A Novel Hybrid DC Transformer Combining Modular Multilevel Converter Structure and Series-Connected Semiconductor Switches

Multicell nonisolated resonant modular multilevel dc‐dc converter with self‐voltage balancing and step‐up conversion

“…The emerging of modular multilevel converter (MMC) provides bright prospect for HVDC system to match grids with different voltage degrees for its modularity, high reliability, and lower harmonic content of output voltage 6,7 . The ac terminals of two submodule (SM) stacks are connected with a transformer to achieve dc‐ac‐dc conversion, then, some isolated modular multilevel dc‐dc converters (IMMDCs) have been put forward 8–13 . Combining the characteristics of LLC resonant converter and MMC, a step‐down resonant modular multilevel dc‐dc converter (MMDC) is proposed in a previous study 8 ; it possesses desirable features of these two converters, for example, high step‐down ratio, soft switching, and flexible fault‐tolerant operation.…”

“…Combining the characteristics of LLC resonant converter and MMC, a step‐down resonant modular multilevel dc‐dc converter (MMDC) is proposed in a previous study 8 ; it possesses desirable features of these two converters, for example, high step‐down ratio, soft switching, and flexible fault‐tolerant operation. To decrease the quantity of SMs, by combining the series‐connected semiconductor switches and the features of MMC, a hybrid dc transformer is presented in Li et al 9 In order to reduce voltage stress, a half‐bridge SM branch connected in series on high‐voltage side is utilized in Jin et al 10 Furthermore, improved operating states and modulation methods of IMMDCs have been proved in previous studies 11–13 . These converters can realize galvanic isolation and improve voltage level by altering transformer turn ratio.…”

“…6,7 The ac terminals of two submodule (SM) stacks are connected with a transformer to achieve dc-ac-dc conversion, then, some isolated modular multilevel dc-dc converters (IMMDCs) have been put forward. [8][9][10][11][12][13] Combining the characteristics of LLC resonant converter and MMC, a step-down resonant modular multilevel dc-dc converter (MMDC) is proposed in a previous study 8 ; it possesses desirable features of these two converters, for example, high step-down ratio, soft switching, and flexible fault-tolerant operation. To decrease the quantity of SMs, by combining the series-connected semiconductor switches and the features of MMC, a hybrid dc transformer is presented in Li et al 9 In order to reduce voltage stress, a half-bridge SM branch connected in series on high-voltage side is utilized in Jin et al 10 Furthermore, improved operating states and modulation methods of IMMDCs have been proved in previous studies.…”