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Parallel Operation of Generators in Island mode- Droop/Isochronous
- Thread starter Selk
- Start date
This applies to ISLAND mode ("small" AC power generation and distribution system).
Ideally there should be one machine (prime mover and generator) operating in Isochronous Speed Control mode and that machine "absorbs" any changes in the load (loadS, actually--the sum of all the lights and motors and tea kettles and televisions and air conditioners and computers and computer monitors, etc.). The Isochronous machine will automatically adjust its load as the load changes (real power) up to the maximum rated load of the prime mover and down to zero watts/kW/MW of the prime mover. (More on that later.)
The other machines should all be in Droop Speed Control mode--that is the operating mode that allows a prime mover and its generator (or prime movers and their generators) to share load with other machines and with the Isochronous machine. Droop Speed Control is straight proportional control--there is nothing in the control system of the prime mover (the prime mover governor) that makes automatic adjustments to return the frequency of the generator to nominal (rated grid frequency) when it drifts above or below nominal. THAT'S the responsibility of the Isochronous machine (the machine with its prime mover operating in Isochronous Speed Control mode).
Let's have an example. A small system operating stably at this instant in time at 25 MW. There are five generators, each rated at 15 MW (the rating of the prime mover; the generator nameplate rating is slightly higher for each machine). One machine's prime mover governor is operating in Isochronous Speed Control Mode at 5 MW. The other four machine's prime mover governors are operating in Droop Speed Control Mode at 5 MW. So, five machine operating at 5 MW each, and the total load on the system is 25 MW. The system frequency is 49.97 Hz--which is pretty good, actually.
Somewhere on the system a large water pump motor is started; it's rated at 1.5 MW. The immediate effect of this motor starting is the system frequency will start to decrease. However, the Isochronous machine immediately senses the decrease in grid frequency (as do the Droop Speed machines) and the Isochronous machine immediately increases the energy flow-rate into the prime mover restoring its generator's (and the system's) frequency to approximately 50 Hz (50.04 Hz--pretty good, still!). The Isochronous machine's generator load increased to 6.5 MW. The other machines, operating in Droop Speed Control, had their frequendies change slightly when the motor started but they all (including the Isochronous machine) went to 50.04 Hz when the system stabilized (which it did very quickly), but their loads remained unchanged at 5 MW each. That's what's supposed to happen and it did so without ANY human operator intervention.
Now, let's say that was early in the morning and as the day progressed the load on the system slowly increased to nearly 44 MW. The Isochronous machine's load increased AUTOMATICALLY to 24 MW and the four Droop machines remained at 5 MW each with no change in electrical output. The system frequency is at approximately 49.91 Hz (still reasonably good)--being maintained automatically by the Isoch machine.
BUT, there's a chance the system load could continue to increase. And that means that the maximum power output of the Isoch machine's prime mover governor could exceed 25 MW. So, the WELL-TRAINED operator, under the guidance of the EXPERIENCED operations supervisor, decides to reduce the load on the Isoch machine. And, to do this the operator goes to one of the Droop machines and increases it's power output to 10 MW. The effect on the system as the load on this Droop machine is being increased is to increase the frequency of system (ALL the generators--including the Isoch machine's generator). BUT the Isoch machines prime mover governor AUTOMATICALLY reduces the load on the Isoch machine by the amount the load was increased on the Droop machine: 5 MW. And the system frequency (and that of all the generators synchronized to the system) was stable at about 50.07 Hz. The load on the Isoch machine is down to 19 MW, the load on one of the Droop machines is up to 10 MW, and the load on the remaining three Droop machines is unchanged at 5 MW--for a total of 44 MW.
I gotta stop here because Control.com has a maximum limit on the number of characters, and if I haven't exceeded the limit yet, I gotta be close.
TO BE CONTINUED.
Ideally there should be one machine (prime mover and generator) operating in Isochronous Speed Control mode and that machine "absorbs" any changes in the load (loadS, actually--the sum of all the lights and motors and tea kettles and televisions and air conditioners and computers and computer monitors, etc.). The Isochronous machine will automatically adjust its load as the load changes (real power) up to the maximum rated load of the prime mover and down to zero watts/kW/MW of the prime mover. (More on that later.)
The other machines should all be in Droop Speed Control mode--that is the operating mode that allows a prime mover and its generator (or prime movers and their generators) to share load with other machines and with the Isochronous machine. Droop Speed Control is straight proportional control--there is nothing in the control system of the prime mover (the prime mover governor) that makes automatic adjustments to return the frequency of the generator to nominal (rated grid frequency) when it drifts above or below nominal. THAT'S the responsibility of the Isochronous machine (the machine with its prime mover operating in Isochronous Speed Control mode).
