Comparison between approximate and accurate calculation of line voltage THD in multilevel inverters with unequal DC sources

Multilevel inverters, the next generation of DC-AC inverters, are widely used in industrial applications especially in high voltage and power. Multilevel inverters include several DC sources which are not equal in practice. Output voltage waveform of these inverters contains harmonic contents. Total Harmonic Distortion (THD) is the most used index to evaluate harmonic content of any waveform. Calculation of line voltage THD by conventional formula leads to an inexact answer, so a novel method is introduced in this paper to calculate accurate line voltage THD of multilevel inverter with non-equal DC sources. This method can be implemented to each multilevel inverter with any number of levels. To show its superiority, line voltage THD of 5, 11 and 15-level inverters are calculated by proposed method and conventional formula and then, compared to each other. To verify the validity of proposed method, harmonics up to 10⁶ order are considered in approximate method to have an accurate index for comparison. MATLAB software is used for these calculations.