Say, there are 4 machines not connected to grid. The four machines all worked in "isochronous load sharing mode" with the PMS and other controllers. What stops the governor from fighting one another in this mode?
It seems all the machines talk to each other as to who will respond to the system variations and when?, but how this is accomplished?

Why is a master controller and a separate controller(for each machine) needed for the "isochronous load sharing mode"? Is PMS alone insufficient?

What makes this mode better than "Droop speed control" with PMS?. The "Isochronous load sharing" seems similar to all the 4 machines operating in Droop mode with PMS control - adjusting the fuel input to increase/decrease load and thereby controlling the frequency but I cannot consolidate the summary and the benefits of "isochronous load sharing" over "droop speed control" in my head.

 

I haven't seen the latest perversion of Isoch Load Sharing that GE has implemented--but, in general, it is a "de-tuned" Isochronous Speed Control function. And, yes, it requires "analog" signals to be shared amongst machines in order for them to understand how much load is being carried on which machines. Some of the implementations were also called "Block Load Control" and this control scheme was running in the GE Mark V operator interfaces/HMIs. With the Mark* VI and Mark* VIe instead of 4-20 mA signals and discrete logic signals all of the control and commands can be done with software. It's also very possible that GE has realized that a "master" controller for plant frequency and load control is better than trying to implement it in multiple turbine controllers.?.?.?

I want to echo a previous contributor to Control.com about GE Belfort control schemes. I presume--but we don't know for sure (because I don't think you've told us what Frame-size machines you are working on)--the control system was provided with the turbines which were manufactured in Belfort, France, at the GE plant there. They have a (bad) habit of re-engineering tried, true and proven control schemes and in the process horribly overcomplicating them. Why? Because they can, and they believe if it wasn't invented in France then it isn't as good as it could be and so it's their national duty to "fix" things that aren't broken and have worked reliably for decades. (There; I said it.)

GE in the USA has a LOT of their engineering work (especially control system retrofits and upgrades) in South Cental Asia. There are some very smart people working there, BUT most of them have never seen, heard, or been around a turbine. They have a tendency to try to fix things that aren't broken, as well, because they don't understand the scheme and they don't have anyone they can ask about why the scheme was implemented in the way that it was. GE also has a problem of testing software functions and control schemes by the same engineer who designed and implemented the functions/schemes. Who is going to write a test that "breaks" their function/scheme? (No one; that's who.) So, they write test plans/programs to basically validate what they wrote without really designing them to prove they are suitable for the application--and work well with other control schemes.

The implementations of older Isoch Load Sharing that I (tried to) commission were not accepted/adopted by the Customers/operators because they didn't operate the way the Customers/operators thought they should operate (and, really, no one had ever given it any thought and thought that when the system made corrections (as it should) that it wasn't working okay). Easy enough to disable. Way too complicated for untrained, inexperienced operators and their supervisors.

Isoch Load Sharing is comparable to multiple machines all operating in Droop Speed Control with a PMS monitoring frequency and sending signals to maintain frequency. BUT, it's very difficult to program and even more difficult to explain without formal training. Having some basic knowledge of AC power generation and very basic fundamentals and someone to ask questions of and help to explain the way the system operates is extremely helpful.

Isoch Load Sharing is basically GE's attempt at providing a system for island frequency control using GE control systems. There was (and still may be) a division of GE that would engineer and construct an entire power plant (meaning the Customer didn't have to engage and architect/engineer and a constructor and assemble a commissioning team) and that group INSISTED that all the controls the Customer wanted were accomplished with GE hardware and software, so, for example, if a Customer wanted island frequency control the GE turbine control system division was charged with developing it using their hardware and software--and they didn't want to design, build, test and document other modules that would rarely, if ever, be sold. Other manufacturers, like Woodward, have very similar modules that perform very similar functions. But, like a lot of Woodward control system functions they are individual modules for specific functions that have to be integrated together to achieve the desired results. They work okay, but the same problem plagues them: Inexperienced and untrained operators and their supervisors. (It doesn't help that their manuals aren't really that much better than GE's....)

 

The machine is LM2500+G4. If the controller is from GE (MarkVie), external controllers are not required?

