High Step Ratio Research Articles

A Bidirectional Current-Fed Isolated MMC with Partial Soft-Switching for High Step Ratio DC-DC Applications

A Bidirectional Current-Fed Isolated MMC with Partial Soft-Switching for High Step Ratio DC-DC Applications

A Step-Up Nonisolated Modular Multilevel DC–DC Converter With Self-Voltage Balancing and Soft Switching

Evolving from the popular modular multilevel ac-dc converter, the single-stage nonisolated modular multilevel dc-dc converter (MMDC) is advantageous for medium- and high-voltage applications. However, exploiting ac circulating power to balance the submodule energy, when utilized for high step ratio applications, existing MMDC topologies suffer from circulating current through the arms and large filter inductor at the low-voltage side. To overcome these issues, this article presents a new power transfer mechanism to balance the submodule energy automatically by reconstructing the half-bridge submodule into a quasi-resonant circuit. Based on this submodule structure, a new MMDC topology for step-up applications is proposed. Compared to the existing MMDCs, the proposed one offers the following advantages. First, the common-mode circulating current through the lower and upper arms is avoided. Second, the self-balancing of the capacitor voltages is guaranteed by the proposed modulation method to insert and bypass adjacent submodules in a complementary manner. Third, the soft-switching operation is achieved for the majority of the switches to alleviate switching losses. Fourth, the voltage stress across the input side inductor is limited to the submodule voltage, thereby reducing the size of the inductor. Simulation analysis and experimental results verify the performance of the proposed MMDC.

Asymmetrical Triangular Current Mode (ATCM) for Bidirectional High Step Ratio Modular Multilevel Dc–Dc Converter

Direct current (Dc) networks have proven advantages in high voltage direct current (HVDC) transmission systems, and now they are expanding to medium- and low-voltage distribution networks. One of the major challenges is to develop reliable dc-dc voltage transformation achieving high efficiency and performance, especially at high voltage and high step ratio. New resonant modular multilevel topologies have arisen as an alternative, mainly because of advantages such as optional use of transformers, natural voltage balance, simple control, and soft-switching capability. However, this type of operation generates a high peak current, does not allow control of power flow in all power range, and has a limited range of voltage variation. This article proposes an asymmetrical triangular current mode applied to high step ratio modular multilevel dc-dc converters. The proposed modulation increases the efficiency and achieves bidirectional control of the power, soft-switching, and a natural balance of the voltage in the cell capacitors. The experimental results show the bidirectional operation and the capacitor voltage balance of the converter under different operating conditions with higher efficiency (97.72%) and lower peak current compared to previous reports of this topology using resonant operation.

Large Step Ratio Input-Series–Output-Parallel Chain-Link DC–DC Converter

High-voltage and high-power dc–dc conversion is key to dc transmission, distribution, and generation, which requires compact and efficient dc transformers with large step ratios. This paper introduces a dc–dc converter with the input-series–output-parallel arrangement of multiple high step ratio subconverter units. Each subconverter unit is an isolated modular dc–dc converter with a stack of half-bridge cells chopping the dc down to low voltage level. The transformer provides galvanic isolation and additional step ratio. The converter achieves a large step ratio due to the combination of the series-parallel configuration, the modular cells, and the isolation transformer. The proposed dc–dc converter is analyzed in a 30 kV–1 kV, 1-MW application to discuss the operation performance, tradeoffs, power efficiency, and selection of components. Finally, the converter is validated through a laboratory downscaled prototype.