we had a trip for one of our frame-5 gas turbine MARK VI and after investigation we found that the cause of the trip was "L12 HFD_P HP OVERSPEED FAULT PROTECTION SPEED INPUT TRBL." this means the difference between TNH_OS and TNH is more than 5%.
In trip log live, I found before one hour of tripping, the difference between both speed is less than 05% (very closer). if you have any previous experience with same issue, please advise me, I have to know why this signal L12 HFD_P triggered to 01 and cause turbine tripped.
The problem was cleared after resetting by operation team, and the turbine started normally without any issue.
What other Diagnostic- and Process Alarms were present and active at the time the trip occurred?
Did anyone check the wiring of the primary- (77NH-n) and emergency (77HT-n) speed pick-ups along the ENTIRE circuit between the Mark VI TTUR and TREG terminal boards and the speed pick-ups at the #1 bearing?
Because, when things like this "mysteriously" happen, it's usually because there was some other condition (Process- and or Diagnostic Alarm) that was being overlooked (ignored) and/or there is a problem with the wiring connecting the signal to the Mark VI. And, if that wiring problem is not found and fixed it could happen again, usually at an inopportune time.
Wiring problems can include loose terminals (crimped connectors; loose terminal board screws), improper shield drain wire earthing/grounding. Failing speed pick-ups can also cause this kind of problem, as can loose speed pick-ups and improperly gapped speed pick-ups.
BUT, if there were no relevant Process- or Diagnostic Alarms before this occurred (like power supply issues, or battery grounds, etc.) the most likely cause is a wiring issue or a speed pick-up issue--either of which can occur again at any time without any notice.
Many thanks for your response, really we appreciate your support.
- I have checked trend trip log live before one hour of GT trip, I found both speed control and protection are very closely (0.03% of difference).
- I found one diagnostic alarm in VPRO RACK 8 CHANEL S Slot 1 (IS200VPRO), FAULT CODE 40, SATUS 01, contact EXCITATION VOLTAGE TEST FAILURE.
This tells there is something amiss with the <P> rack and/or VPROs. It could be the power supply(ies) to the <P> rack. I don't have access to the Mark VI System Guide, GEH-6421, at this writing, but if I recall correctly the VPROs are powered from the <PD> core with 125 VDC, so if there are problems with the 125 VDC power supply that could explain what's happening.
Is this a TMR Mark VI? Or a SIMPLEX?
Please refer to GE bulletin TIL 1905 Sept. 5 2013. It refers to VPRO and VCMI chip and socket issues that can cause some of the issue you have described. We had to upgrade theses cards on all six of our Mark VI control systems.
Thank you, mfrasca12! I was trying to recall the number of this TIL as I was wondering if it was applicable. Sounds like it is or might be, and this is good information, anyway.