On further reading, confused about "Isochronous Load sharing"? Is it (1) or (2)

1) All the machines operated in Droop mode with PMS control
(or)
2) All the machines operated in Isochronous mode with PMS and other controllers.

 

Okay. I’ve been operating on the belief the units were heavy duty gas turbines AND had Mark* turbine control systems (you didn’t say what the turbine control systems are….).

IF they are Mark* turbine control systems then it would most likely be neither 1 or 2. It would most likely be all units operating in a de-tuned Isochronous Speed Control mode with analog and digital signals shared amongst all four controllers and some complicated programming for trying to control frequency and balance (“share”) load without any single unit running at rated load or near zero load. That was my experience.

Many aeroderivatives are packaged with Woodward turbine control systems, they would most likely use one or more Woodward control modules to control frequency and balance (“share”) load. I have no idea if that would mean all turbine governors would be in Isochronous or Droop mode, but I would suspect if any machine was in Isoch mode it would be a de-tuned Isoch mode to prevent the governors from fighting for control of the frequency.

Again, all of this is to avoid any human intervention (and the likelihood of mistakes, or blackouts) of running with one machine in Isoch and the others in Droop. Isochronous Speed Control mode is about very fast response to frequency deviations to return to frequency to rated without “large” frequency swings during the system load change that caused the frequency to be more or less than normal. A single Isoch machine with other machines in Droop can do the same thing—but someone or something must make sure the Isoch machine is not at or near rated load OR at or near zero load by adjusting the load(s) on the Droop machine(s).

One of the Isoch Load Sharing Schemes I tried to commission had one machine in Isoch and one machine in Droop ready to be switched to Isoch if something happened to the “main” Isoch machine. It actually worked well for the short period it was in operation one morning. The Customer DEMANDED all machines be put in Droop with very little load (about 6 MW) and then about 20 MW of load was to be ”thrown on” to the system by switching a large block of residential load from another system to the system being tested. There was no written test plan, no written description of what was being tested and no written definition of what a successful test would be—so basically a recipe for failure. When two of the four machines tripped on under-frequency and the residential load was blacked out the test was declared a failure and I was instructed to disable Isoch Load Sharing. Only one Customer person told me the test failed because it didn't maintain load and frequency with less than a 0.5% deviation--even though the under-frequency trip setpoint was set for 0.35% decrease in frequency! Needless to say I was livid, but the Customer (a major electric utility in the Middle East) was adamant Isoch Load Sharing wasn't configured properly, but the biggest reason it was disabled was because the family of one of the local senior management team was in the area that was blacked out--and it took more than three hours to restore power to the area. So, because a large residential area was blacked out for several hours it was decided to permanently disable the scheme so it would never happen again. The Customer, instead of refusing to pay for the Isoch Load Sharing option accepted the machine with no mention of Isoch Load Sharing's "failure" to satisfy the "test." (When meant I was able to leave the site because commissioning was finished and there were no controls-related punch-list items. (Bittersweet for me, actually. The wife of one of the management team at the site made a mean khusa mahshi (I think that's how it's spelled in English) and I got to eat with the family every Friday (I took a delicious chocolate cake from a local bakery to the dinner--I was living in a hotel and had no ability to cook or prepare anything.) Everybody was happy--most of all me. I will NEVER forget her cooking--NEVER.)

Sorry; a long way of saying Isoch and Droop speed control is very commonly misunderstood. AND, there are many schemes for monitoring and maintaining power system/grid frequency in a small, island system with multiple generators. My suspicion is that "Isoch Load Sharing" is kind of a misnomer, where the word Isoch is used in the name of a scheme which really employs Droop machines all controlled by one (or more) external controllers. Some of the schemes work better than others, some don't work at all.... at least from what I have been told. Some just aren't operated properly because there is no proper written description of how to use the system to achieve a stated goal, and, unfortunately, when operators learn something they NEVER want to be told they could be doing it better or the system would function better by changing what they learned to do. So many operators are fearful of losing their job if they make a mistake they keep doing what they were told in the beginning and think that will be their saving grace if something bad does happen. It's really a bad way to run a small power system/grid--but to them, it's the only way.

