Third-Harmonic Current Suppression for Power Distribution Systems Under Unbalanced Installation of DG Units

The ac/dc front-end converter in low-power applications such as household appliances usually contains diode rectifiers equipped with smoothing dc capacitors, which generate a large amount of harmonic current. As a solution for reducing the harmonic current, distributed generation (DG) units with a harmonic compensation function have become an attractive topic. In three-phase three-wire systems, the effect of the third-harmonic current is not serious, because most of the current is zero-sequence current. However, if many single-phase DG units are unevenly installed in three-phase grids, the third-harmonic currents may be serious. This paper shows that conventional harmonic suppression can increase third-harmonic currents in three-phase three-wire grids. To prevent an increase in the harmonic current, a third-harmonic suppression method called the balancing third-harmonic function is proposed. To validate the proposed control strategy, experimental results using a laboratory system are presented.