In fr9E that has Mark VIe control sys, with 3 turbine bearings (bb1 to bb5) & 2 gen bearings (bb9 to bb12), we had problem with reading of BB5 (39v-3b) & BB11 (39v-5a). Operator reported bb5 reading till 17mm & bb11 till 13mm, on checking trends it was found that multiple spikes came in both at the same time, also proximity reading (39vs-31,32, 101,102) found ok in BN system.
Further it was checked at marshalling tb, cable of bb5 found loose, hence l39vt was forced & cable removed, but on removing it was found that bb11 signal was also generated spike. After some secs bb11 become stable & bb5 io bad in Mark VIe, this activity of removing cable of bb5 done multiple times. All times effect was found in bb1 also along with bb5.
Note:-field jb's of both are quite away from each other,also marshalling tb's are also at a distance, so no chance of inter change.
Request experts to help in resolving issue(can it be due to shield or some interferences)???
Yes, improper grounding of twisted, shielded pair drain wires can definitely cause problems with signals. Though if the unit has been running for some time and this problem just started, that would be odd (unless the wiring was disturbed recently, such as during a maintenance outage).
There have been many threads on control.com about twisted, shielded pair drain wire grounding and the correct ways they must be terminated. You have mentioned JBs and marshaling cabinets--and those are BOTH potential places where the shield drain wires can be improperly terminated.
There is a 'Search' field at the far-right corner of the Menu bar of desktop control.com webpages; for mobile versions of control.com webpages the search field is under the 'Control.com' drop-down. It is suggested you use the Search 'Help' the first couple of times you use Search, as the syntax is a little different than most present-day World Wide Web search engines (but JUST as fast and powerful). There are some crude drawings of various methods of shield drain wire terminations--both good and bad--in some of those previous threads.
Hope this helps! (Again, if this just started happening after a maintenance outage when the vibrations sensor wiring was disturbed, that would be the best place to start your investigation.) You will need to check both circuits ALL THE WAY from the JB closest to the sensor back to the Mark VIe terminal board to be sure the drain wires are properly terminated all along the path.)
Finally, if the functional earth and protective earth are not properly connected, and if devices connected to them are not connected to the proper earth system, this can also cause problems. If the Mark VIe was an upgrade to a previous Mark*/Speedtronic turbine control system, and the plant only has one earth system, then the ground strap which connects the two earths in the Mark VIe cabinetry should be connected--and the protective earth should be properly connected to the station earth grid with a properly sized conductor. Often when older Mark* systems are upgraded to Mark VI or Mark VIe systems this gets overlooked. And, also, unless strict equipment earthing procedures are not followed in the plant after commissioning problems can also be caused.
If you write back for more help, you will also need to provide ALL the Diagnostic Alarms which are being annunciated by the Mark VIe--even the ones you might not think are related to this problem.
Here's a link to a good thread, with some good (and bad) comments above and below the crude drawings of proper drain wire grounding:
It was found by typing:
+"drain wire"into control.com's Search function (it was the first result, but by scrolling through the results one at a time it was found).
As one of the contributors to this thread basically said: Grounding [Earthing] is one of the least understood and least respected construction practices. More or less that's what was said. :-)
I have been to many sites around the world where multiple units were installed at the same time, and the drain wiring of each of the units was found to be different. That's because the electrical supervisors during construction did not ensure the laborers performing electrical terminations understand how to properly terminate the drain wires of shielded cables (both pairs and triads). And, because different laborers can be working at any place along the circuit, the drain wires could be terminated--and were often terminated--differently. It made for fun times at times (NOT!).
Also, a lot of the "drawings" used for electrical termination during construction are no longer drawings, but simple tables of terminations, and they are aren't always done correctly to begin with with respect to drain wire terminations.
Hope this helps! Again, if you need more help after you've investigated and resolved the issue please provide ALL the Diagnostic Alarms which are active, even if they don't seem relevant to you. Actually, it would best to have all the Process Alarms AND Diagnostic Alarms, even if they don't seem relevant. (You would be surprised what can be learned from an analysis of Process- AND Diagnostic Alarms!)