Embodiments of the present disclosure disclose a main bus-bar. The main bus-bar consists of a first elongated flat portion and a second elongated flat portion. A longitudinal side of the first elongated flat portion and a longitudinal side of the second elongated flat portion are arranged to be joined with each other at a predetermined angle. The thickness of each of said the first elongated flat portion and the second elongated flat portion is non-uniform. With embodiments of the present disclosure, the skin effect and proximity effect caused by current carried in the main bus-bar can be reduced greatly. Embodiments of the present disclosure also disclose a transfer connector, a bus-bar assembly and a bus-bar system.