Let's have an example. A small system operating stably at this instant in time at 25 MW. There are five generators, each rated at 15 MW (the rating of the prime mover; the generator nameplate rating is slightly higher for each machine). One machine's prime mover governor is operating in Isochronous Speed Control Mode at 5 MW. The other four machine's prime mover governors are operating in Droop Speed Control Mode at 5 MW. So, five machine operating at 5 MW each, and the total load on the system is 25 MW. The system frequency is 49.97 Hz--which is pretty good, actually.
Somewhere on the system a large water pump motor is started; it's rated at 1.5 MW. The immediate effect of this motor starting is the system frequency will start to decrease. However, the Isochronous machine immediately senses the decrease in grid frequency (as do the Droop Speed machines) and the Isochronous machine immediately increases the energy flow-rate into the prime mover restoring its generator's (and the system's) frequency to approximately 50 Hz (50.04 Hz--pretty good, still!). The Isochronous machine's generator load increased to 6.5 MW. The other machines, operating in Droop Speed Control, had their frequendies change slightly when the motor started but they all (including the Isochronous machine) went to 50.04 Hz when the system stabilized (which it did very quickly), but their loads remained unchanged at 5 MW each. That's what's supposed to happen and it did so without ANY human operator intervention.
Now, let's say that was early in the morning and as the day progressed the load on the system slowly increased to nearly 44 MW. The Isochronous machine's load increased AUTOMATICALLY to 24 MW and the four Droop machines remained at 5 MW each with no change in electrical output. The system frequency is at approximately 49.91 Hz (still reasonably good)--being maintained automatically by the Isoch machine.
BUT, there's a chance the system load could continue to increase. And that means that the maximum power output of the Isoch machine's prime mover governor could exceed 25 MW. So, the WELL-TRAINED operator, under the guidance of the EXPERIENCED operations supervisor, decides to reduce the load on the Isoch machine. And, to do this the operator goes to one of the Droop machines and increases it's power output to 10 MW. The effect on the system as the load on this Droop machine is being increased is to increase the frequency of system (ALL the generators--including the Isoch machine's generator). BUT the Isoch machines prime mover governor AUTOMATICALLY reduces the load on the Isoch machine by the amount the load was increased on the Droop machine: 5 MW. And the system frequency (and that of all the generators synchronized to the system) was stable at about 50.07 Hz. The load on the Isoch machine is down to 19 MW, the load on one of the Droop machines is up to 10 MW, and the load on the remaining three Droop machines is unchanged at 5 MW--for a total of 44 MW.
I gotta stop here because Control.com has a maximum limit on the number of characters, and if I haven't exceeded the limit yet, I gotta be close.
TO BE CONTINUED.
On this particular day, the load doesn't increase too much more and as the day nears its end the load on the system is dropping. As the night wears on the load continues to drop, so the loads are now as follows: Isoch machine--2.5 MW; one Droop Machine is at 10 MW; and the other three Droop machines are at 5 MW--for a total of 27.5 MW. There is a chance the system load could continue to drop more than 2.5 MW, so, again an experienced operations supervisor training a new operator tells the operator to increase the load on the Isochronous machine by reducing the load on the Droop machine with the highest load. So, the operator decreases the load on the Droop machine operating at 10 MW by 5 MW, which causes the load on the Isoch machine to increase by the same amount (5 MW). So, now the Isoch machine is at 7.5 MW, and the four Droop Machines are at 5 MW each.
This is how it's supposed to work. BUT it requires trained and experienced personnel to operate the system. Many people are NOT experienced or trained, and they think the Isoch machine will do everything required without any intervention. OR, worse, they try manually changing the load on the Isoch machine--which won't happen. The only thing they will succeed at is changing the system frequency from what it should be at to something other than the nominal system frequency. So, everyone at the site and in management says, "The Isoch machine isn't working properly!" And nothing could be further from the truth.
Now, there should NEVER be more than one machine who's prime mover governor is operating in Isoch Speed Control Mode. Why? Because they will fight to control frequency as load changes. And that usually leads to wild load swings (of the multiple machines operating in Isoch mode), and that usually leads to black-outs. So, even more people say, "The Isoch machines aren't working properly!" Which again, is untrue.
So, they opt for a PSM--Power System Monitor, or some other name for a third-party control system that will automatically ensure system frequency remains at nominal without ANY human operator intervention. Most often this is done by putting all the generators in Droop Speed Control Mode and sending signals from the PSM (or whatever it's called) to the Droop machines to try to control frequency as load changes. Sometimes this works as intended; most often it does not because the people that programmed the system don't understand AC power generation and transmission or how prime mover governors work. So the PSM (or whatever it's called) gets blamed for not working correctly. Or people just learn to live with larger than normal frequency excursions because the Droop machines can't be loaded or unloaded fast enough. And, it's the prime mover governors that get blamed (falsely) for not working correctly.
So, people invent this scheme called "Standby Isoch Load Control" or "Isoch Load Sharing"--all names for detuned Isoch control. And, it doesn't usually work very well--and if it does in the beginning then over time as loads change/increase in the plant it begins to not work as well as it should or did. And, the prime mover governor control systems get blamed for the problems. Falsely.
