Abstract
Direct current (Dc) networks have proven advantages in high voltage direct current (HVDC) transmission systems, and now they are expanding to medium- and low-voltage distribution networks. One of the major challenges is to develop reliable dc-dc voltage transformation achieving high efficiency and performance, especially at high voltage and high step ratio. New resonant modular multilevel topologies have arisen as an alternative, mainly because of advantages such as optional use of transformers, natural voltage balance, simple control, and soft-switching capability. However, this type of operation generates a high peak current, does not allow control of power flow in all power range, and has a limited range of voltage variation. This article proposes an asymmetrical triangular current mode applied to high step ratio modular multilevel dc-dc converters. The proposed modulation increases the efficiency and achieves bidirectional control of the power, soft-switching, and a natural balance of the voltage in the cell capacitors. The experimental results show the bidirectional operation and the capacitor voltage balance of the converter under different operating conditions with higher efficiency (97.72%) and lower peak current compared to previous reports of this topology using resonant operation.
Highlights
D IRECT current has important advantages over alternating current in the transmission of electrical energy, such as lower power losses, less and smaller cables and absent of reactive power nor peak voltages
The dc-dc conversion that allows the interconnection of high voltage direct current (HVDC), MVDC and LVDC systems is a key element for the development of the networks of the future
This paper presents a new asymmetrical triangular current mode (ATCM) for high step ratio dc-dc modular multilevel converter (MMC), which achieves bidirectional power flow through a simple control law, maintains ZCS operation, reduces the peak current compared to resonant mode, and presents natural balance of the voltage in the cell capacitors without the need of complex control strategies [29]
Summary
D IRECT current has important advantages over alternating current in the transmission of electrical energy, such as lower power losses, less and smaller cables and absent of reactive power nor peak voltages. The FTF-MMC uses two MMCs connected through a transformer and operates at medium-frequency to reduce the size of the transformer and the passive elements of the converter [14] This solution provides full bidirectional power flow control and galvanic isolation, but it requires a complex voltage control and hard-switching operation that reduce efficiency, especially in high step ratio voltage transformation. This paper presents a new asymmetrical triangular current mode (ATCM) for high step ratio dc-dc MMC, which achieves bidirectional power flow through a simple control law, maintains ZCS operation, reduces the peak current compared to resonant mode, and presents natural balance of the voltage in the cell capacitors without the need of complex control strategies [29]. The resistance req represents the total conduction loss resistance in the converter generated by the inductor, switches, capacitors and wire
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