It's related to each generator voltage or reactive power control, If the active power generation of connected generator to grid is constant as the field current is changed, then the distances proportional to the power in the phasor diagram (IA cos θ and EA sin δ) cannot change. When the field current is increased, the flux φ increases, and therefore EA (=K φ↑ ω) increases. If EA increases, but EA sin δ must remain constant, then the phasor EA must “slide” along the line of constant power, as shown below:
Since Vφis constant, the angle of jXS
changes as shown, and therefore the angle and magnitude of IA
change. Notice that as a result the distance proportional to Q (IA
In other words, increasing the field current in a synchronous generator operating in parallel with an infinite bus increases the reactive power output of the generator.