It takes people with experience and knowledge to program and tune these PSMs (or whatever they're called) and there just isn't a lot of that around. When training and experience is all that's normally required for an Island system with one machine operating in Isoch Speed Control and all the others operating in Droop Speed Control to work and be very good at maintaining system frequency without problems.
So, that's it. All your questions should be answered, There shouldn't be any doubts (a really nasty word, as has been said many times before on Control.com).
Need clarification? Just ask. (But not by saying, "I have some doubts.")
This is how it's supposed to work. BUT it requires trained and experienced personnel to operate the system. Many people are NOT experienced or trained, and they think the Isoch machine will do everything required without any intervention. OR, worse, they try manually changing the load on the Isoch machine--which won't happen. The only thing they will succeed at is changing the system frequency from what it should be at to something other than the nominal system frequency. So, everyone at the site and in management says, "The Isoch machine isn't working properly!" And nothing could be further from the truth.
Now, there should NEVER be more than one machine who's prime mover governor is operating in Isoch Speed Control Mode. Why? Because they will fight to control frequency as load changes. And that usually leads to wild load swings (of the multiple machines operating in Isoch mode), and that usually leads to black-outs. So, even more people say, "The Isoch machines aren't working properly!" Which again, is untrue.
So, they opt for a PSM--Power System Monitor, or some other name for a third-party control system that will automatically ensure system frequency remains at nominal without ANY human operator intervention. Most often this is done by putting all the generators in Droop Speed Control Mode and sending signals from the PSM (or whatever it's called) to the Droop machines to try to control frequency as load changes. Sometimes this works as intended; most often it does not because the people that programmed the system don't understand AC power generation and transmission or how prime mover governors work. So the PSM (or whatever it's called) gets blamed for not working correctly. Or people just learn to live with larger than normal frequency excursions because the Droop machines can't be loaded or unloaded fast enough. And, it's the prime mover governors that get blamed (falsely) for not working correctly.
So, people invent this scheme called "Standby Isoch Load Control" or "Isoch Load Sharing"--all names for detuned Isoch control. And, it doesn't usually work very well--and if it does in the beginning then over time as loads change/increase in the plant it begins to not work as well as it should or did. And, the prime mover governor control systems get blamed for the problems. Falsely.
It takes people with experience and knowledge to program and tune these PSMs (or whatever they're called) and there just isn't a lot of that around. When training and experience is all that's normally required for an Island system with one machine operating in Isoch Speed Control and all the others operating in Droop Speed Control to work and be very good at maintaining system frequency without problems.
So, that's it. All your questions should be answered, There shouldn't be any doubts (a really nasty word, as has been said many times before on Control.com).
Need clarification? Just ask. (But not by saying, "I have some doubts.")
Selk, based on your past posts are you programming a PMS (Power Management System) to control frequency and the loads of multiple machines for an islanded installation? In my personal opinion it would still be best to have a single machine operating in Isoch Speed Control Mode to control frequency, and use the PMS to change load(s) on one or more of the Droop Speed Control machines to keep the Isoch machine operating in the "middle" of it's rated load capacity. Let the Isoch machine do the work of controlling frequency--that's what it's purpose is. Again, the load of an Isoch machine can't be changed by an operator OR a third-party control system because it's setpoint is frequency (speed) and its job is to perform pretty tight and fast automatic control of frequency by adjusting the energy input to the Isoch machine's prime mover to keep the system frequency at or very close to desired (nominal). The PMS can issue commands to increase or decrease the load (indirectly by changing the Droop speed control reference) of the Droop machine(s).
Machines don't load very fast when operating in Droop Speed Control Mode. That's one of the big mistakes many make when programming PMSs--they think the prime mover governors of the Droop machines will respond as quickly as if they were in Isoch Speed Control Mode, and that's just not true. I have been to several sites where all of the machines on the power island were operated in Droop Speed Control Mode and the reason I was there was because they weren't responding as fast as the PMS programmer (and plant supervision and management) thought they should be responding, leading to large deviations (+/- 1 Hz) in system frequency. Use the PMS to change loads on the Droop machines and let the Isoch machine respond quickly to load changes (which cause frequency deviations--but because the Isoch machine's prime mover governor's gain is very fast it will limit the frequency changes to small, almost imperceptible changes).
Typically--but NOT always--the machine which is run in Isoch is a machine with a large capability (as much or more than some of the smaller generators which might be running on the island system. The other main consideration when choosing the machine to be the Isoch machine is it's ability to load/unload quickly. Most gas turbines are pretty good for this; some steam turbines are not usually because the "boiler" (the steam source) may not be capable of quickly keeping up with large changes in steam flow which can lead to problems with carry-over and lifting safeties. If the steam turbine is part of a combined cycle plant they usually aren't set up to be operated in Isoch mode--because they are load following and often the steam turbine control valves are wide open even at partial load. And, there's a lag when trying to increase steam flow because the system needs more steam to respond to large increases in power requirement to maintain frequency.
