Thursday, 19 January 2017


Power is referred as the product of voltage and current   P = V x I

In an ac transmission, when the voltage and current go up and down at the same time, i.e. when voltage and current are in phase or in synch, only real power is transmitted and when there is a time shift between voltage and current both active and reactive power are transmitted.
Whenever there is a phase shift between V and I we have:-

a) real power (kW)
b) reactive (imaginary ) power (kVAR)
c) The combination is a complex or apparent power    (kVA)=√(kW)² + (kVAR)².


Reactive power (VARs) is required to maintain the voltage to deliver active power (watts) through transmission lines.
reactive power

Motor loads and other loads require reactive power to convert the flow of electrons into useful work.

When there is not enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines.

The horse and the boat analogy best describe the Reactive Power aspect. Visualize a boat on a canal, pulled by a horse on the bank of the canal.

In the horse and boat analogy, the horse’s objective (real power) is to move the boat straightly.

The fact that the rope is being pulled from the flank of the horse and not straight behind it, limits the horse’s capacity to deliver real work of moving straightly.

Therefore, the power required to keep the boat steady in navigating straightly is delivered by the rudder movement (reactive power).

Without reactive power there can be no transfer of real power, likewise without the support of rudder, the boat cannot move in a straight line.

Reactive power generated by the ac power source is stored in a capacitor or a reactor during a quarter of a cycle and in the next quarter of the cycle it is sent back to the power source.

Therefore the reactive power oscillates between the ac source and the capacitor or reactor  at a frequency equals to two times the rated value (50 or 60 Hz).  So to avoid the circulation between the load and source it needs to be compensated .

Also to regulate the power factor of the system and maintain the voltage stability we need to compensate reactive power .


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