NOC-Based Multiple Low-Order Harmonic Currents Suppression Method

In a microgrid system, the dc bus contains a significant amount of undesired low-order harmonic currents (LHC) when multiple converters of various types and with nonlinear behavior are connected. To address the issue, this article proposes an iterative control strategy based on a negative-order capacitor (NOC) branch on dc bus. In the proposed method, the reference voltage of decoupling capacitors in the NOC circuit is obtained based on the dc bus current. The voltage difference between the two decoupling capacitors is taken as the control objective, leaving little dc difference between the two capacitors and resulting in maximum LHC suppression ability. To refactor the reference voltage, a singularity points hunting strategy is proposed, helping to reduce the current spike. More importantly, the iterative control strategy is employed in the virtual current loop, decreasing steady-state error from device deviation or control deviation. Finally, an 800 W prototype is built to validate the effectiveness of the proposed method.