Its really the "decoupled controller" that can make VSC control the active and reactive power independently. If you understand the d-q transformation, then there are several papers published in the literature regarding this control.
To control the real or active power a VSC can raise or lower its output voltage, to control the reactive power it can vary its phase displacement with respect to the system it is driving. Regards, Orson Cart.
mess123: I'm not quite sure why do D-axis represents the real power flow and Q-axis represents the reactive power. Can you please help me clarify this?