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I've been researching for days about synchronous generators and cannot get my head around how real power output of generators can be adjusted when paralleled to the grid or to each other.

I'm across the reactive power side of things, increasing terminal voltage adjusts amount of reactive power absorbed/produced by the generator (i.e. terminal voltage via excitation increased implies producing VARs and decreased implies absorbing VARs) and I understand how generators operate when connected on their own (i.e. load placed onto generator creates opposing force to slow down prime mover, reducing generator output frequency, more fuel is added to prime mover then generator output frequency restores to nominal).

What I don't understand is how do you control the amount of real power the generator produces, when connected to the grid or in parallel with another generator.

My understanding so far is follows (correct me if I'm wrong):

Let's start with the generator output CB open. Generator is started, rotor is excited by DC voltage. Fuel is added to the prime mover and the prime mover begins to spin. As the prime mover spins, a magnetic field is induced in the stator windings and a generator terminal voltage is produced (the generator is currently unloaded). Synch checks are done to ensure stator output frequency, phase and voltage are the same as the grid.

Generator CB is then closed onto the grid (which is just a whole bunch of other generators). The grid induces a voltage back onto the rotor and hence rotor will slow down. Fuel is added to the prime mover to bring the rotor back up to nominal speed, the same way it would if a resistive load was placed onto it.

Now the prime mover is receiving enough fuel to spin whilst connected to the grid, but as I understand it no power is delivered to the grid yet.

More fuel is added to the prime mover and this is where I am lost. If we add more fuel to the prime mover, does it not begin to accelerate creating a gap between the stator magnetic field (from the grid) and the rotor magnetic field? If this is the case, then the rotor will be phase shifted from the stator magnetic field? Looking at the two wave forms, you would have the grid waveform at nominal frequency and the rotor induced waveform phase shifted by this 'gap'.

If there is a phase shift, let's assume it is 90deg, as I've read that 90deg equates to full load output of the generator. Then won't the connected load see the grid voltage plus the induced voltage from the rotor? Adding two phase shifted waveforms would result in higher RMS voltage across the load? This is bad?

Sorry I'm probably way off here, but thought I could get some help understanding what is going on as it's driving me crazy.