Hello CSA,

Thank you for your answer, it is all clear. If I want to manage my gas turbines I have to put one/two in isoch and other in droop speed Kw. The turbine in droop will keep the load, then the others turbines in isoch will change load when increase o decrease. But in my system is not possible setting the load when the turbine is in droop. Then if the turbines in isoch goes to trip, what's happen in the turbine in droop?

The system is in island

 

Giuseppicciau,

No; it's not really clear. I wouldn't recommend putting two units in true Isochronous Speed Control mode unless you are looking to possibly lose your job after the plant recovers from a black-out.

I tried to be as clear as possible: WITHOUT any kind of external load control (Power Management System), a power island can run with <b>one</b> unit in Isochronous Speed Control mode, and the remaining other units in Droop Speed Control mode. If it wasn't clear in my first reply to your post it is certainly clear in other posts on control.com about islanded power operation and Isochronous Speed Control versus Droop Speed Control modes.

For small power islands without any external power/frequency control systems they can be run quite successfully with one unit in Isochronous Speed Control mode and all the others in Droop Speed Control mode. BUT, if someone switches a second unit into Isochronous Speed Control mode--then all hell will break loose! The two Isoch units will fight each other for control of frequency--and the loads of the two units will likely oscillate back and forth, and the frequency of the system will likely be very unstable, and if the operators don't quickly realize the error of their ways the plant/grid will probably go black. Isochronous Speed Control (without an external load control scheme/system) is NOT designed for more than one unit to be operating in Isochronous Speed Control mode while synchronized with other units. Droop Speed Control mode is the control mode that allows multiple units to be synchronized together to supply a load (or loads) that is larger than any single generator and its prime mover could provide, and at the same time be stable while generating power.

Think of two riders on a tandem bicycle trying to maintain a constant speed over varying terrain and with gusts of wind. If they BOTH try to adjust their pedal pressure/rate to compensate for the changing conditions the speed will probably not be very constant. BUT, if they decide that one rider will adjust his (or her) pedal pressure/rate to compensate for changing conditions and the other rider will just pedal along at a constant pressure/rate then it's very likely the speed will remain much more constant. (NOTE: There is no external speed control in this example!) The rider who was adjusting pedal pressure/rate to compensate for chaging conditions would be akin to an Isochronous Speed Control governor. And the rider who was just pedaling away at a constant pressure/rate regardless of conditions would be the Droop Speed Control unit. It really is that simple--and analagous. Think of two or more railroad engines trying to tow cars over varying conditions at a constant speed. Two or more engines can't be automatically responding to varying conditions at the same time--they will probably "fight" for control of speed, which probably won't be very constant. But, if only one railroad engine is set to respond to varying conditions, and the other(s) are just set to put out the same amount of power regardless of conditions the speed will probably be pretty constant. It's all the same, my friend. Really, it is. It's not that hard--the basics, anyway.

Perhaps if you explained ALL of the modes of the load control system at your plant we might be able to be of more help.

At the small islanded power systems I have worked on with external load control (Power Management Systems) usually if there is more than two generators/prime movers there is some method for setting which unit(s) will be the "lead" unit (responding to frequency changes when the load on the grid is changing), and there is also some method for setting the loads of the "standby" unit(s). I'm not quite clear how your plant is operated, and what modes the governors of the individual prime movers are operating in. Some load control systems operate in Isochronous "Load Sharing" mode--sending signals to governors that are actually in Droop Speed Control Mode. Some governors have the capability to operate in Isochronous Load Sharing mode--but that requires some special logic/sequencing AND some external signals to be shared with the other units participating in the scheme. And we don't know what make of units and control systems and DCS and any load-sharing (load control) scheme that is or might be controlling the loads and also controlling the frequency at your plant are.

A power island without any external load control (load-sharing; Power Management System) and with one unit operating in Isochronous speed control mode and with two other generators operating in Droop Speed Control mode will operate like this. Let's say the load on the grid is 40 MW, Unit 1 (the Isoch unit) is rated for 25 MW; Unit 2 is rated for 15 MW; and Unit 3 is rated for 10 MW. Let's say at the present time Unit 1 is operating at 20 MW; Unit 2 is operating at 12 MW, and unit 3 is operating at 8 MW. Let's say a large pump motor rated at 1 MW at the nearby water treatment facility is started, increasing the load to 41 MW. The IMMEDIATE effect on the grid is that the frequency will start to decrease--BUT, the Isoch unit sees the change in frequency (speed) and IMMEDIATELY increases the energy flow-rate into the prime mover to increase the load of the Isoch unit to 21 MW. Unit 2 remains at 12 MW, and Unit 3 Remains at 8 MW. The Isoch unit can be considered to be the "lead" unit, and it AUTOMATICALLY responds to grid frequency changes when the load on the grid changes and adjusts the load of the Isoch unit's generator to maintain the grid frequency.

