The Error code is 33- " LVDT#1 rms voltage out of limits"
Apart from calibrating the the LVDT, are there other checks to be performed, just to further verify the need to perform a full LVDT calibration.

 

When did this problem start? After a maintenance outage? After an LVDT feedback calibration was performed?

Contrary to extremely popular belief, the answer to every servo-operated device problem <b>is NOT</b> to "calibrate" the LVDT(s).

Have you measured the LVDT voltages (excitation and feedback) for the indicated LVDT at the TSVO using a true AC RMS voltmeter?

Have you looked at the feedback indications for the two LVDTs on the device (presuming there is a dual LVDT arrangement in use) using Toolbox?

This Diagnostic Alarm is indicating there is a problem with LVDT voltage for a particular LVDT. It may indicate a loose wire or termination, or a failure of the LVDT, or a problem with the TSVO card, or a problem with the VSVO card.

If the LVDT feedback was working and suddenly went out of limits, that doesn't indicate a feedback calibration problem. While it might mean that the core of the LVDT (the movable portion) had shifted because of a mechanical problem (such as loose or broken core shaft). But, just performing a "calibration" of the LVDT feedback may or may not solve the problem. And, it's not clear what the problem is.

 

Thanks for this details, but I have measured voltages from LVDT Feedback and excitation respectively, I have 0.84 V and 6.9v respectively engine is stopped.

But I am a bit confused on the source of the the IGV feedback and reference being displayed on the IGV control screen. I have the reference as 25.8 and the feedback as 26.4.

 

So, this alarm is apparently related to the IGV LVDTs? What is the turbine at the site that is experiencing the problem? (Asked because a reference of 25.8 is not typical for most GE-design heavy duty gas turbines.)

The reference is generated in the application code, and the feedback is the scaled value of the LVDT feedback.

There are usually two LVDTs, and the regulator configuration is usually set to use the higher of the two feedback values (something like "Max Select of Two Assigned LVDTs" or something similar).

Or is this an older unit, using a single LVDT for IGV feedback, retrofitted with a Mark VI? If not, you should have two LVDTs connected to the TSVO, which would require two excitation sources (the nominal 7.0 VAC RMS), and two feedback sources (nominally in the 0.700 VAC RMS, +/- 0.02 VAC RMS at 'zero stroke', which should be the minimum mechanical stop). So, unless there's only one LVDT, you should have two sets of readings, each consisting of one excitation reading and one feedback reading.

I don't have a copy of Toolbox any longer, but there may be a configuration setting for the out of limits alarm for the regulator. And, the .084 VAC RMS seems a little high for a zero stroke (non-running) value, but, again, we don't know what the unit/application is.

The error you describe (25.8 DGA (DeGree Angle) vs. 26.4 DGA) may be due to the fact that the reference is calling for a value that can't be achieved because there is no hydraulic pressure or the actuator ring is against the mechanical stop. Again, we don't know the application.

 

What is the allowable deviation between the IGV reference signal from the Feedback?

 

I missed the "short description" for this thread which only appears on the main page of control.com and says the unit is an Hitachi H25, which is something I have no experience with.

Also, I mis-read the whole post because it talks about VSVO and Mark IV. I've never seen a Mark IV with a VSVO, and just ass-u-m-ed that the originator was talking about Mark VI with the VSVO reference. (Mark IVs used HSAA cards for the servo regulators.)

So, I'm going to take back everything I've said to date.

Good luck with this!

 

Sorry, the turbine is a mark VI. Is it a wise idea to mechanically adjust the IGV position to read 25.8DGA?