Turbine Valve Chattering

I am a board operator for an 810 MWG generating Unit with a Westinghouse
turbine and an ABB DCS control system. We have been experiencing problems with the turbine control valves (chattering, as we call it) cycling open and closed. It starts out small maybe a few tenths of a percent of valve position and within a few minutes begins to cycle the valves several percent which causes the megawatts to cycle from a few megawatts to up to 150 megawatts or more in seconds.

Our turbine valves operate in Full Arc Admission. Engineers have tried tuning the turbine master to move slower (no effect), the hydraulic fluid servo valves that control the valves were all replaced (no effect), logic has been gone through (nothing found).

When the unit reaches Valves Wide Open, the problem stops (which makes sense) Most of the time when the valves come out of Valves Wide Open the problem begins and the unit is placed in Boiler Follow with the Turbine Master in Operator Set Point (OSP), this puts the Boiler Master in Manual and we drive from a Fixed Throttle pressure Set Point and Manual turbine valve control.

Other things we have tried when we are not in Valves Wide Open that occasionally work, is to increase the Throttle Pressure Setpoint in Fixed Pressure. Sometimes this works, and sometimes it doesn't. We can't seem to find the correlation as to why it works or doesn't work sometimes and the actual problem. The fact that it works occasionally may just be a coincident as well since we don't actually have proof as to why it works. We also called our regional dispatch center to see if it was something on their end and they don't seem to see anything.
Check the gain setting on the controller, make sure it is not using derivative kicks to speed valve adjustments up. You don't need that kick.

If you have a pressure control loop to stabilize the valve, with remote setpoint fed by a flow controller (steam rate), firing rate drum levels, etc, you have to work through it one system at a time.

You know how the boiler functions and is stable, and that's how you whole system is supposed to work.

Make sure the DCS guy is not trying to save with what he thinks is a better control scheme. If it don't make sense to you, it ain't gonna work...been there.