs aren't all their cracked up to be I speak from experience. I have traveled to several sites where the plant personnel were INSISTING the Mark* turbine controls weren't working correctly. And in every case it was proven the PMS hadn't responded correctly--either because it wasn't programmed correctly OR the operators hadn't made the appropriate matrix selections to allow the PMS to respond correctly. They were ADAMANT and DEMANDING that the Mark* be made to work "properly" when they didn't even know or understand how it should operate and would operate. So, pardon me if I'm not a fan of PMSs. I've been yelled at MORE THAN ONCE because plant personnel were certain the problem was the Mark* and as the OEM field engineer it was my job, therefore, to fix it, and it wasn't the Mark*'s fault. (I was even asked to leave one site because I couldn't convince them otherwise, and it took two more field service people over several weeks to finally convince them it was the site personnel who hadn't operated the PMS correctly. Fortunately, there was an SOE recorder at the plant and it clearly showed no operators had changed PMS matrix selections in weeks! At the end, the PMS supplier had to get their programmer to site, and he agreed (reluctantly, I'm told) that the operators hadn't made the appropriate selections.)

 

You seem to be upset about something....

The conscious, well-trained and experienced operator would adjust the load on the Droop machine to keep the load on the Isoch machine from ever reaching its rated output (or zero) based on the anticipated load on the system. THAT'S what an experienced, well-trained and conscious operator would do.

A well-programmed PMS would do the same. But, even the best PMSs can't always anticipate load changes all the time and require manual intervention from an experienced, well-trained, conscious operator in order to keep the system running at the proper frequency. Automation is only so good--no matter what the salespeople say.

Most PMSs are PACs (Programmable Action Controllers) or PLCs (Programmable Logic Controllers) which can be used in warehouses and roofing factories and chicken processing plants. While there are some VERY knowledgeable and skilled PAC/PLC programmers those that understand AC power systems and frequency response under a variety of conditions, and that's where the problem begins: lack of knowledge and experience with AC power systems using multiple generator sets. This is where experienced, well-trained and conscious operators can help during commissioning and when problems arise--they understand how things should work and can explain (in some cases, anyway) to the programmers what happened and what didn't happen, so that the programming of the PMS can be improved and made more "intelligent" and reliable.

But, plant managers and owners (investors) just see dollar signs when automation salespeople are talking about reducing manpower and training of the operators with "smarter" automation systems. That's possible for a variety of applications, but AC power systems are not as well understood--or documented--as many people think they are. There are many textbooks and references and even control system manufacturer-produced documentation that are blatantly wrong about Droop speed control. (NO machine (prime mover and generator), when synchronized to a grid with other machines (prime movers and their generators), EVER changes speed when being loaded and unloaded, and yet that one of the most pervasive descriptions of Droop Speed Control--and while it may be true for a single generator and its prime mover operating in Droop Speed Control that is experiencing an increase in load on the system it is powering, it's NOT true for machines synchronized to a grid with other generators.

And an experience, well-trained and conscious operator would respond properly to raise the machine/system frequency back to normal.

I say "conscious" operator because they all believe that small, islanded power systems don't require much in the way of manual intervention--and, in fact, many of them are scared to death of losing their jobs if they make a manual change and a unit trips, or worse, the plant blacks out. They will eventually learn--after a few events, including one or three black-outs--that manual intervention is required sometimes, even during normal operation. It's NOT a set-it-and-forget-it thing. The operator(s) can't be reading the newspaper or surfing the Internet and expect the plant/system controls to do everything--even if the salesperson said it could. (You know how to tell when most salespeople are lying, right? When their lips are moving. And, especially when it comes to selling PMSs that someone else configures and programs and has to support.)

I am not upset about anything,I dont get how you arrived in that conclusion.I am on this site to make questions and have asnwers and answer to people if I know something that might help them

 

I want to awaken you to a dirty little secret of single-shaft gas turbines (this includes machines with Reduction Gears between the turbine and generator).

When a gas turbine is running at Base Load (Base Load selected and active) and the grid frequency decreases the power output of the machine will decrease as the grid frequency decreases. This happens because the speed of the machine--including the axial compressor--will slow down as frequency decreases. When the axial compressor slows down the air flow through the machine decreases, which means the axial compressor discharge pressure also decreases, and if the fuel flow-rate remained constant the gas turbine exhaust temperature would rise. So, the turbine control system will sense the decrease in axial compressor discharge and the rising exhaust temperature and start reducing the fuel to prevent an exhaust overtemperature condition--which could lead to a trip of the machine.

This infuriates some people as they've never heard of it, but it's a known fact in the business; it contributed to a couple of near-nationwide blackouts on the Malaysian peninsula a couple of decades ago when MANY machines were running at Base Load and a block of generation was suddenly disconnected from the grid. And, it isn't just GE machines that will operate this way--other gas turbines from other manufacturers use a very similar control scheme to optimize power output (when grid frequency is at rated!) and protect against exhaust overtemperature. Many grid regulators and supervisors are often very surprised to learn about this aspect of single-shaft gas turbine operation, too. When this happens and the gas turbine output decreases it actually hurts grid stability and lowers grid frequency even further--the exact opposite of what everyone wants to happen (and what should happen to support grid stability until the frequency can be returned to normal).

So, don't think that just because a gas turbine running at Base Load--in EITHER Isoch or Droop--will continue to put out rated power when the frequency of the grid it is synchronized to decreases, because it won't (unless there is a specific control scheme to allow it to temporarily do so, or the machine has Peak Load capability AND the experience, well-trained and conscious operator (and the Operations Supervisor) switch the machine to Peak Load operation. Almost everyone (including some TAs) think the gas turbine should INCREASE its output during a grid frequency decrease situation, as it would if the machine were operating at Part Load in Droop Speed Control. But that's not the case.

