Low-Order Harmonic Current Suppression Method Based on Adaptive Fractional-Order Capacitor Considering Parameters Error

In a microgrid system, the current in the dc bus may contain undesired low-order harmonics when multiple converters are employed. The integer negative order capacitor has been verified to be an effective method. However, the performance of harmonic suppression may deteriorate because of the parameter error. To address the issue, this article proposes an adaptive fractional-order capacitor (FOC) based method, in which the order of the capacitor can be modulated, depending on the impedance needed to be compensated. The terminal voltage of adaptive FOC is determined by the external circuit while the current can be adjusted freely, thus, different port impedance characteristics can be performed. The method uses a capacitor as its power storage instead of a dc supply and has nondissipative components to bring no additional loss. To stabilize the internal energy of the adaptive FOC circuit and make the port current accurate, a parallel double closed-loop control is employed. And the parameter error can be calculated from the harmonic content of the bus current. The simulation and experimental results are presented to validate the feasibility of the proposed method. The proposed method opens up an avenue for expanding the application of asymmetric parameter systems.