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.
Highlights
In recent years, with the utilization of marine resources, the island micro-grids based on a medium-voltage DC (MVDC) link have become the key nodes connecting the islands, and have gradually developed into the future smart grid prototype [1], [2]
Compared with the passive distribution networks based on a medium-voltage AC (MVAC) link, this new distribution network has the advantages of high controllability, low transmission loss, and flexible operation [3]ā[8]
EXPERIMENTAL RESULTS To verify the performance of the single-stage control system of the I-modular multilevel converter (MMC), a scaled down prototype with four submodules of each phase is implement
Summary
With the utilization of marine resources, the island micro-grids based on a medium-voltage DC (MVDC) link have become the key nodes connecting the islands, and have gradually developed into the future smart grid prototype [1], [2]. In (8), the first equation is used to control the MVAC currents when the I-MMC-based receiver is operating in the MVDC link off-grid mode. The second equation is used to control the MVAC currents when the I-MMC-based receiver is operating in the MVDC link grid-connected mode. B. MATHEMATICAL MODEL OF MVDC AND LVDC PORTS BASED ON DC MODULATION FREEDOM According to the structure presented, the voltage loop of the MVDC port can be expressed as follows:. Equation (13) is used to control the LVDC current when the I-MMC-based receiver is operating in the MVDC link grid-connected mode. These two equations indicate that D corresponds to the MVDC and LVDC current in different modes respectively. Independent control of the MVDC or LVDC current can be implemented by D in different modes
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