Thread Starter


Dear all,

Frame 6FA, TMR MKVI panel, liquid fuel MNQC, LFBV feedback flow divider magnetic pick up, water injection for NOx reduction, IGV in temperature control, DROOP governor mode, no IBH, no air intake coolers, exhaust to HRSG. Combined cycle operation. Power generation application.

IGV controlled by VSVO in SLOT7. RegType 2_LVposMAX. Mon1: csgv_a, Mon2: csgv_b, Reg3_Fdbk: csgv. ServoOutput3:cagv.

Major inspection in January 2016 conducted by OEM.

Unit running without apparent problems till last week.

21:49:17.547 <R> SLOT 7 VSVO Diagnostic alarm.
21:49:17.547 <S> SLOT 7 VSVO Diagnostic alarm.
21:49:17.547 <T> SLOT 7 VSVO Diagnostic alarm.

Found diagnostic fault code 40 "LVDT # RMS voltage out of limits" active in <R>, <S> and <T>.

No prior process or diagnostic alarms.


Prior to trip unit stable at part load (60 MW). All parameters stable.
After diagnostic alarm was annunciated, trends show quick oscillations of IGV control servo current (cagv) and IGV angle (csgv ranges from 63º to 90º rapidly during incident). Maximum exhaust thermocouple reading reaches 1275 ºF. No record of TTXM but must have gone higher than TTRXB tripping the unit.

After trip, found faulty LVDT1 with bore full of dust. Seems as if the core of the LVDT was displaced and was "rubbing" the bore. Changed LVDT1 and unit is running again without apparent problems.

Anyone has experience with LVDT feedback going high instead of low?

My concern now is if there is a remote possibility to have sudden IGV closing due to IGV feedback going high taking the compressor to a surge event before tripping the unit.

Tried monitoring in Toolbox LVDT1 and LVDT2 feedbacks csgv_a and csgv_b, but with the unit running they show 0. Is there any way to read value of each LVDT independently?

Thanks in advance.

While it's very difficult to predict with great certainty how any instrument or device will fail (including the wiring to/from the device), the predominant failure mode for IGVs is to fail in the open or low condition. That's why the Speedtronic does a high-select of the two LVDT feedback signals, because more often than not when an LVDT fails it's input to the Speedtronic turbine control panel goes low.

Having said that, there is always the possibility that the feedback can fail high--and, yes, I have seen exactly the same thing occur (feedback going high) when the movable core was found to have been bent and rubbed through the armature windings, probably causing a short of some windings. This is why after maintenance outages it's always a good idea when performing LVDT calibration verification checks to check to make sure that the cores are not bent and are not contacting or rubbing the internal passage of the LVDT armature (where the windings are located).

Bent LVDT shafts can be caused by poor re-assembly procedures, or by inadvertent mechanical work causing some piece of equipment to swing into the LVDT shafts (cores) and bend them. And then there are just mechanical staff who are bored and like to fiddle with controls stuff (I know, this never happens--but it does, and while it's infrequent it does cause lots of problems). And, there are mechanical personnel who have little to no regard for any controls device and view conduit as rungs on a ladder to be used to stand on and leverage against (I know, this never happens, either--but it does). This is why pre-start-up inspections of controls devices after any maintenance outage, especially a forced outage where work is fast and furious, are really important. They won't find everything, but they often find things which would have caused major problems.

This is one of those learning experiences, and, again--it's just not always possible to predict failure modes but in my personal opinion GE has done an excellent job of doing so. Is it perfect? No, but then few things ever are.

It was common programming/configuration practice for GE not to assign individual signal names to each LVDT input signal. You can do this in Toolbox, and then you can monitor the signals individually.

Hope this helps!
Dear Rom,

First of all I have to say a very nice job giving all the specifics of your machine, I am surprised CSA did not make mention of this. All kidding aside it is very helpful to us trying to understand the machine and its specifics, and it saves all the back and forth and assumptions.

To answer your question about monitoring each LVDT individually, yes you should be able to monitor them in Toolbox, and on the HMI if the signals are part of an EGD page.

You said "IGV controlled by VSVO in SLOT7. RegType 2_LVposMAX. Mon1:
csgv_a, Mon2: csgv_b, Reg3_Fdbk: csgv. ServoOutput3:cagv."

If I assume someone set things up correctly then you should be able to "monitor" each LVDT using the signal attached to monitor 1 and monitor 2. This is a feature of Toolbox that is not often setup by the engineer or field engineer who commissions the plant, but its the easiest way to see what is going on with each LVDT. The other way is to use the TSM (similar to Telnet) feature, but I personally don't recommend this unless you know what you are doing.

