we have 2 GTG M6001B mark 6 control. capacity 38 MW per unit. we use 36 MW and export to grid 40 MW. When plant islanding 2GTG power very swing. now we set all isoch mode if islanding condition. pleas help me correction this problem.

 

> we have 2 GTG M6001B mark 6 control. capacity 38 MW per unit. we use 36 MW
> and export to grid 40 MW. When plant islanding 2GTG power very swing. now we
> set all isoch mode if islanding condition. pleas help me correction this problem.

The GTGs are bound to swing when islanding as their combined load is dropped by half. You could unload the GTGs so your tie line is neutral (no export or import power) before islanding. Normal configuration for two machines in island operation is to leave both in droop mode (preferred) or select only 1 to isochronous mode (not both).

 

Yes, everyone would like their plant to switch from export- to island mode without a bobble when the tie-line breaker opens suddenly and without warning. People in hell want ice water, too. As the Rolling Stones song goes, "You can't always get what you want. But if you try sometimes, you just might find, you get what you need."

The original poster has not defined the condition of switching from export- to island mode. Is it sudden? Does the tie-line breaker with no forewarning? Most times this happens with no time to reduce load to an export of zero before opening the tie-line breaker, which would make for a pretty smooth transition. There may even be some kind of "Power System Management" scheme the original poster hasn't told us about that is also working to try to stabilize frequency, or some kind of Isochronous load sharing that we don't know about. We also don't know if both machines are being operated at Base Load with BASE LOAD selected and active, or if there is a Pre-selected Load setpoint of say, 40 MW for each unit, or how the units are being operated. It makes a huge difference, and it's necessary to know this information.

Two machines operating in parallel with each and independently of other machines ("island mode") <b>should not</b> both be in Isochronous mode; the governors will fight each other for control of frequency and the result is usually pretty ugly, and usually means one or both GTs will trip on under- or over frequency, or on high exhaust temperature, or on flame-out (because the fuel is cut back too severely by one machine). If the load of one machine can be cut back (say to 50%) <b>very</b> quickly when the tie-line breaker opens and the other machine can be switched to Part Load from Base Load, then both machines can probably remain in Droop control as long as the island load remains pretty stable. But both cannot remain in Droop control with Base Load selected when the tie-line breaker opens. It's just NOT going to work.

If your island load never exceeds the rating of one machine, and you don't anticipate running in island mode for an extended period of time (more than a few hours), then as Sprinter has suggested, you just might want to trip one machine when the tie-line breaker opens and switch the remaining machine to Isoch control automatically when the tie-line breaker opens. If the island load is 36 MW, the one GT should be able to handle the load without reaching rated power output.

An even better scenario would be to open the generator breaker of one machine when a sudden, unannounced tie-line breaker opening occurs and simultaneously switch the other machine to Isoch control. That way, the Isoch machine only has to lose about 2 MW of load to get down to the island load and can do that very easily without much change in frequency. The other machine, still running at FSNL, can then be synch'ed to the Isoch machine, and its load can be increased to reduce the load on the Isoch machine.

If you must have both machines both operating at Base Load with Base Load selected while operating in island mode when separated from the utility, then you're going to have to reduce the load of at least one machine, and if the opening of the tie-line breaker is sudden and without warning, then you're going to have to do it <b>very quickly</b>. And you're going to have to switch the other machine (automatically) to Isoch at the same instant.

So, let's look at an example of a sudden, unannounced opening of the tie-line breaker with both machines operating at Base Load in Droop Control (at 38 MW, each) with one machine automatically switched to Isoch when the tie-line breaker opens and one machine remaining at Base Load (which is not exactly Droop speed control).

If the load suddenly drops from 80 MW to 36 MW, the Isoch machine has to cut it's power output very sharply from 38 MW to 2 MW, and it will do that but there will be a slight time lag and the frequency will be high during that time and it may cut fuel back so quickly that there might be a loss of flame, but then it might not, or it might undershoot for a brief period, and it might even trip on reverse power, but then it might not.

