| High voltage dc(HVDC) transmission has become a mature technology to efficiently transfer large power across long distance. With the development of HVDC transmission system, multi-terminal HVDC has been attracting more and more attentions. To connect multiple HVDC systems with different rated voltage, a dc/dc converter is indispensable. As the voltage level is high, traditional dc/dc converter is not suitable due to the limitation of rated voltage of electronic switch. This thesis proposes a family of dc/dc converters for HVDC system, including the topologies, control strategy and parameters design.This thesis firstly derives two types of 2-chain-link dc/dc converter(2CL-DCC) from the modular multi-level converter(MMC). Based on the 2CL-DCCs, a 3-chain-link dc/dc converter(3CL-DCC) is derived to achieve the function of both step-up and step-down. A special case of 3CL-DCC is proposed, which can eliminate the ac component in the output current and significantly reduce the output-side inductor as compared to the 2CL-DCC. Additionally, a novel operation mode with square ac waveforms is proposed, which offers the merit of smaller ac current component compared with the operation with sinusoid ac waveform.The issue of the voltage balance of the submodule capacitor voltages in 2CL-DCC is analyzed under the carrier phase shifting(CPS) modulation, and based on which, the AC phase difference control strategy and the voltage balancing control strategy are proposed. The AC phase difference control strategy can force the ratio between the average value of the submodule capacitor voltages in two chain links to follow its given value, and the voltage balancing control strategy can ensure the balance of the submodule capacitor voltages in the same chain link.Finally, the simulations of 2CL-DCC and 3CL-DCC are given, and a 1 kW prototype of 2CL-DCC is built. The simulated and experimental results verify the effectiveness of the proposed control strategy. |
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