Multilevel inverter for power system applications highlighting asymmetric design effects from a supply network point of view

This paper highlights some effects of an asymmetrical design on three phase multilevel inverters from a supply network point of view. The investigated topology is based on a series connection of single-phase inverters (partial cell) in each phase. Each partial cell is fed through a three-phase diode rectifier for a unidirectional power flow, fed itself through the secondary of the power transformer (PT). From analytical relationships in continuous and discontinuous current conduction mode, it´s shown that a symmetrical multilevel inverter has a smaller total harmonic distortion than an asymmetrical multilevel inverter (AMI) with the same number of partial cells per phase. An AMI is not more interesting than a classical three-phase inverter. The total harmonic distortion of an AMI is compatible to the recommended IEEE std 519-1992 for all feeding possibilities for a given number of partial cells per phase. In order to exploit the main advantages of an asymmetrical design from a load point of view (generation of a high resolution voltage phasor, reduction of the number of power devices for the same resolution, flexibility for the de-voltage feeding choice), a new solution is investigated: A symmetrical PTfeeds an AMI.