The other machine will continue to run at Base Load (38 MW) and will negatively respond to frequency changes. If the frequency goes high, the unit will actually increase its power output (which isn't what you really want to have happen) and this will cause the load on the Isoch unit to have to drop even further (because there's only 40 MW of load, 38+ being taken by the Base Load machine and the remainder by the Isoch machine). (This is because the Base Load machine is on exhaust temperature control, which is not Droop Speed control.)

Let's say the plant survives this separation, what should happen is that the Isoch unit ends up with 2MW of load, and the Droop Machine with 38 MW of load. At this point, it would be very beneficial to lower the load of the Droop machine to something like 50% of rated output (and the Isoch machine will pick up the load being reduced from the Droop machine). This is a good condition for the plant, with each machine having about 50% of rated load, both running, with a stable grid frequency. As long as there are no large load swings while in island operating the two machines
will run along very smoothly, responding to load changes very nicely.

But, unless there is some kind of Isochronous Load Sharing scheme that has been properly commissioned and tested and is known to work properly then only one machine's control system should be switched to Isoch mode (automatically on tie-line breaker opening) when operating in island mode. The other machine should NOT be at Base Load (because it won't properly respond to any frequency changes).

And until we get more information from the original poster (the information listed in the second paragraph above--all of the information, not just one or two pieces of it), we can't make any more recommendations than these.

 

Okay; I've made a pretty big mistake here, getting my island load and export numbers mixed up. The three paragraphs listed here should have read as below:

>If the load suddenly drops from 80 MW
>to 36 MW, the Isoch machine has to cut
>it's power output very sharply from 38
>MW to 2 MW, and it will do that but
>there will be a slight time lag and the
>frequency will be high during that time
>and it may cut fuel back so quickly that
>there might be a loss of flame, but then
>it might not, or it might undershoot for
>a brief period, and it might even trip
>on reverse power, but then it might
>not.
>
>The other machine will continue to run
>at Base Load (38 MW) and will negatively
>respond to frequency changes. If the
>frequency goes high, the unit will
>actually increase its power output
>(which isn't what you really want to
>have happen) and this will cause the
>load on the Isoch unit to have to drop
>even further (because there's only 40 MW
>of load, 38+ being taken by the Base
>Load machine and the remainder by the
>Isoch machine). (This is because the
>Base Load machine is on exhaust
>temperature control, which is not Droop
>Speed control.)
>
>Let's say the plant survives this
>separation, what should happen is that
>the Isoch unit ends up with 2MW of load,
>and the Droop Machine with 38 MW of
>load. At this point, it would be very
>beneficial to lower the load of the
>Droop machine to something like 50% of
>rated output (and the Isoch machine will
>pick up the load being reduced from the
>Droop machine). This is a good condition
>for the plant, with each machine having
>about 50% of rated load, both running,
>with a stable grid frequency. As long as
>there are no large load swings while in
>island operating the two machines
>will run along very smoothly,
>responding to load changes very nicely.
>

The three paragraphs above should have read:

If the load suddenly drops from 80 MW to 36 MW, the Isoch machine has to cut it's power output very sharply from 38 MW to -2 MW, and it will do that but there will be a slight time lag and the frequency will be high during that time and it may cut fuel back so quickly that there might be a loss of flame, but then it might not, or it might undershoot for a brief period, and it might even trip on reverse power, but then it might not but it probably will.

The other machine will continue to run at Base Load (38 MW, with the Isoch machine at -2 MW and the Base Load machine at 38 MW and the island load at 36 MW) and the Base Load machine will negatively respond to frequency changes. If the frequency goes high, the unit will actually increase its power output (which isn't what you really want to have happen) and this will cause the load on the Isoch unit to have to drop even further (because there's only 36 MW of load, being taken by the Base Load machine. (This is because the Base Load machine is on exhaust temperature control, which is not Droop Speed control.)