Each "monitor" channel can be assigned to an individual LVDT, then a software signal or pin can be attached to that. The caveat to the monitor is that it must be calibrated manually. When you calibrate a gas valve or IGV servo you are teaching the MKVI controller what voltage to expect for the regulator at minimum position, and maximum position. Those learned values (MnLvdt#_Vrms and MxLvdt#_Vrms) are loaded into the configuration of the regulator when you save and download. Whenever you calibrate a regulator, you must then enter those values into the respective monitor that is monitoring the LVDT. I really wish there was a way to copy images into this site (Its my Christmas Wish!!), but for now here is what I can tell you from a toolbox monitor setup. Open up Toolbox and navigate to the card slot #7 and expand monitors, then select monitor #1 and view the configuration in the right pane. I think you should see something like:



MaxPOSValue=91 (this would usually be 100%, but since this is an IGV it is whatever the measured max angle of the IGV's is. Refer the regulator responsible for controlling the IGV actuator and put the same Max value here)

LVDT1input=LVDT1 (This is whatever LVDT you want this monitor to reference)

MnLvdt#_Vrms=.6924 .6928 .6955 (These are the LVDT voltages that equal 0% for the regulator. Copy the values from the regulator responsible for the IGV's and insert them here)

MxLvdt#_Vrms= 3.5289 3.5299 3.5288 (These are the LVDT voltages that equal 91% for the regulator, or whatever value you put for MaxPOSValue. Copy the values from the regulator responsible for the IGV's and insert them here)

Once you have "Calibrated" the values for the monitors you will need to download the configuration to the VSVO by right clicking on the slot 7 VSVO and selection download configuration. You should then see a value displayed for the signals csgv_a and csgv_b.

Please write back and let us know if you were successful. I am curious to know what is configured for the monitors before you make any changes as it sounds like someone set stuff up but missed a step or two.

Thank you very much for your answer. You have been of great help to me. Learning that LVDTs feedback failing high though not common happens and is known tranquilizes me. For sure GE has studied this failure mode and has implemented the necessary safe guards not to put their gas turbines in a dangerous situation.

It's a pleasure reading your posts.

After a few years reading posts in this forum you, CSA and all the experts that answer in this forum have written with fire in my brains that the more information you give of your unit, the better ;).

Navigated to Slot#7 but Monitors isn't expandable (I can expand Configuration, Card Points, Regulators (here I can see the values of the LVDTs set by auto-calibrate) and the terminal boards, but not Monitors).

If I browse through Toolbox, I can see the PIN of csgv_a (IGV angle in deg 1st LVDT FDBK) and csgv_b (IGV angle in deg 2nd LVDT FDBK) . I can also see those two signal put in the EGD but they indicate 0 and I also see them in the Rack but they indicate 0U.

I&#8217;m just a self-learning Operator, lucky enough to lay my hands on some GE manuals and avid reader of the posts published here. Read Only privilege level so I can't download configuration on the MKVI. Don't think I can get any further I'll try notifying the Managers of my Plant and see if they can do anything, but knowing how things work here the answer will be &#8220;If GE left it this way, we leave it this way.&#8221;

Thanks a lot for your answer, you have been of great help to me. If I ever get to a position where I can promote these changes I'll try your procedure.
Dear Rom,

Now I understand better what you have. It sounds to me like the engineer declared the signals by creating the pins, but never attached the pins to a monitor. Also it sounds like the monitors were never configured, if they were then you could expand monitors and you would see each of them that was configured up to 12 per VSVO card.
This monitor feature is an excellent tool in my opinion for monitoring individual LVDT's, but it is not often configured, and I have yet to find any good information in the GE manuals that explains the feature or configuration. Its not difficult to configure and I have helped sites with setting them up for use. But its not always the easiest to do via a forum like this.

In my experience the LVDT's that GE uses on the heavy duty turbines are fairly robust. The LVDT's I see the most issue with are on the steam turbines from GE and Toshiba. These LVDT's are very long, many times outside in the weather, and are subject to a lot of heat. These conditions seem to lead to a higher than normal failure rate as compared to the gas turbine fleet.

I understand the comments from your management related to making any changes. Its unfortunate, but not uncommon. GE seems to have scared(maybe not the right term) many customers into thinking they can't change anything on their machine. It gets complicated with LTSA contracts, warranty and all that. But this is a feature of the control system that has some value in my opinion. Its not ground breaking, or anything, but a good tool to be used, that does not take a lot of effort to configure.

Anyway now you know a little more than you did yesterday, and that ain't a bad thing.
Good Morning,

Sorry to barge into the discussion.

Regarding this issue, though am not an Instrument engineer, there is a catch (most likely). It's to do with the incorrect wiring of the LVDT inside the junction box.

Kindly inspect both drawings and the hardware connections between the different terminals and the grounding. Double check (Actually review) the wiring schematic. I hope you will have a right solution.