MMC with parallel-connected MOSFETs as an alternative to wide bandgap converters for LVDC distribution networks

• Main Authors: Yanni Zhong, Nina M. Roscoe, Derrick Holliday, Stephen J. Finney
• Format: Article
• Language: English
• Published: Wiley 2017-03-01
• Series: The Journal of Engineering
• Subjects: distribution networks; power factor; AC-DC power convertors; gallium compounds; III-V semiconductors; wide band gap semiconductors; power MOSFET; MMC; MOSFET; wide bandgap converters; LVDC distribution networks; low-voltage direct-current networks; conductor utilisation; LVDC networks; DC–AC converters; unity power factor AC-DC converters; AC–DC/DC–AC converter design; conversion loss; power quality; insulated-gate bipolar transistor; two-level converter; metal–oxide–semiconductor field-effect transistor; modular multi-level converter; high-electron-mobility transistor; power loss; reliability; fault tolerance; converter cost; heatsink size; device stress; electromagnetic interference; SiC; GaN
• Online Access: http://digital-library.theiet.org/content/journals/10.1049/joe.2017.0073

Low-voltage direct-current (LVDC) networks offer improved conductor utilisation on existing infrastructure and reduced conversion stages, which can lead to a simpler and more efficient distribution network. However, LVDC networks must continue to support AC loads, requiring efficient, low-distortion DC–AC converters. Additionally, increasing numbers of DC loads on the LVAC network require controlled, low-distortion, unity power factor AC-DC converters with large capacity, and bi-directional capability. An AC–DC/DC–AC converter design is therefore proposed in this study to minimise conversion loss and maximise power quality. Comparative analysis is performed for a conventional IGBT two-level converter, a SiC MOSFET two-level converter, a Si MOSFET modular multi-level converter (MMC) and a GaN HEMT MMC, in terms of power loss, reliability, fault tolerance, converter cost and heatsink size. The analysis indicates that the five-level MMC with parallel-connected Si MOSFETs is an efficient, cost-effective converter for low-voltage converter applications. MMC converters suffer negligible switching loss, which enables reduced device switching without loss penalty from increased harmonics and filtering. Optimal extent of parallel-connection for MOSFETs in an MMC is investigated. Experimental results are presented to show the reduction in device stress and electromagnetic interference generating transients through the use of reduced switching and device parallel-connection.