Output voltage distortion characterization in multilevel PWM converters

One of the main features to consider in the development of new pulsewidth modulations (PWM) for multilevel converters is the high-frequency output-voltage distortion. In this letter, a novel per-switching-cycle figure, the harmonic distortion of order n for switching cycle k(HD_n,k), is introduced to quantitatively characterize the output three-phase voltage harmonic distortion of multilevel converters around all the integer multiples of the switching frequency. This figure allows for the decomposing of the modulation design problem within an output voltage fundamental cycle into an independent set of smaller problems for every switching cycle. The expression of HD_n,k as a function of the switching states´ duty-ratio is presented for the three-level three-phase neutral-point-clamped voltage source inverter and it can be easily obtained for any other multilevel converter. From the evaluation of HD_n,k over 1/6th of the output-voltage fundamental-period the value of HD_n is obtained, providing a measure of the output voltage distortion in a fundamental period. This information is obtained at a lower computational cost than conventional fast Fourier transform (FFT) analysis. The accuracy of the HD_n distortion predictions is verified by comparing it to FFT-based results obtained from simulation and experiments. The expression to compute the total harmonic distortion (THD) as a function of HD_n is also derived.