Contents I. Introduction
By the proliferation of a.c. in the transmission and distribution systems, apparent power was defined as the product of voltage and current rms values to size the system equipment and to be a measure for the system efficiency. Historically, the current of the system was divided into two parts: These are; active current, which transports the net energy from source to the load, and reactive current that is the remaining current component when the active part is subtracted from the total current. According to this resolution, apparent power was expressed as the vector sum of active and reactive powers, which flows due to the active and reactive currents, respectively. Sequently, the ratio of active and apparent powers is named as the power factor, and be utilised to measure the efficiency of the power systems. In addition, conventionally, classical single-phase apparent power is directly extended to three-phase systems by treating each phase individually. Thus, arithmetic apparent power, which is calculated as the arithmetic sum of each phase's apparent power, and vector apparent power, which is calculated as the vector sum of total active and total reactive powers of the system, were constituted for the three-phase systems [1], [2]. Nevertheless, due to the fact that the classical apparent power and its resolution are defined under sinusoidal and balanced conditions, they did not attain their goals in the case of nonsinusoidal and/or unbalanced conditions. Consequently, a number of apparent power definitions and their resolutions have been proposed for nonsinusoidal single-phase and nonsinusoidal & unbalanced three-phase systems to fulfil the gap left out in the classical apparent power concept.