 

Managed to get the functional description;

 

Selk,

This sounds an awful lot like Woodward Governor control's style of writing. Isochronous Speed Control is about one thing: adjusting the energy flow-rate into the prime mover to maintain a constant speed. It's already been established that the frequency of an AC power system is affected by the addition of or the removal of one or more loads, OR the existence of excess generation for the given loads OR the loss of generators (tripping; shut down) reducing the amount of generation below what's required to maintain system frequency. As such, Isochronous Speed Control uses a speed (frequency) reference and the actual machine speed to control the energy flow-rate into the prime mover. The difference with Isochronous Speed Control is that it uses a derivative function to eliminate the error between the speed reference and the actual speed to tightly control speed (frequency). Droop Speed Control is proportional control--as the error between the machine's speed reference and its actual speed change the energy flow-rate into the prime mover changes HOWEVER there is nothing to reduce the error between the machine's speed reference and its actual speed to zero. In fact, Droop Speed Control relies on the existence of an error to control the energy flow-rate. For example, a machine's prime mover governor with 4% Droop Regulation will change the energy flow-rate into the prime mover by 25% of the prime mover's rating for each 1% change in the machine's speed reference--while the machine's actual speed remains steady and constant (on a well-regulated AC power system/grid). So, to produce 75% of rated prime mover output the machine's speed reference has to be increased to 3% and remain at 3% for as long as it's desired for the machine to remain at 75% of rated output. This again relies on the fact that the frequency of the AC power system the machine is synchronized to remains stable and steady at rated frequency (50 Hz or 60 Hz) so that the error will remain at 3% and the power output of the machine will remain at 75% of rated.

So, this machine's prime mover governor is introducing another signal into the equation for Isochronous Speed Control--one based on this particular unit's load and the average of all other units on the AC power system (an islanded power system it is presumed). AND, it would appear that EITHER analog signals from each machine are sent to a "master" device to develop this signal and that signal is sent back to the machine (or machines!) operating in Isochronous Load Sharing Mode (since they are saying two machines can be running in Isoch mode acting as one larger machine in Isoch mode).

But, you are dribbling out information and there's a lot we don't understand about the machines, their governors and this "master" controller. It would seem that this "master" controller's signal is being used to bias (modify) the energy flow-rates into the two machines in response to changes in the loads of the other machines.?.?.? But, without being able to see the control system's program and configuration and without knowing how this "master" controller works we (I) am at a loss to provide any more information. I really don't understand what's driving this line of questioning. Is it that this Isoch Load Sharing scheme is present at a site but it's not in use because people don't understand it or because it doesn't work as it is though it should? Or is this being considered for a site that is already producing power or one that is being designed? (Or, is someone trying to program a PAC or PLC to serve as a power/frequency management system based on how manufacturers accomplish this.?.?.?!!!???!?!)

Again, the passage reads as if it was written by or for a Woodward Isoch Load Sharing module--something I'm not personally experienced with. The basic description above is interesting, but it's a 35,000 foot view of a much more complicated system, I am sure. What happens when the two Isoch Load Sharing machines approach their combined rated output, or one reaches its rated output and the other one is very close to its rated output? Who--or what--takes appropriate action to reduce the loads on the Isoch Load Sharing machines so they can respond to any further system load increases? And the same with the two machines nearing zero output--when the output of one of both machines goes negative that draws power from the other machines on the AC power system and if it goes negative enough the machine(s) will trip on reverse power--and then who or what is going to control frequency in response to AC power system load changes? This ONLY describes how multiple machines (two in this case, it seems) can operate as one larger Isoch machine and participate stably on an AC power system with other machines.

And remember, the loads of the Droop machines synchronized to the same AC power system as the Isoch Load Sharing machines require the AC power system frequency to remain stable at or very near rated in order for them to maintain their power outputs and remain stable, as well....

Unless you can, one, provide more information in one chunk and, two, provide the reasoning for this line of questioning (I have some suspicions, Selk) there's no more assistance I can or will provide. It's a moving target with hidden pieces that just keep popping up. I've given the basics before and will summarize them below again. But, without understanding what's driving these questions, I'm dropping out of this thread.

