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If current leads the voltage in a load, it means the load is mostly capacitive and reactive power is negative. What I don't understand is that reactive power depends on sin(phase of voltage-phase of current) so if current is leading it means I have smaller value for phase of current and sin has a positive value. So how come we alway talk about negative reactive power for current leading loads?

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A load that has a lagging power factor is, by convention, said to be receiving reactive power from the source. A load that has a leading power factor is, by convention, said to be delivering reactive power to the source. In a vector representation of AC circuits, inductors are given the value +jX and capacitors are given the value -jX, so that leads to the the use of +jI for lagging reactive currents and -jI for leading reactive currents. Therefore, lagging reactive power is positive and leading reactive power is negative.

Sometimes a leading power factor is given a negative sign, but that is not a good practice, because power = V X I X pf and negative real power is power that is returned to the source by the load as a motor does during regenerative braking or a utility customer does by generating more power than they use with a renewable energy system.

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Good question.

Wikipedia's article on Power Factor states:

Lagging and Leading Power Factors:

In addition, there is also a difference between a lagging and leading power factor. A lagging power factor signifies that the load is inductive, as the load will “consume” reactive power, and therefore the reactive component Q is positive as reactive power travels through the circuit and is “consumed” by the inductive load. A leading power factor signifies that the load is capacitive, as the load “supplies” reactive power, and therefore the reactive component Q is negative as reactive power is being supplied to the circuit.

I can't quite make sense of the logic here. I suspect that it might be just convention in that most industrial load is inductive and that power-factor measurements were always 0 to 1. For the occasions when it goes capacitive the sign is changed.

I would be happy to be educated further on this.

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    \$\begingroup\$ The reactive component is neither consumed or supplied power. Instantaneously there will be power transfer. With a purely reactive load for half a cycle power will be transferred to the load and for half a cycle power will be recovered from the load back to the source. Reactive loads store power. This is true for capacitive and inductive loads. The only difference is which half cycle. The negative sign is purely a mathematical convention, it all works if you do the maths in the complex plain and use the conventional signs. \$\endgroup\$ – user1582568 Feb 10 '16 at 17:00
  • \$\begingroup\$ What is really supplied is a complementary storage element. If the load is inductive, the supply must have a capacitive storage element that has an equivalent storage capacity. \$\endgroup\$ – Charles Cowie Feb 10 '16 at 17:54

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