Usually many of these PMSs have a means of designating which machines should be loaded/unloaded first, second, third, and so on. Also, there will be a need for at least one other machine to be capable of running in Isoch in the event the "primary" Isoch unit trips or has to be shut down for repairs or maintenance. And, again, the choice of the "secondary" Isoch machine should be based on capacity as well as ability to respond to load changes.
There's a lot involved in setting up a PMS. Lots of analog signals (prime mover speed; system frequency; load shedding; etc.). It's not a short list and involves some design decisions which may or may not limit other choices as well as need to be changed during commissioning. The PMS needs to know how much load each machine can carry, as well as the load on the machine(s) at any given time (realtime). The PMS needs to know about any tieline breaker requirements.
It's not impossible, but I've seen some very smart programmers who didn't really have a grasp of what the Customer wanted (and Customers who didn't properly specify operating parameters and limits) nor of how AC power generation and distribution systems are to be controlled and operated. And that leads to lots of finger-pointing ("smoking fingers" if you get the drift...); talk of withholding money and/or lawsuits; lots of trial and error commissioning (many trips) and a lot of time trying to get it right, again, when the definition wasn't clearly defined in the beginning and might even change during commissioning. It can be very satisfying when it all works correctly and the operators understand the limitations of the PMS (because there WILL be some times when operator intervention is required--automation isn't quite that good yet, especially in these kinds of applications) and how to anticipate and respond appropriately. Usually there is some training necessary, and some detailed write-ups of the system, its intent(s), how it operates, and, again, its limitations.
Machines don't load very fast when operating in Droop Speed Control Mode. That's one of the big mistakes many make when programming PMSs--they think the prime mover governors of the Droop machines will respond as quickly as if they were in Isoch Speed Control Mode, and that's just not true. I have been to several sites where all of the machines on the power island were operated in Droop Speed Control Mode and the reason I was there was because they weren't responding as fast as the PMS programmer (and plant supervision and management) thought they should be responding, leading to large deviations (+/- 1 Hz) in system frequency. Use the PMS to change loads on the Droop machines and let the Isoch machine respond quickly to load changes (which cause frequency deviations--but because the Isoch machine's prime mover governor's gain is very fast it will limit the frequency changes to small, almost imperceptible changes).
Typically--but NOT always--the machine which is run in Isoch is a machine with a large capability (as much or more than some of the smaller generators which might be running on the island system. The other main consideration when choosing the machine to be the Isoch machine is it's ability to load/unload quickly. Most gas turbines are pretty good for this; some steam turbines are not usually because the "boiler" (the steam source) may not be capable of quickly keeping up with large changes in steam flow which can lead to problems with carry-over and lifting safeties. If the steam turbine is part of a combined cycle plant they usually aren't set up to be operated in Isoch mode--because they are load following and often the steam turbine control valves are wide open even at partial load. And, there's a lag when trying to increase steam flow because the system needs more steam to respond to large increases in power requirement to maintain frequency.
Usually many of these PMSs have a means of designating which machines should be loaded/unloaded first, second, third, and so on. Also, there will be a need for at least one other machine to be capable of running in Isoch in the event the "primary" Isoch unit trips or has to be shut down for repairs or maintenance. And, again, the choice of the "secondary" Isoch machine should be based on capacity as well as ability to respond to load changes.
There's a lot involved in setting up a PMS. Lots of analog signals (prime mover speed; system frequency; load shedding; etc.). It's not a short list and involves some design decisions which may or may not limit other choices as well as need to be changed during commissioning. The PMS needs to know how much load each machine can carry, as well as the load on the machine(s) at any given time (realtime). The PMS needs to know about any tieline breaker requirements.
It's not impossible, but I've seen some very smart programmers who didn't really have a grasp of what the Customer wanted (and Customers who didn't properly specify operating parameters and limits) nor of how AC power generation and distribution systems are to be controlled and operated. And that leads to lots of finger-pointing ("smoking fingers" if you get the drift...); talk of withholding money and/or lawsuits; lots of trial and error commissioning (many trips) and a lot of time trying to get it right, again, when the definition wasn't clearly defined in the beginning and might even change during commissioning. It can be very satisfying when it all works correctly and the operators understand the limitations of the PMS (because there WILL be some times when operator intervention is required--automation isn't quite that good yet, especially in these kinds of applications) and how to anticipate and respond appropriately. Usually there is some training necessary, and some detailed write-ups of the system, its intent(s), how it operates, and, again, its limitations.
