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Harmonic emission to the network must be restricted below the predefined limits, avoiding power quality problems. Total harmonic distortion (THD) is a well-known criterion, which is used to determine harmonic content of each waveform, and extracting its exact value is very important. Three-level inverters produce a significant amount of harmonics and the exact closedform expression for their line-to-line voltage THD has not yet been derived. All presented methods in this regard are based on an approximate approach. This study calculates line-to-line voltage THD of a three-level inverter by two different methods and presents their relevant formulas. The first method is usable for any number of controlled PWM switching angles, but the second method is limited to low number of switching angles. By the presented mathematical formulas in this study, the THD is computed simply, accurately and rapidly. The proposed methods are compared with the conventional methods and their advantages are verified by simulation and experimental results.

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