Turbine Tripping on "Combustion Trouble" & "High Exhaust Temp Spread Trip"

We have GE 6FA Gas turbine and Mark-VIe system with TMR configuration. During shutdown of machine load command was changed from 65 MW to 15 MW, and m/c unloaded normally up to 15 MW, but unloading did not stopped at 15 MW. Load further decreased down to 1.89 MW. At same time fuel mode change-over M6BQ to M6Q, as a resultant GCV4 closed. Load started hunting up/down and GCV-1, 2, & 3 also started hunting.

Exhaust thermocouples TTXD1_1, TTXD1_19, TTXD1_20, TTXD1_21 values decreased. It has occurred two times. Please suggest probable reasons.

Is it advisable to check the combustion internals?

It really sounds like there is something wrong with the fuel control system that is causing the problem, and that flame is being lost because of low fuel flow-rate through the nozzles in one or more combustors resulting in the Combustion Trouble and High(-High) Exaust Temperature Spread Trips. If the unit exhaust temperature spreads are normal during normal loaded operation, but this problem only occurs during unloading, then it's mostly likely some kind of problem with the fuel control valves or the sensors monitoring fuel pressures or the hydraulic actuators or possibly the servo-valves (which would be the result of DIRTY oil--not usually a servo-valve problem).

However, it's pretty likely that there are Diagnostic Alarms being annunciated which would help troubleshoot the issue. Can you please provide the Alarm History from one or both events? (Many people post electronic files to web-sharing/hosting sites and then post the link to the files in a reply to the post so we can access the file(s) and examine them).

Wish we could be more help--but we just don't have enough information to say more than the above. Alarms--Process <b><i>AND</i></b> Diagnostic--are always very important when trying to troubleshoot operational issues.

Hope this helps! Please write back to let us know what you find, or to provide more information.