Gas turbines are amazing machines, but they also have to be properly controlled and protected. And most operators and Operations Supervisors and Plant Managers and plant owners/investors don't know all the ins and outs of gas turbine operation during abnormal situations such as grid frequency disturbances. (In the same way that single-shaft gas turbines will actually decrease power output during a grid frequency disturbance when operating with Base Load selected and active, they will INCREASE power output during a grid frequency increase event--because the axial compressor speeds up and the axial compressor discharge pressure increases and the exhaust temperature decreases, so the turbine control system will increase fuel flow-rate (unless there is a specific control scheme to allow temporary operation during this type of grid frequency event). This will actually cause the grid frequency to increase even more--which is the opposite of what everyone wants to happen, even if they think it should remain constant.)

These things will happen if a single-shaft gas turbine is operating at Base Load in Isoch mode, too. Base Load (and Peak Load) only cares about maximizing gas turbine power production by putting as much fuel as possible into the machine. If the machine is already at Base Load and a grid frequency decrease event occurs the gas turbine control system will keep trying to maximize gas turbine power output AND protect the machine if required to prevent exhaust overtemperature.

Most power plant operators think that during a grid frequency disturbance THEIR MACHINE(S) should NOT experience any change in power output--whether they are operating in Isoch or Droop mode, at Part Load or Base (or Peak) Load. And that's simply NOT TRUE. When operating at Part Load in Droop Speed Control mode the grid regulators WANT EVERY machine to be changing load as necessary to support grid stability. And, as we have learned: EVERY SYNCHRONOUS GENERATOR synchronized to a grid with other machines will ALL run at the same frequency--ALWAYS. To think otherwise is simply not understanding basic AC power system fundamentals.

That's all for today!

 

I want to awaken you to a dirty little secret of single-shaft gas turbines (this includes machines with Reduction Gears between the turbine and generator).

When a gas turbine is running at Base Load (Base Load selected and active) and the grid frequency decreases the power output of the machine will decrease as the grid frequency decreases. This happens because the speed of the machine--including the axial compressor--will slow down as frequency decreases. When the axial compressor slows down the air flow through the machine decreases, which means the axial compressor discharge pressure also decreases, and if the fuel flow-rate remained constant the gas turbine exhaust temperature would rise. So, the turbine control system will sense the decrease in axial compressor discharge and the rising exhaust temperature and start reducing the fuel to prevent an exhaust overtemperature condition--which could lead to a trip of the machine.

This infuriates some people as they've never heard of it, but it's a known fact in the business; it contributed to a couple of near-nationwide blackouts on the Malaysian peninsula a couple of decades ago when MANY machines were running at Base Load and a block of generation was suddenly disconnected from the grid. And, it isn't just GE machines that will operate this way--other gas turbines from other manufacturers use a very similar control scheme to optimize power output (when grid frequency is at rated!) and protect against exhaust overtemperature. Many grid regulators and supervisors are often very surprised to learn about this aspect of single-shaft gas turbine operation, too. When this happens and the gas turbine output decreases it actually hurts grid stability and lowers grid frequency even further--the exact opposite of what everyone wants to happen (and what should happen to support grid stability until the frequency can be returned to normal).

So, don't think that just because a gas turbine running at Base Load--in EITHER Isoch or Droop--will continue to put out rated power when the frequency of the grid it is synchronized to decreases, because it won't (unless there is a specific control scheme to allow it to temporarily do so, or the machine has Peak Load capability AND the experience, well-trained and conscious operator (and the Operations Supervisor) switch the machine to Peak Load operation. Almost everyone (including some TAs) think the gas turbine should INCREASE its output during a grid frequency decrease situation, as it would if the machine were operating at Part Load in Droop Speed Control. But that's not the case.

Gas turbines are amazing machines, but they also have to be properly controlled and protected. And most operators and Operations Supervisors and Plant Managers and plant owners/investors don't know all the ins and outs of gas turbine operation during abnormal situations such as grid frequency disturbances. (In the same way that single-shaft gas turbines will actually decrease power output during a grid frequency disturbance when operating with Base Load selected and active, they will INCREASE power output during a grid frequency increase event--because the axial compressor speeds up and the axial compressor discharge pressure increases and the exhaust temperature decreases, so the turbine control system will increase fuel flow-rate (unless there is a specific control scheme to allow temporary operation during this type of grid frequency event). This will actually cause the grid frequency to increase even more--which is the opposite of what everyone wants to happen, even if they think it should remain constant.)

These things will happen if a single-shaft gas turbine is operating at Base Load in Isoch mode, too. Base Load (and Peak Load) only cares about maximizing gas turbine power production by putting as much fuel as possible into the machine. If the machine is already at Base Load and a grid frequency decrease event occurs the gas turbine control system will keep trying to maximize gas turbine power output AND protect the machine if required to prevent exhaust overtemperature.

Most power plant operators think that during a grid frequency disturbance THEIR MACHINE(S) should NOT experience any change in power output--whether they are operating in Isoch or Droop mode, at Part Load or Base (or Peak) Load. And that's simply NOT TRUE. When operating at Part Load in Droop Speed Control mode the grid regulators WANT EVERY machine to be changing load as necessary to support grid stability. And, as we have learned: EVERY SYNCHRONOUS GENERATOR synchronized to a grid with other machines will ALL run at the same frequency--ALWAYS. To think otherwise is simply not understanding basic AC power system fundamentals.

That's all for today!

hmmm interesting bit of information,thank you