Let's say the plant survives this separation, what should happen is that the Isoch unit ends up with -2MW of load, and the Droop Machine with 38 MW of load. At this point, it would be very beneficial to lower the load of the Droop machine to something like 50% of rated output (and the Isoch machine will pick up the load being reduced from the Droop machine). This is a good condition
for the plant, with each machine having about 50% of rated load, both running, with a stable grid frequency. As long as there are no large load swings while in island operating the two machines
will run along very smoothly, responding to load changes very nicely.

Sorry for any confusion caused by my poor proof-reading.

 

Thank you so much .
More data configuration of my plant.We have PMC (Power management control) by YOGOGAWA DCS. normal operation we run 2GTG Preselect load. and we configuration in case islanding follow up

step1: if in house < 36 MW 1GTG will auto FSNL and GTG unit 2 will change from droop to ISOCH mode for maintain load remain. but this function effected to steam supply customer pressure low due to loss of steam form 1HRSG (70 T/H)


Step 2: if in house >36 MW 1GTG will auto Isoch mode and GTG unit 2 still droop mode. by load set point for GTG unit 2 received from bus frequency for raise/low preselect load command. but this function very bad control. 1GTG isoch mode reverse power and trip and GTG unit 2 Droop mode take load and frequency low 48.00 hz effected customer plant back out.

conclusion i want best of configuration in case plant islanding. (2GTG capacity 38 MW/unit in house load 36 MW and export 40 MW)
i want to remain 2 GTG in case islanding. but i don't know how to configuration?

(sorry for my english)
apisit

 

Oh, the plot thickens. (Well, a few minor, important details were left out--like just about all the most important details. But, that's par for the course these days, it seems. Unfortunately.) No worries about the language; we're understanding what you're telling us, when you provide the pertinent information, that is.

And you haven't told us how long the separation occurs. Is it just for a few minutes, or an hour, or two hours, or seven hours, or how long?

You have several problems; I'm only going to be able to address a couple of them.

First, if you lose more than 50% of the exhaust heat what makes you think you will be able to maintain steam pressure, temperature, and flow at the level before the separation occurs? Or even anything close to that? You would be very wise to open the generator breaker of one unit on separation, and simultaneously switch the other to Isoch mode automatically, that way the exhaust temperature will stay as high as possible--at least in the unit that's still on line.

Second, if you're trying to control speed/frequency with RAISE and LOWER commands (discrete inputs to the Speedtronic) from a DCS, it ain't going to work very well. There is a limit to the rate of change in Droop mode, and it's about zero load to full load in approximately 30 seconds (or, about 76 MW/minute, or about 1.267 MW/second, in your case).

<b>And,</b> the maximum loading/unloading rate may be even <b>less</b> if the boilers (HRSGs; WHRBs; whatever they're called at your site) can't be loaded that quickly (if it can't take the sudden temperature swings on a regular basis; you'd need to consult the boiler manufacturer for that information). Yes; boilers on the exhausts of gas turbines have maximum loading/unloading rates, also.

Isoch control is much faster, <b>and</b> you will not be relying on discrete commands from the DCS which are subject to maximum rates of change to raise and lower the turbine speed reference. Now, there will be exhaust temperature swings that will be experienced by the boiler, but you should be able to maintain speed/frequency fairly well with one unit near rated while operating on Isoch (again, presuming the load doesn't increase above rated very often, such as when large motors are started).

Perhaps another contributor here, otised, can make another recommendation about the steam pressure/temperature, but simple physics says you are going to lose at least 50% of steam production if you lose more than 50% of the load upon separation.

If you have duct burners on the boilers you might could fire them up on the unit that's off-line, but there's probably a limit to the amount of steam that can be produced that way as well. And, at least one unit will be lightly loaded.

Again, we don't know enough about the construction of your plant, but you may be able to separate as above (with one unit's generator breaker opening; the other unit being switched to Isoch), then re-synch the off-line unit to the Isoch unit and load it up to 18 MW (50% of island load), and turn on the duct burners in both boilers and get some more steam that way.

