I currently work on a 2+1 combined cycle, with a GE D-11 steam turbine, and two 7FA CTs, utilizing a Mk VI control system.
During a recent start-up of the steam turbine, we experienced an "excessive speed error" alarm when trying to roll the turbine. All permissives were met, the steam turbine was just slightly above turning gear speed, and the RH steam pressure was approximately 150 psi, which is normal for a warm start up.
The alarm persisted, preventing us from rolling the turbine each time we initiated an auto-start, until we tried lowering the RH steam pressure to around 90 psi, at which point we were able to successfully roll the turbine.
My question is, how is the Mk VI determining the "excessive speed error"? From what limited information I've found, speed error seems to be the difference between the turbine's actual speed and reference speed. Given this information, I don't see the correlation between lowering the RH steam pressure and the speed error no longer being excessive?
Any help in this matter would be appreciated!
One kind of turbine control speed error is, indeed, the difference between the turbine speed reference and the actual speed. BUT, that's most likely not the speed error the Mark VI is referring to when the D-11 is on turning gear. Turning gear speed should be fixed (not variable) because of the gearing used on the turning gear. I have no Mark VI steam turbine .m6b files to refer to at this writing, but I'm pretty confident of the statement above (that the speed error of this alarm is not related to any speed difference between a reference and actual speed--but rather that the actual speed is greater than some preset value, even if that's not exactly what the alarm text message says).
Admittedly, I'm no steam turbine expert, but I have been on many combined cycle plants and have been exposed to this issue (though I have not been directly involved with troubleshooting it). Three things have historically been known to cause the problem of the turning gear mechanism disengaging prematurely and the turbine rotor speed to be higher than turning gear speed: "excessive" condenser vacuum (though I've never completely understood this phenomenon); leaking steam valves OR warming valves (around steam valves) mistakenly left open or not closing; and misadjusted turning gear mechanism limit switches.
I'm going to start with the second cause first (because I used to work for GE). If steam valves leak and allow enough steam to get into the steam turbine to cause rotation, that will cause the turning gear mechanism to disengage and the turbine shaft speed to increase. The fact that you decreased the RH (ReHeat) steam pressure to mitigate the problem could possibly indicate leaking RH steam valves. And, many times there are small manual or solenoid-operated valves used for warming steam lines and pipes and steam chests and sometimes the flow through them when the turbine is already warm have caused a similar problem.
Now the first cause, .... I think this would have to be somehow related to the cause above--there would need to be flow into and through the steam turbine from somewhere and into the condenser that would provide the energy for the steam turbine rotor to turn. I don't know if condenser vacuum is normally not very "high" during starting, but I do know that it gets higher as steam starts condensing as flow increases through the steam turbine during starting. I think saying just "excessive vacuum" is not really properly describing the problem, but I may be wrong; again, I'm not a steam turbine expert.
And, finally, the third cause. Again, I don't have any direct experience with this (having worked primarily on the GT side of the combined cycle plant), but I have been on several sites that had serious problems with the steam turbine getting on to and remaining on turning gear because of turning gear limit switch adjustment problems. In a couple of cases when the turning gear mechanism disengaged the unit would begin to "accelerate" instead of going to zero speed. The scuttlebutt was that because there was vacuum on the condenser AND there was steam somehow getting into the steam turbine (through stuck warming valves, I believe) this was causing this problem. To my way of thinking, it was actually more the steam getting into and through the steam turbine to the condenser that was causing the turning gear mechanism to disengage, but, when people get something in their heads about what's causing a problem it's very hard to get them to let go of it without hard physical proof otherwise (and even then, it's sometimes impossible to change a person's mind (remember: You can't reason people out of a position they didn't reason themselves into)).
Myself, I would be looking for some "source" of steam getting into the steam turbine (leaking valves; stuck valves; etc.) that's causing the "acceleration" from turning gear speed--and exceeding the 'allowable' speed while on turning gear (which seems to be a start-check permissive) resulting in the "speed error" alarm. It doesn't seem like the turning gear mechanism should be capable of accelerating the steam turbine above the 'allowable' speed when engaged (again, because the induction motor driving the turning gear mechanism runs at a near constant speed and there are fixed gears in the mechanism). Now, having said that, I suppose there are some turning gear mechanisms with soft-start (variable speed drives) on them and that could be malfunctioning.
Mostly, I just wanted to rebut the statement that the speed error in this alarm text message was related to a difference between a speed reference and the actual speed--because, unless there's a variable speed drive powering the induction motor of the turning gear mechanism (and there may be!) it's hard to think there's a speed reference for turning gear operation. The turbine shaft speed pick-ups are being used to sense shaft speed while on turning gear (at all times, actually), and it could be that one or more shaft speed pick-ups is malfunctioning/intermittent (unlikely; highly unlikely) but there would be Diagnostic Alarms indicating this condition. (Though most Diagnostic Alarms are ignored.)
Anyway, hope the above helps in some small way. If you could write back to let us know what is found (if anything) that would be much appreciated. In the meantime, I will try to get a steam turbine .m6b file.
Thank you for your assistance with this! We've suspected for some time that our steam seals may be a bit worn, do you expect that might account for steam leaking through and causing the roll off turning gear?
As far as the excessive speed error however, at the point where we lowered the RH steam pressure down to 90 psi, the steam turbine was already rolling at around 30 rpm, which is what I found peculiar. Prior to that, we had given an auto start command several times, with the turbine slowly rolling up above turning gear speed (but RH pressure at around 150 psi), and each time we were met with the excessive speed error.
The only other thing I can think of is that perhaps the excessive speed error alarm had not been properly reset, and even though we were within bounds on speed, the Mark VI still had an active alarm in. I'd have to look back through the alarm history to confirm this though, I seem to recall us trying several resets during this whole process, but I'm not 100% sure.
I am no steam turbine expert, and while I think I have heard of leaking (worn) steam turbine seals contributing to unintentional roll-off, I have no personal experience with that phenomenon and so won't make a comment one way or the other. I would seem it would take a LOT of steam to make this happen, but I may be wrong about that. Even when I hear people talk about things like this I always take it with a grain of salt unless I'm personally involved in the troubleshooting and resolution of the problem because some people get things in their heads and won't listen to reason. It makes sense to them and satisfies their suspicions and makes a convenient explanation--but it's entirely wrong. Remember: You can't reason a person out of something they haven't reasoned themselves into. If they just believe it to be so without any thought or consideration they just aren't going to be persuaded otherwise no matter how good the reasoning or logic is.
Usually, though, it doesn't take much to roll a steam turbine if it's already spinning on the turning gear. The initial inertia required to get the shaft spinning was supplied by the turning gear. And, most steam turbine and generator lube oil systems have sufficient flow and pressure (especially if they use jacking oil or lift oil) that they are pretty to spin with very little flow through the steam turbine. With a high vacuum drawing any flow--even air in some cases through seals if not properly working--can cause some smaller steam turbines to spin.
I'm also going to reserve any comment on the Mark VI speed error thing until I get a steam turbine .m6b file and can review it and understand the alarm. And, even if I get a .m6b file pretty soon, I just don't have much time right now to review it and get up to speed to be able to make any comment. Sorry; but I'm just swamped with work these days and reviewing application code can be very time-consuming (at least for me to get a proper understanding).
Please write back to let us know what you find!