Genset Fluctuating Voltage

"technicians have tried doing service but still could not locate the problem"

Did they try,

what is the common causes of the power fluctuation of a generator ...

Fluctuation of Generator Parameters During Low Demand Period ...

Generator voltage fluctuation - Mike Holt's Forum

The tool needed to fix this problem is called a "telephone"; there are so many of them in the world these days it is a wonder why they are not used more often. Use this tool to:

• discuss the problem directly with the original equipment manufacturer's [OEM] technical specialists and, if necessary,
• arrange a call-out service visit from the OEM's technical specialists.

https://www.google.co.uk/search?sourceid=chrome-psyapi2&ion=1&espv=2&ie=UTF-8&q=Adjust%20regulator%20pots%20on%20caterpillar%20genset&oq=Adjust%20regulator%20pots%20on%20caterpillar%20genset&rlz=1C1VFKB_enGB662GB662&aqs=chrome..69i57.21809j0j8

What is the model of the genset?

What is its spec.? How far does its performance deviate from its spec?

Is it just the voltage which fluctuates or is the frequency also unstable?

You have a steam turbine generator and a diesel/or/gas genset: is one considered the master and the other a slave? Do they both need to synch. to the grid?

Have you got the manual that tells you how to adjust the pots on the regulator? Non independent control pots smacks of really bad design especially on an expensive piece of kit: maybe Cat have a newer replacement regulator board on which they have fixed this with more modern technology.

Have the technicians replace the regulator board.

They have a filtering network to prevent oscillations.

What is the serial # of the engine/package?

Is the engine rpm fluctuating, or is the voltage fluctuating by it self?