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In this paper, an overview of three-phase inverter topologies for supplying non-symmetrical, non- linear and single phase loads is presented. Today non-symmetrical and non-linear loads dominate in electric power systems. This is even more evident in the isolated systems like uninterruptible power supply, renewable energy sources operating in island mode or auxiliary power supply in railway vehicles. Three-phase inverters therefore must be able to supply three-phase as well as single-phase loads and at the same time not affect the power quality. There are currently three inverter topologies capable of doing this. A special emphasis is given on a three-phase four- leg inverter topology. In addition, two of the most used modulation techniques for three-phase four-leg inverter are described. Mathematical model in abc and dq0 coordinate system is also derived. At the end, the paper outlines some of the existing control design challenges and their solutions.