Our unit comprises of a single 9FA gas turbine and a single A15 steam turbine. Our shutdown time using the auto stop sequence is about 49 minutes (Auto stop initiated to GT breaker open). We are contracted to shutdown in <40 minutes therefore our customer is penalised for every shutdown. It’s my understanding the 9FA Gas Turbines unloading rate is 0.33% with respect to TNR. During the Auto shutdown when the Steam turbine breaker opens the GT is at about 80MW’s, the GT then takes about 25 minutes to unload to breaker open which is well in excess of the 0.33% unloading rate. To put it simpler the GT should be shedding about 7MW’s every minute but is instead shedding about 3.3MW’s a minute. Why is this happening? When the ST breaker opens a “ST Trip” is generated, if we “Reset” the ST the GT unloads to breaker open in a couple of minutes which is too quick. In the MKVI auto stop sequence what controls TNR when the ST breaker opens and the GT is then unloaded?

 

SteveRoche,

Unfortunately, GE uses the same Control Constant for unloading as for loading when in "Auto" mode--in almost every case. So, if you change the existing "Auto" loading/unloading rate to make the unloading rate faster for the shutdown, it's also going to be faster when loading....

It is possible to program a separate unloading rate, but it would likely take a special "stop" pushbutton to make it work. And, that's no so easy--because of the CIMPLICITY interface. One would have to create a new CPB (Command Pushbutton) in the Speedtronic panel, if it's a Mark VI or Mark VIe you would have to add it to the EGD, then it has to get imported into the CIMPLICITY Signal Database or the Toolbox/ToolboxST .hmb file, then it has to be added to the display, and the logic has to be written to use it and an unused Control Constant for the unique unloading rate.

The loading/unloading rates are based on the Droop setpoint, so if the Droop for the GT is 4% (which it is for most GE-design heavy duty gas turbines), 4/0.33 = 12.12 minutes to go from full load (Base Load) to zero load (approximately). So, I think the value you are quoting is incorrect if you say it's taking about 25 minutes for the shutdown (which would 4/25 = 0.14 %/minute. The 0.33%/minute sounds like the "Manual" loading/unloading rate, which is usually much faster (sometimes double) the Auto loading/unloading rate--which it almost is in this case based on the information provided.

The Control Constant Array which has the setpoints is usually something like TNKR1_n (the name can vary slightly based on the version of Speedtronic), where 'n' is a number from 0-7. There is a whole bunch of logic which controls which array is selected (L83JDn, where 'n' is also a number from 0-7--which corresponds to the TNKR1_n 'n' value). You would have to examine the logic/application code running in the Speedtronic panel at your site to determine which rate constant is being selected during Auto loading/unloading. And, then someone would have to choose an unused loading/unloading rate array value, write some logic to select it when you want a faster Auto unloading rate (probably using a special CPB from an HMI display), and then do all the HMI magic to put the CPB on a display and make it all work.

Hope this helps!

I know this is a LOT of extra work for the operators during a shutdown when it's soooo <b>BUSY</b>, but they could just click continually on LOWER SPEED/LOAD to reduce the load faster than just clicking on STOP, and then when the generator breaker opens, or just before the generator breaker opens, they could click on STOP. But, most operators will complain that it's too much clicking (and will cause repetitive stress injury from all the clicking in such a short period of time--though when they're playing video games they are clicking a lot faster for a longer period of time, but they're not getting paid to do that!). Again, this would work because the Manual unloading rate is usually twice as fast as the Auto unloading rate (with the Control Constants from the factory--which don't get changed very often, but sometimes they do).

An alternative would be to change the Auto loading/unloading rate when you know this kind of shutdown is going to occur, and then when it's over someone just has to remember to change it back.... And, operators will usually forget to do this so this isn't usually a good idea to be changing Control Constants because something usually gets messed up when it's not changed back--or if you get a call to abort the STOP sequence and re-load the machine, and then it will load VERY fast! But, this could be a test--once you know the name and value of the Auto loading/unloading rate Control Constant in the Speedtronic turbine control panel at your site.

Write back to let us know how you fare in working out this issue!

 

Thanks for your reply CSA.

