Submodule Voltage Balancing Technique of Modular Multilevel Dual-Active-Bridge DC-DC Converter for DC Solid-State Transformers

Isolated modular multilevel dual-active-bridge (M2DAB) DC-DC converters are a promising topology for interfacing medium-voltage (MV) DC grids with other DC grids due to their excellent voltage scalability. These converters generate quasi-two-level (Q2L) voltage waveforms to achieve high power density with a medium-frequency transformer (MFT) while mitigating high dv/dt stress that could reduce the lifetime of MFT. Similar to conventional modular multilevel converters (MMCs), maintaining submodule (SM) voltage balance is critical in M2DAB converters. However, since M2DAB converters output voltage and current at the switching frequency, voltage balancing must be integrated into the modulation strategy. Although conventional modulation techniques are used alongside rotation mechanisms to address voltage balancing, non-identical SM characteristics and rapid changes in output power often cause significant SM voltage imbalances. To overcome these challenges, this paper proposes a novel voltage balancing technique using a sorting algorithm with a single carrier modulation (SCM). The proposed method relies on SM voltage magnitudes to achieve consistent balancing even under arm current variations. It also updates PWM duty cycles twice per switching period, enabling faster responses than conventional methods. Performance of the proposed balancing technique is validated through simulation results comparing with other methods.