Journal
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS
Volume 65, Issue 9, Pages 7060-7071Publisher
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/TIE.2018.2793249
Keywords
Compact volume; dc grids; high step-ratio; modular multilevel converter (MMC)
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Funding
- Reconfigurable Distribution Networks project under EPSRC [EP/K036327/1]
- Delta Foundation [DREG2016009]
- Engineering and Physical Sciences Research Council [EP/K036327/1] Funding Source: researchfish
- EPSRC [EP/K036327/1] Funding Source: UKRI
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In multi-terminal dc networks or future dc grids, there is an important role for high step-ratio dcdc conversion to interface a high-voltage network to lower voltage infeeds or offtakes. The efficiency and controllability of dc-dc conversion will be expected to be similar to modular multilevel ac-dc converters. This paper presents a modular multilevel dc-dc converter with a high stepratio for medium-voltage and high-voltage applications. Its topology on the high-voltage side is derived from the halfbridge single-phase inverter with stacks of sub-modules (SMs) replacing each of the switch positions. A near-squarewave current operation is proposed, which achieves nearconstant instantaneous power for single-phase conversion, leading to reduced stack capacitor and filter volume and also increasing the power device utilization. A controller for energy balancing and current tracking is designed. The soft-switching operation on the low-voltage side is demonstrated. The high step-ratio is accomplished by combination of inherent half-bridge ratio, SM stack modulation, and transformer turns ratio, which also offers flexibility to satisfy wide-range conversion requirements. The theoretical analysis of this converter is verified by simulation of a full-scale 40 MW, 200 kV converter with 146 SMs and also through experimental testing of a down-scaled prototype at 4.5 kW, 1.5 kV with 18 SMs.
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