>>>!!CORRECTION!!<<<
I was able to correct an addition error (I should write the numbers down and add them--not try to do them in my head...) in my original post (Post #1). I have since spotted another error--a pretty big one, and it's too late for me to edit the post. I should have said the Isoch machine prime mover in the example was rated for 25 MW--not 15 MW. So the total of the capacity of the five generators in the example is (25+15+15+15+15) 85 MW. It's just an example, and it might be that some part of the loads supplied by the islanded system might be shutdown for plant maintenance, but for some reason the other machines were still running (possibly supplying steam to steam loads, for example). I intended to point to the Isoch machine as having a slightly larger capacity than the other machines (something that is common on smaller islanded systems). I then said the load on the Isoch machine increased to 24 MW during the day and could increase still further and exceed the 25 MW rating of the Isoch machine's prime mover.
I apologize for any confusion.
I was able to correct an addition error (I should write the numbers down and add them--not try to do them in my head...) in my original post (Post #1). I have since spotted another error--a pretty big one, and it's too late for me to edit the post. I should have said the Isoch machine prime mover in the example was rated for 25 MW--not 15 MW. So the total of the capacity of the five generators in the example is (25+15+15+15+15) 85 MW. It's just an example, and it might be that some part of the loads supplied by the islanded system might be shutdown for plant maintenance, but for some reason the other machines were still running (possibly supplying steam to steam loads, for example). I intended to point to the Isoch machine as having a slightly larger capacity than the other machines (something that is common on smaller islanded systems). I then said the load on the Isoch machine increased to 24 MW during the day and could increase still further and exceed the 25 MW rating of the Isoch machine's prime mover.
I apologize for any confusion.
Thanks A lot you cleared all my confusions ..best explanation ever
Thank you for your response ,
Can you also explain why we don't use two generators in Isochronous mode, why only one? As you said that they might fight, but can you give a detail explanation like this ,I shall be very thankful to you.
@afaq,
When a generator's prime mover governor operates in Isochronous Speed Control mode it wants to control the speed of the prime move and generator VERY tightly (for GE-design heavy duty gas turbines that's usually from 59.83 Hz to 60.17 Hz, or from 49.83 Hz to 50.17 Hz (so in the range of +/-0.17 Hz)--and when it senses any change in machine speed (which directly proportional to generator frequency) it acts VERY quickly to bring the machine speed back to nominal (50.0 Hz or 60.0 Hz).
If two generators and their prime movers are both operating in Isochronous Speed Control mode and are synchronized together BOTH m prime mover governors will respond to any change in machine speed (generator frequency) VERY QUICKLY! So, let's say the machine speeds (of the two machines since they are SYNCHRONIZED together and both operate exactly the same frequency!) dropped below 99.66%. BOTH machine prime movers will VERY QUICKLY try to increase the machine speeds (generator frequencies) and what will likely happen is that the system frequency will rise above 100.33% speed. Then both machines will then try simultaneously to decrease machine speeds (generator frequencies) and this leads to some very ROUGH and turbulent power generation which causes severe frequency deviations and usually will result in some system protective relay tripping one or both machines on over- or under-frequency.
There's really very little, or in some cases, no "gently ramping up" or down of fuel by a machine operating in Isochronous Speed Control mode. System frequency is just about the most important aspect of AC power systems. So, governors are designed to respond quickly to even slight deviations, and two machines with no "coordination" between then other than they sense the same speed changes (frequency changes) try to respond simultaneously things can get pretty ugly very quickly.
AC power generation systems were never designed to have more than one machine (generator prime mover) operating in "pure" Isochronous Speed Control mode at the same time while synchronized to the same grid. Yes, there are perversions of Isochronous Speed Control, usually called something like 'Isochronous Load Sharing' or 'Load Sharing' or something like that--but they require extra sensing systems for the machines to know what the other machine is producing and there's where the difficulty comes in because many system programmers really don't understand AC power generation fundamentals and these systems rarely perform well and quite often get abandoned in place (never used and never enabled). UNTIL some manager thinks HE knows how the system should operate and decides to start some testing without knowing how much training and experience the machine operators have to be prepared for what might happen and what to do in the (likely) event things go awry during this test. And once the test is over, it's usually decided NOT to try that again (enabling Isochronous Load Sharing without understanding how it works and what can happen and how operators should respond to system disturbances). Until the next new manager decides HE knows how to make the system work.... And the cycle repeats itself.
This--operating to generators in Isochronous Speed Control Mode--is just one of those things that one should be accepting of the admonition (advice) to never try it. Possibly with some "proven" Isochronous Load Sharing system in place, but "proven" almost never happens in reality because during testing of the Isochronous Load Sharing system usually the plant ends up going black (meaning the lights in the plant--and the surrounding facility/area--also go black) which means SOMEBODY has got some very serious EXPLAINING to do. And that doesn't happen more than once, usually. Owners and Plant Manages and Operations Supervisors are VERY sensitive to not having to make those explanations (because many don't really understand AC power generation fundamentals well enough to explain them to others after a blackout happens--because the only real "explanation" people asking for want (DEMAND) is that it won't happen again!!! (Which means no more testing or tuning, which means the system doesn't get properly configured and adjusted and tested to prove it works as intended.)
