Formulation of line-to-line voltage total harmonic distortion of two-level inverter with low switching frequency

THD is the most common criterion to determine the harmonic content of each waveform. Conventional methods, which are usually used, result in approximate answers due to their inability to take all harmonic orders into account. This paper focuses on the three-phase two-level inverter and proposes two methods, founded on the basic definition of THD, to calculate its line voltage THD, precisely. The first method considers stepped essence of line voltage and the second uses its Fourier coefficients. By proposed methods, all harmonic orders of two-level inverter with any switching angles, named n-degree of freedom, are considered. Another feature of the introduced methods is deriving analytical formulas for line voltage THD. This helps some switching strategies like THD Minimisation (THDM) to get rid of heuristic methods, which find those switching angles that globally minimise THD. Superiority of the proposed methods for one and two degrees of freedom is verified by both numeric and graphic results, simulated in MATLAB. The comparison table is only used for higher degrees of freedom, because of impossibility of plotting n-dimensional space. Finally, experimental results for two sets of switching angles are compared with the results of proposed methods to evaluate the accuracy of introduced methods.