Reactive power is a result of inductive or capacitive reactance in a circuit causing a phase offset between the voltage and current. The overall current supplied to the load can be split into two seperate waveforms -- one in phase with the voltage, and one out of phase by 90 degrees (either leading or lagging the voltage waveform, but typically lagging due to an inductive load). Multiplying the in-phase current waveform by the voltage results in the real power produced by the generator, which is always positive and thus requires torque from the prime mover. However, multiplying the out-of-phase waveform by the voltage results in a power waveform that alternates between positive and negative, with an average of 0. Each phase will absorb power from the generator for 1/4 cycle, and then return the power back to the generator the next 1/4 cycle. If this reactive power is balanced on all phases of a 3-phase system, the reactive power cancels out completely and only results in additional heat due to the increased current in the generator stator and transmission lines, as well as the increased excitation power required to maintain the same voltage.
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