pid+analog
The only reason to vary the excitation of a snychronrous generator would be to vary the power factor of the power being produced. You would need to have a CT and on the conductor the feeds the exciter. You would need two CT's on two of the three phases on the generator output. The phase relationship between the two CT's on the output of the generator will give you the power factor. You need the math of the phase comparison to be done by a math module or what-not. It is a simple proportional control between the current to the exciter and the phase angles of the output of the generator.