Isoch control is about adjusting energy flow-rate to keep the error between the machine's speed reference and its actual speed at or very near zero. The error is supposed to be zero and that's what keeps the frequency of the machine at or very near rated. Droop control is about how much the energy flow-rate into a machine's prime mover (so, it's electrical power output) will change for a given change in the error--and it relies on an error to work correctly. No error, zero load. One controls frequency in response to system load changes; the other is allowed to have an error--indeed it REQUIRES an error--to produce stable power and "play nice" on the AC power system with other machines. Two machines operating in "pure" Isochronous Speed Control mode will FIGHT each other to control the frequency of the AC power system they are synchronized to--they WILL NOT "play nice" with each other--without some form of bias, such as this load signal coming from a "master" controller in the description you provided above.

Again, one mode wants to eliminate the error between the machine's speed reference and its actual speed, and the other mode requires an error to function properly. The second method doesn't really care what the frequency is but it works BEST at or very near rated frequency because it's not the job of this mode to control frequency, only to adjust energy flow-rate in response to the error between the speed reference and the actual speed (which can change if frequency deviates from normal).

You can think of Droop Speed Control like this: increasing the machine's speed reference above 100% WILL NOT cause the machine's actual speed to increase (the AC power system frequency is controlling the machine's actual speed) but it does increase the power output of the machine. Continuing to increase the machine's speed reference, again, will not increase the machine's actual speed; it will only increase the machine's power output. So, while the machine is being commanded to turn faster, it doesn't (under normal conditions), in effect the machine's actual speed is being allowed to lag behind (to "droop below") the speed reference--and it actually relies on the fact that (under normal conditions) the speed reference will NOT be equal to the actual speed. (But IF the machine's actual speed does change--because the frequency of the AC power system the machine is synchronized to changes, Droop Speed Control WILL change the load of the machine in an effort to try and support grid stability--unless the machine is already AT rated output or will reach rated output as a result of the frequency deviation.)

Droop Speed Control REQUIRES the machine's actual speed to be less than--to lag behind; to "droop below"--the machine's speed reference in order for it to work properly, because it relies on the error between the machine's speed reference and its actual speed to change the energy flow-rate into the machine AND to keep the energy flow-rate constant once the error stabilizes (unless the machine is NOT at or near rated power when the machine's actual speed changes because the frequency of the AC power system the machine is synchronized to changes).

Isoch Speed Control is going to do everything it can to quickly eliminate any error between the machine's speed reference and its actual speed (again, unless the machine is at or near rated power, or at or near zero power).

That should be enough for you to understand how the two work and how they differ from each other. Both rely on a speed reference and the machine's actual speed. One works to eliminate the error (make the error equal to zero) which keeps machine speed at or very near rated speed/frequency, and the other REQUIRES an error to stably control and change the energy flow-rate into the machine. One doesn't want the machine's actual speed to differ from the speed reference (which remains at 100%!) and the other needs there to be an error to control the energy flow-rate into the machine.

Tchau!

 

The machine is LM2500+G4. If the controller is from GE (MarkVie), external controllers are not required?

On further reading, confused about "Isochronous Load sharing"? Is it (1) or (2)

1) All the machines operated in Droop mode with PMS control
(or)
2) All the machines operated in Isochronous mode with PMS and other controllers.

Hi Selk. Try to make a potentially complex subject simple... I see two solutions to the isolated grid scenario:

1 - Mixed droop / isoch. Run one machine in isoch (in other words, speed or frequency) control. Run all other machines in droop. The isoch machine is responsible for maintaining the frequency of the isolated grid. The droop machines will still respond to frequency deviations and help with grid stability, but for the most part their load has to be managed by an operator (not ideal). If the operator isn't paying attention, the isoch machine can get pushed to its min or max output as the loads on the grid change (also bad). This solution is simple to implement and doesn't require any extra controllers.

2 - Isochronous load sharing. All machines run in isoch and control the grid frequency together. Something extra has to be added (load sharing logic) to prevent the controllers from fighting with each other. Load sharing logic can be implemented in a separate controller, or it can be integrated into one (or more) of the existing turbine controllers. Different vendors have different solutions for this extra load sharing logic and how it is packaged in hardware. In theory, load sharing should make the operator's job easier. No need to manually set the load of any machine. And you should never see any one machine running to its min or max output because all of the machines are going up and down together.

