How do you control reactive power?

The reactive power depends on the load and not the generator. Reactive power can be detected at the generator, and the load disconnected if there is too much being drawn.

A generators's voltage depends on the speed, and the power produced depends on the current in the field winding.

An alternator running on a bus bar, as in a grid system, runs synchronously with the grid it is connected to. In this case the power produced depends on the rate that steam is supplied to the turbine, and although the speed stays constant, any increase in power output is accompanied by an advance in the phase angle of the rotor.