Now, let's say that another 1 MW pump motor is also started at the water treatment plant adding another 1 MW to the grid load. The Isoch unit immediately increases its power output to 22 MW, while Unit 2 remains at 12 MW and Unit 3 remains at 8 MW.

The power plant supervisor has decreed that once the load on the Isoch unit gets to 22 MW (increasing) the load on the Isoch unit has to be reduced so that it doesn't reach its maximum output power and would not be able to further respond to load increases meaning the grid frequency would decrease. The power plant operator saunters over to the control board for Unit 1 and tries to INCREASE the load on the unit--which is running in Isochronous Speed Control mode (without any external load control, load-sharing or Power Management system!). But, looking at the MW meter for Unit 1, he doesn't see any change. The power plant supervisor kindly tells the power plant operator he is increasing the frequency of Unit 1--and of the entire grid!!! The power plant operator then switches his gaze to the frequency meter, sees it's at 50.37 Hz, and starts pushing on the DECREASE signal to Unit 1, until the frequency meter indicates 50.0 Hz, at which point the power plant supervisor walks away. The load on Unit 1 is STILL unchanged.

The power plant operator hurries over to one of the Droop unit control boards and increases the load on one or both of the Droop units--which will reduce the load on the Isoch unit. The load of Unit 2 is manually INCREASEd to 14 MW, and as the load on Unit 2 is being manually increased by 2 MW the load of the Isoch unit will decrease--automatically--by 2 MW. Unit 2 ends up at 14 MW, and Unit 1 ends up at 20 MW, and Unit 3 remained at 8 MW.

Let's further say that the L.O. temperature of Unit 1 is getting very high and close to the high-high trip temperature, and there is a problem with the L.O. heat exchanger of that unit. The plant mechanic says the load on Unit 1 has to be reduced as much as possible to allow time for the heat exchanger to be repaired. Well, Unit 2 is already at 14 MW out of a 15 MW rating, and Unit 3 is at 8 MW out of a 10 MW rating. The operator raises the load on Unit 2 to 15 MW, and the load on Unit 1 decreases to 19 MW. Next, the operator increases the load on Unit 3 to 10 MW, and the load on the Isoch unit decreases to 17 MW. The mechanic starts to work.

After a few minutes, the mechanic calls the control room to say it's going to take longer to fix than planned, and the L.O. temperature is still continuing to increase slowly. The power plant supervisor calls the water treatment plant to ask if they can shut down any large pumps for a short time because there is a potential serious problem at the power plant they are trying to resolve. The operators at the water treatment plant say, "Yes; we have two pumps running right now that aren't really required and we can shut them down along with another two pumps since we have nearly full tanks and we don't need to be filling them right now. The total decrease in load will be approximately 6 MW; will that be okay? We can run for a couple of hours without these pumps, but please call us back when we can re-start them."

The power plant supervisor thanks the water treatment plant operator, and watches the load on Unit 1 as the pumps at the water treatment plant are shut down. As the motors (pumps) at the water treatment plant are shut down, the load on the Isoch unit decreases--because the decrease in load on the grid IMMEDIATELY causes the grid frequency to increase--BUT the Isoch governor immediately decreases the energy flow-rate into the Unit 1 prime mover which decreases the electrical load on Unit 1. When all the pump motors have been shut down at the water treatment plant, and the load on the grid has decreased from 42 MW to 36 MW, the load on Unit 2 remains at 15 MW, the load on Unit 3 remains at 10 MW, and the load on Unit 1 has decreased to 11 MW.

Is this clear? The operators CANNOT directly change the load on the Isoch unit; they can only do so by changing the load on the Droop units. The Isoch unit will AUTOMATICALLY respond to changes in grid load. The Isoch unit will AUTOMATICALLY respond to changes in the load(s) of the Droop units. The Isoch unit will respond OPPOSITE to changes in load on the Droop units. The Droop units just operate at the load the operator adjusts them to, regardless of the load on the grid. (That's only as long as the Isoch unit can respond to load changes!)

If an operator tried to change the load on the Isoch unit, what would ultimately happen is that the frequency of the grid would change. If the operator tried to increase the load on the Isoch unit by sending a signal to raise load to the Isoch unit's prime mover governor the load would not change--but the frequency of the Isoch unit, and the grid, would increase. The Isoch function is to maintain frequency--which will change as the load on the grid changes. So, the Isoch function monitors frequency (speed) and does whatever it can to try to maintain the frequency--automatically. And any Droop units synchronized to the grid are just along for the ride (as long as the Isoch unit can keep the grid frequency constant).

