Abstract The MMC-H solid-state transformer (SST) uses one high-frequency transformer, greatly reducing the system volume and cost, which is suitable for high to low-voltage conversion scenarios such as distributed energy access and electric vehicle charging stations. However, SST, including MMC-H, typically uses symmetrical square wave modulation to improve transmission power capabilities, leading to the high content of current harmonics. Besides, considering the high voltage level on the MMC side, square wave modulation can make MMC-H experience extremely high dv/dt stress, posing a severe challenge to transformer insulation. Therefore, this paper proposes two asymmetric modulation methods based on MMC topology and studies the current and power variation characteristics of MMC-H SST. Firstly, this paper proposes the sine-square and triangle-square modulation for MMC-H SST. Based on the proposed SST circuit equivalent model, the power and current expressions for MMC-H under the proposed modulation are presented. The variation of transmission current with power is analyzed, and the reasons for the current difference between different modulations are elucidated. Finally, this paper proposes the overall closed-loop control scheme of MMC-H under asymmetric modulation. The control method includes output voltage closed-loop control, MMC high-frequency nearest level modulation method, and MMC submodules voltage balance control. The power characteristic curve and current waveform are verified on Matlab/Simulink. Simulation results show that the average errors of the transmission power model with the proposed two modulations are 2.8% and 3.4%, respectively. A hardware-in-the-loop (HIL) experimental platform based on the FPGA and DSP control board has been constructed. The experiment verifies the voltage, current waveform, current peak, and harmonic content of the system at different transmission power. The average error of the proposed modeling method is 5.71%. In addition, dynamic experiments are conducted on the SST system, and the MMC-H SST still has good dynamic stability and fast response ability at asymmetric modulation. Finally, a comparison is made between the proposed modulation and traditional square wave modulation. Meanwhile, the losses and efficiency of the 1 MW MMC-H SST system are calculated. The following conclusions are drawn from simulation and experimental results. (1) Compared with the traditional square wave modulation, the proposed two modulations reduce the MMC-H system dv/dt stress and current harmonics. (2) The proposed sine-square modulation achieves optimal overall system performance in terms of power variation, current waveform, peak value, and harmonic content. (3) Under steady state and dynamic conditions, the system is stable and consistent with the theoretical waveforms. Therefore, the proposed asymmetric method does not affect the system’s dynamic performance. This paper provides good theoretical support for the optimization and application promotion of the novel MMC-H SST.
Pan Jianyu,Du Yihao,Lin Wenqi等. Power Modeling and Operation Characteristics of Modular Multilevel Converter-H Solid State Transformer under Asymmetric Square Wave Modulation[J]. Transactions of China Electrotechnical Society, 2024, 39(24): 7793-7806.
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2024, Vol. 39 
