A New Approach to Individual-Phase Reactive Power Compensator for Non-Sinusoidal and Unbalanced Three-Phase Systems

Many researchers have attempted to clarify the definitions of active power, reactive power, active current, reactive current, etc. for unbalanced and non-sinusoidal three-phase situations. The so-called pq theory has given a new definition of instantaneous reactive power, and it has been discussed and developed by many authors. In this paper, the merits and demerits of the instantaneous reactive power compensator are discussed. It is shown theoretically that applying instantaneous reactive power compensation to unbalanced three-phase systems has a serious disadvantage in that it causes third-order harmonic currents on the source side, which problem cannot be avoided. To overcome this problem the authors propose a new approach, and name it the “quasi-instantaneous” reactive power compensator. It compensates individual-phase reactive currents. The basic principles of the quasi-instantaneous reactive current compensator are discussed in detail, and its validity is confirmed using digital simulation. In particular the authors show that the power factor of each phase becomes unity on the source side, but the source currents remain unbalanced when the proposed method is applied.