So to answer your question:

"1) All the machines operated in Droop mode with PMS control" - I wouldn't call this isochronous load sharing because none of the machines are actually running in an isochronous mode.

"2) All the machines operated in Isochronous mode with PMS and other controllers." - Yes, this is isochronous load sharing.

Hope that helps.

 

I know some people don't really like the analogy used by a former contributor to Control.com (though I fail to understand why--it's a perfectly relatable analogy), but I'm going to try it from another angle.

Let's say there is a bicycle built for two (a tandem bicycle) and there are two riders riding on a mostly flat (level) well-paved road with some hills and valleys along the way. The bicyclists want to travel at a constant speed regardless of the terrain, let's say 25 km/hr, and as long as the road is level they travel at a relatively constant speed. (The speed is analogous to the speed of a generator's rotor and of its prime mover.)

They approach a long road down to a short valley, and both riders sense the bicycle speed is increasing so both riders decrease the force they are applying to the pedals of their cranks (creating torque), and the decrease is so much that the bicycle actually begins to slow below the desired 25 km/hr. Now, both riders sense the speed is below desired and both increase the force they are applying to the pedals--causing the bicycle to increase in speed. And this continues until they reach the valley and the road levels out and they adjust the forces they are applying to the pedals to get back to a stable 25 km/hr.

Both riders in the above scenario can be said to be in Isochronous Speed Control--each one trying to control the bicycle's speed with no coordination between them leading to speed (frequency) deviations, and probably some yelling and one or five swear words. ;-)

Now, they reach the other side of the valley where the road begins to rise and both riders sense the bicycle speed beginning to decrease and they both increase the forces they are applying to their pedals and the speed of the bicycle increases--to a speed greater than 25 km/hr, when both riders again decrease the forces being applied to their pedals the bicycle speed decreases, to less than 25 km/hr. This continues until they reach the top of the hill when the road again levels out and the riders again get the bicycle speed back to 25 km/hr.

As they ride along they agree that the rider in the front of the bicycle will adjust his pedaling force to maintain the desired 25 km/hr (by providing a variable torque to the bicycle's rear wheel) as road conditions change, and the rider in the back of the bicycle will do his best to just apply a constant pressure to his pedals (producing a constant torque). As they approach a rise in the road again the speed of the bicycle doesn't drift very much from 25 km/hr throughout the hill--a marked improvement in maintaining a constant speed.

In this scenario with one rider adjusting his pedaling force as road conditions change and the other maintaining a relative constant pedaling force the rider in the front can be considered to be in Isochronous Speed Control mode adjusting pedaling force to maintain the desired speed, and the rider in the rear operating in Droop Speed Control mode--providing a constant pedaling torque regardless of road conditions (level; falling; rising). This is what Droop Speed Control does with its proportional control--it just puts out a steady amount of torque to the generator's rotor regardless of the load, and Isochronous Speed Control adjusts its torque to maintain a stable frequency (speed) as load changes. Let's say the bicycle approaches a very steep section of road (up OR down); it might be necessary for the "Droop" rider to nearly or even completely stop applying pressure to his pedals in order for the "Isoch" rider to be able to maintain speed. And with communication between the two riders this can happen easily and smoothly.

You can expand this analogy to cover a bicycle built for three, or even four! You can have one rider (can be any rider) acting to adjust his (or her) pedaling forces to maintain the bicycle's speed constant regardless of road conditions. And the other riders just applying a constant pedaling force regardless of road conditions. There are many other ways this analogy can be used to explain Isoch and Droop (the bicycle is pulling a trailer carrying packages and the number and weight of the packages can change--kind of hokey, but it someone was throwing packages into a passing trailer being towed by a bicycle or someone is tossing package(s) off (again, a little bit of a stretch to believe) it's still a good analogy (changing the load being pulled by the bicycle with multiple riders).

There has also been an analogy using trains with multiple engines trying to maintain a constant speed--same thing. There has to be coordination between each engineer in order to have a stable speed as the terrain changes on the train's journey.

There can even be Isoch Load Sharing schemes where two riders are in "Isoch" but are capable of communicating to be able to stably participate in maintaining a constant speed, but it's going to take communication to coordinate their efforts. (Again, I liken Isoch Load Sharing to be a "de-tuned" Isoch Speed Control mode, where multiple machines respond to load changes (system frequency changes) to maintain a constant frequency.

