An improved power regulation method for a three-terminal hybrid AC/DC microgrid during module failure

Abstract To reduce the number of power conversion stages and achieve the direct connection to medium-voltage (MV) grid without line-frequency transformer integration, a three-terminal hybrid AC/DC microgrid with one AC port and two DC ports has attracted great interests. It is composed of a cascaded H-bridge (CHB) converter based AC grid port and two dual active bridge (DAB) converters based DC microgrid ports. The large number of power modules increases the probability of module failure, which may cause unbalanced capacitor voltages of CHB converter and unstable DC microgrid voltages. This paper proposes an improved power regulation method with a combination of zero-sequence voltage (ZSV) and negative-sequence current (NSC) injection. When module failure occurs, first, ZSV is injected to adjust the modulation margin in the faulty phase; then, based on three-phase power analysis, NSC injection is activated to realize DC voltage balancing control in three clusters. In addition, the constraints ofZSV and NSC injection are respectively investigated, and the selection of injected ZSV and the derived NSC are analyzed. In a three-terminal hybrid AC/DC microgrid, the simulation waveforms validate the effectiveness of the proposed fault-tolerant operation scheme with the stable DC microgrid voltages and balanced DC capacitor voltages. Experimental results on a CHB converter also verify the effectiveness of the proposed scheme.