Selective harmonic elimination in multi-level inverter with zig-zag connection transformers

This study applies the selective harmonic elimination (SHE) technique to determine the switching angles for a multi-level inverter that cooperated with specially connected transformers. With the ability of directly controlling harmonics, the SHE technique has the adaptability to associate with apparatus which are congenitally immune to specific harmonics, such as the zig-zag connection transformers. In this study, the two sets of primary windings of the transformers are supplied by two six-switch full-bridge inverters with 30 electrical degrees phase shift. Harmonics with orders other than 12h ± 1 (h is positive integral) will not appear in the output voltages in the secondary side of the transformer because they are prohibited by the transformers. Then, the SHE technique with half-wave symmetry is employed to eliminate some selected harmonics with orders equal to 12h ± 1, and adjusts the amplitude of the fundamental. This study obtains a set of precise switching angles through offline calculation over a wide range of modulation indices. A digital signal processor is used to implement online calculation of the switching angles by an approximate method derived from the precise solutions. Some selected analysis and experimental results are demonstrated in this paper. A 4 kW prototype is built to verify the validity of the proposed system.