The interest on problems related to non linear devices and their influence on the systems increased considerably since 1980. This is due to the development of new power semiconductor devices and, as a consequence, the development of new converters that increment the non linearity in electric power signals substantially (Arrillaga et al., 1995). Several research institutions have estimated that seventy percent of all electrical power usage passes through a semiconductor device at least once in the process of being used by consumers. The increase on the utilization of electronic equipment modified the sinusoidal nature of electrical signals. These equipments increase the current waveform distortion and, as a consequence, increment the voltage waveform distortion which causes over voltage, resonance problems in the system, the increase of losses and the decrease in devices efficiency (Dugan et al., 1996). In general, quantities used in electrical power systems are defined for sinusoidal conditions. Under non sinusoidal conditions, some quantities can conduct to wrong interpretations, and others can have no meaning at all. Apparent power (S) and reactive power (Q) are two of the most affected quantities (Svensson, 1999). Conventional power definitions are well known and implemented extensively. However, only the active power has a clear physical meaning even for non sinusoidal conditions. It represents the average value of the instantaneous power over a fix period. On the other hand, the mathematical formulation of reactive power may cause incorrect interpretation, aggravated when the analysis is extended to three phase systems (Filipski, 1984; Emanuel, 1999). Although definitions of apparent, active, and reactive power for sinusoidal systems are universally accepted, since IXX century researchers pointed out that the angle difference between voltage and current produces power oscillation between the source and the load. All these research effort remark the importance of the power factor and the reactive power on the optimal economic dispatch. One of the initial proposals consists on dividing the power term into active, reactive and distortion power, and was the most accepted one. In the 80 s the discussion about the definitions mentioned above increased because the use of non linear loads incremented considerably. Although many researchers remark the important implications of non sinusoidal conditions, up today it is very difficult to define a unique power definition for electric networks under distorted conditions. The lack of a unique definition makes that commercial measurement systems utilize different definitions,