Now, when you add a load-sharing (load control; Power Management) system, everything can change. There is this "external" controller that is watching frequency and loads and is sending signals to the governors to achieve the plant design operating scheme. USUALLY, the governors of the units which the load-sharing system is controlling are operating in Droop speed control mode (or, sometimes they are operating in something loosely called 'Isochronous Load-Sharing' mode--which is really just a de-tuned Isochronous Speed Control mode, almost Droop Speed Control). The individual governors of the units under the control of the load-sharing (load control; Power Management) system just get signals to raise or lower load based on how the system is set up. They don't care what the grid frequency is, or what the load on the grid is; they just get signals from the load-sharing (load control; Power Management) system to raise or lower load. Full stop. Period.

The load-sharing (load control; Power Management) system does all of the decision-making about which units change their load and when. The system has to have some kind of input from the operators about which units are to be "lead" and which units are to be "standby"--and the system then decides based on the load on the grid which units will change their load and when. A load-sharing (load control; Power Management) system could be configured and programmed to do jut EXACTLY what the three units did in the example above. BUT, the system has to be capable of accepting inputs from a human operator to be able to respond to conditions such as needing to reduce the load on the Isoch unit. That's all doable--but there must be some kind of input(s) available to the operator to allow that to happen.

It seems from your initial post to this thread that there are two units operating in "Isoch" and one unit operating in "Droop." And when you want to change the loads on the "Isoch" units you have to take them out of "Isoch" mode to change the load, and that, possibly when you put them back in "Isoch" mode they revert to some other load--because the setpoint(s) in the system weren't changed.

THIS is what is wrong with MOST Power Management (load control; load-sharing) systems--they just aren't very flexible and they weren't programmed by people who were knowledgeable in power plant operations, including "unusual" or "emergency" situations. They also might have gotten poor instructions from the power plant designers, and the systems weren't properly tested during commissioning to make adjustments as necessary. So, the operators have to be creative, and come up with "work-arounds" to operate the plant as necessary. OR, maybe the plant operation has changed from the original design--and the load control (load-sharing; Power Management) system hasn't been updated to reflect the new operating philosophy.

For some reason, power plant designers believe that everything in a power plant has to be automated to the nth degreee (fully), and load-sharing (load control; Power Management) systems are a great example. Control systems are very powerful these days--but they are only as powerful as the people who program them. And the people who program them are not usually people who have operated power plants, or who have any experience with operating power plants. They have limited input (usually from the plant operation document given to them by the power plant designers), and no real knowledge of or experience with operating power plants. Most of these systems are capable, but weren't properly configured or programmed.

And lots of power plant operators don't really understand how AC power systems operate, either. They just don't get very good training (most of it is on-the-job (OJT)), and it is delivered to new operators by operators who didn't get much, if any, formal training. And, because the owners and operators of the power plants know that the plants are heavily automated they rely excessively on the automation--quite often to do things it wasn't properly configured or programmed to do. And operations supervision can offer little in the way of guidance and information.

In fairness, there is a LOT lacking in education and training about power plant operation and basic AC power system fundamentals--including two of the most basic fundamentals, Isochronous- and Droop Speed Control. Why? Because for the most part, they just work. It's when the basics try to be controlled by external load control (load-sharing; Power Management) systems that the problems can begin.

If your load-sharing (load control; Power Management) system places two of three units in "Isoch" mode, is there a method for setting the split of the load between them? Can one unit be programmed to take 80% of the load under normal circumstances, and the other 40% of the load under normal circumstances? Or, do they both try to carry the same load at all times? Are they set to carry no more than 50% each of the total load, minus the load on the standby "Droop" unit? Is there a way to adjust the load split between the two "Isoch" units? Is it done using a screen on the plant DCS? Is there a way to adjust the total load carried by the two "Isoch" units?

Most of the above questions are things I have seen load-sharing (load-control; Power Management) systems capable of. Again, I think you need to find and study the documentation for the system at your plant to determine what it's capable of. Because, it doesn't seem to be a "typical" islanded power system, one with<b>out</b> a load-sharing (load control; Power Management) system, which is all I can really describe. There are no "standards" for load-sharing (load control; Power Management) systems--every one is customized for the plant it is configured for. There's just not too much more can be said--I wish it were different. If you can provide more information about the system at your plant and the various modes and screens and setpoints it is has, we MIGHT be able to give you some more answers. But, really, we'd just be guessing.

Finally, I wouldn't suggest trying anything without first understanding what your system is capable of. I think you have at least most of the basics of AC power systems, a basic example of a single Isoch unit and multiple Droop units synchronized to an islanded power system. I could speculate (guess) about what else to suggest, but it would take a LOT more verbiage and text, and I'm really, Really, REALLY tired of Droop- and Isochronous Speed Control. And I seem to be failing miserably at explaining it without pictures and charts or graphs or videos. So, I'm going to give up responding to these kinds of queries. It's all been covered MANY times before on control.com. There is a 'Search' function which can be used to find many, many threads. I can't explain it any more--without writing a book, and using charts and graphs and videos and apps. So, I'm just done. Finished. Finito. Fin.

Ciao!