What is the Reactive Power?

For a "physical" interpretation, reactive current (power/KVA flow), in my opinion is best looked at from the perspective of a generator connected directly to an infinite bus (in LV generators this is the norm).

The generator when connected to the system, "see's/feels" the parallel impedance combination of all other generators (circa 3 ohms each) with respect to ground - which basically parallel to equate to a zero impedance in terms of restriction to any current flow out of our generator.

Post initial synchronization, the system voltage prevents currents from flowing into or out of the generator due to pressure (voltage) balance of our generator matching that of the system voltage.

If you (as the generator operator), try to lift the generator voltage, the result will only be heaps of current output flowing into the system – but with no actual extra power generated!

This is due to the fact that to achieve the extra generator voltage setpoint you desired, the generator must send out enough current into the system impedance to create the back emf required to achieve the new desired generator terminal voltage setpoint.

But because the system impedance to ground is very low (as it actually is) – then despite the extra current sent out in that fruitless attempt, the generator is near impotent to make any substantial effect on raising the "system" voltage – "fruitless" current sent out.

In a DC sense you can equate this to a small DC generator trying to lift the voltage of a load system that has a zener diode installed across that system load.

Back to the AC world, ….that current sent out in the fruitless attempt to lift system voltage must flow through the parallel low impedance of the other connected generators (each of those working against you – lowering their own generator excitation, hell bent on keeping their own same old voltage set points), thwarting our futile attempt to achieve a raise in the system voltage.

All those generators, although collectively of low impedance, compose virtually no resistance, compared to their inductive reactance. Hence all our little generators current flow - in its futile attempt to lift system volts - is virtually purely inductive.

So we have heaps of current flowing out in our attempt to lift generator volts, but because the current is 90 degrees lagging the voltage, the only power imposed on the generator prime mover is that due to the resistance of the generator windings (circa 1% of the full load current rating – hence basically un-noticeable).

Hence the physical interpretation of VAR's, is actually simply a look at the voltage balance perspective of an electricity network. It's the collective attempt of many parallel-connected generators to influence the system voltage – either trying to raise the voltage at a particular node (positive VAR’s) or trying to reduce the voltage at a particular node (negative VAR's flowing back through our generator due to our attempt to lower our generator setpoint – which "lets current in").

Reactive Power is an electrical parameter that exist in a sinusoidal (AC circuits). It maybe zero or a certain magnitude. It maybe capacitive in nature or it maybe inductive nature. In the power triangle, it is the vertical power component (plus or minus / capacitive or reactive). It may be supplied from power sending end (grid or generator) on from the power receiving end (load). A capacitor bank connected on the grid provides capacitive reactive power. An inductor bank connected on the grid provides inductive reactive power. Both of them have magnitude. Reactive power also influences the between phase angle displacement between the voltage and the current. It is power but reactive power.