If for the active power (correctly the energy) exists a net flow from one point of the network to another, for the reactive power there is a continuously flow back and forth (to and fro), but the net flow is zero for a complete cycle, as the amount of energy flowing in one direction for half a cycle is equal to the amount of energy flowing in the opposite direction in the next half of the cycle. The reactive power is exchanged by different parts of the network – capacitors and reactors – permanently, but is never consumed or produced. In reality, we can say that this reactive power is produced once, when the network is energized (after a collapse) and the same reactive power is consumed once, when the network collapses again. In between these two major events, the reactive energy stays constant. Of course the situation is changing when equipment is switched on or off.
It is customary to call the first part of (1) the instantaneous active power and the second part of (1) the instantaneous reactive power. There is a fundamental difference between the two powers. The active power oscillates around an arbitrary average value while the reactive power oscillates around a zero average power as the average value of a cos and sin function is zero. This observation is valid under any conditions as long as the oscillations are sinusoidal. For a non-sinusoidal regime the problem is much more complicated and is not yet fully solved.
As is not practical to work with instantaneous quantities because they are difficult to measure, averaged values are introduced. The average value of the instantaneous active power is called the active power and is given by
P =VI cosø
where V and I are the rms values. This quantity is obtained from averaging in time the first part of (1). Following the same method for the instantaneous reactive power, the average is zero and therefore is useless to introduce a quantity that is always zero.
Instead, another quantity is introduced to describe the instantaneous reactive power that is transferred to the network and this quantity is the maximum of the instantaneous reactive power,
Q = VI sin ø
which is exactly the second part of (1) without the sin of the time. In this way the symmetry between the active and reactive power is broken and therefore P and Q do not have the same meaning as it is inferred in many textbooks. This is the root of the misconceptions about reactive power. The above definitions are summarized in table
It can be seen from the table that the active power describes an average power while the reactive power describes the maximum of the instantaneous power. As a consequence the two concepts cannot be treated on equal foot as they are not similar. The simplest solution to this problem would be to avoid the usage of reactive power term for Q but perhaps this choice would be difficult to be implemented in practice.