"Just don't do it," is very good, sound advice for your career when it comes to trying to operate two generators and their prime movers simultaneously in Isochronous Speed Control mode while synchronized together on the same grid. If there are analog meters in the Control Room when someone tries this they usually jump furiously from left to right and back and forth--until the lights go out. And, depending on the situation and circumstances such a result (a blackout) can be very career limiting.
When a generator's prime mover governor operates in Isochronous Speed Control mode it wants to control the speed of the prime move and generator VERY tightly (for GE-design heavy duty gas turbines that's usually from 59.83 Hz to 60.17 Hz, or from 49.83 Hz to 50.17 Hz (so in the range of +/-0.17 Hz)--and when it senses any change in machine speed (which directly proportional to generator frequency) it acts VERY quickly to bring the machine speed back to nominal (50.0 Hz or 60.0 Hz).
If two generators and their prime movers are both operating in Isochronous Speed Control mode and are synchronized together BOTH m prime mover governors will respond to any change in machine speed (generator frequency) VERY QUICKLY! So, let's say the machine speeds (of the two machines since they are SYNCHRONIZED together and both operate exactly the same frequency!) dropped below 99.66%. BOTH machine prime movers will VERY QUICKLY try to increase the machine speeds (generator frequencies) and what will likely happen is that the system frequency will rise above 100.33% speed. Then both machines will then try simultaneously to decrease machine speeds (generator frequencies) and this leads to some very ROUGH and turbulent power generation which causes severe frequency deviations and usually will result in some system protective relay tripping one or both machines on over- or under-frequency.
There's really very little, or in some cases, no "gently ramping up" or down of fuel by a machine operating in Isochronous Speed Control mode. System frequency is just about the most important aspect of AC power systems. So, governors are designed to respond quickly to even slight deviations, and two machines with no "coordination" between then other than they sense the same speed changes (frequency changes) try to respond simultaneously things can get pretty ugly very quickly.
AC power generation systems were never designed to have more than one machine (generator prime mover) operating in "pure" Isochronous Speed Control mode at the same time while synchronized to the same grid. Yes, there are perversions of Isochronous Speed Control, usually called something like 'Isochronous Load Sharing' or 'Load Sharing' or something like that--but they require extra sensing systems for the machines to know what the other machine is producing and there's where the difficulty comes in because many system programmers really don't understand AC power generation fundamentals and these systems rarely perform well and quite often get abandoned in place (never used and never enabled). UNTIL some manager thinks HE knows how the system should operate and decides to start some testing without knowing how much training and experience the machine operators have to be prepared for what might happen and what to do in the (likely) event things go awry during this test. And once the test is over, it's usually decided NOT to try that again (enabling Isochronous Load Sharing without understanding how it works and what can happen and how operators should respond to system disturbances). Until the next new manager decides HE knows how to make the system work.... And the cycle repeats itself.
This--operating to generators in Isochronous Speed Control Mode--is just one of those things that one should be accepting of the admonition (advice) to never try it. Possibly with some "proven" Isochronous Load Sharing system in place, but "proven" almost never happens in reality because during testing of the Isochronous Load Sharing system usually the plant ends up going black (meaning the lights in the plant--and the surrounding facility/area--also go black) which means SOMEBODY has got some very serious EXPLAINING to do. And that doesn't happen more than once, usually. Owners and Plant Manages and Operations Supervisors are VERY sensitive to not having to make those explanations (because many don't really understand AC power generation fundamentals well enough to explain them to others after a blackout happens--because the only real "explanation" people asking for want (DEMAND) is that it won't happen again!!! (Which means no more testing or tuning, which means the system doesn't get properly configured and adjusted and tested to prove it works as intended.)
"Just don't do it," is very good, sound advice for your career when it comes to trying to operate two generators and their prime movers simultaneously in Isochronous Speed Control mode while synchronized together on the same grid. If there are analog meters in the Control Room when someone tries this they usually jump furiously from left to right and back and forth--until the lights go out. And, depending on the situation and circumstances such a result (a blackout) can be very career limiting.
Thanks a lot for explaining it properly..... The part about new managers thinking “this time we’ll make isochronous load sharing work” and then discovering why everyone before them said don’t touch it was both painful and hilarious. Basically, two generators in pure isochronous without proper coordination is a great way to turn confidence into a blackout
In my maritime phase, I worked on a diesel-electric anchor handling workboat in the Gulf of Mexico (as it was then known). It had 4 identical diesel gensets, numbered 1-4, port to starboard. It was built in the early '80s with the electronics adapted from a drilling rig from the late '70s. Very analog, but it worked as long as you did your part.
