Bifurcate modular multilevel converter for low-modulation-ratio applications

Yiran Chang; Xu Cai; Jianwen Zhang; Gang Shi

Bifurcate modular multilevel converter for low-modulation-ratio applications

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Author(s): Yiran Chang ¹ ; Xu Cai ¹ ; Jianwen Zhang ¹ ; Gang Shi ¹
- Affiliations: 1: Wind Power Research Centre, Department of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai, People's Republic of China
Source: Volume 9, Issue 2, 10 February 2016, p. 145 – 154
DOI: 10.1049/iet-pel.2015.0387 , Print ISSN 1755-4535, Online ISSN 1755-4543

Received 28/05/2015, Accepted 05/01/2016, Revised 05/10/2015, Published 10/02/2016

In this study, a novel modular multilevel converter (MMC) topology for low-modulation applications is proposed. The proposed topology has two three-phase AC ports and is characterised by its bifurcate arms and phase-shifted connection on the AC side. Conventional MMCs have been widely used in high-voltage direct current (HVDC) applications and are able to connect a low-voltage AC source to a high-voltage DC source, which makes them useful in applications such as DC/AC distribution systems and DC collection of renewable energy. However, high arm currents and high energy fluctuation caused by low-modulation ratio will increase converter losses and submodule capacitance. In the proposed bifurcate MMC, each arm has three branches and the branch current is lower than the arm current of the conventional MMC, so the difficulty of designing an MMC with low-modulation ratio is relieved to some extent. Theoretical analysis proves the stability of bifurcate MMCs and provides reference voltage solutions for each branch. A control scheme for a bifurcate MMC is also proposed, and further analysis shows that the circulating current is naturally suppressed under certain conditions. Detailed simulation is performed based on the RT-LAB platform and confirms the analysis results.

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Bifurcate modular multilevel converter for low-modulation-ratio applications

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