1. I read on the internet that the reactive power is negative real power. It is a power that moves from the load to the source (with 90 deg. of phase shift). Why is reactive power measured in VAR not in watt?
A purely inductive load store some watts (a positive power that moves from the supply to the load). After a certain amount of time, It releases them as VARs !! It stores Watts and I think it should releases watts also. Is this just two names of the same unit?
To keep track of the energy budget in a circuit you need to know how much of the energy you are pumping does work, that is energy that is lost to the circuit per second, i.e. watts. The energy that is not doing work is expressed as VAR.
It is important to keep clear the difference between energy and power. Power (Watts) is energy (joules) consumed within a specified time frame(sec), i.e. 1W= 1 J/s. Inductors and capacitors don't store power, they store energy. The next thing to remember when talking about AC power systems is we are generally talking about averages, so not instantaneous power but average power.
"Real" power is power that does work, i.e. turns motors, drives speakers, heats up resistors, emits light..... Volts don't do work unless they are moving a charge in the direction of the EMF field and that moving charge is doing some work, like crashing into atoms generating heat or generating a magnetic field that turns a motor.
So for example, if you had an ideal gasoline 100V AC voltage generator driving an ideal inductor with an impedance of 50 ohms, then the generator is delivering 2A. So how many watts is that? Answer is zero. Instead we say it has supplied 200 VAR. Because no net (average) work is being done. The inductor is storing the energy delivered to it in a magnetic field and when that field collapses it sends all that energy back to the generator -it is never consumed. So the generator would run forever without the need to provide it any more fuel. Actually I guess it would oscillate between being a motor and a generator - but never mind that. Now if you swap in a 50 ohm resistor for the inductor then 200 Watts are consumed as heat by the resistor, which means a gasoline energy equivalent of 200J/sec needs to be supplied to the generator to keep it going. So the guy supplying the gasoline needs to know whether he is supplying Real Power or Reactive Power.
You know the power is 200VA reactive, in the first case, and not 200W because the voltage and current are 90 degrees out of phase. If you take the instantaneous power (watts) at each moment in a cycle and add them up over a complete cycle it adds up to zero; at some instants it is zero, sometimes positive and sometimes negative, but the sum is zero. Then why watts, when doing instantaneous calculations of power? Because when it is positive it is doing work building the magnetic field in the inductor and when it is negative the inductor is doing work generating magnetic field in the generator. Because in AC power systems we talk of averages - no net work is done - so no Watts.