Following diagnostic alarms appeared on running Gas turbine cimplicity screen:

1. Ref-2 voltage out of limit; ID: 167 in PCAA Module of <R> pack.
2. Ref-1 Voltage out of limit ;ID: 168 in PCAA module of <R> pack.
3. Control panel Over temp high

These alarms not getting reset. Checked PCAA module and found ATTN LED RED was blinking.

This time machine was running normally on full load. After 3 hrs, GT goes in shutdown process and following alarms appeared on cimplicity:-

1. Exhaust T/C trouble
2. Combustion control trouble
3. 4 Or More bad change over T/C trouble.

As per logic If 4 Or more bad change over alarm appeared then L94T logic will be true and machine will go in shutdown.

We have started machine again due to crisis of power and again tripped after 3 hrs.

One thing more we have checked all parameters of Exhaust T/C and found only 5 T/C which connected in PCAA<R> Pack increased 80DegF.

PCAA module was replaced with TCAS and alarm normalized.

But i can not get problem route cause why machine will go in shutdown state in MARK-Vie TMR System.

What is calculation of tripping on exhaust T/C?
If any exhaust T/C faulty, then what will impact on machine?

i am completely unknown on exhaust T/C LOGIC. PL explain.

 

vimal gupta,

It would seem there are some new Diagnostic and Process Alarms since I've looked at Mark VIe (not surprising; GE is continually trying to improve reliability). What have you done to try to troubleshoot the two 'Ref. Voltage Out of Limit' Diagnostic Alarms? What does the Diagnostic Alarm Troubleshooting section of the Mark VIe System Guide say about those two Diagnostic Alarms?

I can only comment on what I presume GE's control philosophy might be on this topic. If 4 or more exhaust T/Cs are not reading properly then that means that, one, the detection of exhaust spreads is unreliable, and, two, the average exhaust temperature calculation (TTXM) is likely skewed, and possibly in such a way that the turbine might be over-firing (because it thinks the average exhaust temperature is lower than it actually is because several exhaust T/Cs are 'out of range'). So, it would see appropriate to take some action--if the operator/technician have taken no action--to protect the turbine.

It's just not reasonable to assume that a turbine will be adequately protected against overtemperature and exhaust temperature spreads (combustion trouble) unless the majority of the thermocouples are working properly, and it doesn't sound like that's the case from the information provided. You may need power now and you may feel like it's okay--but if it's really not, when you need power later you might just be rebuilding the turbine.

A '94' device is a shutdown device--meaning that when a logic signal with a 94 in the signal name is active that a normal, fired shutdown is active and the turbine should be unloaded at a pre-programmed ramp rate to reverse power (gen breaker opening), and then the unit goes into a fired shutdown until L94T picks up (usually for a natural gas-fired turbine about 20-30 seconds after decelerating below K60RB (about 20% TNH, usually), and then fuel is shut off and the unit coasts down to cooldown (ratchet; turning gear; barring gear; whatever method is used for cooldown for that particular machine).

So, it would seem--<b>from the information provided</b>--that the turbine is not tripping (an emergency shutdown accomplished by closing the fuel stop valve(s), with no orderly unloading and deceleration--just coastdown from loss of flame caused by sudden loss of fuel). In GE-speak there is a GREAT difference between tripping and shutting down (which is just like an operator-initiated STOP). GE generally doesn't like to trip the turbine if at all possible (the thermal stresses are hard on the hot gas path parts, particularly) so whenever possible they like to initiate what's called an automatic shutdown--indicated by L94AX going to a logic "1", which, again, is an orderly ramp down to breaker opening, and then a fired deceleration to cooldown. Which is much better for the turbine.

Speedtronic turbine controls get a lot of bad "publicity" when they do things like this--because GE is so bad at documenting what's being done and why. They're really trying to protect the turbine, but in doing so without proper documentation they are making it seem like the Speedtronic isn't working properly--when it really is. But, who can say that for sure when there's no proper documentation to refer to.... GE build a fine turbine control system, if only their documentation (and training) was as good.

Without being able to see the exact application code running in your machine it's impossible to say for sure--but from the information provided it's reasonable to presume that the Mark VIe is acting to protect the turbine because of a problem in the control system which has not been properly resolved. It doesn't sound like replacing the TCAS resolved the issue, so more troubleshooting and resolution needs to be done. And, from long, personal experience I can tell you that the Diagnostic Troubleshooting information in the Mark VIe System Guide is far, Far, FAR better than was available on previous Speedtronic turbine control systems. So, make use of it to help your issue.

I would even go so far as to say there are likely other Diagnostic Alarms associated with the TCAS (which is part of the PCAA complement of cards, which includes the TCAT card, as well) that may be indicating deeper problems with the module. But, for certain, if the reference voltages are out of limits and the thermocouple cold junction is out of limits (which is what's typically used to generate the "control panel temp high" alarm!) then there's a problem.

One last thing to note: GE, Salem, were very adamant in the early days of Mark VIe production that the TCAS modules <b>>>>NOT<<<</b> be located in a straight vertical column, with <R>'s TCAS directly above <S>'s, directly above <T>'s. This was because the heat generated by the internal components of the TCAS modules was pretty high and with the heat of two modules directly below <R>, well, that wasn't good for <R>--or even <S>. The modules used to be staggered (offset by a few cm) so as to allow the heat from <T> to rise and dissipate without flowing directly up into <S>, and so forth. But, I've seen pictures recently where that's not being done any longer--and that may be contributing to the problem. Especially if the compartment where the Mark VIe is already not well air-conditioned, and is dusty and humid and not well maintained (good housekeeping is very important to a reliable Speedtronic turbine control system--almost as much as proper temperature and humidity control, and humidity control is almost the most important aspect of the environment for a Speedtronic turbine control system).

Please write back with your findings!