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What have you done to try to troubleshoot the problem?

Have you looked at any of the signals in the Prevote Data Display?

When do these alarms occur? During starting? If so, when? At breaker closure? When the unit is on line? Is the unit a generator drive or a mechanical drive? What kind of turbine is it?

Still, I am yet to change any modules etc. The Prevote display shows that different S core than that of R and T core. i.e., if R and T are 1 then S is zero.

These alarms appear when the conditions occur. for example, the L28FDs appears when flame conditions are satisfied and resets on unit shutdown.

And few alarms are persisting always.
It is a GE Frame 7EA machine.

Regards

Which values in <S> are different from <R> and <T>? Of course, if <S> thinks the unit should be tripped and <R> and <T> don't then it will have different values that <R> and <T>, which is kind of what these alarms are telling you. Some input to <S> is causing <S> to believe that the unit should be tripped. If <S> thinks the unit should be tripped and it sees flame, that's also a problem. Without good data, there's not much we can do to help you.

If you want real help with these alarms via this forum, you are going to have to provide more details. Which alarms are persistent? What are the data values when the alarms are annunciated?

I'm not really clear what you mean by alarms appear when conditions occur and the L28FDs appear when flame conditions are satisfied. Are you trying to say that the L28FD voting mismatches occur when flame is established during starting and then when the unit is shutdown the alarms clear? If so, this is a pretty big clue to the problem! Loss of flame indication to any processor is a pretty big problem.

Have you looked at the cables in the <P> core--the ones that connect the PTBA terminal board to the TCEB and the TCEAs? Are they all inserted correctly? Are the LEDs on the TCEA cards in the <P> core all flashing the same sequences, or does <Y> flash a different sequence than <X> and <Z>? You can see how the cables interconnect the components by looking at the Signal Flow Diagrams in Appendix D of the Mark V Application Manual, GEH-6195.

These types of problems can sometimes be caused by problems with the ribbon cable connectors/pins becoming "corroded" and can be corrected by the application of some conductive grease on the connectors. There are usually tubes of conductive grease provided in the boxes with spare Mark V cards.

I really think you need to have someone knowledgeable in Mark V troubleshooting visit the site and help analyze and resolve the situation. Many times, there's no need to replace components (actaully, there's an awful lot of unnecessary card-swapping and servo replacement going on all the time around the world!).