Design of Modular Multilevel Converters for the Shipnet in Medium Voltage DC All-Electric Ships

This paper presents a Modular Multilevel Converter (MMC) as a potential candidate to supply the onboard Low Voltage (LV) AC Shipnet in All-Electric Ship consisting of Medium Voltage (MV) DC main power system. Models of the MMC are presented together with the LVAC Shipnet including integration of harmonic filter, step-down transformer and LV distribution. A control with circulating current suppression and capacitor voltage balancing is designed for a 1.25 MVA MMC. The simulation results validates the design of control and show that the increased number of Sub Modules (SM) improve the voltage quality while the complexity of the system will also be increased. A detailed comparison of voltage quality at different loads and the system capabilities to handle step load, load shedding, and unbalanced loads are performed using simulation. These give positive indications on the use of the MMC to supply the LVAC Shipnet and guidance of design details.