The Auto loading/unloading rate for our unit is G1:TNKR1_4L and is 0.33%/min. The Manual loading/unloading rate for our unit is G1:TNKR1_2L and is 0.67%/min. I note from looking at the logic tonight the Combined cycle loading rate of array G1:TNKR1R is 0.165%/min.

We shut down in the Auto Stop sequence, the GT unloads from 115MW's to about 80MW's fairly quickly, at this point the ST starts to shutdown whilst all the time the GT stays at the 80MW mark. When the ST breaker opens we some how register a ST trip on the counter. The GT then unloads to breaker open, the GT sheds about 3.4MW's per minute. This is an unloading rate of about 0.16%.min which is the combined cycle loading rate!! If we do a diagnostic reset on the ST when its breaker opens the GT unloads quicker.

Is it possible the ST trip is playing havoc with the GT unloading rate and when the ST is reset this clears? I am in the operations department so this is hard for me to understand but i think there is a glitch in the GT unloading rate caused by the ST registering a trip on shutdown, possible link to temperature matching/combined cycle loading.

 

The combined cycle loading and unloading rates are the same. However, once the steam turbine is tripped, with steam inlet valves closed, you should be able to unload at the gas turbine unloading rate of 0.33%/minute.

Also, if the controls are Mark VIe, and the steam turbine has stress control, it is possible to set up the controls so that the when the steam turbine is running, the steam turbine Mark VIe adjusts the gas turbine loading rate based on the stress control function. With this arrangement, while on combined cycle, the gas turbine loading rate is basically a low value select between the 0.33% rate and the steam turbine stress control rate. This was the method we were using on single shaft 9FA GT's with A15 steam turbines at the time I retired. I don't think you explicitly stated whether this site is single-shaft or multi-shaft, but I got the impression it is multi-shaft. However, I can't think of any reason why the loading rate control should be any different.

 

Stephen Roche,

Based on the signal names (the 'L' suffix at then end of the signal names you quoted) I might suggest that your turbine's logic was modified. I'm going to presume the 'L' suffix stands for 'Lower', which would be the unloading rate. Without being able to see the sequencing/logic/application code running in your turbine control panel it's impossible to say for sure, but the signal names are "unusual" with the 'L' suffix--at least for me; I've never seen the 'L' suffix before.

Loading rates are based on Droop setpoints, as explained before. 0.333% per minute would mean that TNR would be raised/lowered at the rate of 0.333% per minute, and for a Droop setpoint of 4% that would mean 12 minutes from No Load to Full Load (at turbine nameplate ambient conditions with a new and clean turbine), and from Full Load to No Load. The loading rate is expressed as a function of the change in turbine speed reference, and if the machine has a 4% droop the turbine speed reference will change by 4% from No Load to Full Load, or from Full Load to No Load (again, when the unit is in a new and clean condition and the ambient conditions are at or very near turbine nameplate rating).

It is possible that somehow the GT control panels are aware of the condition of the ST, and that somehow it affects the unloading rate (and possibly even the loading rate). Again, without being able to see the sequencing/logic/application code running in the turbine control panel at your site it's virtually impossible to say for sure. But, it's certainly possible, not typical, but possible. If there is more than one GT, and only one ST, then either the ST has to provide two (2) status indications (one to each GT) or some other control system (plant DCS) would have to send two status signals, one to each GT, for the GT controls systems to know the ST status.

I wish I could be more help--but without being able to view and analyze the sequencing/logic/application code running in the turbine control panels at your site. It seems to have been modified from the standard typically provided with a GE-design heavy duty gas turbine, but that's just a guess based on the signal names you provided.

Please write back to let us know how you resolve this problem!

P.S. I'm not familiar with TNKR1R, either. So, again, it would seem your control systems have been modified and that the modification either was not properly tested, the operating mode of the plant has changed, or there was a misunderstanding when the changes were made/implemented.

 

We have the problem solved, when the ST breaker opens the ST latched a trip status. The trip played havoc with the B1 file which in turn selected an unloading rate which extended the GT shutdown period. The logic was modified with the GT now unloading in the Auto unloading rate.

 

Steve Roche,

Great news!

Thanks for the feedback!