Decaying DC and Harmonic Components Detection for Absorbing Impact Load Currents in Weak Grids

Decaying DC (DDC) current due to impact loads causes detrimental consequences in weak grids. In this paper, the active power filter (APF) is enhanced to simultaneously compensate for the undesired harmonics, negative sequence components and the DDC component in the event of a transient. To this end, first, a novel DDC component detection method is mathematically deduced for single-phase transient current, followed by its extension to the three-phase system. With respect to existing techniques, the proposed method has a significantly reduced detection time from 1 grid cycle to approximately 0.5 grid cycle. By further exploiting the transient signal properties in dq-frame, the three-phase fundamental-frequency current in positive sequence can be separated. Finally, the enhanced APF control scheme is developed based on the proposed detection algorithm. Experiments verified the effectiveness and advancement of the proposed technique.