And, I've not seen one power/frequency management/control system that works really well--they are all very complicated--there are LOTS of parameters to consider (the rating of each machine; the percent of rating each machine is producing; which machines should be loaded/unloaded first to keep the Isoch machine(s) from hitting rated load or zero load; which machine becomes the Isoch machine if something happens to the current Isoch machine; etc.); it's not as simple as it looks.) Human operators SHOULD ALWAYS understand what SHOULD and SHOULDN'T be happening--what the power/frequency management/control system should or shouldn't be doing, so that they can take appropriate action or report problems in a timely and correct manner to be investigated and resolved. And, what happens if the power/frequency management system is unavailable.?.?.? Human operators should know what to do to keep the plant from blacking out while still producing power at the proper frequency as system load varies.

I've seen some that can handle basic operational functions, but when a machine trips off line many times these systems just don't know how to react to save the plant or units from tripping on under-frequency. And, sometimes it's also necessary to coordinate system protection relays to allow time for operators to take proper actions to prevent the plant from blacking out.

Isoch is about controlling prime mover energy flow-rate to control frequency (speed). Droop Speed Control is about producing stable power under normal conditions (and supporting grid stability when system frequency deviates from normal until someone or something takes corrective action if the "Isoch" machine(s) can't handle the deviation for one reason or another). This is often referred to as the "load sharing" aspect of Droop Speed Control--how a machine participates stably in producing power when synchronized to a grid with other machines (generators and their prime movers) and does not try to be constantly responding to every system frequency deviation to return system frequency to normal--that's the Isoch machine's job.

On very large ("infinite") power systems/grids with tens or hundreds of machines synchronized together acting as one machine to power a very large "load" (many, many multiple loads) there often isn't even an Isoch machine, or even multiple Isoch machines operating simultaneously. Power system/grid operators (humans--at least until recently (automation can do EVERYTHING, remember?!?!?!!!??!) have to monitor frequency and take appropriate action to maintain system frequency (they often have the ability to directly control the loads of one or more large machines). And, for most situations the Droop Machines can handle most load changes and even some machine(s) or blocks of load being tripped off line--the frequency will change but the system will remain functioning until the power system/grid operators can take appropriate action to return frequency to normal. And, let's not forget, the frequency of ANY power system/grid is never 50.00 Hz or 60.00Hz; it's always very close to these numbers but not exactly during any period.

Anyway, this may or may not help some people to begin to understand the concept of Droop Speed Control. It's really NOT very complicated, but it is loaded with "features" that make it seem complicated. First and foremost, it's a method for defining (predicting) how much machine load will change for an error between the machine's speed reference and the machine's actual speed. This allows the machine to stably participate ("play nice") on a power system/grid with other machines and not be continually changing load to try to correct for frequency deviations. (Yes, it does change load during frequency deviations--but it's NOT in an attempt to return the system to rated frequency, it's only because the error between the speed reference and the actual speed changed, and when that error returns to what it was before the error occurred the machine will be back at the load it was at before the frequency deviation.) Droop Speed Control is how a machine "shares load" with other machines all synchronized to the same power system/grid. Isoch machines do the "heavy lifting" of changing load in response to frequency deviations (caused by system load changes) TO RETURN the system frequency to rated. It does that very quickly. Within the limits of the Isoch machine's rating--and that's what most human operators were never told to monitor or how to adjust the Isoch machine's load to remain inside the machine's limits (by adjusting the load on the Droop machine(s)!!!--not by adjusting the load on the Isoch machine (which just changes the frequency of the power system grid).

And, there are many ways to accomplish this on a small, islanded power system/grid. MANY ways. But I will ALWAYS maintain that even with sophisticated automation to control power system frequency (and load) the human operators SHOULD know what the automation is supposed to do--and what it isn't supposed to do. Without that training and experience, and a lot of LUCK and testing and modification (which may take months or even years as the power system/grid grows), the power system/grid is much more susceptible to problems, up to and including blackouts. Even plant historians (SOE recorders--Sequence of Event recorders and data capture and retrieval) won't always be able to provide the why and what of a blackout (thought they have the capability if properly configured).