Converting synchronous generator in synchronous condenser

The calculations are no different than those for determining how to start a large synchronous motor, it all depends upon the type of generator and how big it is relative to the system that you are connecting it to. Take a look at the SLD (Single Line Diagram) and get the GSU (Generator Step Up) transformer rating and impedance, transmission line impedance to the grid, as well as any information that you have on the generator. Then you can determine how much the voltage will dip if the generator is started DOL (Direct On Line). If it is unacceptable you will need some form of reduced voltage starter.

But it also depends on the construction of the machine, if this old machine with damper windings then it may be possible to start as an induction motor, and have the field energized near synchronous speed.

The small motor (also known as a pony motor) eliminates the above concerns, you just need an induction motor large enough to accelerate the rotor to near synchronous speed and then engage the field. Usually a few hp will do since you don't care how long it takes (within reason) to get to speed. The only problem will be connecting and aligning it with the generator shaft.

As you can see there's a fair amount of engineering involved, and we haven't even discussed the excitation, control and protective system modifications involved.

Twenty-five years ago I played with the same idea for some diesel-generators that were supplying a load with a very low power factor. Today I would definitely try to use a separate LV motor to run up your generator. Even if there is a bit of the turbine in the way physically.

That way you would overcome the expense of installing a MV variable frequency drive. Otherwise you would have to ensure excitation before accelerating, and that would be difficult with most modern brush-less excitation systems, and even with a brush-ed exciter you would have to adjust excitation to the variable frequency drive output.