# Instantaneous Active and Reactive Power

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Q: What's the physical meaning of instantaneous active and reactive power?

A: Reactive power fluctuates as much as active (real) power. For example reactive (inductive) power due to transformer reactance is proportional to the square of current flowing through the unit. Reactive power of a capacitor bank is proportional to the square of voltage across the bank. Therefore any fluctuation of voltage and\or current will result in fluctuation of reactive powers. And, from the physical meaning point of view the inductive power is always stored in magnetic field and capacitive reactive power in electric field.

ABB Inc

This is an excellent basic question. It's easy to lose sight of the physics and math that undergird what we do. Power is the time rate of change (derivative, if you will) of energy. So real power is the rate of change of the electric energy being transformed into mechanical energy (work) or other form, usually heat (losses). Reactive power is the time rate of change of stored energy in the electric field of a capacitor or the magnetic field of an inductor. To simplify the mathematics, we use phasor representation when the frequency is constant, as in a power system at steady state. During transient events reactive power still exists and figures into over-voltages etc. but must be determined by other (non-phasor) mathematics.

Paul Mauldin
Editor