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Study on application of multi-level converter to realize fast current control in DC micro-grid with extremely low impedance interconnections

DC micro-grid has been actively studied as a next-generation small-scale electric power network. In such small-scale DC power grid, individual line impedance between elements such as power supplies, batteries, and loads is very low because of short distance distribution. Moreover, reactive power cannot be utilized to control power flow in the DC grids unlike the AC grids. It causes that large current flows through the lines even by a slight voltage difference. Therefore, power flow controllers with abilities to realize extremely high-speed and high-precision power flow control are required in the small-scale power grid. In this paper, multi-level converters are applied as the power flow controllers and realizes high-speed and high-precision power flow in the DC micro-grid. In the multi-level converter, as the number of the output levels increases, the output filter can become smaller. This paper also presents how to design the LC filter considering the number of the output levels of the converter to satisfy a requirement of current ripple. It is experimentally verified that the multi-level converter with a smaller filter can realize the high-speed and high-precision power flow control under the condition of the low line impedance compared with the 2-level converter.