Abstract
The modular multilevel high-frequency DC transformer (MDCT) plays an important role in the bus connection, voltage conversion, power transmission and electrical isolation in the DC distribution networks. During practical operation, the mismatch between high-frequency-link (HFL) voltages and HFL transformer voltage ratio caused by the variation of DC bus voltages occurs, resulting in the increased reactive power, larger current stress and decreased efficiency in MDCT. To solve this problem, an adaptive modulation strategy (AMS) is proposed in this paper. The proposed AMS adjusts HFL voltages by varying the number of inserted sub-modules adaptively, ensuring the HFL voltages match HFL transformer voltage ratio. Compared with the conventional phase-shift strategies, the proposed strategy obtains higher transmission power capacity and HFL power factor, and it maintains the ideal efficiency of MDCT under light load situation. Besides, the proposed strategy avoids the optimal phase-shift angle calculation containing numerous parameters, reducing computing burden and improving practicality for MDCT. Moreover, the hybrid-wave modulation in proposed strategy makes HFL voltages compose of quasi-square-wave and square-wave, addressing the $\text{d}v/\text{d}t$ stress problem and ensuring the higher DC voltage utilization and power transmission capability simultaneously. The mathematical model, operation principle and control architecture of proposed AMS are investigated, and the experimental results in a MDCT prototype verify the correctness and effectiveness of the analysis and proposed strategy.
Highlights
With the development of distributed renewable power sources, energy storage devices and direct-current (DC) electrical appliances, DC distribution network becomes a preferred solution with higher power conversion efficiency, avoidance of frequency and phase synchronization issues, reduction of power converters, decrease of power loss and component cost and better performances in scalability and stability [1], [2]
As can be seen from (10), under the proposed adaptive modulation strategy, the va can be regulated by varying the number of inserted SMs according to the variation of DC voltages in multilevel high-frequency DC transformer (MDCT) adaptively, ensuring the HFL voltages match HFL transformer voltage ratio, reducing the current stress, HFL reactive power and power loss for MDCT
Considering the HFL voltage adjusting operation with varied number of inserted SMs and the hybrid-wave modulation composed of quasi-square-wave and square-wave under proposed adaptive modulation strategy (AMS), the HFL voltages va and vb can be described as:
Summary
With the development of distributed renewable power sources, energy storage devices and direct-current (DC) electrical appliances, DC distribution network becomes a preferred solution with higher power conversion efficiency, avoidance of frequency and phase synchronization issues, reduction of power converters, decrease of power loss and component cost and better performances in scalability and stability [1], [2]. The terms of staircase rising and falling transition in quasi-square-wave method is small compared with the phase-shift angle between primary and secondary HFL voltages, it dose reduce the DC voltage utilization and degrade power transmission capability of DC transformers [31], [32] To address these problems, benefit from the asymmetric topology of MDCT and its applied occasion connecting the LVDC and MVDC links, a hybrid-wave HFL modulation compose of quasi-square-wave and square-wave modulation is investigated in proposed AMS, addressing the dv/dt stress problem on HFL transformer and ensuring high DC voltage utilization and power transmit capability simultaneously. As can be seen from (10), under the proposed adaptive modulation strategy, the va can be regulated by varying the number of inserted SMs according to the variation of DC voltages in MDCT adaptively, ensuring the HFL voltages match HFL transformer voltage ratio, reducing the current stress, HFL reactive power and power loss for MDCT
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