Minimisations of total harmonic distortion in cascaded transformers multilevel inverter by modifying turn ratios of the transformers and input voltage regulation

A total harmonic distortion (THD) minimisation technique for cascaded transformers multilevel inverters is presented. In the proposed method, turn ratios of the transformers are optimised along with switching angles in order to minimise the THD of the inverter output voltage. Assuming turn ratios of the transformers as variables adds extra degrees of freedom to the equations. To obtain high-quality output voltage with an acceptable THD in lower modulation indices, the input voltage is also regulated. Genetic algorithm is applied to minimise THD as well as satisfy fundamental component of the output voltage. To verify the feasibility of the proposed approach, a nine-level cascaded transformer inverter prototype has been implemented. Simulation and experimental results show the effectiveness of the proposed technique in THD minimisation and generation of the desired fundamental component at output voltage.