A thermodynamic approach to the instantaneous non-active power

Having schematically run through the transition from single-phase to three-phase relationships, the energy balance is formalised Using Park vector notation. The imaginary power notation emerges. Leading back, in the sinusoidal case, to the usual reactive power, it generalises its specific contents in the case of variable states, explaining in particular a typical aspect of three-phase networks: the energy bouncing from one phase to another. This aspect can be seen as an index of power quality. At last, these same considerations tire obtained by means of Lagrangian and thermodynamic approaches that lend a deeper meaning to the energy related quantities.