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We have a problem in one of our Siemens Westinghouse GTs which are frequently tripping on a Fast I/O module fault alarm. A momentary fault occurs causing all analog output signals to fail (in span of milliseconds only and resets itself) and immediately trip the GT. We are suspecting that there might be faulty wire in the field which is affecting the 24 VdC supply to the module.

Can anybody give a suggestion of how to do it without pulling all the wires and whether its safe to megger the instrument cables? Or do you have any suggestion to troubleshoot the problem. Appreciate your help in advance.

Hi JCP,

actually i never work for Siemens control system but using basic concepts of field i will try to help.

- First thing come to my thought from your description is that the module itself could be faulty? why you did not think about it?

- How this module is working? i mean, analog devices are of many types. some are internal powered and others are external powered. please, could you give a brief description on how this module is working?.

- What is the alarm sequence do you have when GT trips?. it's useful to track the first group of alarms that could help narrowing the fault.

- Megger is usually for power cables but for instrumentation cables, we are using current or voltage injection from field side. you can try this in suspected wiring.

- it could be the power source to the module itself that could have this fault. did you check it?

i hope this would be a little helpful.

Thanks and best regards
Max

Many thanks Max for your prompt reply.

Actually the first thing we have thought was that the module was faulty so we replaced it as soon as we received our spare, but we encountered the same fault alarm from the module.

With regards to the type of module, it is a Siemens AddFEM (Front End Module) type used for Gas Turbine Controller; 24VDC load supply to analog outputs supplied internally and for digital outputs supplied externally.

With the alarm sequence, there was a "Turbine Speed Either Channel Fault" alarm prior to the trip and then "I/O Module Fault Trip". During the trip, we have observed that all the analog output signals connected to the module failed at the same time and resets after milliseconds. We have replaced suspected speed transducer with a spare but after some time again GT tripped on the same module fault. Now we are planning to inspect the wires from the module terminals to the field junction box, starting with the suspected speed sensors. However, our unit is still running and we will do it in case we trip again.

I think current or voltage injection to check the condition of the wires, may not work at the moment unless fault is there, am I right? so our better option is to pull out all suspected wires, inspect and replace them.

With the power source possibly failing, we have checked and re-tightened all connections but found all ok. Also we have not noted any faults/alarms from the other modules connected to the same power supply.

Hope these gives you additional ideas about our problem.

If fault occur again, I would recommend to check the LED status on AddFEM. It may help you to clarify the problem.

INTF = Internal fault - Display of internal errors, meaning that the self-diagnostic has recognized erros.

EXTF = External error e.g. short circuit, life zero and etc.

BUSF1 = Bus error 1 - Display of PROFIBUS-DP error at connection X3A.

BUSF2 = Bus error 2 - Display of PROFIBUS-DP error at connection X3B.

Sakravee

Dear Sakravee,

Our problem until now is that the fault automatically resets in seconds and we didn't had enough time to check what was the specific alarm in the module. Is there an event logger for this module wherein we can check all the alarms / events as history?

If the fault is persisting, it will be easier for us to diagnose the problem. At the moment our unit is down and we plan to do megger test for 600V field cables. Can we apply 500V for safely checking the insulation resistance?

Thanks,
JCP

It would be a good idea to check the insulation voltage ratings for the cables. 'Meggering' at 500 V could create insulation failures in cables rated for 300 V. Once again by the 'sound' of the fault, it appears as if the module is faulty.

Regards,
Tomy Zacharia