But, I think you're kind of between a rock and a hard place (in other words: you have a problem). If the plant was built to be able to do what you want it to do and it's not doing that, then you should go back to the AE (Architect-Engineer) and get them to fix the problem.

If you're trying to operate the plant differently than it was designed for, then you need to consider some other methods for generating the steam you seem to require when separated.

<b>OR,</b> how about this? Solve the problem causing the separation?

Now there's a really novel idea. Instead of trying to do something that's not easily doable (again, we likely don't have all of the information about your site!), solve the problem that's creating the need to find a workaround to the problem.

The problem being the separation from the grid.

Best of luck!

 

Dear sir

1. when plant islanding we will re-Synch to grid 5 minutes

2. HRSG (NEM)capacity 140 t/h and duct burner

3. we plan to configuration in case plant islanding by if in house load > 36 MW and plant islanding 2GTG will still droop mode. and we use frequency from bus GIS 115 KV to sent command for raise/low Preselect load set point each GTGs. but i don't sure frequency may be over to 52 hz and effected to customer.

4. if 2GTG can run at 50 % load (18 MW )we can firing duct burner for increase pressure, temp. during islanding

5.we can configuration by item 3 ?

thank you so much my teacher
apisit

 

> 1. when plant islanding we will re-Synch to grid 5 minutes

That's a relatively short period to drop load to less than 50% while trying to use Preselect Load to maintain frequency, <b>and</b> maintain steam production.

> 2. HRSG (NEM)capacity 140 t/h and duct burner

I presume that's with rated load on the GTs.

> 3. we plan to configuration in case plant islanding by if in house load > 36
> MW and plant islanding 2GTG will stilldroop mode. and we use frequency from
> bus GIS 115 KV to sent command for raise/low Preselect load set point each
> GTGs. but i don't sure frequency may be over to 52 hz and effected to customer.

See below.

> 4. if 2GTG can run at 50 % load (18 MW)we can firing duct burner for increase
> pressure, temp. during islanding

Okay.

> 5.we can configuration by item 3 ?

No. Not maintaining frequency using RAISE/LOWER Preselect Load Setpoint. There is a loading/unload rate, and the maximum rate I described previously, which means the frequency is going to be high for at least 15 seconds--<b>PRESUMING</b> the necessary modifications are made to enable the fastest possible rate upon separation. You can set a Preselect Load set point of 18 MW from a load of 38 MW, but the Speedtronic doesn't "jump" to that load it has to ramp to the load at some rate. In other words, the load won't jump to 18 MW when you "jump" the Preselect Load Setpoint from 38 MW to 18 MW.

A Droop characteristic of 5% would translate to roughly 52.5 Hz if the load went from 38 MW from 0 MW, so if it drops by slightly more than 50% then the frequency would increase to approximately 51.3 Hz (for two turbines producing 36 out of 76 MW) before dropping to 50 Hz after the ramp rate that is programmed into the machine. And the fastest that could happen would be approximately 15 seconds <b>IF</b> the necessary modifications were made.

Look, there's no easy way to do what you want, maintaining steam production while maintaining frequency using Preselect Load Control. There's going to be a frequency excursion, and its going to take some time to get back to nominal frequency <b>and</b> get the duct burners going and get steam production up (there will be some residual heat which might help with keeping steam production up).

And you need to get someone knowledgeable about the Speedtronic to site to help with the modifications and testing and tuning if you want to implement a really fast unloading rate upon separation. That's just not something that can be done via a forum like this.

 

A couple of comments:

On a typical combined cycle unit, steam turbine considerations usually limit the loading/unloading rate to a maximum of 5% per minute. This is with a hot steam turbine. You may be able to improve on that if your steam turbine control has a stress limit function which can allow a faster loading rate at least until the stress limit comes into play. Anyway, 5% per minute is 20 minutes from no load to full load, a bit slower than 30 seconds.

