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Unit Trip After 28VDC Loss on MKVIE
Why we're tripped after a failure of 28 vdc power supply network, due to a PDOA fail.

Hi everyone,

In (MKVIE) TMR application, I suppose if we do reboot of one PSVO IOPack ("T" for example), then we continue command VGC Valves normally, isn't it?.

Background:
In our case (Combined Cycle 9FB/15A), the machine is running. we lost one PDOA-"T" IOPack (short-circuited inside), which affect all "T" power supply network, which cause a reboot of the "T" controller, all IONETs, all IOPACKs in "T" power supply network.

Finally, The unit has tripped by VGC-3 doesn't following the reference.

Question:
Regarding the RMS Voltage of LVDTs, and Current of servo, How such a problem (loss of "T" power supply network), could affect functioning of VGC Valves?.

Thanks in advance.

0 out of 1 members thought this post was helpful...

Hi Chemsouhd ,

The most likely cause of problems like this is that one of the servo currents from either <R> or <S> is incorrectly connected to its servo-valve coil. So, the current aids the fail-safe spring in moving the valve to the closed position against the other remaining coil.

Were there any Diagnostic Alarms from any of the PSVO I/O Packs prior to the issue with the PDOA I/O Pack?

What were the servo current values to VGC-3 under steady-state operating conditions PRIOR to the problem with the PDOA I/O Pack?

Check the servo coil polarity, under the control of each controller individually--for ALL servo-operated devices at the earliest possible convenience.

DO NO follow the servo current polarity check procedure provided in the Control Specification if it says anything to the effect of "stroke the device and if it's jerky or not smooth then change the servo current polarity." That's just flat wrong and incorrect. There have been many previous threads here on control.com about the proper way to check servo current polarity; use the 'Search' feature of control.com to find them. If you have questions, ask. We're here to help.

Another possible, though unlikely, cause is that the two LVDTs on VGC-3 are being powered by the same source, and that source is <T>. This is not likely, but should be investigated. NO device with dual LVDTs should have BOTH LVDTs powered by the same source (<R>, <S>, or <T>). Each LVDT of a redundant LVDT pair must be powered by different sources for proper redundant operation. Again, this is unlikely, but stranger things have happened.

I want to be VERY clear here about what is meant by "servo polarity." The electro-hydraulic servo-valves ("servos") used on GE-design heavy duty gas turbines are bipolar devices--meaning they require current that can be either positive or negative. The Mark VIe servo current outputs are designed to be connected to the servo coils in one particular way--so that positive current works to reduce flow through the device the servo is operating, and negative current works to increase flow through the device the servo is operating. (In the case of gas fuel control valves, positive servo current closes the valve, and negative servo current opens the valve.) If the wires connecting the servo output are connected opposite to the way they should be connected, then the polarity of the current being applied to the servo coil is incorrect--and will work to try to close the gas fuel control valve instead of trying to open it.

It's possible to operate a turbine with one coil of a TMR servo having the wrong polarity of current being applied to it. The other two processors will eventually overcome the fail-safe spring and the coil with incorrect polarity, but the servo currents to the three coils will be unbalanced--another indication of possible problems with the wiring connecting the servo outputs to the servo coils. BUT, when one of the two coils with the correct polarity loses its power source, then the device is going to move to shut off flow--and, well, bad things (like turbine tripping) happen.

Hope this helps!

Dear CSA, I was looking for the post..

During a shutdown, I did my self a polarity check, and surprise
the polarity was effectively inversed in GCV-3. I fixed the problem, and it's now under observation.

Many Thanks for your orientations, was really helpful.

Thank you for the feedback! I'm glad you were able to find a post about servo polarity checking and perform the checks--and rectify the problem.

Hi Chemsoud,

Do you have a spare PDOA? Did you check if the PSVO is working fine?