The Theory of Instantaneous Power in Three-Phase Four-Wire Systems and its Applications

This paper discusses "the p-q theory" and "the cross-vector theory" in three-phase four-wire systems, with the focus on similarity and difference between the two theories. They are perfectly identical if no zero-sequence voltage is included in a three-phase three-wire system. However, they are different in definition of the instantaneous active power and instantaneous reactive power in each phase if a zero-sequence voltage or current is included in a three-phase four-wire system. Based on both theory and computer simulation, this paper leads to the following conclusions: An instantaneous reactive-power compensator without energy storage components can fully compensate for the neutral current even in a three-phase four-wire system including a zero-sequence voltage or current, when a proposed control strategy based on the p-q theory is applied: However, the compensator cannot compensate for the neutral current fully, when a conventional control strategy based on the cross-vector theory is applied.