Basically, the lowest numbered generator that was online became the master and set the system frequency. When we paralleled in another generator, we of course had its speed set slightly higher than the online system, but had to quickly lower it slightly once we closed the breaker. If we didn't, then that generator and the one that wanted to be the master would "fight". We would see it first as shivering of the fuel racks, which would gradually get worse until we heard it in the turbos and saw it in the lights flickering. If we didn't catch it in time, Bad Things would happen. A blackout in a shore installation can be pretty bad, but a blackout on a workboat connected to 20k feet of 3" wire rope and 1k feet of rig anchor chain stretched out is....well, probably worse.
If we took the master generator offline, after we stopped pulling hard and didn't need the capacity for a while, we would stand by to make sure that the next lowest number generator took over properly. We would lower the speed setpoints of the higher-numbered generators slightly and alternately until the system frequency drooped just a bit (like we saw the needle twitch) and then tweak the lowest-numbered generator's speed setting up slightly until the system was back at 60Hz. And then we would stand by in the control room until we felt/heard the drive motors change speed/load a few times to make sure it stayed stable.
So, yeah. @WTF? 's comment about experienced, trained operators is critical.
Basically, the lowest numbered generator that was online became the master and set the system frequency. When we paralleled in another generator, we of course had its speed set slightly higher than the online system, but had to quickly lower it slightly once we closed the breaker. If we didn't, then that generator and the one that wanted to be the master would "fight". We would see it first as shivering of the fuel racks, which would gradually get worse until we heard it in the turbos and saw it in the lights flickering. If we didn't catch it in time, Bad Things would happen. A blackout in a shore installation can be pretty bad, but a blackout on a workboat connected to 20k feet of 3" wire rope and 1k feet of rig anchor chain stretched out is....well, probably worse.
If we took the master generator offline, after we stopped pulling hard and didn't need the capacity for a while, we would stand by to make sure that the next lowest number generator took over properly. We would lower the speed setpoints of the higher-numbered generators slightly and alternately until the system frequency drooped just a bit (like we saw the needle twitch) and then tweak the lowest-numbered generator's speed setting up slightly until the system was back at 60Hz. And then we would stand by in the control room until we felt/heard the drive motors change speed/load a few times to make sure it stayed stable.
So, yeah. @WTF? 's comment about experienced, trained operators is critical.
@WTF? thank you for such an insightful explanation on the principles "governing
" generator parallel operation.I would appreciate your point of view and explnation on the preferred method to operate a train of Emergency Diesel Generators in parallel with a train of Gas Turbine Generator for an islanded facility.
Particular scenarios that draw my attention are those happening:
• when starting the GTG (black start) by EDG (what mode to use for the train of EDGs when operating alone and after paralleling with the GTGs and what mode for the GTG pre and post paralleling).
•when orderly Shutting down the GTG and transferring the load (essential only) to the GTGs (what mode to use for the train of GDGs when operating alone and after paralleling with the EDGs and what mode for the EDG pre and post GTG shutdown).
Thanks
@NJay,
The answer to your questions depend on several variables--one of which is the capacities of each of the EDGs. The next is what kind of load/frequency control system is in use at the location you are describing (often called a Power Management System--of which there are MANY types of configurable systems specifically for this purpose and a lot of bespoke (custom engineered) systems.
I can only address one scenario: A group of EDGs being used to start a GTG with ONE EDG operating in Isochronous Speed Control Mode at part load and one or more other EDGs operating at part load or very near rated. The load required by the GTG (and whatever other essential loads are being powered by the EDGs) must not be more than all the operating EDGs combined. AND, the load on the Isoch EDG must NOT be near rated load so that it can provide additional power (load) when the GTG is being started.
I would say the same would apply when orderly shutting down the GTG--the Isoch function (I presume the GT operates in Isoch mode when it's running an providing most of the loads of the site--essential and non-essential) would be transferred to one of the EDGs with sufficient spinning reserve capacity to handle loads/frequency when the GTG is shut down.
Before anyone can understand how an automatic power management system could control a scenario like this one MUST understand how the simplest and most basic of AC power generation systems in an island configuration would have to be operated. I haven't provided and examples of loads because you haven't provided any tangible information, rather just asking a "generic" question for which there might even be a suitable answer without a LOT more information.
As has been written many times before on Control.com, for plants like this there is usually almost always some kind of engineering document which was developed during the planning for the plant--how many machines (prime movers and generators; different types of operating scenarios including normal start-up and orderly shutdown; load swings; frequency control; load shedding for abnormal situations; etc.). Find that document and you will have a lot of site-specific answers. (Normally, those documents are--unfortunately--NOT made available to anyone other than upper level management lest lower level employees become more knowledgeable than upper level management. And, most often, upper level management usually don't have the technical background or even the desire to explain even basic AC power generation fundamentals to others so they can understand them and know how to properly operate and control and protect a plant like you are attempting to describe. It happens WAY more frequently than it should in many places around the world. Knowledge is power.)
