Asymmetric Quasi-Square Wave Modulation for T-Type Transformerless Modular Multilevel DC-DC Converter with Reduced Current Stress

The modular multilevel DC-DC converter (MMDC) has seen increasing interest due to its potential in supporting renewable energy integration. Among the various configurations, the T-type transformerless MMDC offers advantages such as enhanced flexibility in power regulation and effective faultblocking capabilities, making it a suitable solution for connecting DC systems with different voltage levels. In this work, an innovative modulation technique, namely the asymmetric quasisquare wave (AQSW) method, is introduced for the T-type transformerless MMDC. This approach reduces current stress compared to traditional modulation techniques, thus improving the overall system efficiency. The operating principles of the Ttype transformerless MMDC are explained in detail, and the design parameters for the AQSW modulation method are outlined. Additionally, the current stress reduction achieved with the AQSW scheme is analyzed and compared with existing modulation techniques, demonstrating a notable improvement. The theoretical predictions are validated through <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$320 \text{kV} / 100 \text{kV} {@} 0.5 \text{GW}$</tex> simulations.