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Loose Contact in MARK V Control System
Power generation equipment control. topic
Posted by ankarao on 27 February, 2013 - 11:06 pm
One of our gas turbine tripped with some protection/shutdown alarms (Fire in Zone-1 ,2, 3 protection, manual trip, exhaust frame cooling pre low unload, turbine inlet air DP high shut down, IGV control trouble trip,compressor bleed trouble trip..)

DTBA and DTBB card power supply loose contact suspected. Started the unit back and operating normal. How the loose connection can happen? Will it happen again? Any one has same kind of experience?

Please share and suggest to avoid another tripping.


Posted by CSA on 1 March, 2013 - 3:32 pm
ankarao,

There is not really enough information to be of much assistance (which is probably the reason for a lack of response) and I'm just making a presumption about what the problem might have been.

The DTBA and DTBB I/O terminal boards are where discrete (contact) inputs are terminated in the Mark V panel. Based on the alarms mentioned, it would seem the problem occurred on the DTBA/DTBB of <QD1>, one of two possible "critical" discrete (contact) input cores. The overwhelming majority of Mark V discrete (contact) inputs are powered by 125 VDC, which comes from the <PD> core.

For all of these alarms to be annunciated at the same time it sounds like the 125 VDC to power the DTBA/DTBB boards was lost, causing the states of many discrete ("contact") inputs to revert to their de-energized state (which in many cases is the alarm/trip state, for safety reasons). There is a single cable which runs from the <PD> to the DTBA, and then a "jumper" cable connects the DTBA to the DTBB. There is/are fuse(s) upstream of the cable in the <PD>.

A single discrete (contact) input to either the DTBA or DTBB would likely cause an alarm, and if it was from one of a redundant set of trip sensors then certainly an alarm would have been annunciated--but the unit would not trip, and there would not be so many serious indications at the same time.

So, it's presumed the technician(s) said the cable supplying 125 VDC to the DTBA from the <PD> core, and/or the cable from the DTBA to the DTBB was/were not fully inserted in their connectors and were therefore not making good "contact." Or that there was some kind of corrosion found in the connectors preventing good "contact."

How does one prevent loose cables/connectors from causing problems? By making sure during normal maintenance shutdowns they are fully and properly inserted. If there is humidity in the area where the control panel is located, then the connectors should be periodically checked for any corrosion that might develop, and it should be removed and its future formation prevented to the best possible extent.

(This sounds so much like an operations supervisor or -manager asking a question to determine if what he was told by the technician(s) about the reason for a turbine trip was plausible, and if so, how it could be prevented from happening again.)


Posted by ankarao on 2 March, 2013 - 11:49 pm
Dear CSA,

Thank you very much for the reply.
Below is the List of alarms appeared and cleared

GT-1 Compressor inlet thermocouples disagree
Gas fuel temperature high spread
Emergency lube oil pump running
Turbine air inlet filter Diff. Pressure shutdown
Exhaust frame cooling air pressure low.
Generator cubicle trip
Generator breaker tripped
Exhaust frame cooling system trbl.-unload
Inlet gas temperature low
Turb inlet air filter cleaning pres. Low
Gas scrubber#1 level high- alarm
Gas scrubber#1 level high-trip
Manual Trip-Local
Fire in Zone-1,2,3

While checking the 125v DC to DTBA card, same set of alarms appeared. So supply loose contact/disturbance to DTBA (DTBB)is suspected.

I was told/red that <RSTP> are the critical processors.As long as they are healthy, machine do not trip.I under stood that ,QD1 is also equally important.

TCC is air conditioned. So there is no chance of higher humidity.

once again thank you for the advice. Please comment.


Posted by CSA on 3 March, 2013 - 11:53 am
ankarao,

Specifically, what action was taken before the unit was restarted to try to pinpoint or remedy the cause of the alarms?


Posted by mwest on 5 March, 2013 - 2:09 pm
ankarao,

Your mention of the compressor inlet temperature TC disagree alarm points to something other than DTBA to consider.

May I ask what diagnostic alarms occurred during the event?

If you have a historian, was there a specific group of thermocouple inputs and/or other inputs that did weird things, like drop to zero briefly?

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