A Second Harmonic Current Suppressing Method Based on Negative-Order Capacitor

In microgrid system, current in dc bus may contain a lot of undesired low-order harmonics when converters are employed, especially the second harmonic current (SHC). To address the issue, this article proposes a method based on fractional-order capacitor (FOC) to suppress the SHC. The FOC, of which the order α is "−1," has the same characteristics of dc blocking and ac conducting as conventional capacitor; while it can resonate with a conventional capacitor like an inductor, which could be called a negative-order capacitor (NOC) as well. The proposed circuit has good SHC suppression performance, faster dynamic response, and soft switching realization. It parallels on the dc side of an inverter instead of the large bus capacitor, increasing power density and reducing design cost and loss.The secondary power will only circuit between the ac side of inverter and the NOC circuit. In addition, the NOC circuit can adjust zero impedance to specific harmonics; so it can be expanded to suppress harmonic current at other frequencies in the system. The simulation and experimental results are presented to validate the feasibility of the proposed method.