Optimized Control Strategy of Modular Multilevel DC Transformer for High-Frequency-Link Voltages Matching in the Whole Operation Range

High-frequency modular multilevel dc transformer (MDCT) plays an important role in the voltage conversion and power transmission for medium-voltage direct-current (MVdc) power networks application. For MDCT, when high-frequency-link (HFL) voltage amplitudes do not match the HFL transformer ratio, the increase in HFL reactive power and the decrease in efficiency become a bottleneck for its application. To solve this problem, a modified MDCT (M2DCT) is analyzed, and a HFL voltage optimized control (HVOC) for M2DCT is proposed in this paper. Different from the conventional phase shift strategies, the proposed HVOC adjusts the HFL voltage amplitudes by varying the pulsewidth of modulation wave for power switches in submodule (SM), ensuring HFL voltage amplitudes match the HFL transformer ratio in the whole operation range, reducing the HFL reactive power, and improving the efficiency of M2DCT. In addition, the soft switching is achieved in the whole operation range under HVOC. The detailed control method, architecture, SM voltages balancing algorithm, and switching characteristics of HVOC are discussed, and the influences on power transmission capacity, HFL power factor, operating efficiency of M2DCT under HVOC, and conventional HFL phase shift controls are analyzed comparatively. The experimental results verify the correctness and effectiveness of the analysis and proposed strategy.