SRV Fluctuating

we have a problem with the SRV valve, which fluctuate for a load over then 6MW (GCV and load is stable), note that for lower loads the valve is stable.
The turbine is a 6B with an MK6e TMR controler; in a separate electrical grid.
The HP filters, the servovalve and its filter were changed without result.
Thanks in advance for all support.

When did this problem start? After a trip from load? After a maintenance outage? ??? After someone "calibrated" the SRV LVDT(s)?

What process alarms are present for loads below 6 MW?

Which terminal board and associated I/O Pack is (are) the P2 pressure transducer(s) connected to? Have you checked the Diagnostic Alarms for the I/O Pack the P2 pressure transducer(s) is(are) connected to? If so, what are they?

Have you tried recording the SRV position just before it goes unstable, and then, with the gas fuel supply pressure isolated (shut off) have you tried opening the SRV using the Manual Positioning feature of the SRV LVDT Calibration function to move the SRV to that position while observing the valve (placing someone physically next to the SRV to watch the valve position) and then increasing the position reference slowly, all the while monitoring the SRV position? (SRV position should be recorded to a Trend during this operation, too--usually.)

It could be an actuator problem--particularly if the 6B is older and the Mark VIe is newer (an upgrade to an older control system).

It could be an LVDT problem; things like this have been known to happen if the movable core of the LVDT is not straight and rubs against the inside of the stationary armature of the LVDT. How many LVDTs does the SRV have? Have you tried monitoring the feedback from the(both) LVDT(s) and then observing what happens when the load is increased above 6 MW? If so, please describe what you have observed.

Was any mechanical work done on the SRV prior to this problem starting?

Is the SRV contained in the same assembly as the GCV (a large casting housing both valves)? Or, is the SRV separate from the GCV (and if so is it a rotary Fisher Cam Vee-ball valve)? If it's the rotary Fisher valve, does it use LVDT(s) or an RVDT assembly?

Do you know how to use the Verify Position and Verify Current features of the LVDT calibration function?

Has the gas fuel supply pressure changed recently (possibly just before this problem started)?

Does the unit normally burn gas fuel, or is it a multi-fuel unit, and if so, what are the other fuels the unit burns and what percentage of time is each fuel used (roughly--the estimation doesn't have to be exact)?