We are facing an intermittent problem with our temperature processing module of Mark-II based GE gas turbine. “Ground Lamp” of SGDD (Ground Detector) card is flickering “on” and “off” at different intervals. SGDD card has three channels, one for temperature control system “Tx” and remaining two for temperature protection system “OTA” and OTB”. Lamp affiliated with “Tx” is flickering while others are ok.

We have rejected the exhaust thermocouples of temperature control system one by one, whenever indication persists for few minutes, but alarm did not vanish. This apparently shows that problem is not with thermocouples and it might that the SGDD card is faulty. Please guide us in this regard and inform us whether we can change the SGDD card online (Turbine is in running condition). Your early reply would be highly appreciated.

 

I would suggest you check your "TX" wires from field in before resorting to a shut down.

Rejecting the thermocouples one by one may not give you a true indication as to which circuit the problem exist on.

Reason: The SGDE uses a differential op-amp for detecting a ground fault.Any unbalance in -TX or +TX gives an indication. Now, in the thermocouple module, one leg goes through a switch, while the other is connected in parallel through swamping resistors.

It's my belief, your ground fault is on the paralleled leg where it's not isolated by the switch.

OTA and OTB will not suffer this problem, because they are separate circuits, and not a part of the paralleled net work in the thermocouple averaging module.

Disconnecting power from the averaging module to change a card may cause a shut down.