Reactive power definition

To understand what is Reactive power is utmost important for all electrical engineers. Just go to basics of alternate. and basic definition of power P (watts)= V volts (ie Joules/ coulomb ) X I Amps (ie coulomb /sec) = V x I (joules/sec =watts) For Dc system frequency = 0 hence inductive reactance is zero and capacitive reactance is infinity hence only resistive loads dissipates power, mainly in the active is visible form of heat or used for motive load /lighting. this power is conventionally called active power.

The power required to set up magnetic filed /electric filed is called reactive power. Since magnetic filed /electric filed are invisible , it is not possible to sense this power . Further , this power is oscillating at double the system frequency .
with this background if we look at ac alternator driven by prime-mover , at no load current i(t) produced is zero with terminal voltage , v(t) net power = v(t) x i(t) = 0
when reactive load R is connected i(t) =v(t) / R net power = v(t) x v(t) /R
since square of any qty( in this case v(t)) will always be positive net power delivered in resitive load is in unidirectional ie always from source to load. There is no reaction from load to source.
now let us see what happens while delivering pure inductive load Reactance of X=1 ohm ( pf = zero lagging )
The main flux produced by field winding remains same , however current flowing through stator produces field which is 90 degree lagging the main flux /field
power = v( t)x i(t) = Vm Sinὠt x Im(Sinὠt- pi/2) = -Vm X Im ( Sinὠt) X (cos ὠt) =( Vm X Im )/2 X Sin2ὠt) (ὠ is 2pi x frequency)

This power is reactive power oscillating at twice frequency.
Power is not dissipated in a purely reactive load. Though it is alternately absorbed from and returned to the source.
Note that the power alternates equally between cycles of positive and negative. This means that power is being alternately absorbed from and returned to the source. If the source were a mechanical generator, it would take (practically) no net mechanical energy to turn the shaft, because no power would be used by the load. The generator shaft would be easy to spin, and the inductor would not become warm as a resistor would.
Like shortage of active power in system affects the stability, shortage of reactive power also leads to voltage instability and major system, although ample active power may be available in the system.