Compared with conventional low-voltage hybrid AC/DC microgrid, a medium-voltage (MV) hybrid AC/DC microgrid is proposed in this paper. In this proposed microgrid, the MV interlinking converter is composed of front-end cascaded H-bridge (CHB) converters and backend dual active bridge (DAB) converters. In order to make system normal operation with grid fault-tolerant ability, a novel power regulation scheme is proposed. First, the idea of virtual faulty-cell is proposed to bypass certain healthy bridge cells and adjust the power consumption between the three-phase clusters under unbalanced grid conditions. Then, this paper presents a modified modulation strategy combining phase-shifted (PS) PWM and sort and select algorithm, where no carrier reconfiguration is necessary with bypassed bridge cells. The proposed method can generate a balanced grid current and require a lower injected zero-sequence voltage under unbalanced grid conditions. Finally, based on an MV hybrid microgrid system (3 kV/150 kW). Verification results verify the effectiveness of the proposed structure and control method of this MV microgrid. By the proposed scheme, AC current balancing of the three-phase grid, DC capacitor voltage balancing of front-end CHB converters and DC bus voltage stability of DC microgrid can be realized simultaneously.
Keywords Medium-voltage microgrid; hybrid AC/DC microgrid; zero-sequence voltage injection; balancing control; unbalanced grid; grid-voltage sags; stability control