My recollection of GE simple cycle gas turbine controls is that the normal automatic loading rate is 6 minutes from no load to full load. The manual rate used to be 90 seconds from no load to full load, but that was to be used for smaller load changes. It was never intended to be used to go all the way from no load to base load in one shot. There are units sold with a fast load capability, but these were never combined cycle units, and you would shorten the life of the hot gas path parts if you used the fast loading rate.
As far as steam temperature changes go, the steam turbine is likely to be much more sensitive to this than the boiler. The combined cycle controls that I worked on generally had limits on boiler loading during startup on a cold boiler, but once it was hot, the steam turbine limits prevailed.

You may well want to consider turning on the supplemental firing to keep the steam temperature up during this transient. You may have to use a manual mode of burner operation to do this, since automatic operation sometimes requires a high gas turbine load to initiate burner firing. This limitation is there to prevent unnecessary operation at a lower thermal efficiency - as long as the gas turbine is at operating speed there should be sufficient air flow to support the burner operation. It still would be advisable to get approval from the equipment suppliers.

 

Just an additional information to enable you to increase you options. You can set the droop percentage set point for the unit under droop mode to make it less or more sensitive corresponding to you iso droop unit. Assuming the current set point for the droop unit is 6% and the iso droop unit load was pushed to 2MW less than 5 seconds after the separation, then you may consider to increase your droop sensitivity set point to 3%. By doing so you will find that the droop unit will reject load twice of the value as if it was set at 6%. This gives additional load to your iso droop unit to take.

First of all you have to find out whether you unit can handle 3% droop. Be sure too such arrangement valid for system separation only,i.e. the original set point shall be preserved under normal operations. If the set point has already set at 3%, my personal opinion is it is not advisable to reduce it further.

Tq.

 

Just a word of caution. Changing the droop characteristic
can have many unintended knock-on effects, including instability and sanctions from grid regulators.

Droop is not something to be messed with lightly or if you don't understand all of the ramifications and consequences.

Please don't consider modifying the droop characteristic, temporarily or permanently, without consulting someone who can help you understand the effects of even a small change.

The problem here is that the original poster doesn't want to use Isoch mode, which would solve several of his problems. He's looking for an easy solution using Preselected Load Control and it's hard to imagine how that's going to work.

As otised has said, the steam turbine--if it exists--might also be a limiting factor. But I would argue that a separation event is somewhat unusual and shouldn't be a common occurrence. And the original poster has never mentioned a steam turbine, which if driving a generator could also be part of the problem.

We don't know if the plant was designed to be operated this way, or if this is a change to the original plant design and operating philosophy.

From lots of (painful) experience, this kind of problem is very difficult to resolve.

 

Thank you CSA. Many good points you put in. Let me emphasize my points

1. Don't change the droop characteristic. Just change the % set point. The design characteristic shall be preserved. As per design the droop can be set between 0-12% or more. In my country it is set around 4-6%. In the US I think it is around 3-5%. Important note: The droop percentage set point between two parallel generators shall not deviate by a certain limit. Otherwise it can cause governor hunting, See Thomas M. Athay, "Generation, Scheduling and Control" Proceedings of IEEE, 1988.

As far as machine stability is concern, the machine is more stable if the % set point is raised since its dynamic response per 0.1 Hz frequency deviation will become smaller. Thus reducing the possibility of frequency oscillation & overshoot.

2. Yes. You are right. The droop set point shall be sanction by grid operator. I reminded the poster to preserve the original set point under the normal operating condition, i.e. no separation. Probably there is a practicality issue to realize what I was suggesting but I think it is still a valid option.

3. In power generation, all the calculated risks have to be addressed if possible. In this case it is a need for the poster to find the way out.

Thank you.

 

Namatimangan08... adding to CSA's 03-May-11 caveat... didn't the original poster state the GTGs were "swinging" after separating from the grid?

Regards, Phil Corso