Flexible Medium Voltage DC Source Utilizing Series Connected Modular Multilevel Converters

This paper presents the converter configuration acting as a medium voltage DC source. In order to obtain desired voltage across the DC terminals, two series connected modular multilevel converters are used. The use of modular multilevel converters results in high degrees of modularity, flexibility, reliability, current limiting capability during DC faults, as well as the outstanding operational performances. Each modular multilevel converter is connected to the 3.3 kV AC supply, while ensuring arbitrary voltage in the range of +/- 5 kV across its DC terminals. Consequently, series connection of two power units allows for +/- 10 kV to be synthesized across the converter's DC terminals. Moreover, there is a freedom of choice with regards to the way that two modular multilevel converters share the output DC voltage reference. Therefore, studies focusing on converter dynamics and efficiency for various voltage sharing profiles are conducted. As a result, the most optimal voltage sharing profile is generated, covering the whole operating range of the converter.