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Title: MOSFET-based MMC for low-voltage DC distribution networks
Author: Zhong, Yanni
ISNI:       0000 0004 5991 6400
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2016
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The increased load demand and the development of distributed generation technologies present challenges to the existing low-voltage power distribution networks. In particular, numbers of high-capacity power electronic interfaces, such as electric vehicle (EV) chargers and embedded photovoltaic (PV) generation, have increased significantly. In comparison with conventional AC systems, low-voltage DC (LVDC) systems offer several potential benefits, including improved utilisation of cable voltage ratings, and elimination of reactive current and skin effect issues. LVDC distribution also complements the growth of power electronic loads having an implicit DC stage as part of their grid interface. However the DC-AC conversion stage at the customer end is one of the main challenges for LVDC distribution due to the widespread existence of AC loads. To overcome this limit, a high-performance modular multilevel converter (MMC) with parallel-connected MOSFETs is proposed in this thesis. It allows the converter to operate at relatively high voltage with low harmonic content, without the use of large AC filters. MMC also has low switching frequency, and facilitates the use of MOSFETs with the feature of synchronous rectification which provide lower conduction loss and allows parallel-connection to further reduce the losses. Power losses are calculated to show that the efficiency of MMC can exceed that of a conventional 2-level converter. Comparative analysis was carried out for a conventional 2-level converter, a SiC MOSFET 2-level converter, a Si MOSFET MMC and a GaN HEMT MMC, in terms of power loss, power quality, converter cost, and heat sink size. The analysis suggests that the 5-level MMC with parallel-connected Si MOSFETs may be an efficient alternative for this LVDC application. The optimal number of parallel-connected MOSFETs was then investigated. In addition, thermal measurement was developed to verify the loss calculation. A detailed converter design was conducted with current control methods to eliminate circulating current distortion for single-phase MOSFET MMC. Then a single-phase 5-level MMC prototype was built to validate the control methods proposed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral