Suppressing Second-Harmonic Transfer via Fractional-Order Notch Filter and Triple-Active-Bridge Power-Flow Controller for Accurate Measurement

Accurate measurement in DC microgrids (DC MGs) is reduced when second-harmonic current (SHC) propagates to the sensing nodes. To effectively mitigate SHC and improve accurate measurement without compromising the system dynamic performance, a hybrid approach is proposed that combines improvements in both the appended circuit and the fine-tuned control scheme. First, a framework for accurate measurement is established using a triple-active-bridge power-flow controller (TAB-PFC) to decouple bipolar lines and modelling transfer paths via symmetrical component theory. Next, the output impedance under dual-loop control is derived. The analysis demonstrates that reducing the loop gains around SHCs increases the virtual impedance, thereby blocking their transfer. Then, a fractional-order notch filter (FONF) is presented as a SHCs damping controller, with tuning rules that preserve SHC attenuation while enlarging stability margins. Subsequently, the results of SHCs content and total harmonic distortion (THD) at sensing nodes are adopted as measurement-accuracy proxy. Finally, effectiveness is verified by simulations and hardware experiments, which demonstrate fast settling and cleaner DC sensing without additional measurement hardware.