Parallel Operation of Generators in Island mode- Droop/Isochronous
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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.
