What does the alarm "diagnostic alarm TCQA servo current #2 disagrees W/Ref" mean?


Thread Starter


Hi Mark V experts,

i really need some guide in understanding "diagnostic alarm TCQA servo current #2 disagrees W/Ref " in Mark V.

This alarm is coming time to time but it didn't trip the unit.
Hi bil07,

Servo-valves are driven by the TCQA I/O card in location of the control processors (<R>, <S>, <T>) (the digital signal--the reference--comes from the DCC/SDCC card, to the TCQA which converts the reference to an analog signal which is then cabled to the TCQC card in location 4 and then to the QTBA card--of each of the three control processors).

A Mark V has eight (8) servo-valve outputs, and for a GE-design heavy duty gas turbine which burns gas fuel the first servo-valve output is usually assigned to the Stop/Ratio Valve (SRV) electro-hydraulic servo-valve, and the second servo-valve output is usually assigned to the Gas Control Valve (GCV) servo-valve output.

The Mark V "operating system" (in the Mark V turbine control panel) compares the digital reference signal to the amount of actual current being applied to the servo-valve, and if they disagree by some set amount (the amount is NOT field-adjustable, and it's not documented anywhere, either) a Diagnostic Alarm exactly like the one you have described is annunciated.

The problem is likely there's something amiss with the wiring between the Mark V turbine control panel QTBA card(s) and the three-coil electro-hydraulic servo-valve, or there's some issue with the electro-hydraulic servo-valve (it's sticking or one or more of of the coils are failing), or there's some problem with the power supplies of the control processors (the TCPS cards in location 5 of the control processors). If it's the latter, there are usually Diagnostic Alarms to indicate a problem with the internal power supply voltages.

125 VDC Battery grounds, especially intermittent 125 VDC battery ground alarms can cause this problem, as can intermittent grounds in the servo-valve output wiring between the Mark V turbine control panel and the electro-hydraulic servo-valve. Usually, though if it's 125 VDC battery grounds there will be more than one Diagnostic Alarm as well as other Process Alarms <i>(but not always)</i>.

These are the most likely causes for this alarm. Unless there are other Diagnostic Alarms (sometimes they don't seem related--but they can be!), the problem is most likely wiring or the electro-hydraulic servo-valve. Especially if work was done recently on the gas valve assembly, particularly the Gas Control Valve. Sometimes failing or intermittent LVDTs (either one or both) can also cause this alarm, but, again, there are usually other Diagnostic Alarms as well.

Diagnostic Alarms don't trip the turbine--everyone knows and believes that. But, in fact, Diagnostic Alarms can be very indicative of potential or upcoming trips as well as being useful in troubleshooting trips which aren't so obvious, so proper attention to and resolution of Diagnostic Alarms can prevent lots of unwanted trips and lost production. Good on you for being proactive in resolving this Diagnostic Alarm.

Hope this helps!

Please, <b>write back</b> to let us know what you find or if you need further assistance with troubleshooting. "Feedback is the most important contribution!"(c) to control.com, and feedback comes from the original poster of an issue or request for information (that's you in this case). Many people read these posts--both now and in the future--and it's feedback that lets the other people reading these posts whether or not the information provided was useful, particularly when resolving problem (especially Diagnostic Alarms).