Abstract
The isolated modular multilevel dc–dc converter (IMMDCC) constructed by two modular multilevel converters (MMCs) and a medium frequency transformer is attractive for medium- and high-voltage dc-grid applications by offering the following features: the modular multilevel structure can handle high voltages with the series connection of lower voltage submodules; the MMC operating manner reduces the current stress of the switches by dividing the ac currents among the phase-legs; using the square-wave modulation scheme can increase the power capacity meanwhile, achieve zero-voltage switching operation. In the square-wave modulation scheme of IMMDCC, the voltage balancing control of the floating capacitors needs further investigation since the unmatched losses and circuit parameters lead to the deviation of the voltages. In this study, a capacitor voltage balancing control strategy that does not rely on the detection of the arm currents is presented by introducing small delay times among the switching pulses of the submodules, meanwhile dv/dt in the ac link is reduced by doing so. Soft-switching boundary analysis imposes constraints on the balancing method to achieve zero-voltage switching of the converter. Experimental results have confirmed the correctness of the theoretical analysis as well as the effectiveness of the proposed control strategy.
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