Impact of Sampling Frequency on Harmonic Distortion for Modular Multilevel Converter

This paper applies nearest level control (NLC) to the modular multilevel converter (MMC). Since there are a number of submodules (SMs) in high-voltage applications of MMC and all SM voltages are required to be measured and sorted, if the uniform sampling frequency is not high enough, the SM will not be triggered as quickly as possible. Thus, the converter-output voltage levels and voltage harmonics will be affected. A method to systematically analyze the voltage harmonics is presented, considering the uniform sampling frequency. In order to select a proper sampling frequency, two critical values, which significantly influence the output voltage levels and voltage total harmonic distortions, are calculated. Simulation results based on PSCAD/EMTDC proved the validity of the proposed modulation scheme and the importance of a properly selected sampling frequency.