Modular and scalable DC-DC converters for medium-/high-power applications

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Yang, Heng
Saeedifard, Maryam
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In recent years, advanced MVDC (medium-voltage direct-current) and HVDC (high-voltage direct-current) power collection and transmission are becoming increasingly important in power systems. As the DC grids evolve, the interconnection of DC grids will become essential in the future. A DC-DC converter that is suitable for high-voltage/power is a key enabling technology for future DC networks as power flow control and voltage adjustment must be achieved. The DC-DC Modular Multilevel Converter (DC MMC) which has originated from the AC-DC MMC circuit topology, is an attractive converter topology for interconnection of medium-/high-voltage DC grids. The objective of this research is to address the technical challenges associated with the operation and control of the DC MMC. To this end, a mathematical model of the DC MMC is proposed to determine the AC and DC components of the arm current, phase current, and Sub-Module (SM) capacitor voltage ripple in steady state. Based on the proposed model, this thesis presents the design considerations for the DC MMC to meet the electrical specifications while satisfying the design constraints. The accuracy of the developed model and the effectiveness of the design approach are validated based on the simulation studies in the PSCAD/EMTDC software environment. Proper operation of the DC MMC necessitates injection of an AC circulating current to maintain its SM capacitor voltages balanced. The AC circulating current, however, needs to be minimized for efficiency improvement. This thesis proposes a closed-loop control strategy for the half-bridge SM based DC MMC to simultaneously regulate the output DC-link voltage/current, maintain the SM capacitor voltages balanced, and minimize the AC circulating current for arbitrary voltage conversion ratio and power throughput. To address the power derating issue of the DC MMC, an enhanced control strategy is developed, which in conjunction with the full-bridge SMs, increases the power transfer capability and reduces the AC circulating current. A laboratory prototype is developed to experimentally validate the proposed control strategies.
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