Comparative Analysis of Multilevel-High-Frequency-Link and Multilevel-DC-Link DC-DC Transformers Based on MMC and Dual-Active Bridge for MVDC Application

This paper gives a comprehensive comparison of multilevel high-frequency-link (HFL) dc transformer (abbreviated MDCT) based on modular multilevel converter (MMC) and multilevel-dc-link dc transformer (abbreviated ADCT) based on dual-active-bridge (DAB) for medium-voltage dc (MVDC) application. The topology, operation, HFL voltage and current, active and circulating power, characteristic currents, switching behaviors, and power loss are analyzed in detail. Both MDCT and ADCT have fault treatment ability for MVDC application. However, the installation and commissioning of ADCT are more flexible and simpler. In addition, the MDCT needs more switches and high-frequency (HF) inductors than ADCT in the MVDC side with the same voltage level, but the number of HF transformer can be reduced. Compared with MDCT, the ADCT has higher power transfer ability and lower circulating power, lower HFL voltage, and RMS current and peak values with the same transmission power in the MVDC side. However, the arm RMS and average currents of MDCT are lower than those of ADCT in the MVDC side. The switching performance of MDCT deteriorates when the MVDC voltage fluctuates, but the switching behaviors of half bridges are added to ADCT. The loss of ADCT almost keep the same when the MVDC voltage fluctuates, but the loss of MDCT changes a lot. Finally, a small-scale prototype platform with 1 kW/450 V/150 V is built and experimental results verify the correctness and effectiveness of the theoretical analysis.