A Developed Dual MMC Isolated DC Solid State Transformer and Its Modulation Strategy

DC solid state transformer will be a key device in DC power grid in the future. This paper proposes a novel topology of MMC based DC solid state transformer. Unlike the common back-to-back converters, the AC side of the two MMC converters are connected via a middle-frequency AC transformer. A modulation method and corresponding submodule balancing strategy has been presented in the paper. Compared to those sine-type modulation technologies, the proposed Quasi-2-Level (Q2L) modulation has a greater power transmission capacity. Additionally, Q2L modulation can significantly reduce dv/dt stress on the isolated AC transformer, which protects converters from electromagnetic interference. A corresponding submodule balancing strategy based on Q2L modulation is also proposed in this paper, without any effect on AC waveform or requirement on arm current measurement. The effectiveness of the modulation and balancing strategy has been validated by simulation results.