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

Abstract

High-voltage and high-power dc-dc conversion is key to dc transmission, distribution and generation, which require compact and efficient dc transformers with large step ratios. This paper introduces a dc-dc converter with the input-series-outputparallel (ISOP) arrangement of multiple high step ratio subconverter units. Each sub-converter 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 to 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 down-scaled prototype.

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

“…t 4 and t 6 as is shown in Fig. 2, extending the modulation presented in [23] to an asymmetric operation. In order to achieve this, the cells are modulated with the same waveform using a phase shift of T S to reduce the resultant switchingfrequency and to achieve a balanced use of the cells.…”

“…To solve this problem, resonant MMC topologies have been proposed for high step voltage ratio [20]- [23], adding soft-switching operation to decrease the power losses. These proposals used a resonant tank formed by the capacitors of the cells which is excited by a square wave voltage at the resonant frequency, achieving zero-current-switching (ZCS) in the semiconductor devices.…”

Resonant Modular Multilevel DC–DC Converters for Both High and Low Step-Ratio Connections in MVDC Distribution Systems

“…Fig. 1 shows the schematic of the proposed transformercoupled RMMC for MVDC and LVDC connection [37]- [39]. The medium-voltage side utilizes an SM stack of the MMC structure to replace the half-bridge or full-bridge inverter in the classic LLC circuit.…”

“…Also, thanks to the benefits of phase-shift modulation, the frequency of the stack voltage and square-wave voltage will be increased to /( − ) times greater than that of each SM switching frequency , shown in (8), which reduces the required volume of all the passive components in the circuit. The voltage frequency of and is denoted as the effective frequency in this circuit, and it is normally chosen at a value between and ( < < ) to trade-off conduction losses and switching losses for the best overall efficiency [37], [40].…”

“…In addition to the MMC-based circuits mentioned so far, the transformer-coupled resonant modular multilevel dc-dc converter (RMMC) [37]- [39] was proposed for connection between MVDC and LVDC distribution networks where a high step-ratio conversion is needed. It inherits important advantages of both the classic LLC resonant circuit and the MMC structure, and it becomes a promising solution for MVDC distribution systems [40]- [42].…”