I wish I could provide more information but without creating a scenario to try to match what you are describing without knowing more details and information it would just lead to more questions--and ultimately frustration. I have moved on from a high-stress profession to a low-drama life (or at least I wish it to be a lower-drama life, and it's looking like it's a good possibility) and your questions, while good and valid ones, are very technical.
Over the last nearly 30 years there has been a LOT written about Isochronous- and Droop Speed Control on Control.com by many knowledgeable contributors. (One contributor has often remarked if the site should be called DroopSpeedControl.com!) There is a very good Search function here on Control.com. But, there has been so much written about these concepts (which are directly related to your questions) you will have to do a lot of reading and make your own notes to keep up and develop your own understanding. AC power generation fundamentals are very often overlooked because automation is pretty advanced and for most applications it's not really necessary to know all of the details. However, it seems that in your case, though, you need more in-depth knowledge and understanding. I caution you that a lot of textbooks and reference books have extremely poor and in some cases flat-out wrong descriptions of Droop and Isoch speed control. They lack a lot of clarity and very little explanation of all of the nuances of what they are attempting to describe, without much--if any--first-hand experience, or even simulated experience operating islanded power stations (such as on board ships). It's not rocket science--but it's not easily grasped or understood. And, it's even more difficult to be able to explain it to others so they can understand these concepts. Again, for the most part, power plants just work--but there are sites like yours where a much more detailed understanding is required for smooth and reliable operation.
Don't give up!
The answer to your questions depend on several variables--one of which is the capacities of each of the EDGs. The next is what kind of load/frequency control system is in use at the location you are describing (often called a Power Management System--of which there are MANY types of configurable systems specifically for this purpose and a lot of bespoke (custom engineered) systems.
I can only address one scenario: A group of EDGs being used to start a GTG with ONE EDG operating in Isochronous Speed Control Mode at part load and one or more other EDGs operating at part load or very near rated. The load required by the GTG (and whatever other essential loads are being powered by the EDGs) must not be more than all the operating EDGs combined. AND, the load on the Isoch EDG must NOT be near rated load so that it can provide additional power (load) when the GTG is being started.
I would say the same would apply when orderly shutting down the GTG--the Isoch function (I presume the GT operates in Isoch mode when it's running an providing most of the loads of the site--essential and non-essential) would be transferred to one of the EDGs with sufficient spinning reserve capacity to handle loads/frequency when the GTG is shut down.
Before anyone can understand how an automatic power management system could control a scenario like this one MUST understand how the simplest and most basic of AC power generation systems in an island configuration would have to be operated. I haven't provided and examples of loads because you haven't provided any tangible information, rather just asking a "generic" question for which there might even be a suitable answer without a LOT more information.
As has been written many times before on Control.com, for plants like this there is usually almost always some kind of engineering document which was developed during the planning for the plant--how many machines (prime movers and generators; different types of operating scenarios including normal start-up and orderly shutdown; load swings; frequency control; load shedding for abnormal situations; etc.). Find that document and you will have a lot of site-specific answers. (Normally, those documents are--unfortunately--NOT made available to anyone other than upper level management lest lower level employees become more knowledgeable than upper level management. And, most often, upper level management usually don't have the technical background or even the desire to explain even basic AC power generation fundamentals to others so they can understand them and know how to properly operate and control and protect a plant like you are attempting to describe. It happens WAY more frequently than it should in many places around the world. Knowledge is power.)
I wish I could provide more information but without creating a scenario to try to match what you are describing without knowing more details and information it would just lead to more questions--and ultimately frustration. I have moved on from a high-stress profession to a low-drama life (or at least I wish it to be a lower-drama life, and it's looking like it's a good possibility) and your questions, while good and valid ones, are very technical.
Over the last nearly 30 years there has been a LOT written about Isochronous- and Droop Speed Control on Control.com by many knowledgeable contributors. (One contributor has often remarked if the site should be called DroopSpeedControl.com!) There is a very good Search function here on Control.com. But, there has been so much written about these concepts (which are directly related to your questions) you will have to do a lot of reading and make your own notes to keep up and develop your own understanding. AC power generation fundamentals are very often overlooked because automation is pretty advanced and for most applications it's not really necessary to know all of the details. However, it seems that in your case, though, you need more in-depth knowledge and understanding. I caution you that a lot of textbooks and reference books have extremely poor and in some cases flat-out wrong descriptions of Droop and Isoch speed control. They lack a lot of clarity and very little explanation of all of the nuances of what they are attempting to describe, without much--if any--first-hand experience, or even simulated experience operating islanded power stations (such as on board ships). It's not rocket science--but it's not easily grasped or understood. And, it's even more difficult to be able to explain it to others so they can understand these concepts. Again, for the most part, power plants just work--but there are sites like yours where a much more detailed understanding is required for smooth and reliable operation.
Don't give up!
