A new multilevel inverter based on single DC input source and zig-zag connected transformers

This paper proposes an isolated multilevel inverter composed of four standard two-level voltage source inverters (modules), a single dc source and a specially connected three-phase transformer, zig-zag-connected transformer. The proposed configuration can reduce the number of transformers and current and voltage stresses compared with traditional three-phase multilevel inverters with same lower rating. Moreover, only 24h±1 characteristic harmonic components appear in output voltage with each inverter operating in square-wave mode. Therefore, the switching frequency is reduced dramatically. For further improving output waveform, selective harmonic elimination (SHE) strategy with five notch angles is applied to the proposed inverters to eliminate the lowest four 24h ± 1 characteristic harmonics. The solutions of notch angles for a wide range of the modulation index are solved and built in a DSP-based controller for implementing the proposed inverter system. Compared to the traditional SHE strategy that eliminates non-triplen odd harmonics, the proposed inverter provides lower switching frequency and more voltage levels. A 6 kW laboratory prototype is implemented to verify the validity of the proposed configuration.