Three-Phase Four-Wire Step-Down Modular Converter for an Enhanced Interlinking in Low-Voltage Hybrid AC/DC Microgrids

Abstract

This article proposes a three-phase four-wire bidirectional topology that serves as an interlinking converter for hybrid AC/DC microgrids, featuring a single-stage power conversion. The proposed converter configuration comprises three four-switch buck-boost modules connected in a star configuration, with the AC microgrid neutral directly linked to the positive terminal of the DC microgrid. The inclusion of the neutral wire facilitates islanded operation of the hybrid microgrid, ensuring a stable power supply for both single-phase and three-phase AC loads. Moreover, the direct connection between the AC microgrid neutral and the positive terminal of the DC microgrid allows for the grounding of both microgrids, a capability was previously achievable only with isolated topologies. Additionally, the four-wire converters exhibit the ability to control positive, negative, and zero sequence components, providing flexible control during AC voltage imbalances to mitigate their effects and ensure optimal operation. Furthermore, the converter enables independent control of the three phases, facilitating fault-tolerant operation demonstrated in this article under single-phase faults. The performance of the converter is assessed through experimental tests conducted on a 7kW prototype. This prototype has been tested under various operating conditions, including balanced and unbalanced AC voltages, single-module disconnection, and islanded operation.