Single-Stage Control System of I-MMC-Based Island MVDC Link Receiver With Multiple Modulation Freedoms

Abstract

In the medium-voltage AC/DC distribution networks with distributed renewable sources on islands, a multi-port receiver is the key factor for hybrid power conversion. However, the most used modular multilevel converter (MMC)-based multi-port converters (MCs) face the coordination and complexity challenges due to their double-stage control system and voltage-balancing control of capacitors. In particular, the control system is more unstable and complicated when the control of circulating currents is considered. In this paper, an isolated modular multilevel converter (I-MMC) is used as a receiver, and a unified coordinated control scheme based on the multiple modulation freedoms is proposed. Due to the voltage clamping of high-frequency transformers, there is no concern of the capacitors' voltage-balancing control. Based on the proposed single-stage control system, the unified coordinated control scheme solves the coordination problem of the MMC-based MCs. The multiple modulation freedoms corresponding to an AC port, two DC ports, and three-phase circulating currents can independently control respective targets. The control structure is simplified, while the control freedoms are ensured. Experimental results confirming the performance of the designed control system is shown. INDEX TERMS Circulating currents control, isolated modular multilevel converter, medium AC/DC distribution networks, multiple modulation freedoms, single-stage control system, unified coordinated control scheme.

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